Movement of Abscisic Acid into the Apoplast in Response to Water Stress in Xanthium strumarium L

The effect of water stress on the redistribution of abcisic acid (ABA) in mature leaves of Xanthium strumarium L. was investigated using a pressure dehydration technique. In both turgid and stressed leaves, the ABA in the xylem exudate, the `apoplastic' ABA, increased before `bulk leaf' stress-induced ABA accumulation began. In the initially turgid leaves, the ABA level remained constant in both the apoplast and the leaf as a whole until wilting symptoms appeared. Following turgor loss, sufficient quantities of ABA moved into the apoplast to stimulate stomatal closure. Thus, the initial increase of apoplastic ABA may be relevant to the rapid stomatal closure seen in stressed leaves before their bulk leaf ABA levels rise.

Following recovery from water stress, elevated levels of ABA remained in the apoplast after the bulk leaf contents had returned to their prestress values. This apoplastic ABA may retard stomatal reopening during the initial recovery period.

     

Changes in Free Amino Acids in White and Green Tissues of Variegated Tobacco Leaves during Water Stress

Variegated tobacco leaves, white on one side of the midrib andgreen on the other, were detached from the stem and incubatedunder water stress or turgid conditions for 4 days to determineany changes in the levels of free amino acids. Drastic changes in the free amino acid composition occurredin the green tissue during the water stress period, but onlyvery small changes in the white tissue. During that time, themost striking difference between the two tissues was the largeamount of proline accumulated in the green tissue, but not inthe white. An exogenous supply of sucrose increased the prolinecontent in both tissues during water stress. An exogenous supplyof glutamic acid increased the proline and asparagine contentsin the green tissue, but it increased only the asparagine contentin the white tissue during water stress. (Received May 4, 1982; Accepted August 12, 1982)     

PHYSIOLOGICAL STUDIES ON THE VERTICILLIUM WILT DISEASE OF TOMATO

The water loss per unit leaf area of tomato plants was decreased after inoculation with Verticillium albo-atrum. When diseased plants began to wilt water loss temporarily increased, but then rapidly decreased to become less than that of healthy plants grown under conditions of adequate or restricted water supply.
The transpiration of excised leaves from plants grown with a restricted water supply was reduced, but not so severely as that of comparable leaves from infected plants. Water loss from leaves on infected plants was reduced irrespective of any blocking of the petiolar xylem.
The rate of water loss from turgid leaf disks on mannitol solutions, and the rate of water uptake of leaf disks on water was similar for disks cut from wilting or turgid leaves of diseased plants or healthy plants grown with an adequate or restricted water supply.
Disease or poor water supply reduced leaf growth but had no effect on the rate of leaf initiation. Although the density of stomata was higher on leaves of diseased plants the stomatal area was less than on healthy plants.
The resistance to water flow in diseased stems was high and was correlated with vessel blockage. About half the blocked vessels contained hyphae. The severity and localization of symptoms in inoculated plants growing on susceptible or resistant rootstocks was directly related to the extent of invasion by the pathogen and to vessel blockage.
Experiments on the wilting activity of cell-free filtrates from cultures of the pathogen in vitro indicated that it produced a stable substance, not an enzyme, that caused wilting in cut shoots by blocking the end of the stem. It is suggested that an increasing internal water shortage causes major symptoms of disease.     

Stomatal response of cotton to water stress and abscisic Acid as affected by water stress history

The threshold leaf water potential required to initiate stomatal closure in cotton (Stoneville 213) became progressively more negative when plants were subjected to a series of water stress cycles. The shift in the threshold water potential required for induction of stomatal closure was dependent on the number of previous stress cycles and leaf age. The basal level of endogenous abscisic acid (ABA) in fully turgid leaves increased in response to the stress treatments, whereas the amount accumulated in response to a subsequent stress did not differ greatly among plants that had experienced different degrees of stress conditioning.     

