Stomatal conductance,growth and root signaling in Betula pendula seedlings subjected to partial soil drying

Seedlings of Betula pendula Roth were grown with their root systems separated between two soil compartments. Four treatments were imposed: (i) adequate irrigation in both compartments (WW, controls); (ii) adequate irrigation in one compartment and drought in the other compartment (WD); (iii) drought in both compartments (DD); and (iv) half of the root system severed and the remainder kept well-watered (root excision, RE). Predawn leaf water potential, stomatal conductance, soil-to-leaf specific hydraulic conductance, and root and leaf growth decreased in DD-treated seedlings, which also displayed severe leaf shedding (30% loss in leaf area). The DD treatment also resulted in increased concentrations of abscisic acid (ABA) and its glucose ester in the xylem sap of roots and shoots compared to concentrations in control seedlings (about 200 versus 20 nM). Despite the difference in xylem sap concentrations, total ABA flux to the shoots was similar in the two treatments (1-2 pmol ABA m(-2) leaf area s(-1)) as a result of reduced transpiration in the DD-treated seedlings. Compared with root growth in control plants, root growth increased in the RE-treated plants and decreased in the drying compartment of the WD treatment; however, the RE and WD treatments only slightly reduced leaf expansion, and had no detectable effects on shoot water relations or ABA concentrations of the root and shoot xylem sap. We conclude that short-term soil water depletion affecting only 50% of the root system does not cause a measurable stress response in birch shoots, despite root growth cessation in the fraction of drying soil.     

Canopy development and solar conversion efficiency in Acacia auriculiformis under drought stress

A glasshouse experiment was performed with Acacia auriculiformis seedlings to investigate the effects of decreased soil water potential on phyllode extension, abscission and solar conversion efficiency, e. Six-month-old seedlings were subjected for 39 days to one of four treatments: well watered (soil water potential maintained above -0.5 MPa), moderately drought stressed (soil water potential maintained above -1.5 MPa), severely drought stressed (soil water potential maintained above -2.5 MPa) and well watered but pruned to maintain a leaf area approximating that of the severely drought-stressed treatment. Aboveground biomass accumulation decreased by 21% below that of the well-watered controls in the moderately drought-stressed seedlings and by 47% in the severely drought-stressed seedlings as a result of both decreased interception of solar radiation and lower e. Differences in phyllode extension rate, rather than in phyllode abscission, were primarily responsible for the differences in interception of solar radiation among treatments. Decreases in phyllode extension rate and water use occurred simultaneously in response to decreasing soil water potential. Specific leaf area decreased and water use efficiency increased in response to drought stress.     

Geographical variation in water relations, hydraulic architecture and terpene composition of Aleppo pine seedlings from Italian provinces

Ecotypic variations in leaf conductance, soil-to-leaf hydraulic conductance, components of tissue water potential, hydraulic architecture parameters and xylem embolism were examined in greenhouse-grown two-year-old Aleppo pine (Pinus halepensis Mill.) seedlings from six origins representing the geographic range of the species in Italy. Cortical resin composition of the seedlings was also determined. Measurements were made on well-watered seedlings and on seedlings subjected to recurring severe drought. Drought-stressed seedlings had lower mean leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than well-watered seedlings. They also exhibited more negative osmotic potentials, higher relative water deficit at incipient plasmolysis, but a similar maximum modulus of elasticity. Drought-stressed seedlings showed a higher degree of xylem embolism, a lower Huber value, lower leaf specific conductivity and lower specific conductivity than well-watered seedlings. Drought-stressed seedlings of provenances from more xeric habitats (Tremiti, Porto Pino and Mottola) had greater leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than drought-stressed seedlings of provenances from more mesic habitats (Imperia, Otricoli and Vico del Gargano). They also showed higher osmotic adjustment and a lower degree of xylem embolism. Among provenances, there were no significant differences in hydraulic architecture parameters in response to the drought treatment; however, Tremiti and Porto Pino seedlings displayed smaller drought-induced reductions in specific conductivity and leaf specific conductivity, respectively, than seedlings from other provenances. These differences suggest that seedlings from xeric provenances, especially Tremiti, have greater resistance to desiccation than seedlings from mesic provenances. No clear association was found between terpene variability and the other traits investigated, although terpene composition was related to the geographical distribution of the provenances. We conclude that the drought-tolerance responses of Tremiti make it a more suitable provenance than the others for establishment on sites prone to severe soil water deficits.     

