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.     

Water-use efficiency and carbon isotope discrimination in two cultivars of upland rice during different developmental stages under three water regimes

A pot experiment was conducted in a glasshouse to clarify and quantify the effect of plant part, water regime, growth period, and cultivar on carbon isotope discrimination (CID), and to analyze the relationship between CID, stomatal behavior and water-use efficiency (WUE). The experiment was comprised of two upland rice (Oryza sativa L.) cultivars and three water regimes (100, 70, and 40% of saturation moisture) in a completely randomized design. Plants were harvested at tillering, flowering, and maturity. No significant cultivar differences in above-ground dry matter-based WUE (WUE_A) and total dry matter-based WUE (WUE_T) were observed. WUE_A (and WUE_T) increased with water stress up to tillering, but decreased with water stress after tillering. Significant cultivar differences in CID in all the analyzed plant parts were observed at all harvest times. Reduction in CID with water stress was greatest at tillering, and the effect was less pronounced at flowering and at maturity. At each harvest, the effect was most pronounced in newly developed plant parts. Root and grain tended to have the lowest CID values, and stem the highest, at all harvest times. A negative relationship was observed between CID measured at tillering and WUE_A (and WUE_T) measured over the period from seedling to tillering, whereas a reverse relationship was obtained between CID measured at flowering and WUE_A (and WUE_T) measured over the period from tillering to flowering, and an unclear relationship between CID measured at maturity and WUE_A (and WUE_T) measured over the period from flowering to maturity. The ratio of the intercellular and atmospheric concentration of CO₂ (C_i/C_a) were closely associated with CID throughout the water regimes when one cultivar was considered, however, cultivar differences in CID were not related to variations in C_i/C_a. The results indicate that significant cultivar difference existed in CID in all the analyzed plant parts at all harvest times, while corresponding difference in WUE_A (and WUE_T) between the cultivars was not necessarily consistent. Abbreviations: WUE – water-use efficiency; WUEi – instantaneous WUE (or leaf transpiration efficiency); ADM – above-ground dry matter; TDM – total dry matter; WUE_A– ADM-based WUE; WUE_T– TDM-based WUE} CID – carbon isotope discrimination; NL – the newest leaves; FEL – recently fully expanded leaves; FL – flag leaves; P – photosynthesis rate; g – leaf stomatal conductance to water vapor; C_i– intercellular CO₂ concentration; C_a– atmospheric CO₂ concentration; T – transpiration rate; gs – total conductance of CO₂     

Seasonal water-use efficiency and chlorophyll fluorescence response in alpha grass (<Emphasis Type="Italic">Stipa tenacissima</Emphasis> L.) is affected by tussock size

Abstact  Twelve randomly chosen Stipa tenacissima L. individuals were grouped into three tussock size classes, small (ST), medium (MT), and large (LT) with 5.6±0.8, 34.1±4.2, and 631.9±85.8 g of dry green foliar matter, respectively, in three plots with different S. tenacissima cover. Instantaneous (WUE_i) and long-term (WUE_l) water-use efficiencies were measured in two seasons of contrasting volumetric soil water content (early winter 21.0±0.8 % and summer 5.8±0.3 %). Maximum photochemical efficiency of photosystem 2 and stomatal conductance in summer assessed the extent of water and irradiance stress in tussocks of different size. WUE_i was lower in MT and ST “water spender” strategies than in LT during the high water-availability season. In summer net photosynthetic rate and WUE_i were higher and photoinhibition was lower in LT than in MT and ST. Significant spatial variability was found in WUE_i. Water uptake was competitive in stands with denser alpha grass and more water availability in summer, reducing their WUE_i. However, WUE_l showed a rising tendency when water became scarce. Thus it is important to explicitly account for plant size in ecophysiological studies, which must be combined with demographic information when estimating functional processes at stand level in sequential scaling procedures.     

