Contrasting physiological responses of six eucalyptus species to water deficit

BACKGROUND AND AIMS: The genus Eucalyptus occupies a broad ecological range, forming the dominant canopy in many Australian ecosystems. Many Eucalyptus species are renowned for tolerance to aridity, yet inter-specific variation in physiological traits, particularly water relations parameters, contributing to this tolerance is weakly characterized only in a limited taxonomic range. The study tests the hypothesis that differences in the distribution of Eucalyptus species is related to cellular water relations. METHODS: Six eucalypt species originating from (1) contrasting environments for aridity and (2) diverse taxonomic groups were grown in pots and subjected to the effects of water deficit over a 10-week period. Water potential, relative water content and osmotic parameters were analysed by using pressure-volume curves and related to gas exchange, photosynthesis and biomass. KEY RESULTS: The six eucalypt species differed in response to water deficit. Most significantly, species from high rainfall environments (E. obliqua, E. rubida) and the phreatophyte (E. camaldulensis) had lower osmotic potential under water deficit via accumulation of cellular osmotica (osmotic adjustment). In contrast, species from low rainfall environments (E. cladocalyx, E. polyanthemos and E. tricarpa) had lower osmotic potential through a combination of both constitutive solutes and osmotic adjustment, combined with reductions in leaf water content. CONCLUSIONS: It is demonstrated that osmotic adjustment is a common response to water deficit in six eucalypt species. In addition, significant inter-specific variation in osmotic potential correlates with species distribution in environments where water is scarce. This provides a physiological explanation for aridity tolerance and emphasizes the need to identify osmolytes that accumulate under stress in the genus Eucalyptus.     

Molecular mapping of quantitative trait loci for drought tolerance in maize plants

Drought tolerance is one of the most important but complex traits of crops. We looked for quantitative trait loci (QTLs) that affect drought tolerance in maize. Two maize inbreds and their advanced lines were evaluated for drought-related traits. A genetic linkage map developed using RFLP markers was used to identify QTLs associated with drought-related traits. Twenty-two QTLs were detected, with a minimum of one and a maximum of nine for drought-related traits. A single-QTL was detected for sugar concentration accounting for about 52.2% of the phenotypic variation on chromosome 6. A single-QTL was also identified for each of the traits root density, root dry weight, total biomass, relative water content, and leaf abscisic acid content, on chromosomes 1 and 7, contributing to 24, 0.2, 0.4, 7, and 19% of the phenotypic variance, respectively. Three QTLs were identified for grain yield on chromosomes 1, 5, and 9, explaining 75% of the observed phenotypic variability, whereas four QTLs were detected for osmotic potential on chromosomes 1, 3, and 9, together accounting for 50% of the phenotypic variance. Nine QTLs were detected for leaf surface area on chromosomes 3 and 9, with various degrees of phenotypic variance, ranging from 25.8 to 42.2%. Four major clusters of QTLs were identified on chromosomes 1, 3, 7, and 9. A QTL for yield on chromosome 1 was found co-locating with the QTLs for root traits, total biomass, and osmotic potential in a region of about 15 cM. A cluster of QTLs for leaf surface area were coincident with a QTL for osmotic potential on chromosome 3. The QTLs for leaf area also clustered on chromosome 9, whereas QTLs for leaf abscisic acid content and relative water content coincided on chromosome 7, 10 cM apart. Co-location of QTLs for different traits indicates potential pleiotropism or tight linkage, which may be useful for indirect selection in maize improvement for drought tolerance.     

Identification of drought-inducible genes and differentially expressed sequence tags in barley

Drought limits cereal yields in several regions of the world and plant water status plays an important role in tolerance to drought. To investigate and understand the genetic and physiological basis of drought tolerance in barley, differentially expressed sequence tags (dESTs) and candidate genes for the drought response were mapped in a population of 167 F₈ recombinant inbred lines derived from a cross between Tadmor (drought tolerant) and Er/Apm (adapted only to specific dry environments). One hundred sequenced probes from two cDNA libraries previously constructed from drought-stressed barley (Hordeum vulgare L., var. Tokak) plants and 12 candidate genes were surveyed for polymorphism, and 33 loci were added to a previously published map. Composite interval mapping was used to identify quantitative trait loci (QTL) associated with drought tolerance including leaf relative water content, leaf osmotic potential, osmotic potential at full turgor, water-soluble carbohydrate concentration, osmotic adjustment, and carbon isotope discrimination. A total of 68 QTLs with a limit of detection score 2.5 were detected for the traits evaluated under two water treatments and the two traits calculated from both treatments. The number of QTLs identified for each trait varied from one to 12, indicating that the genome contains multiple genes affecting different traits. Two candidate genes and ten differentially expressed sequences were associated with QTLs for drought tolerance traits.     

