Variation In Carbon Isotope Discrimination Research Articles

Spatio-Temporal Variations in Carbon Isotope Discrimination Predicted by the JULES Land Surface Model.

Stable carbon isotopes in plants can help evaluate and improve the representation of carbon and water cycles in land-surface models, increasing confidence in projections of vegetation response to climate change. Here, we evaluated the predictive skills of the Joint UK Land Environmental Simulator (JULES) to capture spatio-temporal variations in carbon isotope discrimination (Δ13C) reconstructed by tree rings at 12 sites in the United Kingdom over the period 1979-2016. Modeled and measured Δ13C time series were compared at each site and their relationships with local climate investigated. Modeled Δ13C time series were significantly correlated (p<0.05) with tree-ring Δ13C at eight sites, but JULES underestimated mean Δ13C values at all sites, by up to 2.6‰. Differences in mean Δ13C may result from post-photosynthetic isotopic fractionations that were not considered in JULES. Inter-annual variability in Δ13C was also underestimated by JULES at all sites. While modeled Δ13C typically increased over time across the UK, tree-ring Δ13C values increased only at five sites located in the northern regions but decreased at the southern-most sites. Considering all sites together, JULES captured the overall influence of environmental drivers on Δ13C but failed to capture the direction of change in Δ13C caused by air temperature, atmospheric CO2 and vapor pressure deficit at some sites. Results indicate that the representation of carbon-water coupling in JULES could be improved to reproduce both the trend and magnitude of interannual variability in isotopic records, the influence of local climate on Δ13C, and to reduce uncertainties in predicting vegetation-environment interactions.

Mixing oak and pine trees does not improve the functional response to severe drought in central French forests

Mixing sessile oak and Scots pine in central France to reduce intraspecific competition for water resources did not improve the ability of these two species to withstand severe drought during the summer. In order to reduce the impact of increasingly extreme droughts on forests, managers must adapt their practices to future climate conditions. Maintaining a greater diversity of tree species in temperate forest ecosystems is one of the recommended options. We addressed how interactions between sessile oak and Scots pine in mixed forests in central France affect their functional response to drought. We characterized the carbon isotope composition (δ13C) in the tree growth rings formed during wet (2001, 2007) or dry (2003, 2004) summers for each of the two species growing both in pure and in mixed stands in order to compare the effect of stand composition on variations in carbon isotope discrimination (Δ13C) among contrasted years. The severe drought in 2003 induced a strong decrease in Δ13C for all trees and in all stands as compared to 2001. This decrease was greater in pine than in oak. There was no significant difference between pure and mixed stands in the response of either species to drought. Mixing sessile oak and Scots pine in stands in central France does not improve the ability of either species to withstand severe drought during the summer.

Determination of leaf carbon isotope discrimination in C4 plants under variable N and water supply

Understanding the mechanisms underlying variations in carbon isotope discrimination (Δ) in C4 plants is critical for predicting the C3/C4 ratio in C3/C4 mixed grassland. The value of Δ is determined by bundle sheath leakiness (Ф) and the ratio of intercellular to ambient CO2 concentration (Ci/Ca). Leaf nitrogen concentration (Nleaf) is considered a driver of Δ in C4 plants. However, little is known about how Nleaf affects Ф and Ci/Ca, and subsequently Δ. Here leaf carbon isotope composition, Nleaf, Ф, and leaf gas exchange were measured in Cleistogenes squarrosa, a dominant C4 species in the Inner Mongolia grassland. Δ remained relatively stable under variable N and water supply. Higher N supply and lower water supply increased Nleaf, stimulated photosynthesis and further decreased Ci/Ca. High N supply increased Ф, which responded weakly to water supply. Nleaf exerted similar effects on Ci/Ca and on Ф in the field and pot experiments. Pooling all the data, Nleaf explained 73% of the variation in Ci/Ca. Overall, both Ф and Ci/Ca determined Δ; however, the contribution of Ф was stronger. Nleaf influenced Δ primarily though Ci/Ca, rather than Ф. Ф should be considered in estimating Δ of C4 endmember.