Water-use efficiency of a mallee eucalypt growing naturally and in short-rotation coppice cultivation

This study compared mature Eucalyptus kochii subsp. plenissima trees in inner regions or edges of natural bushland to young trees belt-planted through cleared agricultural land as uncut saplings or regenerating coppice over 2.7 years at Kalannie, Western Australia (320 mm annual rainfall). We assessed the ability of the species to alter its gas exchange characteristics, leaf physical attributes, and water-use efficiency of foliar carbon assimilation (WUE _i) or of total dry matter production (WUE _DM). Stomatal conductance (g _s) varied five-fold between treatment means, with coppices exhibiting greatest values and mature bush least. Photosynthetic rates followed this trend. Leaf photosynthetic capacity estimated by chlorophyll content varied 1.3-fold parallel with variations in leaf thickness, with coppices rating lowest and mature edge trees most highly. WUE _i varied 1.5-fold between treatments and was greatest in mature inner-bush and edge trees. Leaf photosynthetic capacity and g _s were both correlated with WUE _i. Carbon isotope composition (δ¹³C values) of new shoot dry matter produced early in a seasonal flush were similar to those of root starch but when averaged over the whole season correlated well with WUE _i and gas exchange characteristics of trees of each treatment. Coppices showed poorest WUE _i and most negative shoot tip δ¹³C but their WUE _DM was high. This discrepancy was suggested to relate to carbon allocation strategies in coppices favouring fast growth of replacement shoots but not of roots. Physiology of coppice growth of E. kochii is usefully geared towards both rapid and water-use efficient production of woody biomass in water limited environments.     

Dendroecological and genetic insights for future management of an old-planted forest of the endangered Mediterranean fir Abies pinsapo

Pinsapo fir (Abies pinsapo Boiss.) is an endangered Mediterranean conifer that has raised strong conservation interest as a paradigmatic example of species characterized by small and fragmented populations. We studied an old reforestation stand composed of A. pinsapo, Pinus nigra and Pinus sylvestris established in the 1910s in central-eastern Spain (about 500 km north of the species native distribution range), with the aim of evaluating the stand’s suitability as an ex situ conservation unit for the fir. To this end, we investigated the long-term performance of the stand and assessed genetic diversity of A. pinsapo. Tree-ring width (TRW) and carbon isotope discrimination (Δ¹³C) were used to characterise growth dynamics and intrinsic water-use efficiency (WUE_i), respectively. Furthermore, 42 pinsapo firs were genotyped at five microsatellite loci to compare their genetic variation with published data on natural populations. A. pinsapo showed ca. two-fold higher radial growth than pines in the last 80 years; however, a growth decrease was observed for all species from the 1990s onwards. Indexed TRW was positively associated with Δ¹³C at the species level, denoting inter-annual growth dependence on water availability. Overall, Δ¹³C was higher for A. pinsapo compared to pines, indicating lower WUE_i, but Δ¹³C significantly decreased over the last 50 years for all species, likely as the result of tighter stomatal regulation of water loss, resulting in WUE_i increases of about 25 %. Recently, however, A. pinsapo showed reduced WUE_i increase in concord with growth slowdown, suggesting a threshold response for stomatal regulation. Although genetic diversity of A. pinsapo was about half of natural populations, the old-planted stand could be important for the conservation of this endemic species considering its good long-term growth and physiology. The latest decrease in performance of A. pinsapo, however, asks for urgent management measures aimed at reducing the competition for water and promoting growth and natural regeneration. This study illustrates the potential of combining tree-ring-based long-term physiological information with genetic data to ascertain the prospects of artificial stands for conservation purposes.     

