Hydraulic properties of rice and the response of gas exchange to water stress

We investigated the role of xylem cavitation, plant hydraulic conductance, and root pressure in the response of rice (Oryza sativa) gas exchange to water stress. In the field (Philippines), the percentage loss of xylem conductivity (PLC) from cavitation exceeded 60% in leaves even in watered controls. The PLC versus leaf water potential relationship indicated diurnal refilling of cavitated xylem. The leaf water potential causing 50 PLC (P(50)) was -1.6 MPa and did not differ between upland versus lowland rice varieties. Greenhouse-grown varieties (Utah) were more resistant to cavitation with a 50 PLC of -1.9 MPa but also showed no difference between varieties. Six-day droughts caused concomitant reductions in leaf-specific photosynthetic rate, leaf diffusive conductance, and soil-leaf hydraulic conductance that were associated with cavitation-inducing water potentials and the disappearance of nightly root pressure. The return of root pressure after drought was associated with the complete recovery of leaf diffusive conductance, leaf-specific photosynthetic rate, and soil-leaf hydraulic conductance. Root pressure after the 6-d drought (61.2 +/- 8.8 kPa) was stimulated 7-fold compared with well-watered plants before drought (8.5 +/- 3.8 kPa). The results indicate: (a) that xylem cavitation plays a major role in the reduction of plant hydraulic conductance during drought, and (b) that rice can readily reverse cavitation, possibly aided by nocturnal root pressure.     

Embolized conduits of rice (Oryza sativa, Poaceae) refill despite negative xylem pressure

Embolism reversal in rice plants was studied by testing the plant's ability to refill embolized conduits while xylem pressures were substantially negative. Intact, potted plants were water-stressed to a xylem pressure of -1.88 ± 0.1 MPa and a 66.3 ± 3.8% loss of xylem conductivity (PLC) by cavitation. Stressed plants were carefully rewatered, allowing xylem pressure to rise, but not above the theoretical threshold of c. -0.15 MPa for embolism collapse. Despite xylem pressures being more negative than this threshold, the PLC fell significantly (28.5 ± 5.6%), indicating the refilling of vessels. Above c. -1.0 MPa, almost all plants regained their maximum hydraulic conductivity. Dye uptake experiments showed the same pattern of embolism refilling despite negative pressure. Refilling was prevented in plants that were light-starved for 5 d, suggesting the unknown mechanism is dependent on metabolic energy. Results are among the first showing that herbaceous plants can reverse embolism without bulk xylem pressures rising near or above atmospheric.     

Cavitation fatigue. Embolism and refilling cycles can weaken the cavitation resistance of xylem

Although cavitation and refilling cycles could be common in plants, it is unknown whether these cycles weaken the cavitation resistance of xylem. Stem or petiole segments were tested for cavitation resistance before and after a controlled cavitation-refilling cycle. Cavitation was induced by centrifugation, air drying of shoots, or soil drought. Except for droughted plants, material was not significantly water stressed prior to collection. Cavitation resistance was determined from "vulnerability curves" showing the percentage loss of conductivity versus xylem pressure. Two responses were observed. "Resilient" xylem (Acer negundo and Alnus incana stems) showed no change in cavitation resistance after a cavitation-refilling cycle. In contrast, "weakened" xylem (Populus angustifolia, P. tremuloides, Helianthus annuus stems, and Aesculus hippocastanum petioles) showed considerable reduction in cavitation resistance. Weakening was observed whether cavitation was induced by centrifugation, air dehydration, or soil drought. Observations from H. annuus showed that weakening was proportional to the embolism induced by stress. Air injection experiments indicated that the weakened response was a result of an increase in the leakiness of the vascular system to air seeding. The increased air permeability in weakened xylem could result from rupture or loosening of the cellulosic mesh of interconduit pit membranes during the water stress and cavitation treatment.     