Inhibition of proline oxidation by water stress

The conversion of proline to glutamic acid and hence to other soluble compounds (proline oxidation) proceeds readily in turgid barley (Hordeum vulgare) leaves and is stimulated by higher concentrations of proline. This suggests that proline oxidation could function as a control mechanism for maintaining low cellular levels of proline in turgid tissue. In water-stressed tissue, however, proline oxidation is reduced to negligible rates. These results are consistent with the idea that proline accumulation results from inactivation by water stress of normal control mechanisms. It seems likely that inhibition of proline oxidation is necessary in maintaining the high levels of proline found in stressed barley leaves.     

Xylem ABA controls the stomatal conductance of field-grown maize subjected to soil compaction or soil drying

Stomatal conductance of individual leaves was measured in a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration in the same leaves. Stomatal conductance showed a tight relationship with xylem ABA, but not with the current leaf water status or with the concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non-compacted and irrigated soil, and to plant-to-plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Leaf hydraulic architecture correlates with regeneration irradiance in tropical rainforest trees

The leaf hydraulic conductance (K(leaf)) is a major determinant of plant water transport capacity. Here, we measured K(leaf), and its basis in the resistances of leaf components, for fully illuminated leaves of five tree species that regenerate in deep shade, and five that regenerate in gaps or clearings, in Panamanian lowland tropical rainforest. We also determined coordination with stomatal characters and leaf mass per area. K(leaf) varied 10-fold across species, and was 3-fold higher in sun- than in shade-establishing species. On average, 12% of leaf hydraulic resistance (= 1/K(leaf)) was located in the petiole, 25% in the major veins, 25% in the minor veins, and 39% outside the xylem. Sun-establishing species had a higher proportion of leaf resistance in the xylem. Across species, component resistances correlated linearly with total leaf resistance. K(leaf) correlated tightly with indices of stomatal pore area, indicating a coordination of liquid- and vapor-phase conductances shifted relative to that of temperate woody species. Leaf hydraulic properties are integrally linked in the complex of traits that define differences in water use and carbon economy across habitats and vegetation zones.     

Leaf and root control of stomatal closure during drying in soybean

The stomatal conductance of an illuminated 2.5 cm² area of an intact soybean leaflet was the same whether the rest of the shoot was in light or darkness. This was true throughout soil drying cycles. Water potential of tissue immediately outside the illuminated area consistently decreased about 0.3 MPa upon illumination of the shoot. This erroneously suggested that stomatal conductance during soil drying did not respond to diurnal reductions in leaf water potential, but was controlled by root or soil water status. Tests showed that the water potential of tissue in the illuminated area did not change in the steady-state upon illumination of the rest of the shoot. Water potentials of shaded sections of leaves were not different from predawn water potentials, and were higher than leaf xylem pressure potentials as determined with a pressure chamber. These steep local gradients of leaf water potential suggest that there is minimal interchange of water among xylem elements leading from roots to different sections of leaves. The relationship between stomatal conductance and leaf water potential was the same whether leaf water potential was reduced by soil drying, application of polyethylene glycol (PEG) to the root system, lowering root temperature, or leaf excision. In the root cooling experiment, there was no soil drying, and with leaf excision, there was no root drying. The similarity of stomatal responses to leaf water potential in all cases strongly suggests control of conductance by a signal produced by local leaf water potential rather than root or soil water status in these experiments.     

Allometric co-variation of xylem and stomata across diverse woody seedlings

Leaf stomatal density is known to co-vary with leaf vein density. However, the functional underpinning of this relation, and how it scales to whole-plant water transport anatomy, is still unresolved. We hypothesized that the balance of water exchange between the vapour phase (in stomata) and liquid phase (in vessels) depends on the consistent scaling between the summed stomatal areas and xylem cross-sectional areas, both at the whole-plant and single-leaf level. This predicted size co-variation should be driven by the co-variation of numbers of stomata and terminal vessels. We examined the relationships of stomatal traits and xylem anatomical traits from the entire plant to individual leaves across seedlings of 53 European woody angiosperm species. There was strong and convergent scaling between total stomatal area and stem xylem area per plant and between leaf total stomatal area and midvein xylem area per leaf across all the species, irrespective of variation in leaf habit, growth-form or relative growth rate. Moreover, strong scaling was found between stomatal number and terminal vessel number, whereas not in their respective average areas. Our findings have broad implications for integrating xylem architecture and stomatal distribution and deepen our understanding of the design rules of plants' water transport network.     