Sex-specific physiological and growth responses to water stress in <Emphasis Type="Italic">Hippophae rhamnoides</Emphasis> L. populations

Dioecious plant species and those occupying diverse habitats may present special analytical problems to determine effects of environmental stress. Here, sex-specific physiological and growth responses of two contrasting sea buckthorn (Hippophae rhamnoides L.) populations were recorded after exposure to different watering regimes. The populations used were from wet and dry climate regions in China, respectively. In the semi-controlled environmental study, the well-watered and water-deficiency plants which were watered to 100 % and 50 % field capacity were used, respectively. Sexual differences in height growth (HT), dry matter accumulation (DMA), root/shoot ratio (RS), specific leaf area (SLA), net photosynthesis (A), transpiration (E), instantaneous water use efficiency (WUE_i) and carbon isotope composition (δ¹³C) between the male and female individuals were detected under water-deficiency treatment in both populations tested. However, these sexual differences were not detected under well-watered treatment. On the other hand, compared with the wet climate population, the dry climate population showed lower HT, DMA, SLA, A and E, and higher RS under both watering regimes. The dry climate population also showed higher WUE_i and δ¹³C as affected by water deficit than the wet climate population. These morphological and physiological responses to drought showed that the different populations and the different sexual individuals may employ different survival strategies under environmental stress. The male individuals and the dry climate population would have a conservative water-use strategy in response to drought stress.     

Water-use responses of ��living fossil�� conifers to CO2 enrichment in a simulated Cretaceous polar environment

Background and Aims

During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO₂-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO₂] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO₂] reduces plant water use over the growing season.

Methods

Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) ‘living fossil’ coniferous species after 3 years'' growth in controlled-environment simulated Cretaceous Arctic (69°N) conditions at either ambient (400 µmol mol⁻¹) or elevated (800 µmol mol⁻¹) [CO₂].

Key Results

Stimulation of whole-plant WUE (WUE_P) by CO₂ enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUE_L) and δ¹³C of foliage (tWUE_L). This response was driven largely by increased rates of carbon uptake, because there was no overall CO₂ effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUE_L differed from those measured for iWUE_L. The results suggest caution against over simplistic interpretations of WUE_P based on leaf isotopic composition.

Conclusions

The data suggest that the efficiency of whole-tree water use may be improved by CO₂ enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO₂ in the living fossil species investigated.Key words: Water-use efficiency, elevated CO₂, living fossil plants, conifers, paleoecology, ancient polar forests, stable carbon isotopes, stomatal conductance, canopy transpiration     

Stories hidden in tree rings: A review on the application of stable carbon isotopes to dendrosciences

During the last 25 years, the dendrochronology community has benefited from the application of stable carbon isotopes in tree rings to answer a number of research questions, which has resulted in an exponential burst in the number of publications related to this discipline. Here, we perform a literature review of the most influential topics (scope, methodology, targeted species, and climatic signals) that have shaped the research agenda and its impact on the scholarly output over the recent decades. Based on a total of 550 publications, we observed that: (1) conifers are the most investigated tree functional type, being included in over two-thirds of studies; (2) although being a subject of debate, the material of choice for carbon isotope analyses is still cellulose, for which the extraction procedure has been refined with methodological improvements over the years; (3) a shift in the application of carbon isotopes has occurred from proxies of climatic information for paleoclimatic reconstructions to tools for the understanding of short- and long-term tree functioning; and (4) such shift in research scope is apparently linked to an increasing number of publications showing drought-related signals governing isotope time series in the context of current climate change. Besides, we also assessed the number and outcome of publications analyzing ¹³C-derived intrinsic water-use efficiency (WUE_i) trends within the framework formulated by Saurer et al. (2004). We found that leaf intercellular CO₂ concentration responding proportionally to the increase in atmospheric CO₂ is the most common scenario reported across continents, but with changing importance relative to other scenarios depending on the biome examined. We also detected that climate plays a dominant role over any potential CO₂ fertilization linked to increasing WUE_i. Looking for new and thrilling research avenues, future studies should further examine δ¹³C fluctuations at intra-annual resolution, explore its links to wood characteristics, and disentangle δ¹³C signals of the distinct wood constituents for a better understanding of tree functioning at varying spatiotemporal scales, also benchmarking vegetation models of different complexities.     

