Factors involved in alleviating water stress by partial crop removal in pear trees

We studied the relief of water stress associated with fruit thinning in pear (Pyrus communis L.) trees during drought to determine what mechanisms, other than stomatal adjustment, were involved. Combinations of control irrigation (equal to crop water use less effective rainfall) and deficit irrigation (equal to 20% of control irrigation), fruit load (unthinned and thinned to 40 fruits per tree) and root pruning (pruned and unpruned) treatments were applied to pear (cv. 'Conference') trees during Stage II of fruit development. Daily patterns of midday stem water potential (Psi(stem)) and leaf conductance to water vapor (g(l)) of deficit-irrigated trees differed after fruit thinning. In response to fruit thinning, gl progressively declined with water stress until 30 days after fruit thinning and then leveled off, whereas the effects of decreased fruit load on Psi(stem) peaked 30-40 days after fruit thinning and then tended to decline. Soil water depletion was significantly correlated with fruit load during drought. Our results indicate that stomatal adjustment and the resulting soil water conservation were the factors determining the Psi(stem) response to fruit thinning. However, these factors could not explain differences in daily patterns between g(l) and Psi(stem) after fruit thinning. In all cases, effects of root pruning treatments on Psi(stem) in deficit-irrigated trees were transitory (Psi(stem) recovered from root pruning in less than 30 days), but the recovery of Psi(stem) after root pruning was faster in trees with low fruit loads. This behavior is compatible with the concept that the water balance (reflected by Psi(stem) values) was better in trees with low fruit loads compared with unthinned trees, perhaps because more carbon was available for root growth. Thus, a root growth component is hypothesized as a mechanism to explain the bimodal Psi(stem) response to fruit thinning during drought.     

Response of winter root starch concentration to severe water stress and fruit load and its subsequent effects on early peach fruit development

Effect of water stress during stage III of peach fruit development on winter root starch concentration (RSC) and subsequent reproductive development was studied. Two irrigation treatments were applied in two consecutive seasons (2003-2004): full irrigation (FI) and no irrigation during stage III of fruit development until visible leaf wilting (LWI), which occurred when midday stem water potential reached -1.80 MPa. Three fruit thinning intensities were applied within each irrigation treatment. The year 2005 was a recovery year in which all trees received full irrigation and commercial fruit thinning. Water deficit and high fruit loads in the previous season significantly reduced the concentration of winter RSC. Fruit set and fruit growth from full bloom to 30 days after full bloom (30 DAFB) increased with increasing winter RSC before other factors, such as inter-fruit competition and availability of carbon from current photosynthesis, came into play. Consequently, severe water stress reduced the total number of fruits and fruit dry mass growth 30 DAFB. However, during the recovery year and after fruit thinning, fruit loads were similar between irrigation treatments and yield capacity remained unaffected. Peach fruit production recovered quickly from the deleterious effects of two consecutive years of water stress because of a combination of two factors: (1) reduced initial fruit set that was still adequate to achieve a commercial crop; and (2) the low sensitivity of fruit growth 30 DAFB to winter RSC.     

Responses of apple fruit size to tree water status and crop load

The combined effects of irrigation rate and crop load on apple yield and fruit size were examined in two commercial apple orchards (cv. Golden Delicious) in a semi-arid zone. The irrigation rates applied were 1, 3 and 7 mm day(-1), and the two fruit thinning treatments involved adjusting crop load to 100 and 300 fruits per tree at Ortal and 50 and 150 fruits per tree at Matityahu. Unthinned trees served as the control. The fruit from each tree was picked separately, and fruit size distribution was determined with a commercial grading machine. Midday stem water potentials varied from -0.9 to -2.8 MPa, crop load varied from 80,000 to 1,900,000 fruit ha(-1) and crop yield varied from 10 to 144 Mg ha(-1). Midday stem water potential decreased with increasing crop load in all irrigation treatments at Matityahu, but only in the 1 mm day(-1) treatment at Ortal. The extent of the lowering of midday stem water potential by crop load decreased with increasing soil water availability. At both orchards, a similar response of total crop yield to crop load on a per hectare basis was observed. Mean fruit mass and relative yield of fruit > 70 mm in diameter increased with midday stem water potential, with the low crop loads having similar but steeper slopes than the high crop load. The responses of mean fruit mass and relative yield of fruit > 70 mm in diameter to midday stem water potential were similar at both orchards, perhaps indicating that thresholds for irrigation scheduling are transferable to other orchards within a region. Factors that may limit the transferability of these thresholds are discussed.     