Variation and inheritance of carbon isotope discrimination in wheat (Triticum aestivum)

Significant small variations exist in the proportion of the heavy isotope (13C) in organic and inorganic materials. The amounts of variations depend on the discrimination against or (in favor of) the heavy isotope through equilibrium reactions or kinetic processes particularly during CO2 fixation in plants. These variations have also paved the way to measure carbon isotope discrimination in plant materials as a surrogate for water-use efficiency. Thus looking for variation and understanding the inheritance of this character is a prerequisite for improving wheat genotypes showing high water use efficiency and tolerance to drought. The main objectives of this review is to provide an overview of various aspects in which inheritance of ∆ is documented and of variation for carbon isotope discrimination reported in wheat under different circumstances. The relationship between carbon isotope discrimination and drought tolerance is also discussed exclusively in wheat.

Environmental and physiological determinants of carbon isotope discrimination in terrestrial plants

Stable carbon isotope ratios (δ(13) C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ(13) C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (ci /ca ), coordination between stomatal conductance and photosynthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of ci /ca on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night-time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

Variation in carbon isotope discrimination and its relationship with harvest index in the reference collection of chickpea germplasm.

Terminal drought is a major constraint to chickpea productivity. Carbon isotope discrimination (Δ13C), an integrator of plant behaviour influencing transpiration efficiency (TE), is an important component of yield under drought. The variation in Δ13C and its association with yield was assessed in the reference collection of chickpea germplasm. Drought stress reduced shoot biomass by 36-39% and grain yield by 23%. Mean Δ13C was low and the range of genetic variation was high under drought stress. Largely, high Δ13C accessions were early in flowering (40-50 days), moderate in shoot biomass, high in seed yields and high in harvest index (HI). Δ13C was positively correlated with seed yield in both the years under drought stress, only in 2008-09 under optimal irrigation. This positive association was very close with HI. Among the yield components, Δ13C was closely associated with pod numbers per unit area and seed size under drought stress. Path coefficients showed no direct association of Δ13C with grain yield but an indirect negative association through shoot biomass at maturity and a close positive association through HI. The closest association of HI or shoot biomass was seen in the maturity group of accessions that experienced the optimum terminal drought stress.

Variation in carbon isotope discrimination in Cleistogenes squarrosa (Trin.) Keng: patterns and drivers at tiller, local, catchment, and regional scales.

Understanding the patterns and drivers of carbon isotope discrimination in C4 species is critical for predicting the effects of global change on C3/C4 ratio of plant community and consequently on ecosystem functioning and services. Cleistogenes squarrosa (Trin.) Keng is a dominant C4 perennial bunchgrass of arid and semi-arid ecosystems across the Mongolian plateau of the Eurasian steppe. Its carbon isotope discrimination (13Δ) during photosynthesis is relatively large among C4 species and it is variable. Here the 13Δ of C. squarrosa and its potential drivers at a nested set of scales were examined. Within cohorts of tillers, 13Δ of leaves increased from 5.1‰ to 8.1‰ from old to young leaves. At the local scale, 13Δ of mature leaves varied from 5.8‰ to 8.4‰, increasing with decreasing grazing intensity. At the catchment scale, 13Δ of mature leaves varied from 6.2‰ to 8.5‰ and increased with topsoil silt content. At the regional scale, 13Δ of mature leaves varied from 5.5‰ to 8.9‰, increasing with growing-season precipitation. At all scales, 13Δ decreased with increasing leaf nitrogen content (Nleaf). Nleaf was positively correlated with grazing intensity and leaf position along tillers, but negatively correlated with precipitation. The presence of the correlations across a range of different environmental contexts strongly implicates Nleaf as a major driver of 13Δ in C. squarrosa and, possibly, other C4 species.