Relative rates of delivery of xylem solute to shoot tissues: Possible relationship to sequential leaf senescence

The rates of delivery of regulatory solutes such as cytokinins and mineral ions from the roots to competing shoot tissues can influence rates of metabolism and development. A 15 min pulse of a synthetic xylem mobile and phloem-immobile solute, acid fuchsin, was used to quantify relative rates of solute delivery to competing organs on excised transpiring bean shoots (Phaseolus vulgaris L. cv. Contender) at different stages of development. Stem, flower and fruit tissues received comparatively low rates of solute delivery. The relative rate of solute delivery to newly opened leaves was initially low, but increased during rapid leaf expansion and then declined progressively as the leaves exceeded 70% of their final area. The relative rate of solute delivery to tissues of the basal primary leaves declined progressively from 2 weeks onwards. This decline appeared to be caused by progressive internally regulated increases in both stomatal resistances and hydraulic resistances to xylem flow up to and into the leaf blade. Thus combined abaxial and adaxial stomatal resistance values in the primary leaves (Rs) increased from 3 to ≥ 7 s cm^?1 between 2 and 5 weeks. Similarly, mean values for the connection resistances (Rc) to hydraulic flow into the primary leaves rose from 7 to 13 TPa · s · m^?1 between 2 and 4 weeks. In the same period pathway resistance from stem to primary leaf petioles (Rp), as determined by direct pressure flow assay, increased from 7 to 15 TPa · s · m^?1. Senescence-associated declines in protein and chlorophyll levels in the primary leaves were initiated in parallel with, or after, declines in relative rates of solute delivery. The provision of extra illumination at the basal leaf level between 2 and 5 weeks did not prevent declines in chlorophyll and soluble protein or increases in stomatal resistance. We suggest that internally programmed changes in the hydraulic architecture of the plant progressively divert xylem-transported root supplies of nutrients and cytokinins from basal to more apical leaves and thus regulate the progressive senescence of leaves from the base upwards.     

赤皮青冈幼苗叶片解剖结构对干旱胁迫的响应

以浙江庆元、湖南洞口和湖南靖州3个种源的赤皮青冈1年生幼苗为材料,采用盆栽称量法控制土壤水分含量,设置土壤相对含水量分别为75%～80%(对照)、55%～60%(轻度干旱胁迫)、45%～50%(中度干旱胁迫)和30%～35%(重度干旱胁迫),研究不同程度干旱胁迫对赤皮青冈幼苗叶片解剖结构和蒸腾作用的影响.结果表明: 干旱胁迫下,3个种源赤皮青冈幼苗叶片的总厚度、上下表皮厚度和栅栏组织厚度均显著降低.栅栏组织厚度与海绵组织厚度比(栅海比)、气孔长度和宽度随干旱程度的增强而显著减小,而气孔密度随干旱胁迫程度的增强而显著增加.干旱处理对叶片主脉厚度无显著影响,木质部厚度的变化因种源而异.叶片结构的变化引起生理功能改变,随着胁迫程度的加强,赤皮青冈幼苗的蒸腾速率显著降低.对照和各干旱处理湖南洞口种源幼苗叶片的下表皮厚度、栅海比、气孔密度显著大于其他2个种源,蒸腾速率显著小于其他2个种源,表明湖南洞口赤皮青冈对干旱胁迫的适应性较强.     