Crop load affects maximum daily trunk shrinkage of plum trees

We studied the effects of low fruit load (3-4 fruits cm(-2) of trunk cross-sectional area (TCSA), and high fruit load (6-7 fruits cm(-2) TCSA) on maximum daily trunk shrinkage (MDS) and trunk growth rates (TGR) over two seasons in plum (Prunus salicina Lindell) trees receiving full irrigation or deficit irrigation. Seasonal changes in MDS and TGR were compared with those in midday stem water potential (Psi(s)) and leaf stomatal conductance (g (s)). Crop load increased g (s) in fully irrigated trees approaching harvest. Although crop load did not affect plant water status in either watering regime, there were considerable differences in both MDS and TGR as a function of crop load. Compared with low-cropping [corrected] trees, MDS was 34% higher and TGR was 48% lower in high-cropping [corrected] trees. The differential responses of MDS and Psi(s) to crop load were a consequence of a higher MDS for a given Psi(s) in the high-cropping trees compared with the low-cropping trees. There was a linear increase in MDS with crop load, with a slope of 15.2 microm MPa(-1) per unit increment of crop load. In the fully irrigated trees, day-to-day variations in MDS were related to evaporative demand; however, the slope of the relationship between MDS and evaporative demand increased with crop load, indicating that different reference equations must be used to adjust for tree crop load when using MDS to determine plant water status and irrigation requirements.     

Effects of irrigation deprivation during the harvest period on nonstructural carbohydrate and nitrogen contents of dormant, mature almond trees

Effect of irrigation deprivation during the harvest period on the nonstructural carbohydrate (NC) content of dormant, mature, field-grown almond (Prunus dulcis (Mill.) D.A. Webb cv. Nonpareil) trees was studied. Roots, trunk, branches, spurs and stems of 12 trees were subsampled in February 1997, across a gradient of irrigation treatments (FI = fully irrigated, MS = moderately stressed and SS = severely stressed) to relate NC concentration to the degree of water stress experienced by individual trees during the previous (1996) harvest period. To assess the effect of water stress on whole-tree NC content, three dormant FI trees and three dormant SS trees were excavated on December 10, 1997, and dry weights and NC and N concentrations of the tree components were determined. Whole-tree biomass did not differ significantly between FI and SS trees, although SS trees tended to have less total dry weight. Although roots constituted just 13% of tree biomass, they stored 36 and 44% of tree NC and N contents, respectively. There were negative relationships between the seasonal minimum values of both midday (Psi(ms)) and predawn (Psi(pd)) stem water potentials during the harvest period and root NC content of dormant trees. Severe water stress during the harvest period resulted in a 26% reduction in NC content and a 50% reduction in biomass of current-year stems (> 5 cm in length) per tree. The reduction in NC content is consistent with the previously reported late season reductions in leaf function and persistence. The SS trees exhibited a reduction in NC content but not in N content per tree, indicating that late season accumulation of NC and N were uncoupled in trees subjected to severe harvest-period water stress.     

Effects of irrigation deprivation during the harvest period on leaf persistence and function in mature almond trees

In nut tree orchards in California, irrigation is typically withheld during the harvest period to reduce the likelihood of bark damage during mechanical shaking of the trees. The ensuing water stress, however, may result in premature defoliation and subsequent yield declines. Our objective was to establish and quantify the water stress resulting from irrigation deprivation and determine its impact on leaf function and persistence in mature almond trees (Prunus dulcis (Mill.) D.A. Webb cv. Nonpareil) during a 3-year field experiment. The severity of the water stress was characterized by measurements of predawn leaf (Psi(pd)) and midday stem (Psi(ms)) water potentials, stomatal conductance (gs), net CO2 assimilation rate (A) and leaf abscission. During 1995, Psi(ms) of fully irrigated (FI) trees was maintained above -1.0 MPa. In trees in the moderate- (MS) and severe-stress (SS) treatments, Psi(ms) was reduced to -1.4 to -2.0 MPa and -2.0 to -2.6 MPa, respectively. After 18 days of irrigation deprivation, A was reduced by 32 and 58% at midday and early afternoon, respectively, compared with morning values. A significant decrease in morning values of A only occurred after 30 days of irrigation deprivation. Water-use efficiency and A declined as evaporative demand increased from morning to afternoon. Assimilation also declined seasonally as leaves aged. Midday stem water potential was highly correlated with A, but less so with gs. The coefficient of determination between Psi(ms) and gs improved considerably when vapor pressure deficit and wind were multiply regressed with Psi(ms). Although A recovered rapidly when MS trees were irrigated, recovery in SS trees was slower and incomplete. Integrating the MS and SS effects for an extended period during 1995 resulted in 14 and 30% declines in A, and 6 and 20% declines in gs, respectively. The apparent Psi(ms) threshold for leaf abscission was -1.8 MPa. Daily canopy light interception declined with decreasing Psi(ms) as a result of premature defoliation (and perhaps altered leaf angles) from 67.9% in FI trees to 61.4 and 60.7% in MS and SS trees, respectively.     

