Experimental Study and Mathematical Simulation of Water Use Efficiency in Wheat Leaves Affected by Some Environmental Factors

By incorporating Ball-Berry model of stomatal conductance into the models of photosynthesis and transpiration, a model of leaf water use efficiency (WUE) as affected by several environmental variables [irradiance (Ⅰ), vapor pressure deficit (VPD) and atmospheric CO2 concentration (Ca) ] was constructed. Because the environmental variables influenced the photosynthetic rate and transpiration rate in different ways, the changes of leaf WUE with these factors were quite complicated. The rates of photosynthesis and transpiration of wheat leaves were also measured in the phytotron where the environmental variables were kept within certain ranges, and leaf WUE was calculated therefrom. The results of simulation fit quite well with the measurements except at high Ca.     

Leaf Epicuticular Waxes of the Eceriferum Mutants in Arabidopsis

Wild-type Arabidopsis leaf epicuticular wax (EW) occurs as a smooth layer over the epidermal surface, whereas stem EW has a crystalline microstructure. Wild-type EW load was more than 10-fold lower on leaves than on stems. Compared with the EW on wild-type stems, EW on wild-type leaves had a much higher proportion of their total EW load in the form of alkanes and 1-alcohols; a large reduction in secondary alcohols, ketones, and esters; and a chain-length distribution for major EW classes that was skewed toward longer lengths. The eceriferum (cer) mutations often differentially affected leaf and stem EW chemical compositions. For example, the cer2 mutant EW phenotype was expressed on the stem but not on the leaf. Compared to wild type, the amount of primary alcohols on cer9 mutants was reduced on leaves but elevated on stems, whereas an opposite differential effect for primary alcohols was observed on cer16 leaves and stems. Putative functions for CER gene products are discussed. The CER4 and CER6 gene products may be involved in fatty aldehyde reduction and C26 fatty acylcoenzyme A elongation, respectively. CER1, CER8, CER9, and CER16 gene products may be involved in EW substrate transfer. The CER3 gene product may be involved in release of fatty acids from elongase complexes. CER2 gene product may have regulatory functions.     

环境因子对小麦叶片水分利用效率影响的实验研究和数值模拟

根据现有的光合作用和蒸腾作用的模型,利用Ｂａｌｌ－Ｂｅｒｒｙ 的气孔导度模型,将叶片的光合作用模型和蒸腾作用模型结合起来,建立了光强（Ｉ）、叶片－大气水汽饱和差（ＶＰＤ）和大气ＣＯ２ 浓度（Ｃａ）等环境因子对小麦叶片水分利用效率（ＷＵＥ）影响的模型。由于这３ 种环境因子对光合、蒸腾的影响方式上的差异,作为两者之比的叶片ＷＵＥ随各环境因子的变化出现复杂的图景,同时,在人工气候箱内分别于这３ 个因子变化时对光合、蒸腾的变化作了测定,计算了叶片的Ｗ ＵＥ。测定结果与模拟结果的对比表明,在多数情况下两者符合程度良好,但在高Ｃａ下有较大的偏离     

EFFECT OF LEAF ANGLE AND ORIENTATION ON PHOTOSYNTHESIS AND WATER RELATIONS IN SILPHIUM TEREBINTHINACEUM

Leaf angle and orientation were measured for 217 leaves from two populations of Silphium terebinthinaceum Jacq., a prairie forb with large, unlobed leaves. Seventy-five percent of leaves measured had an angle of deviation from horizontal of more than 60°, and 60% were oriented within 15° of North. Incident Photon Flux Density (PFD), leaf temperature, photosynthesis, stomatal conductance to CO₂, internal CO₂ concentration, transpiration, and water use efficiency (WUE) were measured on 67 pairs of leaves with the axes oriented in either a North-South (N–S) or East–West (E–W) direction. Leaves with axes oriented N–S intercepted higher levels of PFD during morning and afternoon and exhibited higher diurnal rates of photosynthesis and WUE. Leaf temperature was reduced in N–S leaves during midday as compared to E–W leaves, and was lower in vertical leaves than in those in a horizontal position. Therefore, it was concluded that leaf orientation and verticality enhance carbon gain and minimize water loss—characteristics which may have adaptive significance in a hot, stressful prairie environment.     