Association genetics of carbon isotope discrimination, height and foliar nitrogen in a natural population of Pinus taeda L

Loblolly pine, Pinus taeda L., is one of the most widely planted, commercially and ecologically important tree species in North America. We took an association genetics approach, using an unimproved population of 380 clonally replicated unrelated trees, to test 3,938 single nucleotide polymorphisms (SNPs) in as many genes for association with phenotypic variation in carbon isotope discrimination, foliar nitrogen concentration and total tree height after two growing seasons. Best linear unbiased prediction (BLUP) was used with a spatial adjustment to remove environmental variation from phenotypic data derived from a common garden experiment. After correction for multiple testing, a total of 14 SNPs were associated with the traits of carbon isotope discrimination (n = 7), height (n = 1) and foliar nitrogen concentration (n = 6) using 380 clones. Tails of the population phenotypic distribution were compared for allele frequency differences, revealing 10 SNPs with allele frequency in at least one tail significantly different from the overall population. Eight associated SNPs were in sequences similar to known genes, such as an AP2 transcription factor related to carbon isotope discrimination and glutamate decarboxylase associated with foliar nitrogen concentration, and others were from unknown genes without homologs in Arabidopsis.

Variation in carbon isotope discrimination during growth and at different organs in sugar beet ( Beta vulgaris L.)

It is widely documented that higher carbon isotope discrimination (Δ) is generally associated with lower water use efficiency. As a consequence, Δ has been evaluated and used as a possible selection criterion in several crops. Its variation in time and between plant organs has not however been very much studied in sugar beet, despite the likelihood of improving the effectiveness of selection strategies. The research reported here was carried out over 3 years, 2000–2002, with the objective of investigating the variation of Δ in plant organs of sugar beet, as affected by growth phase and water regime. Results showed that Δ can change significantly over time and among organs. Leaf had the highest values of Δ (on average 21.2‰) and root, the lowest (18.8‰). Furthermore, root Δ showed a relatively low variance (i.e. 0.31), being weakly affected by water regime and time. In contrast, leaf Δ changed drastically between young and mature plants. There was a significant ‘time × water regime’ interaction in leaf A in 2001: in irrigated plants, leaf Δ increased over time, while in rainfed plants it decreased, which may be due to the persistence of photosynthetic active leaves. Root Δ was related positively to sucrose content (Brix%) and negatively to root biomass accumulation ( r = 0.46 ** and −0.48 **, respectively). These results suggest that the variation of Δ among organs or with plant age, if ignored, can lead to an unwanted increase of source of variance. Selecting on root Δ may be the most effective criterion for the lowest unwanted source of variance.

Variation in carbon isotope discrimination in relation to plant performance in a natural population of Cryptantha flava

Few studies of phenotypic selection have focused on physiological traits, especially in natural populations. The adaptive significance of plant water-use efficiency, the ratio of photosynthesis to water loss through transpiration, has rarely been examined. In this study, carbon isotopic discrimination, Delta, an integrated measure of water-use efficiency, was repeatedly measured in juveniles and adults in a natural population of the herbaceous desert perennial Cryptantha flava over a 4-year period and examined for plasticity in Delta, consistency between years in values of Delta, and evidence for selection on Delta phenotypes. There was significant concordance in Delta values among the 4 years for adult plants and significant correlations in Delta values measured in different years for juveniles and adults combined. The wettest year of the study, 1998, proved an exception because Delta values that year were not correlated with Delta values in any other year of the study. Consistency in Delta measured on the same plants in different years could indicate genotypic variation and/or consistency in the water status of the microhabitats the plants occupied. Two forms of plasticity in Delta were also evident; mean seasonal values were correlated with precipitation the preceding autumn, and Delta values also declined with plant size, indicating increasing water-use efficiency. Phenotypic selection was evident because in the first year of the study juvenile plants that would survive until year five averaged lower Delta values than did those that failed to survive. During the driest year, 2000, Delta was significantly negatively correlated with adult plant size, measured as the number of leaf rosettes, but the negative relationship between Delta and the number of flowering stalks, a more direct measure of fitness, was not significant. These results suggest that the direction of phenotypic selection on Delta changes as plants grow.

