A large nitrogen supply often causes increased partitioning of biomass above-ground and more of this to leaves Charles-Edwards In a study by Boot and den Dubbelden with two perennial grasses showed that both species invested significantly less dry matter in their leaves and more in their roots root weight ratio, RWR when grown at low nitrogen supply.
Kwai reported a significant increase in total dry weight of four poplar clones as a result of the addition of N. She also reported a significant increase in shoot dry weight of the same clones resulting from increased N supply, and mentioned that as the rate of nitrogen increased so did the proportion of total tree weight that was allocated to the foliage. Among the four poplar clones she used in the study, P. Relative to accumulation of dry weight in the unstressed, control plants, restriction in the accumulation of dry weight in leaves in the nitrogen-stressed treatment was noticeable.
Accumulation of dry weight in the stem was also adversely affected. In contrast with the restriction of growth in the shoots, dry weight accumulation in roots was enhanced in the nitrogen-stress treatment Rufty et al. In their study of cotton Wullschleger and Oosterhuis indicated that N deficiency significantly reduced all the dry matter components of canopy development.
Most of the above findings are in agreement with the well established phenomenon that plants invest relatively more in their roots and less in their shoots when soil resources are growth- limiting Brouwer , and Bradshaw et al. However, in some cases there was little or no effect of N supply on the partitioning of biomass. For example Boot and den Dubblden observed modest differences in dry matter partitioning due to differences in nitrogen supply in a perennial grass species, while Sheriff and Namibar and Sheriff found little or no differences in partitioning of dry matter in some Eucalyptus trees due to N treatments.
The research on biomass allocation patterns has been refined by not only studying the allocation of biomass to shoots and roots, but also the morphological characteristics of leaves and roots Aerts et al. N availability and nitrogen use efficiency NUE Nutrient-use-efficiency is usually expressed as the amount of growth or the assimilation rate divided by the concentration of the nutrient in the appropriate tissue, single leaf for instantaneous photosynthetic measurements, all leaves for growth indices or biomass Chapin and Van Cleve Nitrogen use efficiency as an index of the efficient use of nitrogen by the plant or foliage in accumulating carbon has been the subject of many studies.
Pastor et al. They also mentioned that it seems logical that nutrients would be used more efficiently in nutrient-poor sites and that this efficient nutrient use could be important in the survival of individuals in such sites. NUE can change according to internal cycling of N. For instance Millard and Proe reported a reduction in NUE after the first flush of spring growth, when remobilization of N terminated, then NUE increased again during the second flush of growth, and finally decreased again in the autumn during the period of rapid N accumulation.
In growth analysis, growth is defined as the increase in dry weight of the plant or stand investigated Kvet et al. Various growth characteristics that describe the growth of the plants and their various parts as well as the relationship between the assimilatory apparatus and dry matter production are calculated from the primary values on which growth analysis is based.
Kvet et al. Relative growth rate RGR Relative growth rate at a given instant of time is the increase in plant material per unit of material present per unit of time. Kwai reported a significant increase in the relative growth rate of four poplar clones as a result of the addition of N. The organic plant nitrogen content was found to increase with increasing RGR Poorter Also he concluded that species with a high RGR allocate more biomass to the leaves which tend to have a higher N content than the stem and roots. Moreover, Boot and den Dubbelden found higher RGRs at high rather than at low nitrogen supply in two perennial grasses.
Net assimilation rate NAR Net assimilation rate also called unit leaf rate is a measure of the efficiency of the photosynthesis system or the rate of dry weight increase at any instant expressed on a leaf area basis. Unit leaf rate is a complex parameter involving biomass allocation, carbon content of the plant, and components of the carbon balance Evans and Poorter But some evidence exists for positive effects of mineral nutrients supply, especially of potassium on NAR Blackman In a study of the response of four poplar clones to added nutrients Kwai showed that net assimilation rates were significantly increased by phosphorus plus potassium but not by nitrogen.
Leaf area ratio LAR The other component of relative growth rate is leaf area ratio LAR which characterizes the relative size of the assimilatory apparatus. The leaf area per unit plant weight appears to be a crucial attribute determining the potential RGR of a species Poorter Leaf area ratio is considered a useful measure of differences between plants or stands, resulting from genetic or environmental factors, or from different treatments.
Seasonal changes in LAR usually reflect the interaction of ontogenetic factors average leaf age and position with respect to leaf area with environmental effects increase of LAR with shading, high levels of nitrogen and water supply, etc. The leaf area ratio LAR and leaf area index LAI showed a pronounced seasonal pattern and were for all species higher at high rather than at low nitrogen supply Aerts et al. Leaves with a low investment of dry weight per unit area will almost automatically have a low investment in photosynthetically active compounds and as a result, photosynthetic activity per unit area will be relatively low.
It is therefore not likely that these plants have a high NAR. He also mentioned that a low SLA may result in a high N concentration per unit leaf area and consequently a higher photosynthetic capacity. A low SLA is often the result of the accumulation of secondary compounds, like tannins, phenols and the quantitively most important lignin, and of starch Waring et al. Konings Also Reich and Walters found significant relationships between leaf nitrogen per unit leaf area and SLA in 18 of 23 Amazonian tree species which they studied, where all the 18 significant relationships had negative slopes, indicating that leaf nitrogen per unit leaf area increases with increasing leaf "thickness" decreasing SLA.