Comparison of drought resistance among Prunus species from divergent habitats

Root and shoot characteristics related to drought resistance were compared among cultivated peach (Prunus persica (L.) Batsch.), P. andersonii (Nevada Desert almond), P. besseyi (western sand cherry), P. maritima (beach plum), P. subcordata (Sierra or Pacific plum), and P. tomentosa (Nanking cherry). In all species, shoot characteristics were more closely associated with drought adaptation than root characteristics. The most xeric species, P. andersonii, had the lowest specific leaf area, smallest leaves, highest stomatal conductance (before stress), highest rate of carbon assimilation (A), high root length/leaf area and root weight/leaf area ratios, and the highest leaf nitrogen content on an area basis. Root hydraulic conductivity was similar for all species, indicating a lack of importance of this parameter for drought resistance. During a 5-7 day drought, water use efficiency (WUE) increased as shoot water potentials (Psi) declined to -3.0 to -4.0 MPa for the xeric P. andersonii and P. subcordata, whereas after an initial increase, WUE decreased with declining Psi in the -1.5 to -3.0 MPa range for the more mesic P. maritima, P. persica and P. tomentosa as a result of non-stomatal limitations to A. Carbon assimilation rate decreased linearly with Psi during drought in all species, but the Psi at which A reached zero was not associated with drought adaptation. We conclude that the variation in leaf characteristics among Prunus species could be exploited to improve the drought resistance of commercial cultivars.     

Stomatal and mesophyll limitations of photosynthesis in black spruce seedlings during multiple cycles of drought

Container-grown black spruce (Picea mariana (Mill.) B.S.P.) seedlings were planted in trays containing a sand and peat mixture, and placed in a climate-controlled greenhouse. One group of seedlings was kept well-watered, and another group was subjected to three cycles of drought. Gas exchange analysis showed that mesophyll photosynthetic function was largely unimpaired by drought. In contrast, stomatal conductance was sensitive to drought, although it became less sensitive with each drought cycle. Both stomatal and mesophyll conductances increased with time in control and drought-stressed seedlings, but mesophyll conductance increased with time more rapidly than did stomatal conductance. Limitation of photosynthetic rate was dominated by the mesophyll. In control seedlings, relative stomatal limitation increased from 6 to 16% by the end of the experiment. In drought-stressed seedlings, relative stomatal limitation of photosynthesis reached 40% during the first drought, but decreased to near control values immediately after rewatering. Because the third, most severe drought had only a minor effect on stomatal conductance, relative stomatal limitation of photosynthesis was similar to that in control seedlings by the end of the experiment. Inhibition of ontogenetic change during drought stress may be responsible for the apparent acclimation of mesophyll photosynthetic processes. We conclude that it would be more effective to select for high photosynthetic capacity than for reduced stomatal sensitivity when breeding for increased drought resistance in black spruce seedlings.     

Variation in drought resistance, drought acclimation and water conservation in four willow cultivars used for biomass production

Growth and water-use parameters of four willow (Salix spp.) clones grown in a moderate drought regime or with ample water supply were determined to characterize their water-use efficiency, drought resistance and capacity for drought acclimation. At the end of the 10-week, outdoor pot experiment, clonal differences were observed in: (1) water-use efficiency of aboveground biomass production (WUE); (2) resistance to xylem cavitation; and (3) stomatal conductance to leaf-specific, whole-plant hydraulic conductance ratio (g(st)/K(P); an indicator of water balance). Across clones and regimes, WUE was positively correlated with the assimilation rate to stomatal conductance ratio (A/g(st)), a measure of instantaneous water-use efficiency. Both of these water-use efficiency indicators were generally higher in drought-treated trees compared with well-watered trees. However, the between-treatment differences in (shoot-based) WUE were smaller than expected, considering the differences in A/g(st) for two of the clones, possibly because plants reallocated dry mass from shoots to roots when subject to drought. Higher root hydraulic conductance to shoot hydraulic conductance ratios (K(R)/K(S)) during drought supports this hypothesis. The same clones were also the most sensitive to xylem cavitation and, accordingly, showed the strongest reduction in g(st)/K(P) in response to drought. Drought acclimation was manifested in decreased g(st), g(st)/K(P), osmotic potential and leaf area to vessel internal cross-sectional area ratio, and increased K(R), K(P) and WUE. Increased resistance to stem xylem cavitation in response to drought was observed in only one clone. It is concluded that WUE and drought resistance traits are inter-linked and that both may be enhanced by selection and breeding.     

Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars

We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (Ks) or leaf specific conductivity (Kl) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in Ks, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (PsiPLC50) occurred at a water potential of -4.4 MPa in the well-watered treatment, and at -6.0 MPa in the moderate drought treatment. In the other species, PsiPLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between -6.4 and -7.5 MPa in both irrigation treatments. During severe drought, Ks of twigs and stomatal conductance (g(s)) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of Ks was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum g(s)) was reached at a predawn water potential (Psi(pd)) of -2.5 MPa in C. atlantica (Luberon) and at -3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Psi(pd) values. Cedrus brevifolia, with a Psi(pd) (-3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed.     

Effects of phenology,water availability and seed source on loblolly pine biomass partitioning and transpiration

One-year-old loblolly pine (Pinus taeda L.) seedlings from four seed sources (Arkansas, Georgia, Texas and Virginia) grown in 1-m-deep sand-filled pits in two water regimes (well-watered and drought) were studied, to gain insight into the process of seedling establishment. Whole-plant transpiration was measured biweekly from July to December. Whole-plant harvests were conducted at 6-week intervals from April to December. Whole-plant transpiration and transpiration per unit leaf and root area were affected by treatment, seedlot and phenology. Seedlings of the Arkansas seedlot maintained significantly higher transpiration rates per unit leaf and root area during drought than seedlings of the Virginia, Georgia or Texas seedlots, but did not accumulate greater biomass. The high transpiration rates of the Arkansas seedlings were attributed to their deep root systems. Allometric relationships indicated that, relative to the whole plant, biomass allocation to needles of drought-treated seedlings was enhanced during the summer (allometric ratio 1.09), whereas allocation to roots was enhanced in the spring and fall (allometric ratios of 1.13 and 1.09, respectively). Relative to the whole plant, biomass allocation to needles of well-watered seedlings was enhanced throughout the experiment (allometric ratio of 1.16 declining to 1.05), whereas the allometric ratio of root to total biomass was 0.89 or less throughout. Allometric relationships also indicated variation in biomass partitioning to roots in three soil layers (0-30, 30-60 and 60-100 cm), which differed among harvests in each soil layer. Root growth in both well-watered and drought-treated seedlings was concentrated in the top soil layer in the spring, shifted to the middle and bottom soil layers in the summer, and then increased in the top soil layer in the fall. Compared with well-watered seedlings, drought-treated seedlings had higher rates of root growth in the bottom soil layer in the fall, a characteristic that would confer tolerance to future periods of limited soil water availability.     

Adaptations to soil drying in woody seedlings of African locust bean, (Parkia biglobosa (Jacq.) Benth.)

Stomatal conductance, transpiration and xylem pressure potential of African locust bean (Parkia biglobosa (Jacq.) Benth.) seedlings subjected from the sixth week after emergence to four weeks of continuous soil drought did not differ from those of well-watered, control plants until two-thirds of the available soil water had been used. In both well-watered and drought-treated plants, stomatal conductance was highest early in the day when vapor pressure deficits were low, but decreased sharply by midday when evaporative demand reached its highest value. There was no increase in stomatal conductance later in the day as vapor pressure deficit declined. The relationship between transpiration rate and xylem pressure potential showed non-linearity and hysteresis in both control and drought-treated plants, which seems to indicate that the plants had a substantial capacity to store water. The rate of leaf extension in African locust bean seedlings subjected to six consecutive 2-week cycles of soil drought declined relative to that of well-watered, control plants, whereas relative root extension increased. It appears that African locust bean seedlings minimized the impact of drought by: (1) restricting transpiration to the early part of the day when a high ratio of carbon gain to water loss can be achieved; (2) utilizing internally stored water during periods of rapid transpiration; (3) reducing the rate of leaf expansion and final leaf size in response to soil drought without reducing the rate of root extension, thereby reducing the ratio of transpiring leaf surface area to absorbing root surface area.     