Acclimation to Drought in Acer pseudoplatanus L. (Sycamore) Seedlings

A glasshouse experiment was conducted with well-watered andwater-stressed seedlings of sycamore (Acer pseudoplatanus L.)grown in soil columns. Water was withheld when the seedlingswere 82-d-old. Effects of soil drying on stomatal behaviour,water relations, xylem cavitation, and growth of leaves androots were evaluated. Stomatal conductance declined well before any observable changein bulk leaf water potentials, and was correlated with soilwater status. At seven weeks, osmotic potential had declinedby 0·51 MPa and 0·44 MPa at full and zero turgor,respectively. Drought significantly increased both bulk elasticmodulus and leaf dry weight to turgid weight ratio of water-stressedplants. Drought had no effect on relative water content at zeroturgor. Water cavitation in the xylem was detected as ultrasonic acousticemissions (AE). Water-stressed plants displayed significantlyhigher rates of AE than well-watered plants. Maximum rate ofAE coincided with the minimum level of stomatal conductanceand apparent rehydration of the leaves. Drought caused changes in the root distribution profile andit increased the root weight. The increase in root weight wasmainly due to a substantial shift in assimilates allocated infavour of roots with total biomass being unaffected. Leaf growthwas maintained for six weeks without any significant declinein expansion rate. However, the development of severe waterstress reduced both leaf production and expansion.     

Effect of water stress on proline synthesis from radioactive precursors

Barley (Hordeum vulgare L. var. Prior) leaves converted more ¹⁴C-glutamic acid to free proline when water-stressed than when turgid; neither decreased protein synthesis nor isotope trapping by the enlarged free proline pools found in wilted tissue seemed to account for the result. This apparent stimulation of proline biosynthesis in wilted leaves was not observed when radioactive ornithine or P5C (Δ¹-pyrroline-5-carboxylate, an intermediate following glutamate in proline synthesis) were used as proline precursors unless proline levels were high as a result of previous water stress. We interpret this to mean that any stimulation of proline synthesis by water stress must act on P5C formation rather than its reduction to proline. Experiments showing greater apparent conversion of ¹⁴C-glutamate to proline do not unequivocally prove that proline synthesis is stimulated by water stress, as P5C feeding studies show that proline oxidation is inhibited under comparable conditions. This inhibition could account, at least in part, for increased proline labeling, and must be considered an alternate possibility.     

In Situ Estimates of Variable Plant Resistance to Water Flux in Ilex opaca Ait

Xylem pressure potentials and stomatal diffusion resistances were measured in the field in Ilex opaca Ait. during days which differed in temperature and vapor pressure deficit. Water flux into leaves was calculated by combining the field data with laboratory determinations of the relation between tissue water deficit and water potential. Estimates of apparent plant resistance were then calculated from fluxes and differences between soil water potential and xylem tension. The resistance depended strongly on water flux, dropping by a factor of over 7 from low to high water flow rates. This extends the generality of variable plant resistances measured in controlled environment studies to I. opaca as it occurs naturally in the field. The relation of apparent plant resistance to water flux as estimated in this study can be useful in simulation models which calculate water uptake to leaves as a flux driven by a difference in soil and leaf water potentials across a resistance between the bulk soil and the leaf.     

Osmoregulation in Cotton in Response to Water Stress : II. LEAF CARBOHYDRATE STATUS IN RELATION TO OSMOTIC ADJUSTMENT

Diurnal changes in tissue water potential components, photosynthesis, and specific leaf carbohydrates were examined in water stress-adapted and nonadapted cotton plants. Adapted plants exhibited lower daily minimum leaf water potentials and maintained turgor to lower leaf water potentials than nonadapted plants. Because of this turgor maintenance, photosynthesis continued in adapted plants at leaf water potentials that inhibited photosynthesis in nonadapted plants. Adapted plants exhibited lower rates of photosynthesis than did nonadapted plants when leaves were fully turgid. The inhibition was not due to stomatal restriction of CO₂ diffusion because leaf conductances of nonadapted and adapted leaves were similar at high leaf water potentials.     