Photosynthesis and transpiration of the flag leaf in four spring-wheat cultivars

Co₂ exchange and transpiration rates of the flag leaves of four spring wheat (Triticum aestivum L.) cultivars, namely Glenlea, Neepawa, Opal and Kolibri, were compared using infra-red gas-analysis technique. The plants were grown in a controlled environment under an 18-h photoperiod, with day and night temperatures of 20 and 15° C, respectively. The time course of the CO₂-exchange rate (CER) of the flag leaf differed among cultivars. CER began to decrease rapidly some 2 weeks after ear emergence in Glenlea, Neepawa and Kolibri, but only after 4 weeks in Opal. The decline in CER of Glenlea, Neepawa and Opal was continuous throughout the period of grain development whereas in Kolibri CER was maintained at a constant level between the 4th and 6th weeks after ear emergence. The transpiration rates of the flag leaves of the 4 cultivars did not change markedly until 6–7 weeks after ear emergence, indicating that the reduction in CER was not primarily a response to increased stomatal resistance to the diffusion of CO₂. Removing the ear of the main shoot of intact plants failed to depress CER of the subtending flag leaf until 5 weeks after ear removal. Removing the ears of all the tillers of plants in which all but 3 tillers had been removed at ear emergence did not depress CER until 4 weeks after ear emergence, but removal of the ear of the main shoot of plants where all the tillers had been removed at ear emergence reduced the CER of the flag leaf 2 weeks after ear removal. Removal of tillers at ear emergence had a marked effect on the time course of CER and transpiration rates of the flag leaf. Both CER and transpiration rates of a 4-tiller plant were maintained at a higher level throughout ear development as compared to those of a one-tiller plant. The transpiration rate of the flag leaf of Glenlea increased during the later part of the life of the leaf even for one-tiller plants with no ear, indicating that such a stomatal response may be part of the normal course of leaf aging and not a response to a feedback stimulus from the ear.     

Water use efficiency as a function of leaf age and position within the cotton canopy

The gas exchange properties of whole plant canopies are an integral part of crop productivity and have attracted much attention in recent years. However, insufficient information exists on the coordination of transpiration and CO₂ uptake for individual leaves during the growing season. Single-leaf determinations of net photosynthesis (Pn), transpiration (E) and water use efficiency (WUE) for field-grown cotton (Gossypium hirsutum L.) leaves were recorded during a 2-year field study. Measurements were made at 3 to 4 day intervals on the main-stem and first three sympodial leaves at main-stem node 10 from their unfolding through senescence. Results indicated that all gas exchange parameters changed with individual main-stem and sympodial leaf age. Values of Pn, E and WUE followed a rise and fall pattern with maximum rates achieved at a leaf age of 18 to 20 days. While no significant position effects were observed for Pn, main-stem and sympodial leaves did differ in E and WUE particularly as leaves aged beyond 40 days. For a given leaf age, the main-stem leaf had a significantly lower WUE than the three sympodial leaves. WUE's for the main-stem and three sympodial leaves between the ages of 41 to 50 days were 0.85, 1.30, 1.36 and 1.95 μmol CO₂ mmol⁻¹ H₂O, respectively. The mechanisms which mediated leaf positional differences for WUE were not strictly related to changes in stomatal conductance (g_s·H₂O) since decreases in g_s·H₂O with leaf age were similar for the four leaves. However, significantly different radiant environments with distance along the fruiting branch did indicate the possible involvement of mutual leaf shading in determining WUE. The significance of these findings are presented in relation to light competition within the plant canopy during development.     

Water use by the desert cucurbit Citrullus colocynthis (L.) Schrad.

Summary The rates of water use and leaf surface conductance of Citrullus colocynthis (Cucurbitacea) were evaluated from measurements of the surface temperature and microenvironment of leaves. At desert sites in Saudi Arabia the transpiration rates reached 0.13–0.17 g m^-2 s^-1 and the leaf temperatures were always close to air temperature. Leaf models (dry) placed in the canopy were considerably warmer than the air. To investigate responses over a wider range of conditions, plants were grown in a controlled environment room. It was found that when conditions were made hotter than those that occurred in the desert, the stomatal conductance increased greatly. Transpiration rate attained 0.6 g m^-2 s^-1 and the leaves were up to seven degrees cooler than the air. The results suggest a finely-tuned control mechanism working like a switch when the leaves experience extreme conditions, and enabling the plant to avoid lethal temperatures.     