A morpho-physiological approach differentiates bread wheat cultivars of contrasting tolerance under cyclic water stress

Leaf micromorphological traits and some physiological parameters with potential relevance to drought tolerance mechanisms were investigated in four selected winter wheat varieties. Plants were subjected to two cycles of drought treatment at anthesis. Yield components confirmed contrasting drought-sensitive and -tolerant behavior of the genotypes. Drought tolerance was associated with small flag leaf surfaces and less frequent occurrence of stomata. Substantial variation of leaf cuticular thickness was found among the cultivars. Thin cuticle coincided with drought sensitivity and correlated with a high rate of dark-adapted water loss from leaves. Unlike in Arabidopsis, thickening of the cuticular matrix in response to water deprivation did not occur. Water stress induced epicuticular wax crystal depositions preferentially on the abaxial leaf surfaces. According to microscopy and electrolyte leakage measurements from leaf tissues, membrane integrity was lost earlier or to a higher extent in sensitive than in tolerant genotypes. Cellular damage and a decline of relative water content of leaves in sensitive cultivars became distinctive during the second cycle of water deprivation. Our results indicate strong variation of traits with potential contribution to the complex phenotype of drought tolerance in wheat genotypes. The maintained membrane integrity and relative water content values during repeated water limited periods were found to correlate with drought tolerance in the selection of cultivars investigated.     

Approaches to increasing the salt tolerance of wheat and other cereals

This review describes physiological mechanisms and selectable indicators of gene action, with the aim of promoting new screening methods to identify genetic variation for increasing the salt tolerance of cereal crops. Physiological mechanisms that underlie traits for salt tolerance could be used to identify new genetic sources of salt tolerance. Important mechanisms of tolerance involve Na+ exclusion from the transpiration stream, sequestration of Na+ and Cl- in the vacuoles of root and leaf cells, and other processes that promote fast growth despite the osmotic stress of the salt outside the roots. Screening methods for these traits are discussed in relation to their use in breeding, particularly with respect to wheat. Precise phenotyping is the key to finding and introducing new genes for salt tolerance into crop plants.     

New QTLs identified for plant water status, water-soluble carbohydrate and osmotic adjustment in a barley population grown in a growth-chamber under two water regimes

Quantitative trait locus (QTL) analysis was carried out with 167 recombinant inbred lines (RILs) of barley derived from a cross between Tadmor and Er/Apm to identify the genomic regions controlling traits related to plant water status and osmotic adjustment (OA). The experiment was conducted in a growth chamber using a random incomplete block design (nine blocks). Relative water content (RWC) and leaf osmotic potential (ψ_π) were measured at 100% and 14% of the field capacity on 105 RILs in each block. In addition, the water-soluble carbohydrate concentration (WSC) was measured in the four first-blocks. The leaf osmotic potential at full turgor (ψ_π100), the water-soluble carbohydrate concentration at full turgor (WSC100), and also OA, the accumulation of water-soluble carbohydrates (dWSC100), the contribution of a change in water content to OA (CWC) and of the net solute accumulation to OA (SA) have also been calculated. In a previous paper (Teulat et al. 1998), 12 QTLs were identified for RWC, ψ_π, ψ_π100 and OA with adjusted means (block effects and pot-within-block effects fixed) with an incomplete genetic map. In the present paper, a more-saturated and improved map is described. A new QTL analysis as been performed with adjusted means. The new QTLs identified for previous evaluated traits, as well as the QTLs for the new traits, are presented. Eight additional regions (22 QTLs) were identified which increased to 13 the total number of chromosomal regions (32 QTLs) controlling traits related to plant water status and/or osmotic adjustment in this barley genetic background. The results emphasise the value of the experimental design employed for the evaluation of traits difficult to assess in genetic studies. The putative target regions for drought-tolerance improvement are discussed combining arguments on the consistency of QTLs and, when possible, the physiological value of QTLs (trait relevance, syntenic relationships and clustering of QTLs). Received: 8 March 2000 / Accepted: 18 October 2000     