Stomatal factors and vulnerability of stem xylem to cavitation in poplars

The relationships between the vulnerability of stem xylem to cavitation, stomatal conductance, stomatal density, and leaf and stem water potential were examined in six hybrid poplar (P38P38, Walker, Okanese, Northwest, Assiniboine and Berlin) and balsam poplar (Populus balsamifera) clones. Stem xylem cavitation resistance was examined with the Cavitron technique in well-watered plants grown in the greenhouse. To investigate stomatal responses to drought, plants were subjected to drought stress by withholding watering for 5 (mild drought) and 7 (severe drought) days and to stress recovery by rewatering severely stressed plants for 30 min and 2 days. The clones varied in stomatal sensitivity to drought and vulnerability to stem xylem cavitation. P38P38 reduced stomatal conductance in response to mild stress while the balsam poplar clone maintained high leaf stomatal conductance under more severe drought stress conditions. Differences between the severely stressed clones were also observed in leaf water potentials with no or relatively small decreases in Assiniboine, P38P38, Okanese and Walker. Vulnerability to drought-induced stem xylem embolism revealed that balsam poplar and Northwest clones reached loss of conductivity at lower stem water potentials compared with the remaining clones. There was a strong link between stem xylem resistance to cavitation and stomatal responsiveness to drought stress in balsam poplar and P38P38. However, the differences in stomatal responsiveness to mild drought suggest that other drought-resistant strategies may also play a key role in some clones of poplars exposed to drought stress.     

Variation in embolism occurrence and repair along the stem in drought‐stressed and re‐watered seedlings of a poplar clone

Root pressure and plasma membrane intrinsic protein (PIP) availability in the xylem have been recognized to participate in the refilling of embolized conduits, yet integration of the two mechanisms has not been reported in the same plant. In this study, 4‐month‐old seedlings of a hybrid poplar (Populus alba × Populus glandulosa) clone 84K were subjected to two contrasting soil‐water treatments, with the drought treatment involving withholding of water for 17 days to reduce the soil‐water content to 10% of the saturated field capacity, followed by a re‐watering cycle. The percentage loss of stem hydraulic conductance (PLC) sharply increased, and stomatal conductance and photosynthesis declined in response to drought stress; these processes were gradually restored following the subsequent re‐watering. Embolism was most severe in the middle portions of the stem, followed by the basal and top portions of the stems of seedlings subjected to drought stress and subsequent re‐watering. Although drought stress eliminated root pressure, re‐watering partially restored it in a short period of time. The expression of PIP genes in the xylem was activated by drought stress, and some PIP genes were further stimulated in the top portion after re‐watering. The dynamics of root pressure and differential expression of PIP genes along the stem coincided with changes in PLC, suggesting that root pressure and PIPs work together to refill the embolized vessels. On the basis of the recovery dynamics in PLC and g_smax (maximum stomatal conductance) after re‐watering, the stomatal closure and xylem cavitation exhibited fatigue due to drought stress.     

不同水分条件下两个树种木质部栓塞对P素添加的响应

 在两种水分供给(干旱胁迫和适宜水分,土壤含水量分别为田间持水量的30%～40%和70%～80%)下,研究了耐旱树种元宝枫(Acer truncatum)和 中生树种女贞(Ligustrum lucidum )木质部栓塞(以导水率(Percentage loss of hydraulic conductivity, PLC)损失程度衡量)对P素添加的 响应。结果发现,两个树种PLC的日变化均呈现出先上升后降低的规律,表明木质部栓塞的形成与恢复是植物体的一种平常事件;除适宜水分条 件的女贞外,P素可以显著提高元宝枫和遭受干旱胁迫时女贞的PLC;两种水分条件下,干旱胁迫时元宝枫木质部栓塞明显高于适宜水分供给时 。女贞的PLC在两种水分状况下无显著差异;树种间,干旱胁迫促进了元宝枫木质部的栓塞形成,明显高于同等水分条件下的女贞。该研究结果 证实了“木质部限流耐旱假设”。     

Cryo-scanning electron microscopy observations of vessel content during transpiration in walnut petioles. Facts or artifacts?