梯田堰边老龄低产花椒树更新改造研究

以梯田堰边老龄低产花椒为试材,采用随机区组设计进行不同修剪试验。发现刻芽、中截、重截、疏枝修剪处理均能促进新梢生长,提高叶绿素含量,显著提高花椒单株产量、穗重,增加花椒出皮率。以疏枝处理对花椒新梢生长、果实产量品质影响最显著。平茬更新处理的花椒树的叶色浓绿、叶片肥厚、生长壮旺,果实产量和枝量明显高于对照树,复壮效果明显。     

Regulated deficit irrigation and the recovery of water relations in pistachio trees

Recovery of water status in water-stressed pistachio trees (Pistacia vera L. cv. Kerman) was investigated by subjecting trees to regulated deficit irrigation (RDI) (60% of crop evapotranspiration rate, ET(c)) during stages I and II of fruit development (FD) followed by full irrigation during FD stage III (kernel-filling). Trees irrigated at 100% ET(c) throughout FD stages I, II and III served as controls. Water-stress severity was characterized by changes in soil water content and midday stem water potential (Psi(md)). Midday leaf conductance (g(1)) and trunk diameter variation (TDV) were also measured. In RDI trees, the lowest Psi(md) value, -1.8 MPa, occurred at the end of the RDI period. The corresponding value for the control trees was around -1.1 MPa. Although the RDI treatment affected gas exchange later than Psi(md), the greatest reductions in gas exchange (60% of control values) also appeared at the end of the RDI period. There were significant differences in TDV between control and RDI trees at the end of the RDI period. Although plant water status recovered within 20 days of resuming irrigation, the TDV values indicated a longer period might be necessary for complete recovery. Recovery of g(1) was faster than that of Psi(md), although differences in TDV between control and RDI trees indicated that gas exchange recovered later than Psi(md). The slow recovery of pistachio trees during FD stage III from water stress imposed during FD stages I and II suggests that irrigation should exceed 100% ET(c) during FD stage III or that more extensive irrigation should commence before the end of FD stage II.     

Crop load and water stress effects on daily stem growth in peach (Prunus persica)

We investigated crop load and water stress effects on diurnal stem extension growth of field-grown peach (Prunus persica (L.) Batsch) trees. Neither the presence of fruit nor reduced irrigation significantly altered the timing of diurnal fluctuations in stem growth rate. Stems with subtending fruit had significantly reduced growth compared to stems with no subtending fruit. Crop load had no significant effect on relative stem extension rates and the majority of the reduction in absolute growth was the result of a smaller zone of elongation in fruit-bearing stems than in stems with no subtending fruit. Fruit removal did not increase growth rates within 24 h. When irrigation was reduced, the length of the stem elongation zone and total daily stem growth were significantly decreased relative to well-irrigated controls and the decreases were highly correlated with stem water potential. Compared with well-irrigated controls, relative stem extension rates of water-stressed trees were reduced at several times during the 24-h period, but the degree of reduction was not proportional to the difference in stem water potentials between the treatments.     

Effects of water stress cycles on turgor maintenance processes in pear leaves (Pyrus communis)