Water relations of loblolly pine seedlings from diverse geographic origins

Leaf conductance at three absolute humidity deficits (AHDs) (7, 14 and 21 g m(-3)), hydraulic conductance and components of tissue water potential were measured in one-year-old loblolly pine seedlings from six origins representing the geographic range of the species. Measurements were made on seedlings grown (a) with ample water (moist regime) and (b) with recurring severe drought (dry regime). However, all seedlings were well-watered prior to and during measurements. Seedlings grown in the moist regime had greater mean leaf conductances (0.30 versus 0.13 cm s(-1)) and greater responses to AHD than seedlings grown in the dry regime. They also exhibited greater hydraulic conductances (0.53 versus 0.35 microg cm(-2) s(-1) MPa(-1), less negative osmotic potentials (-1.45 versus -1.57 MPa) and higher relative water contents at turgor loss (0.72 versus 0.65). Seed source differences in water relations characteristics were detected only in seedlings grown in the moist regime. In these, trees from the three interior origins had greater mean leaf conductances than those from the three coastal sources (0.32 versus 0.28 cm s(-1)), but no differences in response to changing AHD were observed. Seedlings from North Carolina had lower osmotic potentials at turgor loss than those from Florida, Georgia or Texas. These differences in water relations characteristics are not clearly related to the observed greater survival ability of trees from interior origins compared with those from coastal origins.     

Survival and growth of drought hardened <Emphasis Type="Italic">Eucalyptus pilularis</Emphasis> Sm. seedlings and vegetative cuttings

Forestry requires low mortality of transplanted seedlings. Mortality shortly after planting is often associated with inadequate hydration of transplants. Seedlings can be hardened to the drought conditions they may experience after transplanting by exposing them to controlled drought conditions in the nursery. Eucalyptus pilularis Sm. seedlings were drought hardened by providing nil (severe treatment) or half (mild treatment) the daily irrigation routinely received (control treatment) for up to two non-consecutive days per week during the last 4 weeks of growth in the nursery. Drought hardening reduced stem diameter, seedling leaf area, leaf area per root biomass and seedling quality measured by the Dickson quality index, but increased root:shoot ratio. Hardened seedlings had lower stomatal conductance and leaf water potential on the days they received less irrigation that the control treatment. Hardened seedlings had greater stomatal conductance and were less water stressed than seedlings experiencing drought for the first time indicating hardened seedlings had adjusted physiologically to drought. Survival after transplanting in the controlled drought environment in a glasshouse was enhanced by the hardening treatments. Non hardened seedlings that had had their upper leaves manually removed immediately prior to transplanting to reduce leaf area (top-clipped) had similar survival to hardened seedlings. Stomatal conductance and leaf water potential after transplanting were higher in hardened and top-clipped seedlings than unhardened control seedlings or vegetative cuttings. Survival in the field trial was over 95% for all treatments, possibly as rain fell within 4 days of planting and follow-up rain occurred in the subsequent weeks. Neither the hardened or top-clipped seedlings planted in the field trial had reduced growth, increased propensity to form double leaders or worse stem form than control seedlings when measured at age 3 years.     

Root water flow and growth of aspen (Populus tremuloides) at low root temperatures

Effects of root zone temperature on growth, shoot water relations, and root water flow were studied in 1-year-old aspen (Populus tremuloides Michx.) seedlings. Seedlings were grown in solution culture and exposed to day/night air temperatures of 22/16 degrees C and solution culture temperatures of 5, 10, or 20 degrees C for 28 days after bud flush. Compared with root growth at 20 degrees C, root growth was completely inhibited at 5 degrees C and inhibited by 97% at 10 degrees C. The 5 and 10 degrees C treatments severely reduced shoot growth, leaf size, and total leaf area. Root water flow was inhibited by the 5 and 10 degrees C treatments. However, when seedlings were grown for 28 days at 5 degrees C and root water flow was measured at 20 degrees C, there was an increase in flow rate. This increase in root water flow was similar in magnitude to the decrease in root water flow observed when seedlings were grown for 28 days at 20 degrees C and root water flow was measured at 5 degrees C. Reduced root water flow of seedlings grown at 5 and 10 degrees C resulted in decreased stomatal conductance, net assimilation, and shoot water potentials. Root water flow was positively correlated with leaf size, total leaf area, shoot length, and new root growth. Transferring seedlings from 5 to 20 degrees C for 24 h significantly increased root water flow, shoot water potential, and net photosynthesis, whereas transferring seedlings from 10 to 20 degrees C resulted in only a slightly increased shoot water potential. Transferring seedlings from 20 to 5 degrees C greatly reduced root water flow, stomatal conductance, and net photosynthesis, whereas shoot water potential decreased only slightly.     