For instance Waring et al. The dry matter and N partitioning are linearly related to the nitrogen concentration of the whole plant and that specific leaf area is positively correlated with nitrogen content per unit leaf area Konings In a study of the response of four poplar clones to added nutrients, Kwai found that specific leaf area SLA was higher when N and PK were not applied.
The extent to which specific leaf area as well as leaf weight ratio and leaf area ratio is affected by the environment is different in different species Geyger Leaf weight ratio LWR Leaf weight ratio represents the average fraction of the total dry weight in the form of leaves. Leaf weight ratio increased with nitrogen and decreased with phosphorous plus potassium Kwai Boot and den Dubbelden reported a positive relationship between leaf weight ratio and N supply. Response of plants to N treatment Differences in the response of plants to N nutrition are determined by the N requirement for plant processes, the allocation of N within the plant, and the ability of the plant to take up N from the soil Bowman The composition of the plant reflects its nutritional status and the adequacy of its nutrient supplies and it has been shown that the percentage of nitrogen in the whole plant provides a useful index of nitrogen status.
For example the concentration of nitrogen in foliage was significantly increased by nitrogen treatment in poplar seedlings Kwai and in Douglas-fir trees Brix On the other hand, Wullschleger and Oosterhuis found no significant differences in N concentration of N-sufficient and N- deficient cotton leaves. C: N ratio Thornley mentioned that as N supply increased the roots will increase in size and the C: N ratio in the roots decreases.
This will be followed by a greater export of nitrogen from the roots to the shoots and, because the C: N ratio in the shoots then decreases, a reduced transport of assimilates to the roots Konings A high C: N ratio in the plant leads to an increase in carbon export to the roots but at a low C: N ratio leaf growth is promoted Brouwer and Trewavas The importance of water to plants Kramer summarized the four main functions for which water is essential for plants: 1.
Water is a reagent, essential in photosynthesis and in other hydraulic processes such as digestion of starch to sugar. Water is a solvent in which salts, gases, and other materials move into the plant and through the cell walls and xylem tissues to create a more or less continuous solvent system throughout the plant. Water is essential for turgidity, cell growth, maintenance of leaf form, operation of leaf stomata, and structural movement of the plant.
In order to build up a comprehensive understanding of the importance of water to plants we should give consideration to the effects of water deficits on plant growth. Kramer and Kozlowski concluded that the distribution of forests, their species composition and productivity are controlled chiefly by too little or too much water. Water deficit reduces plant growth and crop yield more than all the other stresses combined, because of its ubiquitous nature Kramer Development of water stress in plants The higher plants have developed specilized tissues and organs for the rapid uptake and transport of water.
Continuous absorption of water is essential to the growth and even the survival of most plants Kramer The water and solutes are absorbed by the root system and are conducted to all parts of the plant. Across parenchyma water passes by osmosis, while in the xylem there is a mass movement of solutions Thomas et al. Some of the water is retained, but much is lost by transpiration so that the internal water balance of plants is controlled by the relative rates of water absorption and water loss.
In moist soil, water absorption is controlled by the rate of transpiration, but in drying soil it is gradually reduced by the decreasing difference in water potential between roots and soil and the increasing resistance to water movement toward roots through drying soil Kramer The continuity of water in the conducting system provides an essential communication network between roots and shoots that keeps the rate of absorption and transpiration in balance. Thus, when transpiration increases, the demand for an increased water supply to the leaves is transmitted to the roots by a decrease in water potential in the xylem sap, causing an increase in absorption.
Conversely, when water absorption is reduced, the information quickly reaches the leaves as a decrease in water potential in the xylem sap, causing loss of guard cell turgor and closure of stomata.
This results in a compensating decrease in water loss by transpiration Kramer Leaf water potential has been used as a measure or index of water balance, and it is defined as the difference between the free energy status of water in the plant tissue and that of pure water at the same temperature and under normal atmospheric pressure. At zero water potential the plant's tissue are fully turgid and as more water is lost than absorbed the water potential becomes negative and decreases.
The turgor pressure of the cells also decreases, and when the water potential reaches the level at which cell turgor pressure is zero, the cell is flaccid and severely stressed Kramer According to Crafts plant water potential values depend on three parameters a excess of transpiration over absorption b soil moisture potential and c height of the plant. For absorption to occur, a gradient of decreasing negative water potential must exist from the root to leaf. Thus the plant water potential cannot be higher than the soil water potential.
As the soil dries out there is a corresponding drop in the level of plant water potential Slatyer The lower water potential in leaves provides a driving force for water movement out of adjacent organs, resulting in subsequent loss of water from the stem followed by the roots, and such a decline of water potential can lead to the development of water stress Muhiuddin It is impossible to grow plants without their experiencing some degree of water stress; even plants growing in a moist environment develop small water deficits during normal transpiration Rook He also asserted that water stress can arise not only from a soil water deficit, but also from too much water.
Moreover, Grace claimed that all land plants face some degree of water stress. In a Picea sitchensis stand water potential of He also added that only reliable indicator of water status of plants is measurements made on the plants themselves. Available water is taken to be that between field capacity and wilting percentage, within the rooting depth of the plant. Movement of water through the system The soil-plant-atmosphere system can be considered as a continuum through which water moves along a path of decreasing water potential Kozlowski The resistance of wet soil to water movement is low because only small forces are necessary to move water through water-filled soil pores.
As the soil dries the resistance to water transport increases, sometimes leading to the situation when soil resistance becomes the greatest resistance in the soil-plant system Nnyamah et al. Also, Running a concluded that root resistance may account for half to two-thirds of the total resistance to water movement.