Leaf shape linked to photosynthetic rates and temperature optima in South African <Emphasis Type="Italic">Pelargonium</Emphasis> species

The thermal response of gas exchange varies among plant species and with growth conditions. Plants from hot dry climates generally reach maximal photosynthetic rates at higher temperatures than species from temperate climates. Likewise, species in these environments are predicted to have small leaves with more-dissected shapes. We compared eight species of Pelargonium (Geraniaceae) selected as phylogenetically independent contrasts on leaf shape to determine whether: (1) the species showed plasticity in thermal response of gas exchange when grown under different water and temperature regimes, (2) there were differences among more- and less-dissected leafed species in trait means or plasticity, and (3) whether climatic variables were correlated with the responses. We found that a higher growth temperature led to higher optimal photosynthetic temperatures, at a cost to photosynthetic capacity. Optimal temperatures for photosynthesis were greater than the highest growth temperature regime. Stomatal conductance responded to growth water regime but not growth temperature, whereas transpiration increased and water use efficiency (WUE) decreased at the higher growth temperature. Strikingly, species with more-dissected leaves had higher rates of carbon gain and water loss for a given growth condition than those with less-dissected leaves. Species from lower latitudes and lower rainfall tended to have higher photosynthetic maxima and conductance, but leaf dissection did not correlate with climatic variables. Our results suggest that the combination of dissected leaves, higher photosynthetic rates, and relatively low WUE may have evolved as a strategy to optimize water delivery and carbon gain during short-lived periods of high soil moisture. Higher thermal optima, in conjunction with leaf dissection, may reflect selection pressure to protect photosynthetic machinery against excessive leaf temperatures when stomata close in response to water stress.     

Phosphorus concentrations in the leaves of defoliated white clover affect abscisic acid formation and transpiration in drying soil

Increased leaf phosphorus (P) concentration improved the water-use efficiency (WUE) and drought tolerance of regularly defoliated white clover plants by decreasing the rate of daily transpiration per unit leaf area in dry soil. Night transpiration was around 17% of the total daily transpiration. The improved control of transpiration in the high-P plants was associated with an increased individual leaf area and WUE that apparently resulted from net photosynthetic assimilation rate being reduced less than the reductions in the transpiration (27% vs 58%). On the other hand, greater transpiration from low-P plants was associated with poor stomatal control of transpirational loss of water, less ABA in the leaves when exposed to dry soil, and thicker and smaller leaf size compared with high-P leaves. The leaf P concentration was positively related with leaf ABA, and negatively with transpiration rates, under dry conditions ( P < 0.001). However, leaf ABA was not closely related to the transpiration rate, suggesting that leaf P concentration has a greater influence than ABA on the transpiration rates.     

Evaluation of physiological traits for improving drought tolerance in faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.)

Among grain legumes, faba bean is becoming increasingly popular in European agriculture due to recent economic and environmental interests. Faba bean can be a highly productive crop, but it is sensitive to drought stress and yields can vary considerably from season to season. Understanding the physiological basis of drought tolerance would indicate traits that can be used as indirect selection criteria for the development of cultivars adapted to drought conditions. To assess genotypic variation in physiological traits associated with drought tolerance in faba bean and to determine relationships among these attributes, two pot experiments were established in a growth chamber using genetic materials that had previously been screened for drought response in the field. Nine inbred lines of diverse genetic backgrounds were tested under adequate water supply and limited water conditions. The genotypes showed substantial variation in shoot dry matter, water use, stomatal conductance, leaf temperature, transpiration efficiency, carbon isotope discrimination (Δ¹³C), relative water content (RWC) and osmotic potential, determined at pre-flowering vegetative stage. Moisture deficits decreased water usage and consequently shoot dry matter production. RWC, osmotic potential, stomatal conductance and Δ¹³C were lower, whereas leaf temperature and transpiration efficiency were higher in stressed plants, probably due to restricted transpirational cooling induced by stomatal closure. Furthermore, differences in stomatal conductance, leaf temperature, Δ¹³C and transpiration efficiency characterized genotypes that were physiologically more adapted to water deficit conditions. Correlation analysis also showed relatively strong relationships among these variables under well watered conditions. The drought tolerant genotypes, ILB-938/2 and Melodie showed lower stomatal conductance associated with warmer leaves, whereas higher stomatal conductance and cooler leaves were observed in sensitive lines (332/2/91/015/1 and Aurora/1). The lower value of Δ¹³C coupled with higher transpiration efficiency in ILB-938/2, relative to sensitive lines (Aurora/1 and Condor/3), is indeed a desirable characteristic for water-limited environments. Finally, the results showed that stomatal conductance, leaf temperature and Δ¹³C are promising physiological indicators for drought tolerance in faba bean. These variables could be measured in pot-grown plants at adequate water supply and may serve as indirect selection criteria to pre-screen genotypes.     