Effects of atmospheric CO(2) on longleaf pine: productivity and allocation as influenced by nitrogen and water

Longleaf pine (Pinus palustris Mill.) seedlings were exposed to two concentrations of atmospheric CO(2) (365 or 720 micro mol mol(-1)) in combination with two N treatments (40 or 400 kg N ha(-1) year(-1)) and two irrigation treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) in open-top chambers from March 1993 through November 1994. Irrigation treatments were imposed after seedling establishment (i.e., 19 weeks after planting). Seedlings were harvested at 4, 8, 12, and 20 months. Elevated CO(2) increased biomass production only in the high-N treatment, and the relative growth enhancement was greater for the root system than for the shoot system. In water-stressed trees, elevated CO(2) increased root biomass only at the final harvest. Root:shoot ratios were usually increased by both the elevated CO(2) and low-N treatments. In the elevated CO(2) treatment, water-stressed trees had a higher root:shoot ratio than well-watered trees as a result of a drought-induced increase in the proportion of plant biomass in roots. Well-watered seedlings consistently grew larger than water-stressed seedlings only in the high-N treatment. We conclude that available soil N was the controlling resource for the growth response to elevated CO(2) in this study. Although some growth enhancement was observed in water-stressed trees in the elevated CO(2) treatment, this response was contingent on available soil N.     

Effects of two Glomus species on the growth and physiological performance of Sophora davidii seedlings under water stress

Sophora davidii is an important leguminous scrub that is widely used for revegetation in the semiarid Loess Plateau and other arid valley areas of China, where it usually suffers drought stress. This study investigated the effects of arbuscular mycorrhizal (AM) fungi (Glomus mosseae and Glomus constrictum) and water stress on the growth and physiological performance of S. davidii seedlings under greenhouse pot conditions. Two soil water availability treatments (well-watered (WW) ?0.10 MPa; water-stressed (WS) ?0.86 MPa) were applied for 61 days. At the end of this experiment, G. mosseae and G. constrictum had colonized the roots of S. davidii seedlings. Water stress inhibited AM colonization, plant growth, chlorophyll concentration, gas exchange and chlorophyll fluorescence of S. davidii seedlings. Mycorrhizal seedlings had greater shoot dry weight, root dry weight, plant height, root length, instantaneous water use efficiency (iWUE), net photosynthetic rate (Pn), stomatal conductance (g_s), maximal photochemical efficiency of PSII photochemistry (Fv/Fm), lower intercellular CO₂ concentration and photochemical quenching values (qP), when compared with non-mycorrhizal seedlings under both WW and WS conditions. Furthermore, G. constrictum was found to be more efficient at improving the shoot and root mass, plant height, iWUE, Pn, g_s, qP, and ΦPSII of S. davidii seedlings, when compared with G. mosseae under both WW and WS conditions. Our results demonstrate that AM Glomus symbiosis enhanced S. davidii seedling resistance by improving its growth and physiological performance under water stress conditions. This suggests that Glomus inoculation is a potential tool for enhancing outplanting performance of S. davidii in semiarid areas of China.     