Assessing the Suitability of Various Physiological Traits to Screen Wheat Genotypes for Salt Tolerance

Success of improving the salt tolerance of genotypes requires effective and reliable screening traits in breeding programs.The objective was to assess the suitability of various physiological traits to screen wheat genotypes for salt tolerance.Thirteen wheat genotypes from Egypt,Germany,Australia and India were grown in soil with two salinity levels(control and150mmol/L NaCl)in a greenhouse.The physiological traits(ion contents in leaves and stems,i.e.Na~ ,Cl~-,K~ ,Ca~(2 )),theratios of K~ /Na~ and Ca~ /Na~ in the leaves and stems,net photosynthesis rate,stomatal conductance,transpiration rate,chlorophyll content(SPAD value),and leaf water relations,were measured at different growth stages.The physiologicaltraits except for Na~ and Cl~-in stems and the leaf transpiration rate at 150 mmol/L NaCI showed a significant genotypicvariation,indicating that the traits that have a significant genotypic variation may be possibly used as screening criteria.According to the analysis of linear regression of the scores of the physiological traits against those of grain yield,however,the physiological traits of Ca~(2 )and Ca~(2 )/Na~ at 45 d and final harvest with the greatest genotypic variation were ranked atthe top.From a practical and economic point of view,SPAD value should be considered to be used as screening criteriaand/or there is a need to develop a quick and practical approach to determine Ca~(2 )in plant tissues.     

Assessing the Suitability of Various Physiological Traits to Screen Wheat Genotypes for Salt Tolerance

Success of improving the salt tolerance of genotypes requires effective and reliable screening traits in breeding programs. The objective was to assess the suitability of various physiological traits to screen wheat genotypes for salt tolerance. Thirteen wheat genotypes from Egypt, Germany, Australia and India were grown in soil with two salinity levels （control and 150 mmol/L NaCI） in a greenhouse. The physiological traits （ion contents in leaves and stems, i.e. Na^＋, Cl^-, K^＋, Ca^2＋）, the ratios of K^＋/Na^＋ and Ca^＋/Na^＋ in the leaves and stems, net photosynthesis rate, stomatal conductance, transpiration rate, chlorophyll content （SPAD value）, and leaf water relations, were measured at different growth stages. The physiological traits except for Na^＋ and Cl^- in stems and the leaf transpiration rate at 150 mmol/L NaCI showed a significant genotypic variation, indicating that the traits that have a significant genotypic variation may be possibly used as screening criteria. According to the analysis of linear regression of the scores of the physiological traits against those of grain yield, however, the physiological traits of Ca^2＋ and Ca2^＋/Na^＋ at 45 d and final harvest with the greatest genotypic variation were ranked at the top. From a practical and economic point of view, SPAD value should be considered to be used as screening criteria and/or there is a need to develop a quick and practical approach to determine Ca^2＋ in plant tissues.     

Mapping QTLs for chlorophyll content and chlorophyll fluorescence in wheat under heat stress

Heat stress, one of the major abiotic stresses in wheat, affects chlorophyll fluorescence and chlorophyll content and thereby photosynthesis. To identify quantitative trait loci (QTLs) associated with these traits under terminal heat stress, 251 recombinant inbred lines (RILs) derived from a cross HD 2808/HUW510 were phenotyped. Using composite interval mapping, 40 QTLs were identified; 17 were related to conditions after timely sowing and 23 to heat stress after late sowing. The various parameters of chlorophyll fluorescence were associated with 23 QTLs, which were located on chromosomes 1A, 2A, 3A, and 2D and explained 3.67 to 18.04 % of phenotypic variation, whereas chlorophyll content was associated with 17 QTLs on chromosomes 2A, 2B, 2D, 5B, and 7A explaining 3.49 to 31.36 % of phenotypic variation. Most of the identified QTLs were clustered on chromosome 2D followed by 2A and 1A. The QTL Qchc.iiwbr-2A for chlorophyll content linked with marker gwm372 was stable over conditions and explained 3.81 to 18.05 % of phenotypic variation. In addition, 7 epistatic QTL pairs were also detected which explained 1.67 to 11.0 % of phenotypic variance. These identified genomic regions can be used in marker assisted breeding after validation for heat tolerance in wheat.     