The Physiology of Heterosis in Sorghum with Respect to Environmental Stress

The hypothesis that heterosis in biomass production of sorghum(Sorghum bicolor L. Moench) may be ascribed to stability incarbon exchange rate (CER) over a wide range of environmentalconditions was evaluated. This hypothesis was based on previousresults from detached leaves that hybrids sustained greaterCER over a wider temperture range than their parents. Two grain sorghum hybrids (ATx378/RTx430 and ATx378/RTx434)and their parental lines were grown in the greenhouse in a gradientof ambient temperatures under two water regimes (well-irrigatedand drought up to heading). Plant water-use (estimated by weighingpots), leaf area, leaf gas exchange, grain yield, and above-groundbiomass were determined. Significant heterosis was found for biomass, grain yield perplant, and grain number per panicle. No heterosis occurred forharvest index, indicating that heterosis in grain yield wasdue to heterosis in biomass. Neither growth duration nor leafarea could explain heterosis in biomass. CER and stomatal conductancefor hybrid ATx378/RTx430 in the controls were greater than forboth its parents at leaf temperatures above 38 °C. This,however, was not observed in the other hybrid which was lessheterotic for biomass and grain yield in the controls. WhenCER data were subjected to a stability analysis by joint linearregression, the two hybrids had greater CER than their respectiveparents especially under conditions favouring high CER. Whenextreme stress conditions developed, the hybrid's performancedepended on its genetic background more than on heterosis. Sorghum, Sorghum bicolor L. Moench., heterosis, hybrids, photosynthesis, transpiration, stomata, drought, heat, temperature     

Wind increases leaf water use efficiency

A widespread perception is that, with increasing wind speed, transpiration from plant leaves increases. However, evidence suggests that increasing wind speed enhances carbon dioxide (CO₂) uptake while reducing transpiration because of more efficient convective cooling (under high solar radiation loads). We provide theoretical and experimental evidence that leaf water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, thus improving plants' ability to conserve water during photosynthesis. Our leaf‐scale analysis suggests that the observed global decrease in near‐surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric CO₂ concentrations. However, there is indication that the effect of long‐term trends in wind speed on leaf gas exchange may be compensated for by the concurrent reduction in mean leaf sizes. These unintuitive feedbacks between wind, leaf size and water use efficiency call for re‐evaluation of the role of wind in plant water relations and potential re‐interpretation of temporal and geographic trends in leaf sizes.     

The relationship between the soil water storage and water-use efficiency of seven energy crops

The aim of this work was to determine two types of photosynthetic water-use efficiency in order to examine their utility as selection criteria for tolerance of energy crops to soil water deficit. Furthermore, effects of crop cultivation on soil water content and storage were investigated. Seven energy crops were examined: miscanthus, prairie cordgrass, willow, thorn-free rose, Virginia mallow, Bohemian knotweed, and topinambour. The highest values of instantaneous (WUE) and intrinsic (WUE_i) water-use efficiencies were found for miscanthus and prairie cordgrass. The reduction of WUE and/or WUE_i was caused mainly by a rapid rise in the transpiration rate and a greater stomatal conductance, respectively. Principal component analysis showed that neither WUE nor WUE_i could be recommended as universal selection criteria for the drought tolerance in different energy crops. The proper localization of soil with a good supply of water is most the important condition for energy crop plantations.     