The current controversy about the "cohesion-tension" of water ascent in plants arises from the recent cryo-scanning electron microscopy (cryo-SEM) observations of xylem vessels content by Canny and coworkers (1995). On the basis of these observations it has been claimed that vessels were emptying and refilling during active transpiration in direct contradiction to the previous theory. In this study we compared the cryo-SEM data with the standard hydraulic approach on walnut (Juglans regia) petioles. The results of the two techniques were in clear conflict and could not both be right. Cryo-SEM observations of walnut petioles frozen intact on the tree in a bath of liquid nitrogen (LN(2)) suggested that vessel cavitation was occurring and reversing itself on a diurnal basis. Up to 30% of the vessels were embolized at midday. In contrast, the percentage of loss of hydraulic conductance (PLC) of excised petiole segments remained close to 0% throughout the day. To find out which technique was erroneous we first analyzed the possibility that PLC values were rapidly returned to zero when the xylem pressures were released. We used the centrifugal force to measure the xylem conductance of petiole segments exposed to very negative pressures and established the relevance of this technique. We then analyzed the possibility that vessels were becoming partially air-filled when exposed to LN(2). Cryo-SEM observations of petiole segments frozen shortly after their xylem pressure was returned to atmospheric values agreed entirely with the PLC values. We confirmed, with water-filled capillary tubes exposed to a large centrifugal force, that it was not possible to freeze intact their content with LN(2). We concluded that partially air-filled conduits were artifacts of the cryo-SEM technique in our study. We believe that the cryo-SEM observations published recently should probably be reconsidered in the light of our results before they may be used as arguments against the cohesion-tension theory.     

A wilty mutant of rice has impaired hydraulic conductance

The rice CM2088 mutant is the wilty phenotype and wilts markedly under well-watered sunny conditions. The leaf water potential and epidermal (mainly stomatal) conductance of CM2088 plants decreased significantly under conditions that induced intense transpiration, as compared with those of wild-type plants, revealing that the wilty phenotype was not the result of abnormal stomatal behavior but was due to an increase in resistance to water transport. The resistance to water transport was dramatically elevated in the node and the sheath and blade of a leaf of the mutant, but not in the root or stem. The diameter of xylem vessels in the large vascular bundles of the leaf sheath and the internode tended to be small, and the numbers of vessel elements with narrowed or scalariform perforation plates in the leaf blade and sheath were greater in the mutant than in the wild type. Most xylem vessels were occluded, with air bubbles in the leaf sheath of the mutant during the midday hours under intense transpiration conditions, while no bubbles were observed in plants that were barely transpiring, revealing that the significant increase in resistance to water transport was a result of the cavitation. The additive effects of cavitation in xylem vessels and the decreased diameter and deformed plates of vessel elements might be responsible for the wilty phenotype of CM2088.     

Water loss regulation to soil drought associated with xylem vulnerability to cavitation in temperate ring-porous and diffuse-porous tree seedlings

Key message

Sustainable stomatal opening despite xylem cavitation occurs in ring-porous species and stomatal closure prior to cavitation in diffuse-porous species during soil drought.

Abstract

To elucidate the relationship between water loss regulation and vulnerability to cavitation associated with xylem structure, stomatal conductance (g _s), defoliation, vulnerability curves, and vessel features were measured on seedlings of ring-porous Zelkova serrata and Melia azedarach, and diffuse-porous Betula platyphylla var. japonica, Cerasus jamasakura and Carpinus tschonoskii. Under prolonged drought conditions, the percentage loss of hydraulic conductivity (PLC) increased and g _s decreased gradually with decreasing predawn (Ψ_pd) or xylem water potential (Ψ_xylem) in Z. serrata. During the gentle increase of PLC in M. azedarach, g _s increased in the early stages of dehydration while leaves were partly shed. A sharp reduction in g _s was observed before the onset of an increase in the PLC for drying plants of the three diffuse-porous species, suggesting cavitation avoidance by stomatal regulation. In the ring-porous species, xylem-specific hydraulic conductivity (K _s) was higher, whereas the vessel multiple fractions, the ratio of the number of grouped vessels to total vessels, was lower than that in the diffuse-porous species, suggesting that many were distributed as solitary vessels. This may explain the gradual increase in the PLC with decreasing Ψ_xylem because isolated vessels provide less opportunity for air seeding. Different water loss regulation to soil drought was identified among the species, with potential mechanisms being sustainable gas exchange at the expense of xylem dysfunction or partial leaf shedding, and the avoidance of xylem cavitation by strict stomatal regulation. These were linked to vulnerability to cavitation that appears to be governed by xylem structural properties.     

Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water status

The long-term interaction between elevated CO₂ and soil water deficit was analysed in N₂-fixing alfalfa plants in order to assess the possible drought tolerance effect of CO₂. Elevated CO₂ could delay the onset of drought stress by decreasing transpiration rates, but this effect was avoided by subjecting plants to the same soil water content. Nodulated alfalfa plants subjected to ambient (400 μmol mol^?1) or elevated (700 μmol mol^?1) CO₂ were either well watered or partially watered by restricting water to obtain 30% of the water content at field capacity (ampproximately 0.55 g water cm^?3). The negative effects of soil water deficit on plant growth were counterbalanced by elevated CO₂. In droughted plants, elevated CO₂ stimulated carbon fixation and, as a result, biomass production was even greater than in well-watered plants grown in ambient CO₂. Below-ground production was preferentially stimulated by elevated CO₂ in droughted plants, increasing nodule biomass production and the availability of photosynthates to the nodules. As a result, total nitrogen content in droughted plants was higher than in well-watered plants grown in ambient CO₂. The beneficial effect of elevated CO₂ was not correlated with a better plant water status. It is concluded that elevated CO₂ enhances growth of droughted plants by stimulating carbon fixation, preferentially increasing the availability of photosynthates to below-ground production (roots and nodules) without improving water status. This means that elevated CO₂ enhances the ability to produce more biomass in N₂-fixing alfalfa under given soil water stress, improving drought tolerance.     

Arbuscular mycorrhizal influence on leaf water potential, solute accumulation, and oxidative stress in soybean plants subjected to drought stress

This study investigated several aspects related to drought tolerance in arbuscular mycorrhizal (AM) soybean plants. The investigation included both shoot and root tissues in order to reveal the preferred target tissue for AM effects against drought stress. Non-AM and AM soybean plants were grown under well-watered or drought-stressed conditions, and leaf water status, solute accumulation, oxidative damage to lipids, and other parameters were determined. Results showed that AM plants were protected against drought, as shown by their significantly higher shoot-biomass production. The leaf water potential was also higher in stressed AM plants (-1.9 MPa) than in non-AM plants (-2.5 MPa). The AM roots had accumulated more proline than non-AM roots, while the opposite was observed in shoots. Lipid peroxides were 55% lower in shoots of droughted AM plants than in droughted non-AM plants. Since there was no correlation between the lower oxidative damage to lipids in AM plants and the activity of antioxidant enzymes, it seems that first the AM symbiosis enhanced osmotic adjustment in roots, which could contribute to maintaining a water potential gradient favourable to the water entrance from soil into the roots. This enabled higher leaf water potential in AM plants during drought and kept the plants protected against oxidative stress, and these cumulative effects increased the plant tolerance to drought.     

In vivo observation of cavitation and embolism repair using magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to noninvasively monitor the status of individual xylem vessels in the stem of an intact, transpiring grape (Vitis vinifera) plant over a period of approximately 40 h. Proton density-weighted MRI was used to visualize the distribution of mobile water in the stem and individual xylem vessels were scored as either water or gas filled (i.e. embolized). The number of water-filled vessels decreased during the first 24 h of the experiment, indicating that approximately 10 vessels had cavitated during this time. Leaf water potentials decreased from -1.25 to -2.1 MPa during the same period. Watering increased leaf water potentials to -0.25 MPa and prevented any further cavitation. Refilling of xylem vessels occurred as soon as the lights were switched off, with the majority of vessels becoming refilled with water during the first 2 to 3 h in darkness. These measurements demonstrate that MRI can be used to monitor the functional status of individual xylem vessels, providing the first method to study the process of cavitation and embolism repair in intact plants.     

Does freezing and dynamic flexing of frozen branches impact the cavitation resistance of Malus domestica and the Populus clone Walker?