Photosynthetic responses to vapour pressure deficit in temperate and tropical evergreen rainforest trees of Australia

Rainforests occur in high precipitation areas of eastern Australia, along a gradient in seasonality of precipitation, ranging from a summer dry season in the temperate south to a winter dry season in the tropical north. The response of net photosynthesis to increasing vapour pressure deficit (VPD) was measured in a range of Australian rainforest trees from different latitudes to investigate possible differences in their response to atmospheric drought. Plants were grown in glasshouses under ambient or low VPD to determine the effect of growth VPD on the photosynthetic response. Temperate species, which experience low summer precipitation, were found to maintain maximum net photosynthesis over the measurement range of VPD (0.5–1.9 kPa). In contrast, the tropical species from climates with high summer precipitation showed large reductions in net photosynthesis with increasing VPD. Temperate species showed higher intrinsic water-use efficiencies under low VPD than the tropical species, whereas their efficiencies were similar under high VPD. Growing plants under a low VPD had little effect on either the photosynthetic response to VPD or the intrinsic water-use efficiency of the species. These different responses of gas exchange to VPD shown by the tropical and temperate rainforest species may reflect different strategies to maximise productivity in their respective climates.     

A genetic link between leaf carbon isotope composition and whole‐plant water use efficiency in the C4 grass Setaria

Genetic selection for whole‐plant water use efficiency (yield per transpiration; WUE_plant) in any crop‐breeding programme requires high‐throughput phenotyping of component traits of WUE_plant such as intrinsic water use efficiency (WUE_i; CO₂ assimilation rate per stomatal conductance). Measuring WUE_i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE_i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ¹³C_leaf) has been suggested as a potential time‐integrated proxy for WUE_i that may provide a tool to screen for WUE_plant. However, a genetic link between δ¹³C_leaf and WUE_plant in a C₄ species has not been well established. Therefore, to determine if there is a genetic relationship in a C₄ plant between δ¹³C_leaf and WUE_plant under well watered and water‐limited growth conditions, a high‐throughput phenotyping facility was used to measure WUE_plant in a recombinant inbred line (RIL) population created between the C₄ grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for δ¹³C_leaf were found and co‐localized with transpiration, biomass accumulation, and WUE_plant. Additionally, WUE_plant for each of the δ¹³C_leaf QTL allele classes was negatively correlated with δ¹³C_leaf, as would be predicted when WUE_i influences WUE_plant. These results demonstrate that δ¹³C_leaf is genetically linked to WUE_plant, likely to be through their relationship with WUE_i, and can be used as a high‐throughput proxy to screen for WUE_plant in these C₄ species.     

Climate-related variability in carbon and oxygen stable isotopes among populations of Aleppo pine grown in common-garden tests

The Aleppo pine (Pinus halepensis Mill.) is found in the Mediterranean under a broad range of moisture and thermal conditions. Differences in severity and duration of water stress among native habitats may act as selective forces shaping the populations’ genetic make-up in terms of contrasting drought strategies. We hypothesised that these strategies should translate into intraspecific variation in carbon isotope composition (δ¹³C, surrogate of intrinsic water-use efficiency, WUE_i) of wood holocellulose, and such variation might be linked to changes in oxygen isotope composition (δ¹⁸O, proxy of stomatal conductance) and to some climatic features at origin. Thus, we evaluated δ¹³C, δ¹⁸O, growth and survival for 25 Aleppo pine populations covering its geographic range and grown in two common-garden tests. We found intraspecific variability for δ¹³C and growth, with high-WUE_i populations (which showed ¹⁸O-enriched holocellulose) having low growth. These results suggest stomatal regulation as common control for δ¹³C and productivity. We also detected sizeable relationships between δ¹³C and climate factors related to the magnitude and timing of drought such as the ratio of summer to annual rainfall. The main climate variable associated with δ¹⁸O was minimum temperature, but only in the coldest trial, suggesting differences in growth rhythms among sources. Overall, slow growing populations from highly-seasonal dry areas of the western Mediterranean exhibited a conservative water-use, as opposed to fast growing sources from the northernmost distribution range. The particular behaviour of the Mediterranean Aleppo pine as compared with other conifers demonstrates different selective roles of climate variables in determining intraspecific fitness.     