Plant transpiration at high elevations: Theory,field measurements,and comparisons with desert plants

Summary The influence of elevational changes on plant transpiration was evaluated using leaf energy balance equations and well-known elevational changes in the physical parameters that influence water vapor diffusion. Simulated transpirational fluxes for large leaves with low and high stomatal resistances to water vapor diffusion were compared to small leaves with identical stomatal resistances at elevations ranging from sea level to 4 km. The specific influence of various air temperature lapse rates was also tested. Validation of the simulated results was accomplished by comparing actual field measurements taken at a low elevation (300 m) desert site with similar measurements for a high elevation (2,560 m) mountain research site. Close agreement was observed between predicted and measured values of transpiration for the environmental and leaf parameters tested.Substantial increases in solar irradiation and the diffusion coefficient for water vapor in air (D _wv) occurred with increasing elevation, while air and leaf temperatures, the water vapor concentration difference between the leaf and air, longwave irradiation, and the thermal conductivity coefficient for heat in air decreased with increasing elevation. These changes resulted in temperatures for sunlit leaves that were further above air temperature at higher elevations, especially for large leaves. For large leaves with low stomatal resistances, transpirational fluxes for low-elevation desert plants were close to those predicted for high-elevation plants even though the sunlit leaf temperatures of these mountain plants were over 10°C cooler. Simulating conditions with a low air temperature lapse rate (0.003° C m^-1 and 0.004° C m^-1) resulted in predicted transpirational fluxes that were greater than those calculated for the desert site. Transpiration for smaller leaves decreased with elevation for all lapse rates tested (0.003° C m^-1 to 0.010° C m^-1). However, transpirational fluxes at higher elevations were considerably greater than expected for all leaves, especially larger leaves, due to the strong influence of increased solar heating and a greater D _wv. These results are discussed in terms of similarities in leaf structure and plant habit observed among low-elevation desert plants and high-elevation alpine and subalpine plants.     

The combined effect of constant water deficit and nitrogen supply on WUE, NUE and Δ13C in durum wheat potted plants

Water scarcity and nitrogen shortage are the main constraints on durum wheat productivity. This paper examines the combined effects of a constant water deficit and nitrogen supply (NS) on growth, photosynthesis, stomatal conductance (g_s) and transpiration, instantaneous and time‐integrated water use efficiency (WUE) and nitrogen use efficiency (NUE) and carbon isotope discrimination (Δ¹³C) in durum wheat genotypes grown in pots under greenhouse conditions. Three water levels (40%, 70% and 100% container capacity), two nitrogen doses (high and low N) and four genotypes were assayed in a total of 24 experimental treatments. Water and nitrogen treatments were imposed 2 weeks after plant emergence. The growth, nitrogen content and Δ¹³C of the shoot and the gas exchange in the flag leaf were determined about 2 weeks after anthesis. As expected, both water and NS had a strong positive effect on growth. However, a reduction in water supply had low effect decreasing photosynthesis and transpiration, Δ¹³C and NUE and increasing WUE. On the contrary, increasing the level of nitrogen supplied had a significant negative effect on g_s, which decreased significantly the ratio of intercellular to ambient CO₂ concentrations and Δ¹³C, and increased both instantaneous and time‐integrated WUE. In addition, a higher N level also negatively affected the instantaneous and time‐integrated NUE. The Δ¹³C of shoots correlated significantly and negatively with either instantaneous or time‐integrated measurements of WUE. Moreover, within each NS, Δ¹³C also correlated negatively with the integrated NUE. We concluded that under our experimental conditions, Δ¹³C gives information about the efficiency with which not just water but also nitrogen are used by the plant. In addition, this study illustrates that a steady water limitation may strongly affect biomass without consistent changes in WUE. The lack of effect of the different water regimes on gas exchange, WUE and Δ¹³C illustrate the importance of how stress is imposed during growth.     

SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO₂ Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.     

Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO₂ assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO₂ (g_c) and H₂O (g_s), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO₂ assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 mol s^–1 m^–2. Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher g_c and g_s than sun-grown leaves. Shade leaves of taro had greater g_c and g₃ rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.     