Drought tolerance of clonal Malus determined from measurements of stomatal conductance and leaf water potential

We examined tolerance to soil drying in clonally propagated apple (Malus domestica Borkh.) rootstocks used to control shoot growth of grafted scions. We measured leaf conductance to water vapor (g(L)) and leaf water potential (Psi(L)) in a range of potted, greenhouse-grown rootstocks (M9, M26, M27, MM111, AR69-7, AR295-6, AR360-19, AR486-1 and AR628-2) as the water supply was gradually reduced. Irrespective of the amount of available water, rootstocks that promoted scion shoot growth (M26 and MM111) generally had higher g(L) and more negative Psi(L) than rootstocks that restricted scion shoot growth (M27 and M9). After about 37 days of reduced water supply, there were significant decreases in g(L) and Psi(L) in all rootstocks compared with well-watered controls. In all treatments, the slope of the relationship between log (g(L)) and Psi(L) was positive, except for rootstocks AR295-6, AR628-2 and AR486-1 in the severe-drought treatment, where the drought-induced change in the relationship suggests that rapid stomatal closure occurred when leaf water potentials fell below -2.0 MPa. This drought response was associated with increased root biomass production. Rootstock M26 showed little stomatal closure even when its water potential fell below -2.0 MPa, and there was no effect of drought on root biomass production. We conclude that differences among rootstocks in the way that g(L) and Psi(L) respond to drought reflect differences in the mechanisms whereby they tolerate soil drying. We suggest that these differences are related to differences among the rootstocks in their ability to control shoot growth.     

Planting stress, water status and non-structural carbohydrate concentrations in Corsican pine seedlings

Two-year-old Corsican pine (Pinus nigra ssp. laricio var. Corsicana) seedlings were either well watered or subjected to a moderate drought for one month before being lifted from the nursery bed on October 9 and transplanted. Well-watered, non-transplanted seedlings served as controls. Needle predawn water potential (Psi(wp)), non-structural carbohydrate concentrations and plant development (survival, bud break, shoot elongation) were assessed before and during the first growing season after transplanting. On April 16, just before bud break, Psi(wp) was lower for the well-watered + transplanted and drought-conditioned + transplanted seedlings (Psi(wp) = -1.45 and -1.83 MPa, respectively) than for the controls (Psi(wp) = -0.56). There was a close relationship between the Psi(wp) measured on April 16 and bud break, shoot elongation and plant survival during the following growing period. Above a Psi(wp) of -1.1 MPa, all plants developed normally. Between -1.1 MPa and -1.6 MPa, bud break, and thus shoot elongation, did not occur in all plants. Between -1.6 MPa and -2.1 MPa, the plants were characterized by the absence of shoot growth, but mortality was zero. Below -2.1 MPa, there was a large increase in plant mortality. On April 16, starch concentrations were markedly lower in the roots of transplanted seedlings than in the controls. There was a positive correlation between Psi(wp) and root starch concentration. The Psi(wp) (-2.3 MPa) at which complete starch depletion was observed in the roots corresponded to the Psi(wp) below which plants did not survive. These results suggest that mechanisms specifically linked to altered water status and metabolic processes associated with altered carbohydrate status are involved in transplanting stress; however, it was not possible to disentangle the two effects. Drought conditioning did not lead to a marked increase in soluble carbohydrate concentrations, as reported for other species, and did not increase plant tolerance to transplanting stress.     

Steady-state nutrition of Pinus resinosa seedlings: response to nutrient loading, irrigation and hardening regimes

To test effects of fertilization on late-season growth and nutrient content of container-grown red pine (Pinus resinosa Ait.), seedlings were subjected to a factorial combination of two fertilization regimes (exponentially increasing concentrations providing 25 (1N) and 75 (3N) mg N seedling(-1), respectively) and two irrigation regimes (well-watered and water-stressed) followed by drought-hardening or nonhardening. The fertilization treatments gave rise to steady-state nutrition in the seedlings during the prehardening phase. The 3N treatment increased macronutrient uptake significantly more than dry matter production, particularly in the roots, giving rise to what has been called luxury consumption of nutrients, or nutrient loading. Nutrient loading was higher in well-watered seedlings than in water-stressed seedlings. Among well-watered seedlings, relative growth rate of nutrient-loaded seedlings was more sensitive to drought during the hardening phase than that of nonloaded seedlings. However, when watering was resumed at the end of the hardening treatment, the relative growth rate of the nutrient-loaded seedlings (421%) exceeded that of the nonloaded seedlings (213%). Nitrogen accumulation was also higher in nutrient-loaded seedlings than in nonloaded seedlings during the post-hardening phase.     