On the other hand, Taylor and Klepper postulated that the resistance of the plant has often been assumed to remain constant while the soil dries. The last result confirmed Newman's conclusion a, b that the plant becomes the greatest resistance to water flow as soil dries. Blizzard and Boyer in their important study with Glycine max plants stated that as the soil dried water flow through the soil and plant decreased greatly and both soil resistance and the plant resistance increased.
Significantly, plant resistance was greater than the soil resistance over a wide range of soil moisture contents. Taylor and Klepper found much of the increased total soil resistance results from an increase at the soil-root interface. Increased soil-root interface was associated with a decrease in soil-root contact as a result of root shrinkage Weatherley Some of the increased plant resistance to water transport as soil water is depleted results from an increased tension of water in xylem vessels, and appears to be associated with cavitation of the water column Mulburn and Johnson while Kramer cited that changes in plant resistance may occur in the roots as the soil dries, probably from root suberization and losses in viability cf.
Kozlowski Leaf water potential is subjected to diurnal and seasonal variations. Typically, leaf water potential decreases from a high value in the morning to a midday minimum followed by an increase in the late afternoon Kozlowski Slatyer described the progressive daily changes in soil water potential and plant water potential. Kozlowski mentioned that larger amplitude and earlier daytime minima in twig water potential of four Pinus sylvestris clones were found during the summer than during the winter. Decreasing turgor in the leaves may produce a steeper water potential gradient within the plant to increase water extraction from dry soil Hinckley et al.
In another study with Acer pseudoplatanus Khalil and Grace observed that the bulk leaf water potential of the water-stressed seedlings showed no response to water stress until day 23, after which water potential fell significantly by day Thereafter, there was a recovery in the water potential of stressed plants, although soil water content of the non-irrigated column fell steadily. Mohiuddin found that the bulk leaf water potential in poplar declined during the drying period along with the initial decline of soil water content, although there was no substantial change of turgor potential.
However, with further decline of soil water the leaf water potential increased close to that of the control plants. On the other hand, Ranney et al. Stomata in relation to water stress Stomata are important because most of the water lost by transpiration escapes through them and most of the carbon dioxide used in photosynthesis enters through them Kramer Stomatal aperture appears to be controlled by complex mechanisms which operate to maintain a variable balance between allowing CO2 uptake to proceed, while restricting the loss of water vapor Schulze and Hall The operation of opening and closing the stomatal aperture depends on the two guard cells bordering the aperture.
The size of the aperture increases with increasing turgor of the guard cells and decreases as their turgor decreases.
Invasive Species Compendium
Measurement of the size of stomatal opening in terms of stomatal conductance is important because of its pivotal role in controlling the uptake of carbon dioxide and water loss. Overall stomatal closure and transpiration reduction in response to water deficits have been established Hsiao The traditional view of this response is that the reduction in water uptake by plants from dry soil induces lower leaf water potentials which in turn cause a decline in turgor potential of the stomatal guard cells.
As a result, stomatal conductance is reduced Muhiuddin Reduction of stomatal conductance, therefore, is the consequence of stomatal closure when their guard cells turgor decreases. Stomatal activity is affected by numerous internal factors, including leaf water status, leaf temperature, internal and external CO2 concentration, growth regulators, irradiance, and ambient humidity Kramer Thus, contradictory reports about stomatal behaviour usually occur in the literature.
Response of stomata to soil water status The direct response of stomata to soil water status before any evident change in leaf water potential has been reported. For example, with progressive soil drying, stomatal closure occurred in cow-pea while leaf water potentials were not very different from those of well- watered plants Osnubi Stomatal conductance in six species of birch decreased after irrigation was withheld Ranney et al.
Recently, Khalil and Grace found that reduction in stomatal conductance of sycamore seedlings was strongly correlated with the changes in soil water status. However, decreases in stomatal conductance were not accompanied by a particular decline in bulk leaf water potentials. Other greenhouse and field studies have shown that stomatal conductance decreases with low soil water availability Comstock and Ehleringer and, Reich and Hinckley Other authors have also reported on the direct or prior response of stomatal closure to soil water status e.
Blckman and Davies ; Growing et al. This effect is transduced through a root-to-leaf signal under soil water stress Gollan et al. In other words, a direct signal from the roots to the stomata appears to regulate leaf conductance as the soil dries but prior to leaf wilting Schulze Evidence for direct effects of soil drying on stomatal conductance, not mediated by any change in leaf water status, has been reported Blachman and Davies and, Gollan et al.
Response of stomata to plant water stress Specific leaf water potential values are seldom associated with a specific degree of stomatal aperture, even though a general relationship exists Slatyer The responsiveness of stomata to leaf water potentials varies with species Turner For example rapid stomatal closure occurred in Populus grandidentata at Stomatal conductance in both beans and poplar seedlings was more closely related to the predawn leaf water potential Mohiuddin Braatne et al.
In Pinus sylvestris stomata closed at There are direct or indirect indications that stomata of other species may be sensitive to small water deficits Hsiao For instance Ranney et al. Mild drought favoured stomatal opening of poplar Liu and Dickmann and, in sweet cherry and plum optimal stomatal conductance was obtained at moderate water deficits Yoon and Richer A decline in leaf water potential leading to stomatal closure has been reported for many plant species e. El-Sharkawy and Hesketh ; Ficher et al. Differences among other tree species in sensitivity of stomata to leaf water deficits have been reported see Kozlowski , and also among some other plant species see Hsiao However, the critical leaf water potential for stomatal closure reported for different species should not be taken too seriously because the value varies for different clones and cultivars Pallardy and Kozlowski b.