Genotypic variations in carbon isotope discrimination, transpiration efficiency, and biomass production in rice as affected by soil water conditions and N

Genotypic and environmental (soil water regime and N level) variation in carbon isotope discrimination (CID) in relation to the gas exchange, transpiration efficiency (A/T), and biomass production were investigated in field experiments using eleven rice (Oryza sativa L.) genotypes. The results showed that genotype was more dominant for variation in CID than in total biomass. Genotypic ranking in CID was consistent across environments because of small genotype × environment interactions. Japonica genotypes tended to have lower CID than indica genotypes. Higher soil water and lower N rate significantly increased CID. Variation in CID was slightly smaller for water regime than for genotype. There was a negative correlation between CID andA/T among genotypes within water regimes. Genotypic variation in CID was associated mainly with variation in stomatal conductance under all soil water regimes and with photosynthetic capacity in late growth stages under aerobic soil conditions. The decrease in CID at higher N was probably due to lower stomatal conductance under aerobic soil conditions and to higher photosynthetic rates under submerged soil conditions. The correlation between biomass and CID was not clear in aerobic soil, whereas it was positive in submerged soil, which indicated that the significance of lower or higher CID for improving biomass productivity may differ under different soil water regimes. Overall, the results implied a possible use of CID as a selection criterion for genotypic improvement inA/T and productivity in rice.

Genetic variation in carbon isotope discrimination in six European populations of Castanea sativa Mill. originating from contrasting localities

The objective of this study was to evaluate the variability of physiological performances of Castanea sativa Mill. in relation to drought tolerance, among and within European populations coming from contrasting environmental conditions. Forty-eight open-pollinated families from a stratified sample (temperature/precipitation) of six naturalized populations from Spain, Italy and Greece were grown for one growth period under two temperature regimes (25 and 32 degrees C), in combination with two watering regimes in growth chambers. Complementary to growth traits analysed in a previous study, carbon isotope discrimination (Delta), a complex physiological trait involved in acclimation and adaptive processes, was studied. anova indicated significant Delta variability for C. sativa populations across Europe and, thereby, variation in adaptedness to drought. The European pattern of Delta variability matches the previously reported one for the centre of origin of C. sativa (Ponto-Caucasian region). This suggests that common mechanisms of drought adaptedness, involving both genetic and physiological determinants, give C. sativa the capacity to colonize a wide range of site conditions. The highest Delta values, indicating the lowest water-use efficiency (WUE), were found within each treatment for populations originating from Mediterranean drought-prone sites. These populations also had the highest phenotypic plasticity of Delta. Significant among-family genetic variation in Delta was found. The heritability based on the joint anova was estimated at 0.31 +/- 0.07. The estimates of the coefficients for the additive variance varied in the range 2.6-4.0%, suggesting possibilities for selection on WUE and adaptedness to drought. The genetic correlations between Delta and growth traits were generally strong and negative, especially in the two high temperature treatments.