Improvement in drought tolerance in bread wheat is related to an improvement in osmolyte production,antioxidant enzyme activities,and gaseous exchange

Water deficit stress negatively affects wheat growth, physiology, and yield. In lab and hydroponic experiments, osmotic stress levels (control, −2, −4, −6 and −8 Bars) created by PEG-6000, caused a significant decline in germination, mean germination time, root, shoot, and coleoptile length in both wheat genotypes examined. Germination was inhibited more in Wafaq-2001 than in Chakwal-50. Wafaq-2001 showed a higher susceptibility index based on root and shoot dry weight than did Chakwal-50. Wheat plants exhibited osmotic adjustment through the accumulation of proline, soluble sugars, soluble proteins, and free amino acids, and increased antioxidation activities of superoxide dismutase, peroxidase, catalase, and malondialdehyde. Increasing water deficit stress caused a linear decline in chlorophyll contents, leaf membrane stability, and relative water content in all wheat plants, with Wafaq-2001 showing a more severe negative impact on these parameters with increasing stress levels. The results suggest the possibility of utilizing some of these parameters as quantitative indicators of water stress tolerance in plants. Gas exchange measurements (photosynthesis, transpiration, stomatal conductance), leaf osmotic potential, water potential, and yield attributes decreased more abruptly with increasing water deficit, whereas leaf cuticular wax content increased in both genotypes, with more severe impacts on Wagaq-2001. More reduction in biochemical, physiological, and yield attributes was observed in Wafaq-2001 than was observed in Chakwal-50. Based on these results, we can conclude that Chakwal-50 is a more drought-tolerant genotype, and has excellent potential for future use in breeding programs to improve wheat drought tolerance.     

Growth,photosynthesis and water relations as affected by different drought regimes and subsequent recovery in <Emphasis Type="Italic">Medicago laciniata</Emphasis> (L.) populations

The aims of this study were to investigate the effects of water deficit and recovery on growth, photosynthesis and water relations in four Medicago laciniata populations from saharian (Ml-90), inferior arid (Ml-204), superior arid (Ml-306) and semi-arid (Ml-376) Tunisian regions. After 28 d of sowing with ample irrigation, the plants were subjected to 4 water regimes: optimal irrigation (100% of field capacity, FC), moderate drought (75% FC), severe drought (35% FC) and rewatering (plants submitted to 35% FC during 7 d, afterwards the plants were rewatered to 100% FC). Harvest was carried out after 28 d of treatments. The drought tolerance in M. laciniata populations was found to be increased particularly with increasing temperatures of collection site of the population. The Ml-204 and Ml-90 populations used mainly physiological strategies for survival under moderate water shortage. Higher severe drought tolerance in both signaled populations would be related to their lower photosynthesis metabolic impairment, relatively higher leaf RWC and greater osmotic potential decrease. The results suggest that plants with low values of leaf features are likely to maintain higher leaf RWC under sever drought. The largest decrease of osmotic potential was found associated with the solute accumulations such as proline and K⁺.     

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.     

Locating QTL for osmotic adjustment and dehydration tolerance in rice

Research was conducted to identify and map quantitative traitloci (QTL) associated with dehydration tolerance and osmoticadjustment of rice. Osmotic adjustment capacity and lethal osmoticpotential were determined for 52 recombinant inbred lines grownin a controlled environment under conditions of a slowly developedstress. The lines were from a cross between an Indica cultivar,Co39, of lowland adaptation and a Japonica cultivar, Moroberekan,a traditional upland cultivar. The QTL analysis was conductedusing single marker analysis (ANOVA) and interval analysis (Mapmaker/QTL).The measurements obtained and the QTL identified were comparedto root traits and leaf rolling scores measured on the samelines. One major locus was associated with osmotic adjustment. Theputative locus for osmotic adjustment may be homoeologous witha single recessive gene previously identified for the same traitin wheat. The putative osmotic adjustment locus and two of thefive QTL associated with dehydration tolerance were close tochromosomal regions associated with root morphology. In thispopulation, osmotic adjustment and dehydration tolerance werenegatively associated with root morphological characters associatedwith drought avoidance. High osmotic adjustment and dehydrationtolerance were associated with Co39 alleles and extensive rootsystems were associated with Moroberekan alleles. To combinehigh osmotic adjustment with extensive root systems, the linkagebetween these traits will need to be broken. Alternatively,if the target environment is a lowland environment with onlybrief water deficit periods, selection for drought tolerancecharacteristics without consideration of the root system maybe most appropriate. Key words: Drought, rice, osmotic adjustment, dehydration tolerance, molecular markers, QTL, breeding     

Several QTLs involved in osmotic-adjustment trait variation in barley (Hordeum vulgare L.)