不同年代推出的冬小麦品种叶片气体交换日变化的差异

选择 6 0年来北京地区广泛种植的 3个冬小麦 (TriticumaestivumL .)品种 ,在相同的环境条件下种植。为了研究它们的产量与单位叶面积的净光合速率 (Pn)的关系 ,测定了不同生育期Pn、蒸腾速率 (Tr)的日变化 ,并用Pn/Tr计算叶片瞬时的水分利用率 (WUE)。结果表明 :单位叶面积净光合速率与产量之间的关系随生育期不同而变化。在拔节期高产品种“京冬 8号”(九十年代推出 )的光合速率和蒸腾速率在一天中总是最高 ,一天中差异最大时 ,分别比低产品种“燕大 1817”(四十年代推出 )高 77%和 6 9%。而其水分利用率却小于低产品种。这种差异随小麦的生长发育而变化 ,一般上午 10 :0 0前“京冬 8号”的光合速率较高 ,而 10 :0 0后“燕大 1817”的光合速率较高。到腊熟期 ,低产品种“燕大 1817”的光合速率在一天中始终最高。蒸腾速率的变化规律与光合速率相似 ,然而“燕大1817”叶片的水分利用率一般最高。与现代推出的品种不同 ,老品种“燕大 1817”叶片的光合作用午休现象不明显 ,说明它可能具有一定的抗光氧化性。我们认为 ,在品种改良的过程中 ,叶片光合作用的潜力可能有所提高 ,但它的抗光氧化性可能减弱。     

Progressive Soil Drought Promotes the Export Function in the Birch (Betula platyphylla) Leaf

A greenhouse experiment, which imitated a short (4-day-long) and progressive (3-week-long) soil drought, was employed to assess, with an IR gas analyzer, leaf CO₂ exchange rate (CER) in intact one-year-old seedlings of Betula platyphylla as related to the flux of photosynthetically active radiation ranging from 0 to 1400 E/(m² s). The registered indices comprised leaf temperature, leaf transpiration conductivity, and the average daily increment of the leaf area. Within a week period following the transition from the short severe soil drought (20% H₂O per soil weight) to the conditions of sufficient water content (35–40%), the plants completely regained the initial leaf CER. Under the progressive soil drought, leaf CER was reduced by 30–35%, as compared to the conditions of sufficient water content, evidently due to a 3.7-fold drop in the transpiration conductivity as compared to the control plants. The apparent constant of Rubisco carboxylation and leaf respiration in the light were not affected by the drought period. The rate of leaf growth under the progressive drought was reduced by 64% as compared to the sufficient moisture conditions. Thus, under the progressive drought, the diminished stomatal conductivity reduced CO₂ concentration inside the leaf and lowered carbon photosynthetic assimilation. Meanwhile, the leaf source activity considerably increased in spite of diminished photosynthesis.     

Stomatal control of transpiration in the canopy of a clonal Eucalyptus grandis plantation

 Predawn leaf water potential, stomatal conductance and microclimatic variables were measured on 13 sampling days from November 1995 through August 1996 to determine how environmental and physiological factors affect water use at the canopy scale in a plantation of mature clonal Eucalyptus grandis Hill ex-Maiden hybrids in the State of Espirito Santo, Brazil. The simple ”big leaf” Penman-Monteith model was used to estimate canopy transpiration. During the study period the predawn leaf water potential varied from –0.4 to –1.3 MPa, with the minimum values observed in the winter months (June and August 1996), while the average estimated values for canopy conductance and canopy transpiration fell from 17.3 to 5.8 mm s^–1 and from 0.54 to 0.18 mm h^–1, respectively. On the basis of all measurements, the average value of the decoupling coefficient was 0.25. During continuous soil water shortage a proportional reduction was observed in predawn leaf water potential and in daily maximum values of stomatal conductance, canopy transpiration and decoupling coefficient. The results showed that water vapour exchange in this canopy is strongly dominated by the regional vapour pressure deficit and that canopy transpiration is controlled mainly by stomatal conductance. On a seasonal basis, stomatal conductance and canopy transpiration were mainly related to predawn leaf water potential and, thus, to soil moisture and rainfall. Good results were obtained with a multiplicative empirical model that uses values of photosynthetically active radiation, vapour pressure deficit and predawn leaf water potential to estimate stomatal conductance. Received: 10 June 1998 / Accepted: 20 July 1998