Slatyer cite that water deficit, in itself, may not affect the stomatal aperture until a critical value is reached and then, as the water deficit increases, progressive decreases in stomatal aperture occur until almost complete closure exists. Moreover, the leaf water potential at which stomata begin to close depends on plant age, growth condition, stress pre-history and rate of stress Turner and, Jones and Rawson Turner et al.
Water potential of leaves can vary over a considerable range without a marked effect on stomatal aperture Jarvis Stomatal closure without any change of leaf water status has been reported in several more recent studies e. Gollan et al. These results suggest that leaf conductance is not solely affected by leaf turgor, but may also be influenced directly by the soil or root water status Turner On the other hand, several studies show that stomatal behaviour is insensitive to water stress e.
Duniway ; Kanemasu and Tanner and, Boyer Effects of water stress on photosynthesis Attempts to correlate photosynthetic activity with tissue water potential have been complicated by the fact that stomata are photoactive as well as hydroactive Crafts There are extensive data in the literature suggesting that leaf carbon assimilation can be limited by stomatal closure either in response to a decrease in plant water potential or to an increase in the water vapour difference between the leaf and the air Chaves In other words, photosynthesis generally is reduced by both water deficits and excesses.
This reduction is caused by stomatal closure, reduction in leaf turgidity, and dehydration of the protoplasm Schneider and Childers and, Brix The availability of CO2 to chloroplasts is limited by various resistances in its inward diffusion path, including those associated with the boundary layer, cuticle, stomata, and mesophyll Kozlowski or with the upset of the balance of growth regulators leading to more abscisic acid ABA accumulation in the leaves Zhang and, Liu and Dickmann It is now well established that the rate of CO2 assimilation in the leaves is depressed at moderate water deficits, mostly as a consequence of stomatal closure Chaves As drought develops the stomata close and consequently photosynthesis and transpiration decrease.
Reduction in leaf area by water stress is an important cause of reduced crop yields because the reduced photosynthetic surface persists of the stress is relieved Kramer Photosynthesis in relation to stomatal behaviour Regehr et al. The close correlation often observed between stomatal conductance and rate of photosynthesis is caused by stomatal closure Kozlowski et al. Pieters and Zima asserted that the reduction of the light saturated photosynthesis rate by water stress of poplar leaves exposed to normal air is determined mainly by stomatal closure.
Ranney et al. Also Ranney et al. The relationship between photosynthesis and leaf conductance of mature northern red oak was stable throughout the period of drought indicating a high degree of coordination between photosynthesis and leaf conductance Weber and Gates Recently, Liu and Dickmann concluded that poplar plants subjected to progressive drought gained full recovery upon rewatering, indicating that the restriction of CO2 through stomatal closure was a major constraint to photosynthesis. On the other hand, Schulze and Hall stated that there is plenty of evidence that leaf conductance and CO2 assimilation change proportionally during variations of certain habitat factors, e.
Nonetheless, this proportionality does not hold with respect to changing plant water status Schulze Photosynthesis in relation to soil moisture The discussion of leaf gas exchange, as one major component in the whole-plant water and carbon flux, shows that stomata are regulated by two 'feedforward' responses namely air humidity and soil water status Schulze He also asserted that photosynthesis is inhibited by soil water content, but the magnitude of this effect depends on the pretreatment.
Whole-tree photosynthesis and photosynthesis per unit of needle weight of 3-year-old Scots pine seedlings increased with increasing soil moisture Schultze and Gatherum In many mesic species the rate of photosynthesis begins to decline when leaf water potential becomes only slightly negative Kozlowski Pieters and Zima found that a lowering of the water content of the detached leaves of poplar Populus euramericana Dode Guinier cv. Robusta causes a reduction of the photosynthetic rate at light saturation and at light limitation.
They also mentioned that measurements of water content and water potential in the poplar leaves revealed that the relation between these parameters in the range studied was approximately linear. In Acer negundo from a streamside habitat, photosynthesis ceased at shoot water potential values 1. Quercus gambelli Dian and Klukoff There are many studies emphizising the correlation between reduced photosynthesis and negative plant water potential in tree species as well as the other plant species e.
Al-Ani et al. However, there are considerable variations in the photosynthetic response to leaf water potential because of stomata behaviour. Photosynthesis in the same species may be affected differently by stomatal closure because of variations in response of plant growth in different environments Van Volkenburgh and Davies Because stomata of various Populus clones respond differently to water stress Pallardy and Kozlowski b, Pallardy photosynthesis may also be expected to vary Kozlowski On the other hand, there are several studies which show a lack of correlation between photosynthesis and plant water status.
Recently, Kleiner et al. Furthermore, photosynthesis in two hybrid Populus clones was not affected early in the drought cycle Liu and Dickmann It is difficult to generalize about the effect of water or other stresses on the rate of photsynthesis because of differences among species and within species, and depending on past treatments Kramer Effect of water stress on respiration Since most if not all of the multitude of chemical reactions that make up the process of respiration are enzymatic, and since the function as well as, the structure of enzymes are affected by the water status of plants, it seems that respiration should be subjected to regulation by the hydration of tissues Crafts McCree noted that both growth and maintenance respiration should be reduced as a result of chronic water stress, the reduction of growth respiration in proportion to the reduction in photosynthesis input and the reduction in maintenance respiration in parallel with a general reduction in the level of metabolic activity.