Effects of water deficits on leaf turgor maintenance processes were analyzed for pear trees (Pyrus communis L. cv. "Barlett") grown in 120-liter containers. Four irrigation treatments were applied: a well-watered control treatment, a spring water stress cycle (Sp), a summer water stress cycle (Su), and a spring plus summer water stress cycle (Sp + Su). For the Sp treatment, water application was progressively reduced from 100 to 20% of the control dose over a period of 27 days in spring. For the Su treatment, water application was progressively reduced over 23 days in summer, from 100 to 20% of the control dose. The Sp + Su treatment comprised both the spring and summer drought stress cycles. Pressure-volume (P-V) curves were constructed and stomatal conductances were determined for pear leaves from each treatment during the spring and summer stress cycles. Leaf water potential (Psi(pi) (0)) and relative water content (R(0)) at the turgor loss point of control leaves tended to decrease from spring to summer. Changes in leaf osmotic water potential at full turgor (Psi(pi) (100)) and in symplast water fraction (R(s)) did not explain the seasonal decrease in Psi(pi) (0). The water stress treatments had no effect on Psi(pi) (100), but R(s) was reduced by the water stress treatments, particularly during the summer stress cycle of the Su and Sp + Su treatments. The decrease in R(s) was correlated with an increase in the slope of the linear region of the P-V curve. Such a coupled adjustment would lead to increased water uptake capacity of water-stressed trees only under non-turgor conditions. Furthermore, pear leaves did not actively accumulate solutes. We conclude, therefore, that changes in leaf tissue water relations as a result of leaf acclimation to water stress are unlikely to facilitate maintenance of fruit productivity under drought.     

Influence of branch autonomy on fruit,scaffold, trunk and root growth during stage III of peach fruit development

We studied the influence of branch autonomy on the growth of reproductive and vegetative organs by establishing different patterns of fruit distribution within and between large branch units (scaffolds) in mature peach trees (Prunus persica (L.) Batsch cv. 'Elegant Lady'). Different patterns of fruit distribution were established by defruiting either whole scaffolds (uneven fruit distribution between scaffolds; US) or several selected hangers (small fruiting branches) per tree (uneven fruit distribution between hangers; UH). The effects of these patterns were compared with the effects of an even fruit distribution treatment (EVEN) in which fruits were thinned to achieve maximum uniformity of fruit distribution within the canopy. The desired fruit loads were obtained by differentially thinning the remaining bearing parts. On a tree basis, the response of mean fruit mass to fruit load was strongly affected by fruit distribution. The steepest mean fruit mass to fruit load relationship was found in US trees, whereas the relationship in UH trees was intermediate between the US and EVEN trees. On a scaffold basis, differences in fruit size between EVEN and US trees with similar fruit loads, though statistically significant, were relatively small, indicating that scaffolds were almost totally autonomous with respect to dry matter partitioning to fruit during the final stage of peach fruit growth. Hangers also appeared to exhibit significant autonomy with respect to the distribution of dry matter during the final phase of fruit growth. Branch autonomy was evident in scaffold growth: defruited scaffolds in the US treatment grew more than fruited scaffolds, and fruit distribution treatments had little impact on scaffold cross-sectional area on a tree basis. On the other hand, as observed for fruit growth, branch autonomy did not appear to be complete because the fruited scaffolds grew more in US trees than in EVEN trees under heavy cropping conditions. However, the effect of fruit distribution occurred only over short distances, and was negligible on organs located farther away from the source of heterogeneity (fruits), such as the trunk and roots.     

Whole-plant adjustments in coconut (Cocos nucifera) in response to sink-source imbalance

Coconut (Cocos nucifera L.) is a perennial tropical monocotyledon that produces fruit continuously. The physiological function of the large amounts of sucrose stored in coconut stems is unknown. To test the hypothesis that reserve storage and mobilization enable the crop to adjust to variable sink-source relationships at the scale of the whole plant, we investigated the dynamics of dry matter production, yield and yield components, and concentrations of nonstructural carbohydrate reserves in a coconut plantation on Vanuatu Island in the South Pacific. Two treatments were implemented continuously over 29 months (April 2002 to August 2004): 50% leaf pruning (to reduce the source) and 100% fruit and inflorescence pruning (to reduce the sink). The pruning treatments had little effect on carbohydrate reserves because they affected only petioles, not the main reserve pool in the stem. Both pruning treatments greatly reduced dry matter production of the reproductive compartment, but vegetative growth and development were negligibly affected by treatment and season. Leaf pruning increased radiation-use efficiency (RUE) initially, and fruit pruning greatly reduced RUE throughout the experiment. Changes in RUE were negatively correlated with leaflet soluble sugar concentration, indicating feedback inhibition of photosynthesis. We conclude that vegetative development and growth of coconut show little phenotypic plasticity, assimilate demand for growth being largely independent of a fluctuating assimilate supply. The resulting sink-source imbalances were partly compensated for by transitory reserves and, more importantly, by variable RUE in the short term, and by adjustment of fruit load in the long term. Possible physiological mechanisms are discussed, as well as modeling concepts that may be applied to coconut and similar tree crops.     