Mesophyll CO2 conductance and leakiness are not responsive to short‐ and long‐term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C₄ crops for enhanced whole‐plant water‐use efficiency (WUE_plant) is needed for sustainable agriculture. WUE_plant is a complex trait and an efficient phenotyping method that reports on components of WUE_plant, such as intrinsic water‐use efficiency (WUE_i, the rate of leaf CO₂ assimilation relative to water loss via stomatal conductance), is needed. In C₄ plants, theoretical models suggest that leaf carbon isotope composition (δ¹³C), when the efficiency of the CO₂‐concentrating mechanism (leakiness, ?) remains constant, can be used to screen for WUE_i. The limited information about how ? responds to water limitations confines the application of δ¹³C for WUE_i screening of C₄ crops. The current research aimed to test the response of ? to short‐ or long‐term moderate water limitations, and the relationship of δ¹³C with WUE_i and WUE_plant, by addressing potential mesophyll CO₂ conductance (g_m) and biochemical limitations in the C₄ plant Sorghum bicolor. We demonstrate that g_m and ? are not responsive to short‐ or long‐term water limitations. Additionally, δ¹³C was not correlated with gas‐exchange estimates of WUE_i under short‐ and long‐term water limitations, but showed a significant negative relationship with WUE_plant. The observed association between the δ¹³C and WUE_plant suggests an intrinsic link of δ¹³C with WUE_i in this C₄ plant, and can potentially be used as a screening tool for WUE_plant in sorghum.     

Photosynthesis, water use efficiency and stable carbon isotope composition are associated with anatomical properties of leaf and xylem in six poplar species

Although fast‐growing Populus species consume a large amount of water for biomass production, there are considerable variations in water use efficiency (WUE) across different poplar species. To compare differences in growth, WUE and anatomical properties of leaf and xylem and to examine the relationship between photosynthesis/WUE and anatomical properties of leaf and xylem, cuttings of six poplar species were grown in a botanical garden. The growth performance, photosynthesis, intrinsic WUE (WUE_i), stable carbon isotope composition (δ¹³C) and anatomical properties of leaf and xylem were analysed in these poplar plants. Significant differences were found in growth, photosynthesis, WUE_i and anatomical properties among the examined species. Populus cathayana was the clone with the fastest growth and the lowest WUE_i/δ¹³C, whereas P. × euramericana had a considerable growth increment and the highest WUE_i/δ¹³C. Among the analysed poplar species, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance (g_s) and lowest WUE_i/δ¹³C. Moreover, significant correlations were observed between WUE_i and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUE_i and δ¹³C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUE_i. It is anticipated that some poplar species, e.g. P. × euramericana, are better candidates for water‐limited regions and others, e.g. P. cathayana, may be better for water‐abundant areas.     

Hyposensitive canopy conductance renders ecosystems vulnerable to meteorological droughts