Results are often conflicting, showing no change, an increase, or a decrease in dark respiration with water stress Hsiao Slatyer stated that as water stress is imposed, an increase in respiration rate may first be observed, followed by a reduction in rate as the plant adapts to stress. These two phases may correspond to Stocker's reaction and restitution phases.
If stress is imposed gradually, the first phase may not become apparent with a progressive decline in respiration rate and increasing stress being observed. If stress is imposed rapidly, however, the initial increase can be quite marked. Leaf respiration Stocker reported that leaves which become flaccid from rapid transpiration on warm days may exhibit high rates of respiration. Severe dehydration of leaves often leads to a burst in respiration as a result of hydrolysis of starch to sugar Kramer and Kozlowski Furtheremore, other studies showed an increase in respiration with decreasing osmotic potential increasing water stress e.
Takaoki and, Iljin Brix found an initial decrease in top respiration of loblolly pine seedlings at moderate water stress, then an increase at severe stress to levels above the control, and finally a decline again at extremely severe stress. As photosynthesis decreased before there was an effect on respiration, he proposes that the reduction in concentration of respiratory substrates resulting from lowered photosynthesis might be responsible for the initial decrease in respiration.
The subsequent increase in respiration may result from an increase in sugars produced by hydrolysis of starch. Slatyer states that the effects of water deficits on respiration rates can result in significant changes in rates of apparent phtosynthesis, since the relative significance of even a constant respiration rate increases rapidly as photosynthesis is reduced by water stress.
The decline in respiration rate is associated with decreased availability of respirable substrates, due to either a decreased rate of photosynthesis, or an increased demand for sugars for accumulation of organic solutes, or to a combination of both Lambers Schultz and Gatherum reported on a linear increase in the rate of respiration per gram dry weight of needle and per seedling basis with an increase in soil moisture in Scots pine.
In leaves, the decrease in the rate of dark respiration is less than the decrease in photosynthesis Boyer , but for whole plants, the decrease is similar Boyer a. Dark respiration generally has been observed to decline as the plant water potential falls, but the amount of suppression typically is not very large Bradford and Hsiao On the other hand, Deng et al. Root respiration Although much data are available in respect of the effect of water deficits on plant respiration, less is available regarding root respiration in relation to plant water status or soil moisture.
Most of the studies have been on leaf respiration and little on a whole plant basis. Jarvis and Jarvis measured root respiration in relation to osmotic potential in the medium using apical root segments of pine, spruce, birch and aspen. Root respiration in all four species was negatively correlated with osmotic potential of the medium. These results, however, differ from those of Takaoki which show an increase in respiration with decreasing osmotic potential cf. Crafts This is because water deficits influence all phases of plant growth and are probably responsible for more growth loss than all other causes combined Kramer This is not surprising as water deficit affects almost every aspect of plant physiology and morphology Kozlowski et al.
Kozlowski also considered that water deficit reduces growth both directly through effects on cell turgor and indirectly through intermediation of seed germination, photosynthesis, respiration, mineral nutrition, enzymatic activity, hormone relation and nitrogen metabolism. Cell division and enlargement Growth of any part of a plant is usually the outcome of an increase in size which results from cell division and enlargement.
Hsiao argued that cell enlargement generally is regarded as more sensitive than cell division to water deficit. However Doley and Leyton claimed that there is some evidence that cell size affects cell division, because cells do not divide until they have attained a certain size. Because cell enlargement depends on cell turgor, the elongation of cells is very sensitive to desiccation, so water stress directly and physically reduces growth by lowering cell turgor Kozlowski There were significant effects of imposed water deficit on cell length, width and area of Eucalyptus globulus leaves Metcalfe et al.
Growth of the leaves Vegetative growth in general and leaf expansion, in particular, are severely inhibited by relatively moderate water stress Kramer Leaf production Zahner pointed out that mild droughts during mid- and late-summer reduced the number of needle primordia of Pinus strobus L. The number of needle fascicles on Pinus resinosa Ait.
Myers and Landsberg observed a reduction in the average number of leaves per seedling of both Eucalyptus maculata Hook and E. The severe development of water stress significantly reduced both leaf initiation and total leaf area of water-stressed Acer pseudoplatanus L. Leaf expansion Water deficit has a negative effect on both leaf expansion rate and final leaf size Takami et al. Severely water-stressed P. Don trees produced smaller needles compared with irrigated trees Linder et al.
Leaf expansion in dicotyledonous species is especially sensitive to reduced leaf water potential Boyer He also mentioned a, b that desiccation for long period results in small leaves. The rate of needle elongation in Pinus radiata was closely related to osmotic potential of the rooting solution Sands and Correl Roden et al. Van Volkenburgh and Boyer observed a marked decrease in the rate of maize leaf elongation with increasing soil water stress.
Boyer a, b , On the other hand, mentioned that normal diurnal changes in leaf hydration do not greatly affect final leaf size. Leaf area One of the damaging effects of water stress is a reduction in leaf area, which reduces the loss of water, but unfortunately also reduces the surface that carries on photosynthesis, thus decreasing the amount of photosynthate available for growth Kozlowski et al.