 Osmotic adjustment (OA) was previously demonstrated to be an important adaptive mechanism of drought tolerance in cereals. In order to determine which genomic regions are involved in OA variation, 187 barley (Hordeum vulgare L.) recombinant inbred lines (RILs) derived from a cross between Tadmor (drought tolerant) and Er/Apm (susceptible) were studied in a growth chamber for their OA capacity (through correlated traits and by calculation), at an early growth stage and under two water treatments (soil moisture of 14% and 100% of field capacity). The continuous distribution of the traits and their broad-sense line heritabilities, ranging from 0.04 to 0.44, indicated that OA and related traits should have a polygenic nature. A subset of 167 RILs were also genotyped using 78 RFLP, 32 RAPD and three morphological markers and a linkage map was constructed. Despite strong environmental effects acting on the traits, interval mapping and single-marker ANOVA allowed the detection of three QTLs for relative water content (RWC), four QTLs for osmotic potential (ψ_π), two QTLs of osmotic potential at full turgor (ψ_π100) and one QTL for osmotic adjustment at a soil moisture of 14% field capacity. For the irrigated treatment, only two QTLs were detected: one for RWC and one for ψ_π100. Two chromosomal regions were involved in several OA-related trait variations and could be considered as regions controlling OA; these were present on chromosome 1 (7H) and chromosome 6 (6H), whereas other regions were specific for one trait. No major QTL was found. However, the genomic region involved in OA-related traits on chromosome 1 (7H) in barley seemed to be conserved for OA variation among cereals. Epistatic effects, with or without additive effects, acted on the traits. Received: 15 July 1997 / Accepted: 29 October 1997     

The evolutionary rate of leaf osmotic strength drives diversification of Primulina species in karst regions

Leaf water storage capacity and osmotic strength are important traits enabling species to adapt to environments that are often moisture limited. However, whether these drought tolerance traits are correlated with the species diversification rate (DR) of plant lineages is yet to be determined. In this study, we selected a species-rich genus (Primulina) of plants widely distributed in karst regions in which species frequently experience variable periods of drought. We measured water storage capacity-related traits (including leaf thickness and water content per mass) and saturated osmotic strength in the leaves of 58 Primulina species growing in a common garden. Subsequently, using phylogenetic methods, we examined the relationships between the rate of species diversification and the drought tolerance traits and between the species DR and evolutionary rates of these traits. We found that neither water storage capacity nor saturated osmotic strength showed significant relationships with the rate of species diversification. However, the evolutionary rate of saturated osmotic strength showed a significant correlation with the species DR, although no comparable significant relationship was detected regarding the evolutionary rate of water storage capacity. Our study indicates that the diversification among Primulina species has typically been accompanied by an extensive divergence of leaf osmotic strength but not a divergence in leaf water storage capacity. These findings will enhance our current understanding of how drought tolerance influences the diversification of plant species in karst regions.     

QTL for relative water content in field-grown barley and their stability across Mediterranean environments

A quantitative genetics approach was developed to identify the genomic regions that control relative water content (RWC) in field-grown barley. The trait was previously demonstrated to be a relevant screening tool of drought-tolerance in cereals, as well as a good indicator of plant water-status. The trait was measured at the heading stage on flag leaves recorded from 167 recombinant inbred lines grown in several Mediterranean sites (Montpellier, France; Meknès, Morocco; Le Kef, Tunisia). The results obtained confirmed that several genomic regions are implicated in the total phenotypic variation of RWC. A total of nine chromosomal regions were identified. One region situated on the long arm of chromosome 6H contains the most-consistent QTL obtained in the present study. This region was previously identified as controlling RWC, as well as leaf osmotic potential under water stress and osmotic adjustment, from an experiment conducted in growth-chamber conditions with the same genetic background. The confirmation of the role of this region in the genetic control of water and turgor status underlined its interest for breeding purposes in the Mediterranean area. In addition, the presence of several dehydrin loci in the same chromosomal area reinforce its interest for genomics analyses to confirm, or not to confirm, the implication of these genes in the variation of RWC.Communicated by H. F. Linskens     