Increased meteorological drought intensity with rising atmospheric demand for water (hereafter vapor pressure deficit [VPD]) increases the risk of tree mortality and ecosystem dysfunction worldwide. Ecosystem-scale water-use strategy is increasingly recognized as a key factor in regulating drought-related ecosystem responses. However, the link between water-use strategy and ecosystem vulnerability to meteorological droughts is poorly established. Using the global flux observations, historic hydroclimatic data, remote-sensing products, and plant functional-trait archive, we identified potentially vulnerable ecosystems, examining how ecosystem water-use strategy, quantified by the percentage bias (δ) of the empirical canopy conductance sensitivity to VPD relative to the theoretical value, mediated ecosystem responses to droughts. We found that prevailing soil water availability substantially impacted δ in dryland regions where ecosystems with insufficient soil moisture usually showed conservative water-use strategy, while ecosystems in humid regions exhibited more pronounced climatic adaptability. Hyposensitive and hypersensitive ecosystems, classified based on δ falling below or above the theoretical sensitivity, respectively, achieved similar net ecosystem productivity during droughts, employing different structural and functional strategies. However, hyposensitive ecosystems, risking their hydraulic system with a permissive water-use strategy, were unable to recover from droughts as quickly as hypersensitive ones. Our findings highlight that processed-based models predicting current functions and future performance of vegetation should account for the greater vulnerability of hyposensitive ecosystems to intensifying atmospheric and soil droughts.     

Responses of young ‘Pink lady’ apple to alternate deficit irrigation following long-term drought: growth, photosynthetic capacity, water-use efficiency, and sap flow

We studied photosynthetic capacity, growth, sap flow, and water-use efficiency in young trees of ‘Pink Lady’ apple (Malus domestica) that were exposed to 60 d of moisture stress. Three irrigation schemes were tested in the greenhouse: well-watered control; drought; or alternate deficit irrigation (ADI). Compared with the drought-stressed plants, those treated via ADI showed better height growth, larger scion diameters, and greater total leaf area, as well as significantly increased gains in dry biomass and rootstock diameters. However, their performance was still significantly lower than that demonstrated by continuously well-watered plants. Sap flow was greater under ADI than under drought, but less than under control conditions. The average rate of net photosynthesis, total amount of irrigation water applied, and dry biomass gain had highly significant and positive linear correlations with long-term water-use efficiency (WUE_L). The same was true between average stomatal conductance and WUE_L. By contrast, instantaneous water-use efficiency (WUE_I) was very significantly and negatively correlated with WUE_L. In addition, values for WUE_L were much higher from well-watered plants when compared with either drought-stressed trees or those treated per ADI. Therefore, our results indicate that, although ‘Pink Lady’ apple normally has high WUE, it still consumes a large amount of water. Therefore, the practice of ADI following a period of long-term drought could be used to improve growth and WUEL by this cultivar.     

Increase in water-use efficiency and underlying processes in pine forests across a precipitation gradient in the dry Mediterranean region over the past 30 years

Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W _i) with environmental conditions in Pinus halepensis. We used 30-year (1974–2003) tree-ring records of basal area increment (BAI) and cellulose ¹³C and ¹⁸O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280–700 mm) in Israel. The results show a clear trend of increasing W _i in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W _i trends were better correlated with the increase in atmospheric CO₂ concentration, C _a, than with the local increase in temperature (~0.04°C year⁻¹), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W _i from EW and BAI were dependent on the interannual variations in P. From reconstructed C _i values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W _i sensitivity to C _a between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C _i/C _a towards a homeostatic set-point observed at the most mesic site, with a decrease in the W _i response to C _i with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W _i driven, and there was an increase in the W _i sensitivity to C _a with aridity and a decreasing sensitivity of C _i to C _a. The apparent C _a-driven increases in W _i can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.     

Photosynthetic photon flux density,carbon dioxide concentration,and vapor pressure deficit effects on photosynthesis in cacao seedlings

Independent short-term effects of photosynthetic photon flux density (PPFD) of 50–400 μmol m⁻² s⁻¹, external CO₂ concentration (C _a) of 85–850 cm³ m⁻³, and vapor pressure deficit (VPD) of 0.9–2.2 kPa on net photosynthetic rate (P _N), stomatal conductance (g _s), leaf internal CO₂ concentration (C _i), and transpiration rates (E) were investigated in three cacao genotypes. In all these genotypes, increasing PPFD from 50 to 400 μmol m⁻² s⁻¹ increased P _N by about 50 %, but further increases in PPFD up to 1 500 μmol m⁻² s⁻¹ had no effect on P _N. Increasing C _a significantly increased P _N and C _i while g _s and E decreased more strongly than in most trees that have been studied. In all genotypes, increasing VPD reduced P _N, but the slight decrease in g _s and the slight increase in C _i with increasing VPD were non-significant. Increasing VPD significantly increased E and this may have caused the reduction in P _N. The unusually small response of g _s to VPD could limit the ability of cacao to grow where VPD is high. There were no significant differences in gas exchange characteristics (g _s, C _i, E) among the three cacao genotypes under any measurement conditions.     