It is well known that trees respond to a period of severe water stress by reducing their leaf area Linder et al. Reduction in leaf area appears to be largely affected by soil water status and root hydration Termaat et al. Reduction in leaf area due to drought can happen through a reduction in leaf initiation or leaf expansion or both or by leaf shedding. Some workers attribute the reduction in leaf area to a reduction in the number of leaves or inhibition of leaf initiation, while others argue that the reduction in leaf area is chiefly a result of the reduction in leaf size, or both number of leaves and leaf size.
For example Kozlowski mentioned that water deficits reduce leaf area by inhibiting initiation of leaves as well as their subsequent enlargement. Begg reviewed that water stress reduces leaf area by accelerating the rate of senescence of the physiologically older leaves and through its effects on leaf shedding. Applied water stress reduced total leaf area of both Eucalyptus malacata and E. Decreasing water potential in both birch and aspen seedlings caused a decrease in leaf area, and the smaller leaf areas in water-stressed treatments were the result of lower rates of leaf production, earlier senescence and abscission, and smaller size of leaves Jarvis and Jarvis Also Roden et al.
Reduction in leaf area appears to be a common response of poplars to low soil water content Braatne et al. However they concluded that leaf area reductions appear unrelated to drought-induced reductions in leaf size. On the other hand, Metcalfe et al. Growth of the shoots Shoot growth of seedlings is very sensitive to water stress and is much slower if soil is periodically allowed to dry below field capacity than if it is kept near field capacity Zahner Both height and diameter growth are subject to the effects of water stress. Elongation Height growth and elongation of internodes of lateral shoots are often affected differently to that of leaf expansion by water deficits Kozlowski Waring and Schlesinger suggested that decreasing predawn leaf water potential is well correlated with decreasing tree height at maturity cf.
Height growth of Eucalyptus globulus seedlings was restricted by drought treatment, but not as severely as leaf growth Metcalfe et al. Also Steinberg et al. For example, in poplar seedlings Mohiuddin and in Pinus radiata trees Snowdon and Benson Radial growth Stem diameter growth was among the first processes influenced by drought Rook et al. Fritts suggested that evidence for extreme sensitivity of cambial activity to water deficits comes from correlations of xylem increment with rain fall or available soil water.
Water stress has either long- or short-term effects on diameter growth. Long-term increases in stem diameter result from an increase in number and size of xylem cells, but short-term variations are caused by reversible shrinkage and swelling of existing cells, chiefly those of the inner bark and cambial region Kozlowski et al. Short-term stem growth of Pinus radiata was very sensitive to tree water stress at mild to moderate levels of stress Benson et al. Moreover, Kozlowski concluded that water deficits not only decrease the number of xylem cells formed in the annual ring but also prevent expansion of xylem cells.
Cambial activity is inhibited directly by water deficits when cell turgor is low enough to prevent hormonal growth regulators from acting Whitmore and Zahner Indirect effects of water deficits are mediated by reduced synthesis and downward transport in the stem of hormonal growth regulators Kozlowski Linder et al. Poplar trees in non-irrigated plots produced less stem volume and leaf area than irrigated trees Roden et al.
In general, shoot growth is reduced more than root growth because more severe water deficits develop in the transpiring shoots and probably persists longer Kramer Growth of the roots Water deficits reduce the rate of root growth, root branching, and cambial growth Kramer Root elongation and radial expansion must be restricted through restricted cell division, cell enlargement, and tissue differentiation just as are shoot extension and cambial activity in the stem and branches Zahner Kozlowski has shown that a large root system develops in tree seedlings when grown in soil maintained close to field capacity, in contrast to sparse root development in soil allowed to dry almost to permanent wilting before rewatering cf.
Zahner All the above findings are correct because of the necessity of soil moisture to root growth. Gingrich and Russell , asserted that the rate of root elongation is highly dependent on available water because root growth is in part a hydration process. However, Newman found some root growth of flax when the soil water potential was below bars, which is more than the permanent wilting point. The failure of roots to grow into dry soil is probably more the result of physical impedance than soil water stress per se Zahner because soil strength increases sharply as the soil dries resulting in physical resistance to penetration by a root tip Taylor and Gardner Slowing or cessation of root growth in drying soil decreases water absorption because it reduces the invasion of previously unoccupied soil and is accompanied by an increase in the proportion of root surface that is suberized Kozlowski et al.
In general, root growth is decreased or stopped by soil water stress and roots tend to become suberized to their tips, reducing their capacity to absorb water Kramer Showing the important role of the behaviour of roots in soil water stress conditions Kummerow stated that root systems show a high degree of morphological plasticity that enables them to cope with the most variable soil and soil moisture content. However, because of the time lag in transmission of water stress from leaves to roots, the roots are the last tissues to be stressed Kozlowski et al.
Although most findings showed decreased root growth as a result of water stress Khalil and Grace in their study with Acer pseudoplatanus seedlings found soil drying altered the root distribution profile and increased root growth in water stress treatments relative to well- watered plants. Effects of water stress on relative growth rate and its components Relative growth rate of birch seedlings decreased with decreasing water potential whereas the aspen rates were not significantly affected Jarvis and Jarvis For both species, decreasing water potential also caused decreased leaf area and leaf area ratio.
The reduction in leaf area ratios were accompanied by an equivalent reduction in specific leaf area. However, Buchlard found no change in specific leaf area with decreasing soil water potential in a range of Eucalyptus species. Crafts showed that during wilting, relative growth rate and net assimilation rate of tomato plants were considerably and significantly reduced and following rewatering, however, growth rates rose to levels significantly greater than those of the control.