节能温室栽培矮甜李修剪方法研究

通过夏季摘心处理和冬剪方法的对比得出:夏季摘心处理生长量大,果树成形早,定植第1、2 年夏剪生长量是冬剪的2~3倍,随着树龄的增大,两者生长量差距减少。中、短、花束状果枝比例也随树龄增大比例增加。     

Effects of cone-induction treatments on black spruce (Picea mariana) current-year needle development and gas exchange properties

Both drought and root pruning (RP) increased the number of cones induced when black spruce (Picea mariana (Mill.) B.S.P.) grafts were injected with gibberellins A(4/7) (GA), but their effects on predawn shoot water potential and current-year needle development differed. Drought decreased predawn shoot water potential (Psi(pd)), but only during the period when irrigation was withheld, and it had no effect on the growth or gas exchange properties of current-year needles. Conversely, root pruning had little effect on Psi(pd), but it resulted in trees with smaller current-year needles that had lower nitrogen and chlorophyll concentrations and reduced rates of gas exchange up to the later stages of shoot elongation compared with needles of control trees. These findings are discussed in relation to potential effects on the development of induced cones in the following growth cycle.     

夏秋季节水肥调控综合措施对油茶促花保果效果的影响

为了提高盛果期油茶林的产量,针对当前成林油茶落花落果率高而保花保果率低的问题,就不同的保墒、根外追施微肥与植物生长调节剂及冬季修剪等水肥调控综合技术措施对油茶保花保果效果的影响情况进行了试验研究。结果表明:(1)采用覆草、根外追施0.1 g/L的尿素与100 mg/L的GA3的坐果率最高;修剪与保水措施对坐果率的影响较大。(2)对油茶成林4月与7月保果率的影响最大的分别为修剪强度与保水措施,追施0.1g/L的硼肥有利于提高油茶4月与7月的保果率。(3)滴灌、喷施0.10 g/L的硼肥与100 mg/L的GA3及轻度修剪后油茶花芽分化数量最高;植物生长调节剂对油茶7月花芽分化的影响最大。     

Precommercial thinning of beech (Fagus sylvatica L.): Early effects of stump height on growth and natural pruning of potential crop trees

Abstract

This paper reports the early effects of stump height on the growth and natural pruning of potential crop trees after precommercial thinning of a young stand of naturally regenerated beech (Fagus sylvatica L.) in Denmark. The experiment comprises five treatments based on combinations of three grades of precommercial thinning and three stump heights. Treatments include the unthinned control, thinning only of whips and wolf trees (using low stumps), and thinning for potential crop trees using low, medium or high stumps. All treatments were replicated three times. Stump heights averaged 10, 90 or 230 cm, respectively. Pretreatment stem number (live trees) varied from 17,500 to 41,000 ha^?1. In the potential crop tree treatment, post-treatment stem number ranged from 4750 to 9500 ha^?1. Following two growth seasons, the quantity of stump regrowth increased with increasing stump height, the rate of stump regrowth increased with increasing stump height, the diameter growth of potential crop trees increased with decreasing stump height, the increase in stand height did not depend on stump height or post-treatment stem number, and the natural pruning of potential crop trees increased with increasing stump height. It remains to be seen whether these trends hold in the long run, and whether additional economic return from the increase in wood quality with increasing stump height compensates for the reduction in diameter growth.     

Physiological and morphological adaptations of the fruit tree Ziziphus rotundifolia in response to progressive drought stress

The physiological basis of drought resistance in Ziziphus rotundifolia Lamk., which is an important, multipurpose fruit tree of the northwest Indian arid zone, was investigated in a greenhouse experiment. Three irrigation regimes were imposed over a 34-day period: an irrigation treatment, a gradual drought stress treatment (50% of water supplied in the irrigation treatment) and a rapid drought stress treatment (no irrigation). Changes in gas exchange, water relations, carbon isotope composition and solute concentrations of leaves, stems and roots were determined. The differential rate of stress development in the two drought treatments did not result in markedly different physiological responses, but merely affected the time at which they were expressed. The initial response to decreasing soil water content was reduced stomatal conductance, effectively maintaining predawn leaf water potential (Psi(leaf)), controlling water loss and increasing intrinsic water-use efficiency, while optimizing carbon gain during drought. Carbon isotope composition (delta13C) of leaf tissue sap provided a more sensitive indicator of changes in short-term water-use efficiency than delta13C of bulk leaf tissue. As drought developed, osmotic potential at full turgor decreased and total solute concentrations increased in leaves, indicating osmotic adjustment. Decreases in leaf starch concentrations and concomitant increases in hexose sugars and sucrose suggested a shift in carbon partitioning in favor of soluble carbohydrates. In severely drought-stressed leaves, high leaf nitrate reductase activities were paralleled by increases in proline concentration, suggesting an osmoprotective role for proline. As water deficit increased, carbon was remobilized from leaves and preferentially redistributed to stems and roots, and leaves were shed, resulting in reduced whole-plant transpiration and enforced dormancy. Thus, Z. rotundifolia showed a range of responses to different drought intensities indicating a high degree of plasticity in response to water deficits.     