Effect of water stress on growth, osmotic adjustment, cell wall elasticity and water-use efficiency in Spartina alterniflora

In the northern spring–summer season of 2004–2005, vegetative propagated plants of Spartina alterniflora were grown under control and water stress conditions on the Mediterranean sea shore of the south-east of Tunis. Control plants were irrigated every week and water stress plants were irrigated until the soil achieved 50% (mild stress) and 25% (severe stress) field capacity (FC). Dry and fresh weight at the whole plant level (g plant⁻¹), shoot to root ratio on dry and fresh weight, photosynthesis (A), transpiration rate (E), instantaneous water-use efficiency (WUE_i), leaf water potential (Ψw), leaf water content (WC), osmotic potential at full turgor (Ψs¹⁰⁰), osmotic potential at turgor loss point (Ψs⁰), osmotic adjustment (OA), proline, sugars, inorganic compounds and cell wall elasticity (CWE) were evaluated during a period of 6 days period between 82 and 90 days after the beginning of treatment (DAT). Plants grown under severe and mild-water stress showed lower Ψw than in control plants with values that averaged −3.1, −1.6 and −0.9 MPa, respectively. S. alterniflora plants submitted to mild-water stress exhibited OA and a decrease in CWE. However, under severe water stress the OA was not observed and CWE also decreased, but it was higher than in the mild-water stress. OA was mainly explained by the accumulation of nitrates, sugars and at a lesser degree, proline. S. alterniflora had a strong decline of the dry and fresh weight of the whole plant associated to a marked decrease of photosynthesis (A) and transpiration (E) in response to water stress, although WUE_i was increased. These results suggest that OA and WUE_i can be important components of the water stress adaptation mechanism in this species, but they are not sufficient enough to contribute to resistance to water stress.     

Transpiration efficiency and apparent cuticular transpiration in some c3 and c4 plants

Amphistomatous C₃ (Nicotiana tabacum L., Datura stramonium L.) and C₄ (Sorghum saccharatum Pers. and Zea mays L.) species were examined to find how (if at all) their inherent differences in water-use economy are reflected in apparent cuticular transpiration or vice versa. Transpiration efficiency (TE) was calculated from steady state photosynthesis (A) and transpiration (E) rates estimated for the upper side of the leaf after light induction of stomata opening. Apparent cuticular transpiration (’E_c) was measured as the part of transpiration which was not eliminated by convective counteraction of the air stream passing across the amphistomatous leaf: total pressure difference (AP) across the leaf was increased and the minimal value of E_ΔPτ0 was taken as the apparent cuticular transpiration rate (’E_c). ’E_c was treated relative to E at AP equal to zero (E_GDP=0), E’_cr. Measurements were carried out under two leaf-air vapour pressure differences (VPD). E_r (i.e. E_GDPτ0/E_GDP=0) versus GDP patterns differed qualitatively between the investigated C₃ and C₄ plants. TE increased and ’E_cr decreased from tobacco, stramony, maize to sorghum for both VPD of air. ’E_cr and TE were approximately linearly related, the slope being dependent on VPD. The increase in VPD resulted in larger E and slightly smaller epidermal conductance (g) at GDP equal to zero. Both E’_cr and E’_cr decreased markedly at the same time especially, for species with high TE. The results were considered as an indirect confirmation that E’_c values estimated by the technique used reflect species-specific differences in external peristomatal and cuticular vapour loss, at least in a relative sense.