Is proline involved in stomata regulation of Commelina communis plants recovering from salinity stress?

Plants of Commelina communis L. were grown in culture solution to which NaCl was added for 48 h. The solutions were then replaced with normal medium, so that the plants could recover from the stress. The water potential increased almost to that of the controls during 4 h of recovery, but stomatal resistance stayed high. Cytokinin treatment of leaf discs failed to enhance recovery of stomatal aperture, although it enhanced stomatal recovery of identically treated epidermal tissue. Proline levels in leaves correlated well with stomatal resistance. Incubation of epidermal tissue in D-proline inhibited stomatal opening. NaCl and benzyladenine interacted with the effect of proline, and the effect of abscisic acid and was additive to that of proline.     

Unraveling the Effects of Plant Hydraulics on Stomatal Closure during Water Stress in Walnut

The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia x nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (Eplant) and stomatal conductance (gs). Potted trees were exposed to a soil water depletion to alter soil water potential (Psisoil), soil resistance (Rsoil), and root hydraulic resistances (Rroot). Soil temperature was changed to alter Rroot alone. Embolism was created in the trunk to increase shoot resistance (Rshoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P(rachis)) above -1.4 MPa and the leaf water potential (Psileaf) above -1.6 MPa. The same dependence of Eplant and gs on P(rachis) or Psileaf was always observed. This suggested that stomata were not responding to changes in Psisoil, Rsoil, Rroot, or Rshoot per se but rather to their impact on P(rachis) and/or Psileaf. Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa. The minimum Psileaf values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P(rachis) might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.     

How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponically were exposed to 5 degrees C for up to 4 d; stem girdling was used to inhibit the phloem transport from the leaves to the roots. After initial water stress, stomatal closure and an amelioration of root water transport properties allowed the plants to rehydrate and regain turgor. Chilling augmented the concentration of abscisic acid (ABA) content in leaves, roots and xylem sap. In intact plants stomatal closure and leaf ABA accumulation were preceded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdling did not affect the influence of chilling on plant water relations and leaf ABA content, but it reduced slightly the alkalinization of xylem sap and, principally, prevented the massive ABA accumulation in root tissues and the associated transport in the xylem that was observed in non-girdled plants. When the plants were defoliated just prior to chilling or after 10 h at 5 degrees C, root and xylem sap ABA concentration remained unchanged throughout the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humidity), there were no significant variations in endogenous ABA levels. The increase in root hydraulic conductance in chilled plants was a response neither to root ABA accretion, nor to some leaf-borne chemical signal transported downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with plants chilled at 100% relative humidity. It was concluded that the root system contributed substantially to the bean's ability to cope with chilling-induced water stress, but not in an ABA-dependent manner.     

Sequential leaf senescence and correlatively controlled increases in xylem flow resistance

In bean, Phaseolus vulgaris L. (Contender), the directly measured hydraulic resistance of the xylem pathway between roots and primary leaf pulvinal junctions increased rapidly and progressively from 21 to 28 days after planting. These increases in xylem resistance (+390%) were specifically located in the pulvinal junction of the primary leaf. Moreover, they occurred just prior to the onset of primary leaf yellowing. Developmental increases in xylem hydraulic flow resistance and stomatal resistance, as well as subsequent primary leaf yellowing, were completely prevented by detopping the shoots above the primary leaves at 21 days. Thus, the onset of these senescence-associated symptoms was correlatively controlled. In short-term investigations of the mechanisms involved, flow between petiole and cut tip of excised leaves was rapidly reduced by infiltration of 20 picomoles of soluble dextran into the xylem. Moreover, imbibition of approximately 120 picomoles of dextran by excised leaves increased stomatal resistances. A programmed secretion of hormonal concentrations of similar polysaccharides into specific xylem sections in vivo might provide a mechanism for regulating the partitioning of essential xylem supplies between leaves, thus inducing sequential leaf senescence.