Response to the interaction of thinning and pruning of pine species in Mediterranean mountains

Pruning allows knot-free timber to be obtained, thereby increasing the value of the highest-value wood products. However, the effect of pruning on growth is under discussion, and knowledge about the tree response to the simultaneous development of thinning and pruning is scarce. The objective of this study was to analyze the effect of the interaction of thinning and pruning on tree and stand level and the annual radial growth of two pine species native to Mediterranean mountains. We used long-term data of three trials installed in pine stands where several combinations of pruning and thinning were developed. Five inventories were carried out for each trial, and the mean dasometric features of the different treatments were compared using linear mixed models including a competition index. In addition, we collected cores from ten trees per plot in order to evaluate the annual response of trees to the thinning and pruning. We analyzed the annual radial growth using a semiparametric approach through a smooth penalized spline including rainfall and temperature covariates. Pruning did not show any effect on growth. However, larger diameter and increased annual radial growth were found in thinned plots, both with and without pruning, as compared to unthinned plots. Also, we found significant effects of climate on annual radial growth. We recommend the application of thinning and pruning in stands of Mediterranean mountains in order to get knot-free timber since growth reduction was not found in thinned stands.     

Water stress integral-a link between short-term stress and long-term growth

Water stress integral (S(Psi)), the cumulative integral of pre-dawn leaf water potential over any chosen period of time, was estimated from measurements of pre-dawn water potential made every two weeks in a Pinus radiata D. Don plantation near Canberra, Australia. Also measured were final length of current-season needles and annual stem basal area increment. Data were gathered over a 4-year period from a control plot, a fertilized plot, an irrigated plot, and two plots that were both fertilized and irrigated. Among years and treatments, annual basal area increment varied over a threefold range. Of this variation, 91% was accounted for by variation in S(Psi) for the entire year, during every month of which stem diameter growth occurred. Of variation in annual needle elongation, 90% was accounted for by variation in S(Psi) from late August to late February, which was the period of needle growth. In dry years, the annual value of S(Psi) in non-irrigated plots was mainly determined by soil water content, but in wet years in non-irrigated plots, and in all years in irrigated plots, it was closely correlated with tree nutrient status (r(2) = 0.81).     

Leaf osmotic potential of Eucalyptus hybrids responds differently to freezing and drought, with little clonal variation

Concentrations of solutes, and thus leaf osmotic potential (Psi pi), often increase when plants are subject to drought or sub-zero (frost) temperatures. We measured Psi pi and concentrations of individual solutes in leaves of 3-year-old Eucalyptus camaldulensis Dehn., E. globulus Labill., E. grandis W. Hill ex Maid. and 29 hybrid clones on a site subjected to both summer drought and winter frost. We sought to characterize seasonal and genetic variations in Psi pi and to determine whether Psi pi or leaf turgor is related to bole volume increment. Leaf osmotic potential at full turgor (Psi pi(100)) was 0.7 MPa more negative in winter than in late summer, and this trend was uniform across genotypes. Soluble carbohydrates were confirmed as key contributors to Psi pi, accounting for 40-44% of total osmolality. The seasonal trend in Psi pi(100) was facilitated by changes in leaf morphology, such as reduced turgid mass:dry mass ratio and increased apoplastic water fraction in winter. Cell wall elasticity increased significantly from winter to summer. Our results suggest that elastic adjustment may be more important than osmotic adjustment in leaves exposed to drought. Although Psi pi(100) was a reasonable predictor of in situ osmotic potential and turgor, we found no relationship between any physiological trait and bole volume increment. Clone-within-family variation in Psi pi(100) was small in both summer and winter and was unrelated to bole volume increment. We conclude that, for the study species, tree improvement under water-limited conditions should concentrate on direct selection for growth rather than on indirect selection based on osmotic potential.