Responses to water stress in two Eucalyptus globulus clones differing in drought tolerance

We evaluated drought resistance mechanisms in a drought-tolerant clone (CN5) and a drought-sensitive clone (ST51) of Eucalyptus globulus Labill. based on the responses to drought of some physiological, biophysical and morphological characteristics of container-grown plants, with particular emphasis on root growth and hydraulic properties. Water loss in excess of that supplied to the containers led to a general decrease in growth and significant reductions in leaf area ratio, specific leaf area and leaf-to-root area ratio. Root hydraulic conductance and leaf-specific hydraulic conductance decreased as water stress became more severe. During the experiment, the drought-resistant CN5 clone maintained higher leaf water status (higher predawn and midday leaf water potentials), sustained a higher growth rate (new leaf area expansion and root growth) and displayed greater carbon allocation to the root system and lower leaf-to-root area ratio than the drought-sensitive ST51 clone. Clone CN5 possessed higher stomatal conductances at moderate stress as well as higher hydraulic conductances than Clone ST51. Differences in the response to drought in root biomass, coupled with changes in hydraulic properties, accounted for the clonal differences in drought tolerance, allowing Clone CN5 to balance transpiration and water absorption during drought treatment and thereby prolong the period of active carbon assimilation.     

Tolerance of Japanese larch to drought is modified by nitrogen and water regimes during cultivation of container seedlings

Improving drought tolerance of container seedlings of Japanese larch is of high importance to afforestation. We hypothesized that adequate nitrogen (N) and limited water supply would increase the tolerance of container seedlings to water-deficit stress, circumventing photoinhibition, by means of (i) enhanced photosynthetic capacity with higher leaf N and (ii) decreased water loss from leaves with lower biomass allocation into aboveground parts. Container seedlings of Japanese larch were grown under the treatment combinations of adequate (+?N: 300 mg N container^?1) or limited (??N: 150 mg N container^?1) N and adequate (+?W: daily irrigation) or limited (??W: twice-a-week irrigation) water. Then, seedlings were subjected to a progressive drought treatment. Higher leaf N was observed in container seedlings grown under?+?N and???W. During progressive drought, lower stomatal conductance and net photosynthetic rate were observed in leaves with higher leaf N at a given predawn leaf water potential. Furthermore, the maximum efficiency of PSII photochemistry (F_v/F_m) was lower in leaves with higher leaf N, suggesting that higher leaf N might impair intrinsic tolerance to drought at the leaf level contrary to expectations. Conversely,???N and???W seedlings with lower shoot biomass delayed soil drying as a whole-plant response via a reduction in leaf transpiration, leading to delayed photoinhibition as indicated by a decline in F_v/F_m. To circumvent stress at the initial stage of water deficit, lower leaf N via limited N regime and smaller shoot biomass driven by limited N and water regimes would be important.

     

Regulation of transpirational water loss in Quercus suber trees in a Mediterranean-type ecosystem

Sap flux density in branches, leaf transpiration, stomatal conductance and leaf water potentials were measured in 16-year-old Quercus suber L. trees growing in a plantation in southern Portugal to understand how evergreen Mediterranean trees regulate water loss during summer drought. Leaf specific hydraulic conductance and leaf gas exchange were monitored during the progressive summer drought to establish how changes along the hydraulic pathway influence shoot responses. As soil water became limiting, leaf water potential, stomatal conductance and leaf transpiration declined significantly. Predawn leaf water potential reflected soil water potential measured at 1-m depth in the rhizospheres of most trees. The lowest predawn leaf water potential recorded during this period was -1.8 MPa. Mean maximum stomatal conductance declined from 300 to 50 mmol m(-2) s(-1), reducing transpiration from 6 to 2 mmol m(-2) s(-1). Changes in leaf gas exchange were attributed to reduced soil water availability, increased resistances along the hydraulic pathway and, hence, reduced leaf water supply. There was a strong coupling between changes in soil water content and stomatal conductance as well as between stomatal conductance and leaf specific hydraulic conductance. Despite significant seasonal differences among trees in predawn leaf water potential, stomatal conductance, leaf transpiration and leaf specific hydraulic conductance, there were no differences in midday leaf water potentials. The strong regulation of changes in leaf water potential in Q. suber both diurnally and seasonally is achieved through stomatal closure, which is sensitive to changes in both liquid and vapor phase conductance. This sensitivity allows for optimization of carbon and water resource use without compromising the root-shoot hydraulic link.