Frost damage to the xylem conduits of trees is a phenomenon of eco-physiological importance. It is often documented in terms of the percentage loss of conductivity (PLC), an indicator of air filling of the conduits. However, trees that refill their conduits in spring could be impacted more by damage to the conduits that reduce cavitation resistance, making them more susceptible to future drought events. We investigated whether ice formation, dynamic flexing of frozen branches or freeze–thaw events could reduce the cavitation resistance (cause “frost fatigue”) in first-year shoots of apple (Malus domestica) and clonal hybrid cottonwood (Walker). Frost fatigue was measured in terms of P₅₀ (the negative xylem pressure required to cause a 50 % loss of conductivity). All treatment groups showed significant frost fatigue, with the exception of the pre-flushed, constantly frozen poplar branches. The P₅₀ following freeze treatments was approximately 50 % of the pre-freeze values. The effect tended to be greater in freeze–thawed branches. Dynamic bending of the branches had no effect on either PLC or P₅₀. In three out of four cases, there was a significant correlation between P₅₀ and PLC. Frost fatigue occurred in both apple and poplar, two unrelated species with different drought and frost tolerances, suggesting that it may be a widespread phenomenon that could impact the ecophysiology of temperate forests.     

Impact of long-term drought on xylem embolism and growth in Pinus halepensis Mill.

 The present study was carried out to elucidate the response mechanisms of 50-year-old Pinus halepensis Mill. trees to a long-term and severe drought. The amount of water available to trees was artificially restricted for 12 months by covering the soil with a plastic roof. Over the short term a direct and rapid impact of drought was evident on the water relations and gas exchanges of trees: as the soil dried out in the Spring, there was a concurrent decrease of predawn water potential; transpiration was strongly reduced by stomatal closure. Seasonal changes in the water volume fractions of twig and stem xylem were observed and interpreted as the result of cavitation and refilling in the xylem. When droughted trees recovered to a more favourable water status, refilling of embolized xylem was observed; twig predawn water potentials were still negative in the period when the embolism was reversed in the twig xylem. A few months after the removal of the covering, no differences in whole plant hydraulic resistance were observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees; no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. An after-effect of the drought on growth was observed. Received: 4 August 1997 / Accepted: 1 October 1997     

Pod set related to photosynthetic rate and endogenous ABA in soybeans subjected to different water regimes and exogenous ABA and BA at early reproductive stages

BACKGROUND AND AIMS: The physiological reasons for reduced pod set in soybean (Glycine max) caused by drought during anthesis are not established. The objective of this study was to investigate the involvement of photosynthetic rate (A), pod endogenous abscisic acid (ABA) and exogenously applied ABA and 6-benzylaminopurine (BA) in regulating pod set in soybean during drought. METHODS: Two pot experiments were done in a controlled-environment glasshouse. In expt I, soybeans were either well-watered (WW) or droughted by withholding water from 4 d before to 4 d after anthesis (DAA). In expt II, soybeans were drought-stressed (DS) from -4 to 4 DAA. From -2 to 4 DAA, some of the WW and DS plants were treated with 0.1 mm ABA or 1 mm BA. KEY RESULTS: Drought stress decreased A, but increased pod ABA concentration ([ABA]). Pod set decreased only when A had decreased by 40 %, and pod [ABA] had increased 1.5-fold. Beyond the thresholds, pod set correlated positively with A and negatively with pod [ABA]. Exogenously applied ABA decreased A and pod set in WW plants, whilst it increased A and pod set in DS plants; exogenous BA had opposite effects. In these plants, pod set correlated linearly with A. CONCLUSIONS: Drought-induced decrease in A is significant in inducing pod abortion, probably as a consequence of carbohydrate deprivation. The effects of ABA and BA on pod set may be partially due to their effects on photosynthate supply.     

Vessel-diameter distribution in roots of grapevines (Vitis vinifera L. cv. Shiraz)

The distribution frequency patterns of diameter of xylem vessels and percentage of total predicted axial conductances were studied in 190-day and 212-day-old main roots of grapevine (Vitis vinifera L. cv. Shiraz) grown under well-watered and stressed conditions. The protoxylem were the first to mature and were responsible for most of the theoretical conductance in root segments between the tip and 2.5 cm from the tip. Some large xylem vessels retained cross walls and protoplasm up to 22.5 cm from the tip. Statistical tests using the Kolmogorov-Smirnov two sample test showed that the pattern of distribution frequency of xylem vessels classified in different diameter classes varied with distance from the root tip. The distribution frequency of xylem vessels was similar in both well-watered and stressed plants from the tip up to 15 cm from the tip. At distances further from the tip the distribution frequency of xylem vessels of well-watered plants was significantly different from that of stressed plants, with the former having more larger vessels than the latter. The pattern of vessel distribution frequency was different from that of percent total axial conductance (K_h) predicted with fewer large vessels carrying most of the axial flow.     