Genetic Variation for Carbon Isotope Discrimination in Sunflower

Plants accumulate isotopes of carbon at different rates because of discrimination against 13C relative to 12C. In plants that fix carbon by the C3 pathway, the amount of discrimination correlates negatively with transpiration efficiency (TE) where TE is the amount of dry matter accumulated per unit water transpired. Therefore, carbon isotope discrimination (Δ) has become a useful tool for selecting genotypes with improved TE and performance in dry environments. Surveys of 161 sunflower (Helianthus spp.) genotypes of diverse origin revealed a large and unprecedented range of genetic variation for Δ (19.5–23.8‰). A strong negative genetic correlation (rg) between TE and Δ (rg = −0.87, P &lt; 0.001) was observed in glasshouse studies. Gas exchange measurements of field grown plants indicated that Δ was strongly correlated with stomatal conductance to water vapor (g), (rg = 0.64, P &lt; 0.01), and the ratio of net assimilation rate (A) to g, (rg = −0.86, P &lt; 0.001), an instantaneous measure of TE. Genotype CMSHA89MAX1 had the lowest TE (and highest Δ) of all genotypes tested in these studies and low yields in hybrid combination. Backcrossing studies showed that the TE of this genotype was due to an adverse effect of the MAX1 cytoplasm, which was inherited from the diploid perennial H. maximiliani Schrader. Overall, these studies suggested that there is an excellent opportunity for breeders to develop sunflower germplasm with improved TE. This can be achieved, in part, by avoiding cytoplasms such as the MAX1 cytoplasm.

Seasonal and interannual variations in carbon isotope discrimination in a maritime pine (Pinus pinaster) stand assessed from the isotopic composition of cellulose in annual rings.

Stable carbon isotope composition (delta; per thousand) was measured on cellulose extracted from maritime pine (Pinus pinaster Aït.) tree rings to investigate inter-tree and interannual variability (7 trees, 20 rings per tree, each ring divided into early and late wood). A model of stand primary production coupled to water balance was used to calculate the stand annual water-use efficiency (WUE). Inter-tree variability in discrimination (maximum 2.88 per thousand in late wood in 1989, 2.69 per thousand in early wood in 1983) was as large as interannual variation (maximum 2.72 per thousand in late wood, 2.05 per thousand in early wood). Tree size did not explain these differences. Relationships were found between annual discrimination and climate variables such as annual rainfall, summer temperature and vapor pressure deficit (VPD). Higher correlations were found with late wood discrimination. Early wood discrimination was shown to be related to previous-year late wood discrimination. Late wood discrimination was also related to soil water availability. Stand annual WUE was only weakly related to tree ring carbon discrimination.

Carbon isotope discrimination and its relationship to drought resistance under field conditions in genotypes of Eucalyptus globulus Labill.

The aim of this study was to investigate the relationship between carbon isotope discrimination (Δ), survival and growth, to determine the value of Δ for screening for yield improvement under drought conditions. Intra-specific variation in carbon isotope discrimination (Δ) was examined in Eucalyptus globulus. Wood samples were collected from 3 to 7 year-old populations from three different field-sites at Huelva (south-western Spain). Differences between study sites were related to the availability of water resulting from soil physical properties. Average carbon isotope discrimination ranged from 16.7 to 18.1‰. There was a 2.2‰ maximum range in Δ values among different trees of the same age. A strong, positive correlation was observed between average carbon isotope discrimination and average tree diameter at breast height (DBH ob). Average carbon isotope discrimination was also strongly correlated with survival at the Valdeoscuro site ( r 2=0.95) and with the percentage of failures in neighbouring trees ( r 2=0.65) at the San Sebastián site. These results suggest a strong effect of availability of water on Δ values. Implications for the use of carbon isotope discrimination in selecting for better adapted genotypes are discussed.

Genotypic variation in carbon isotope discrimination and gas exchange of ponderosa pine seedlings under two levels of water stress