Restricting water supply caused large differences in growth rates of Eucalyptus maculata and Eucalyptus brockwayi which were related to large differences in total leaf area. In both species net assimilation rate g dry matter m-2 day-1 of severely water- stressed seedlings was slightly lower than that of well-watered plants Myers and Landsberg Recently, Khalil and Grace cited that water stress reduced leaf area ratio of sycamore seedlings due to a reduction in specific leaf area with leaf weight unaffected.
Effect of water stress on dry matter production Total dry weight Studies with seedlings have shown consistently that dry matter production is related directly to frequent irrigation. Gates a, b and in his experiments with tomato found that when the water deficit was more severe dry weight was retarded about twice as much as it was under moderate water deficit, thus he interpreted the pattern of dry weight accumulation in a whole plant subjected to water stress as a tendency toward senescence during wilting and return towards a more juvenile condition upon rewatering. Response of dry matter production to water stress appears to be more pronounced in conifers than broad-leaved tree species.
For instance dry weight of Pinus sylvesris and Picea abes was reduced by about one third in soil that dried to 1. They also mentioned that this because the external resistance to vapour loss from the leaves of conifers was about one third that from leaves of angiosperms, which causes mutual interference between leaves of the same plant resulting in less water deficit within the broad-leaved species.
Byman and Gumbs found that irrigation resulted in increased dry matter yield of Digitaria decumbens Stent. The water stress treatments caused large differences in growth rate, leading to significant differences in dry matter produced after six weeks for Eucalyptus maculata and after ten weeks for E. In their study with seven ecotones and eucalyptus forest species, Barrett and Ash showed that total plant dry weight increased significantly at high water treatments in Eucalyptus sieberi L.
Also total biomass production of poplar trees was significantly reduced by limited water supply Mohiuddin Recently, Goenaga reported a linear increase in total dry weight of Tanier Xanthosoma spp. On the other hand, the total biomass of sycamore seedlings was unaffected by soil drying Khalil and Grace Morton and Watson found that growth of sugar beet leaves in terms of dry matter increment was reduced under water stress cf. Gates Compared with control plants, drought-treated Eucalyptus globulus seedlings produced little more than half the dry mass of new leaves Metcalfe et al.
Mean leaf dry weight of seven tree species from ecotone and eucalyptus forests increased significantly under a high water treatment. In a study of above-ground biomass production of Pinus radiata, Snowdon and Benson showed that irrigation resulted in a steady but small increase, compared with the control, in foliar biomass amounting to about 1.
In another study, foliage biomass production of Pinus radiata trees tended to decline with increasing water stress integral during the period of needle growth Raison et al. Moreover, Mohiuddin reported on a substantial decrease in leaf dry weight of poplar trees when grown in water stress treatments. Shoot dry weight Since water deficits reduce stem elongation, diameter growth and consequently stem volume, a decrease in stem dry matter will be expected.
In red oak, shoot weights were lower in the low water treatment than in the high water treatment Kleiner et al. Many workers reported on decreased stem dry weight of tree species as a consequence of water stress, For example, Roden et al. Root dry weight Water stress adversely affects root growth. Sharp and Davies reported on a reduction in root dry matter as a result of water stress. Root biomass production of poplar trees was reduced significantly by soil water stress Mohiuddin Nevertheless, increasing root weight in absolute terms or relative to shoots of plants under water stress has been reported.
Also Khalil and Grace cited that soil drying increased total root dry weight of sycamore plants in absolute terms under water stress relative to well-watered plants while Osonubi and Fasehun reported on an increase in root dry weight relative to shoot growth. These results, however, could be correct if the root structure in a water stressed condition was modified.
For instance, Nobel and Sanerson stated that suberization of root surfaces to prevent water loss from the root into very dry strata of the soil profile might increase root weight. Osonubi and Davies , reported that the increased root growth of water-stressed Quercus robura was due to a change in production from short, thick roots to a profusion of long, thin roots that penetrated deeper into the soil. Nevertheless root weight is less important than root surface area because most absorption occurs through fine roots, which contribute little to dry weight Newman Effect of water stress on dry matter partitioning Water stress not only decreases the total dry matter production of plants but also alters the partition of dry matter between the different plant organs.
Leaf weight ratio LWR Fraction of biomass allocated to foliage leaf weight ratio seems to be affected somewhat by water stress since the reports on a positive effect of water stress are rather less compared with those on no effect. Gates found that more than half the dry matter of the tomato plants at the beginning of the experiments was in the leaf lamina, then the dry matter partitioned to the lamina decreased during wilting below those of the controls, and after rewatering lamina weight ratio increased to values above those of the controls.
However, in birch and aspen seedlings LWR was largely unaffected by water-stress treatments Jarvis and Jarvis Barrett and Ash in their study of the responses of seven tree species from rainforest, ecoton and eucalyptus Australian forests to water and nutrient treatments concluded that the proportion of plant mass partitioned to leaves did not change between experimental treatments. Leaf weight ratio of sycamore seedlings was reduced by water deficits, although the reduction was not statistically significant Khalil and Grace Moreover, the LWR of poplar trees remained unchanged in all three water supply regimes Mohiuddin Linder and Axelsson cited that the overall effect of irrigation compared with non- irrigation was to reduce the proportion allocated to stems.
Moreover he cited the finding of Strothmann's study who found that large, healthy seedlings of Pinus resinosa resulted when released of root competition from surrounding vegetation, and the ratio of roots to shoots was not different in the large seedlings than in the small seedlings under severe competition. Hsiao and Acevedo stated that when water supply is limiting allocation of assimilates tends to be modified in favour of root growth which leads to increased root weight and consequently the root to shoot ratio increases.