Frost fatigue and spring recovery of xylem vessels in three diffuse‐porous trees in situ

Frost has been shown to cause frost fatigue (reduced cavitation resistance) in branch segments in the lab. Here, we studied the change in cavitation resistance and percent loss of conductivity (PLC) from fall to spring over 2 consecutive years in three diffuse‐porous species in situ. We used the cavitron technique to measure P₂₅, P₅₀ and P₉₀ (the xylem pressure causing a 25, 50 and 90% conductivity loss) and PLC and stained functioning vessels. Cavitation resistance was reduced by 64–87% (in terms of P₅₀), depending on the species and year. P₂₅ was impacted the most and P₉₀ the least, changing the vulnerability curves from s‐ to r‐shaped over the winter in all three species. The branches suffered an almost complete loss of conductivity, but frost fatigue did not necessarily occur concurrently with increases in PLC. In two species, there was a trade‐off between conduit size and vulnerability. Spring recovery occurred by growth of new vessels, and in two species by partial refilling of embolized conduits. Although newly grown and functioning conduits appeared more vulnerable to cavitation than year‐old vessels, cavitation resistance generally improved in spring, suggesting other mechanisms for partial frost fatigue repair.     

Effects of water stress on gas exchange and water relations of a succulent epiphyte,Kalanchoë uniflora

Summary Measurements of gas exchange, xylem tension and nocturnal malate synthesis were conducted with well-watered and droughted plants of Kalanchoë uniflora. Corresponding results were obtained with plants grown in 9 h and 12 h photoperiods. In well-watered plants, 50 to 90% of total CO₂-uptake occurred during the light period. Nocturnal CO₂-uptake and malate synthesis were higher and respiration rate was lower in old leaves (leaf pairs 6 to 10) compared to young leaves (leaf pairs 1 to 5). Within four days of drought distinct physiological changes occurred. Gas exchange during the light period decreased and CO₂-uptake during the dark period increased. Nocturnal malate synthesis significantly increased in young leaves.Respiration rate decreased during periods of drought, this decrease being more pronounced in young leaves compared to old leaves. Restriction of gas exchange during the light period resulted in a decrease of transpiration ratio from more than 100 to about 20. The difference between osmotic pressure and xylem tension decreased in young leaves, indicating a reduction in bulk leaf turgor-pressure.We conclude that both the CAM-enhancement in young leaves and the decrease of respiration rate are responsible for the increase of nocturnal CO₂-uptake during water stress. During short drought periods, which frequently occur in humid habitats, the observed physiological changes result in a marked reduction of water loss while net CO₂-uptake is maintained. This might be relevant for plant growth in the natural habitat.Abbreviations LP light period - DP dark period - CAM crassulacean acid metabolism     

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.     

Abscisic acid concentrations and fluxes in droughted conifer saplings

We present the first study of abscisic acid (ABA) concentrations and fluxes in the xylem sap of conifers during a drought cycle. In both Pinus sylvestris and Picea sitchensis the concentration of ABA in the sap rose 11-fold as the drought progressed. There were clear diurnal trends in this concentration, which reached its maximum (6–8.ininol ABA m^?3) near the middle of the day. The fluxes of ABA were calculated by multiplying the xylem ABA concentration by the sap flow rate. The ABA fluxes in the droughted plants in the middle of the day were usually no higher than those of the controls, as a result of the very low sap flow in the droughted plants at that time. However, the ABA flux in the droughted plants was higher than in the controls in the morning, and we postulate that the stomata are responding to these ‘morning doses’ Stomatal conductance, g_s, could not be related statistically to leaf turgor or to the ABA flux. However, £s did display a negative exponential relationship with ABA concentration in the xylem. Pinus displayed more acclimation to drought than Picea, Its ABA concentration rose and its stomatal conductance fell at day 6 of the drought, as opposed to day 17 for Picea, and its osmotic potential fell during the drought treatment.