As part of a program to select ponderosa pine (Pinus ponderosa Dougl. ex Laws.) genotypes for improved drought tolerance, we examined physiological and morphological characteristics of 12 half-sib families of ponderosa pine from four seed sources; New Mexico, South Dakota, Nebraska, and Wyoming. We analyzed genetic variation in carbon isotope discrimination (Δ), photosynthetic gas exchange, needle morphology, and growth of 2-year-old seedlings from the four seed sources grown under two levels of moisture availability. To gain a better understanding of within-provenance variation and identify opportunities to refine selection strategies, we also examined family within seed source variation in the traits. Water stress significantly (P &lt; 0.05) reduced net photosynthesis (A), needle conductance to water vapor (gwv), carbon isotope discrimination (Δ), and growth of the seedlings as compared to well-watered seedlings. However, instantaneous water use efficiency (A/gwv) did not differ between water treatments. Seedlings from New Mexico had significantly lower gwv and higher A/gwv than seedlings from the other sources. Carbon isotope discrimination was lowest for seedlings from New Mexico and Nebraska. Families within seed sources varied significantly in A, gwv, stomatal density, needle length, height increment, and Δ. Carbon isotope discrimination was significantly correlated with gwv but not with A, supporting results from mature trees suggesting that variation in Δ in ponderosa pine is more related to gwv than to A. Seed source × water treatment interactions were not observed for any of the traits analyzed. These results support our previous assertion that genotype × environment interaction in Δ of mature ponderosa pine trees from these sources grown in Nebraska and Oklahoma was related to factors other than moisture availability.

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland.

Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of 13CO2 uptake relative to 12CO2), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ.

Provenance variation in carbon isotope discrimination of mature ponderosa pine trees at two locations in the Great Plains

We analyzed genotypic variation in carbon isotope discrimination (Δ), photosynthetic gas exchange, and needle morphology of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees from four seed sources growing in two 26-year-old provenance plantings near Plattsmouth, Neb., and Norman, Okla. The populations studied were from South Dakota, New Mexico, Wyoming, and Nebraska. Net photosynthesis (A) and needle conductance to water vapor (gwv), were measured during the growing season of 1994. Specific leaf area, stomatal density, and Δ were analyzed in needles grown from 1991 to 1994. The southernmost source (New Mexico) had the highest intrinsic water-use efficiency (A/gwv) among the sources studied. Carbon isotope discrimination was correlated with growth (r = -0.81, P &lt; 0.05), A/gwv (r &lt; -0.54, P &lt; 0.001), and gwv (r &gt; 0.46, P &lt; 0.05) but not A. Variation in Δ was significant among seed sources and years (P &lt; 0.001). We observed a strong genotype × environment interaction in Δ resulting from geographic location but not moisture availability within locations. We hypothesize that the genotype × environment interaction is related to variation in growth phenology among the seed sources. Improving water-use efficiency or growth of ponderosa pine via Δ will require an understanding in genotypic variation in growth rhythms.

Provenance variation in carbon isotope discrimination of mature ponderosa pine trees at two locations in the Great Plains

We analyzed genotypic variation in carbon isotope discrimination (Δ), photosynthetic gas exchange, and needle morphology of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees from four seed sources growing in two 26-year-old provenance plantings near Plattsmouth, Neb., and Norman, Okla. The populations studied were from South Dakota, New Mexico, Wyoming, and Nebraska. Net photosynthesis (A) and needle conductance to water vapor (gwv), were measured during the growing season of 1994. Specific leaf area, stomatal density, and Δ were analyzed in needles grown from 1991 to 1994. The southernmost source (New Mexico) had the highest intrinsic water-use efficiency (A/gwv) among the sources studied. Carbon isotope discrimination was correlated with growth (r = -0.81, P < 0.05), A/gwv (r < -0.54, P < 0.001), and gwv (r > 0.46, P < 0.05) but not A. Variation in Δ was significant among seed sources and years (P < 0.001). We observed a strong genotype × environment interaction in Δ resulting from geographic location but not moisture availability within locations. We hypothesize that the genotype × environment interaction is related to variation in growth phenology among the seed sources. Improving water-use efficiency or growth of ponderosa pine via Δ will require an understanding in genotypic variation in growth rhythms.

Genotypic variation in carbon isotope discrimination and gas exchange of ponderosa pine seedlings under two levels of water stress

As part of a program to select ponderosa pine (Pinus ponderosa Dougl. ex Laws.) genotypes for improved drought tolerance, we examined physiological and morphological characteristics of 12 half-sib ...