Cold tolerance,water relations and accumulation of osmolytes in <Emphasis Type="Italic">Bituminaria bituminosa</Emphasis>

We determined the cold (freezing) tolerance of five Spanish populations of the perennial shrub Bituminaria bituminosa (L.) C.H. Stirton (Fabaceae), as the temperature at which 50 % of leaf electrolytes are released (LT₅₀) using leaves of field-grown plants, obtained in two winters and one spring. The freezing tolerance was greater in winter and reflected the minimum temperatures at the original sites from which the populations were obtained. Tolerance in vitro was related to osmotic adjustment in the leaves; more negative osmotic potential values and more positive pressure potential values (MPa) were associated with greater tolerance. Tolerance and osmotic potential were not related to leaf cation contents but to leaf amino acids, soluble sugar and proline contents.     

QTLs affecting morph-physiological traits related to drought tolerance detected in overlapping introgression lines of rice (Oryza sativa L.)

Plant photosynthetic traits such as net photosynthetic rate (P_n), stomata conductance (g_s), transpiration rate (T_r), and intercellular CO₂ concentration (C_i), are known to relate to drought tolerance in plants, but the genetic basis of these traits remains largely uncharacterized because of the difficulty in phenotyping physiological traits in a large mapping population. In this study, a set of 55 overlapping introgression lines (ILs) in the Teqing (indica) background were used to genetically dissect several morph-physiological traits and their relationship with grain yield under water stress and non-stress conditions. These traits included specific leaf weight (SLW), chlorophyll content (CC), leaf stomata frequency (SF), P_n, g_s, T_r, and C_i. A total of 40 QTLs affecting the measured traits were identified and mapped to 21 genomic regions in the rice genome. Clustered QTLs affecting P_n, g_s, T_r, and C_i in the same genomic regions suggest common genetic bases for the physiological traits. Low or no phenotypic correlations between leaf morphological traits and photosynthetic traits and between morph-physiological traits and grain yield (GY) appeared to be due to inconsistence in QTL effect for clustered QTLs, unlinked QTLs affecting different traits, and to possible epistasis that could not be adequately addressed in this study. Our results indicate that improving drought tolerant (DT) of rice by selecting any single secondary traits is not expected to be effective and the identified QTLs for GY and related morph-physiological traits should be carefully confirmed before to be used for improving DT in rice by MAS.     

Coordination between leaf and stem traits related to leaf carbon gain and hydraulics across 32 drought-tolerant angiosperms

We examined 15 traits in leaves and stems related to leaf C economy and water use for 32 co-existing angiosperms at ridge sites with shallow soil in the Bonin Islands. Across species, stem density was positively correlated to leaf mass per area (LMA), leaf lifespan (LLS), and total phenolics and condensed tannins per unit leaf N (N-based), and negatively correlated to leaf osmotic potential and saturated water content in leaves. LMA and LLS were negatively correlated to photosynthetic parameters, such as area-, mass-, and N-based assimilation rates. Although stem density and leaf osmotic potential were not associated with photosynthetic parameters, they were associated with some parameters of the leaf C economy, such as LMA and LLS. In the principal component (PCA) analysis, the first three axes accounted for 74.4% of total variation. Axis 1, which explained 41.8% of the total variation, was well associated with parameters for leaf C and N economy. Similarly, axis 2, which explained 22.3% of the total variation, was associated with parameters for water use. Axis 3, which explained 10.3% of the total variation, was associated with chemical defense within leaves. Axes 1 and 2 separated functional types relatively well, i.e., creeping trees, ruderal trees, other woody plants, C₃ shrubs and forbs, palms, and CAM plants, indicating that plant functional types were characterized by similar attributes of traits related to leaf C and N economy and water use. In addition, when the plot was extended by two unrelated traits, leaf mass-based assimilation rates and stem density, it also separated these functional types. These data indicate that differences in the functional types with contrasting plant strategies can be attributed to functional integration among leaf C economy, hydraulics, and leaf longevity, and that both leaf mass-based assimilation rates and stem density are key factors reflecting the different functions of plant species.