Although growth of both roots and shoots decreases under drought conditions the root-shoot ratio generally increases Kramer and Kozlowski Conversely, poplar root weight ratio, that reflects the magnitude of root growth relative to the whole plant, was smaller in unwatered and rewatered treatments than in the watered control Mohiuddin Chlorophyll content Chlorophyll destruction is hastened by water stress Kramer Reports on the effect of water stress have been ranged from negative to positive effect. Beadle and Jarvis cited that chlorophyll in Stika spruce did not change significantly until needle water potential reached bar after slow drying.
However, Alberte et al. Bhardwaj and Singhal reported a similar effects on greening of barley. Water stress readily inhibits light-induced chlorophyll formation in etiolated leaves but has little or no effect on chlorophyll contents in green leaves Jones b, Boyer , cf. Hsiao Similarly, Hoffmann states that chlorophyll and carotenoid content of the desiccation tolerant plants is unchanged during desiccation and rehydration. On the other hand, Bourque and Naylor mentioned that small water deficits can have marked effects on chlorophyll accumulation. Nielsen found that the larger, the water supplies plants, was the lighter in colour so that he concluded that a dark green colour might indicated water deficiency.
Soil water-nutrient relationship Although nutrient and water absorption are independent processes in roots, the necessity for available water in both the plant and soil for growth and nutrient transport makes them intimately related Viets In addition, water stress apparently inhibits the mechanisms of uptake and of nutrient unloading into the xylem more directly Bradford and Hsiao Crafts et al. The immediate effect of adding a salt to a soil low in salt content is to increase the diffusion pressure deficit of soil water, reducing the gradient between the soil solution and the root cells Magistad Little research, however, has been done on the effect of mineral deficiencies on water absorption Kramer In soil, water in the suction range of about 0.
Thus the water content of soil affects nutrient transport to the root surface by affecting the rate of diffusion and the mass flow of water to roots Viets Effect of water stress on nutrient availability In water stress studies which include various degrees of watering or soil water depletion and rewatering through several drying cycles, the results are sometimes difficult to interpret in terms of nutrient availability.
Nielsen says "It is difficult to evaluate whether the decrease in yield, possibly accompanied by a compensating increase in root yield, are caused by the increasing water stress or by the decreasing nitrogen effect due to water stress". Grunes states that one precaution in the interpretation of such experiments is if the water regime favours more growth, then a greater proportion of fertilizer nutrient as opposed to native soil nutrient is absorbed, so that the effect of moisture regime on nutrient availability may be over emphasized. Also Viets cited that such experiments are difficult to interpret because much of the nutrient uptake may occur soon after the soil is rewetted.
Moreover, Slatyer mention that "The effect of water stress on mineral nutrition is difficult to resolve clearly Data from comparisons of nutrient uptake in different soil water regimes provide useful information on nutrient availability in relation to water status. However, various indices of nutrient availability, such as the amount of an element extractable from soil or even the amount of fertilizer applied, are often confused with nutrient availability. Viets comments on two measures of nutrient availability often used: total uptake and percentage composition. If water stress decreases growth and total nutrient content also decreases, then by definition water stress also decreases nutrient availability.
If the nutrient concentration in plants grown with different water supplies remains constant but drought restricts growth, then total nutrient uptake is less and so nutrient availability by definition is less. An increase in nutrient concentration of the plant with increasing water stress may be considered to be nutrient absorption at a faster rate than an increase in dry weight water stress inhibits growth more than nutrient uptake. A decrease in nutrient concentration means that net assimilation and growth have not been affected as much as nutrient availability.
The preponderance of evidence indicates that drought decreases nutrient availability to plants as measured by total nutrient uptake and sometimes in reduced concentration. Nitrogen availability and water stress The importance of nitrogen to plant growth has been well established and mentioned elsewhere in this review. Nitrogen is among the major elements N, P and K that have received most attention. Jensen postulated that water and nitrate follow uniform rates of uptake, and an increase in transpiration will increase rate of uptake of both water and nitrate.
Meyer and Gingrich found that the osmotic stress obtained by use of colloidal suspensions of Carbowax applied to half the root system of wheat reduced the rate of P and N uptake, resulting in decreases in N and P content of the whole plant within 24 h. The rate of nitrogen uptake and other mineral elements increased considerably with increasing water supply to Lolum perenne plants Nielsen Moreover, Raison et al.
In both fertilised and irrigated-fertilised Pinus radiata stands, concentration of nitrogen in foliage increased rapidly in the year of fertilisation, and then declined over the subsequent three years to levels equal to the control Crane and Banks Rapid N uptake coincided with abundant soil moisture, high concentrations of mineral N in the soil and rapid needle growth in pine Linder et al.
On the other hand, Lahiri stated that studies undertaken on a number of pearl millet varieties indicated that the rate of nitrogen uptake remained virtually unaffected over a wide range of soil water potentials. Also Muchow observed that the water regime had no effect on net above-ground N uptake g m-2 in kenaf Hibiscus cannabinus L. Furtheremore nitrogen concentration per unit of leaf dry weight in Eucalyptus globulus was unaffected by irrigation and addition of fertilizers Pereira et al. Also Raison et al. Journal of Turkish Phytopathology, 24 3 ; 8 ref. Dandelion control in cereal and oilseed crops.
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