Variation In Water Use Efficiency Research Articles

Effective rainfall seasons for interannual variation in δ13C and tree-ring width in early and late wood of Chinese pine and black locust on the Loess Plateau, China

We studied the importance of effective rainfall for interannual variation in water use efficiency (WUE) and tree-ring growth of Chinese pine (Pinus tabulaeformis Carr.) and black locust (Robinia pseudoacacia L.) by examining correlations of seasonal precipitation with annual values of stable carbon isotope ratio (δ13C) and tree-ring width in early and late wood. The correlations with precipitation were examined for each month and for periods of all possible lengths from 2 to 22 months starting from January of the previous year to October of the current year. The period with the highest correlation was adopted as the most effective rainfall season for interannual variations in WUE and tree-ring width. In early wood, precipitation during the dry season (October to May) before the growing season was negatively correlated with δ13C in pine trees and positively correlated with ring width in pine and locust trees. In late wood, rainfall during the growing season in the current year was negatively correlated with δ13C in pine and locust trees, and positively correlated with ring width in locust trees. Our results demonstrated the differences in the water use strategies of pine and locust trees. The δ13C in pines indicated higher WUE and more conservative water use than in locust trees. Precipitation during the dry season affected the interannual variation in WUE and tree-ring growth in pine and locust trees, indicating that rainfall during the dry season is important for carbon gain and tree-ring growth during the following growing season.

Association of root, specific leaf area and SPAD chlorophyll meter reading to water use efficiency of peanut under different available soil water

Drought is the major abiotic constraint affecting peanut productivity and quality worldwide. There is a pressing need to improve the water use efficiency (WUE) of rain-fed peanut production. Breeding varieties with higher water use efficiency is seen as providing part of the solution. The objectives of this work were to (i) evaluate genetic variation in WUE, harvest index, root dry weight, specific leaf area (SLA) and SPAD chlorophyll meter reading (SCMR) among peanut genotypes in response to different available soil water levels and (ii) assess the relevance of root dry weight, SLA and SCMR to WUE in peanut under receding soil moisture levels. Two greenhouse experiments were conducted in the dry and rainy seasons in 2002/2003. The 11 peanut genotypes (ICGV 98300, ICGV 98303, ICGV 98305, ICGV 98308, ICGV 98324, ICGV 98330, ICGV 98348, ICGV 98353, Tainan 9, KK 60-3 and Tifton-8) and three soil moisture levels [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW] were laid out in a factorial randomized complete block design (RCBD) with six replications. At 37, 67, and 97 day after sowing (DAS), data were recorded for SLA and SCMR. Root dry weight, harvest index (HI) and WUE were recorded at harvest. Drought reduced WUE, root dry weight and HI. Across both seasons, Tifton-8 and ICGV 98300 had high WUE and also had large root systems under drought conditions. ICGV 98324 and Tifton-8 had low SLA and high SCMR under stressed and non-stressed condition. Under drought conditions, ICGV 98324 had high HI and Tifton-8 had low HI. Root dry weight had a greater contribution to WUE under well-watered and mild drought (2/3 AW). Under severe drought (1/3 AW), SLA showed a more important contribution to WUE than the other traits. Traits that were associated to high WUE under drought conditions were different among different peanut genotypes. ICGV 98300 maintained high root dry weight under 2/3 AW and ICGV 98324 maintained low SLA and high SCMR under 1/3 AW. Tifton-8 had both large root systems and low SLA associated with high WUE.

Variability in water use efficiency at the leaf level among Mediterranean plants with different growth forms

Assessing natural variability of leaf water use efficiency in plants adapted to extreme conditions of the Mediterranean climate represents an important step in the evaluation of the usefulness of some plant ecophysiological traits under water stress. Eleven Mediterranean species naturally inhabiting the Balearic Islands and corresponding to different growth forms (herbs, semi-deciduous shrubs, woody evergreen shrubs and woody evergreen semi-shrubs) were subject to progressive soil water depletion. Leaf intrinsic water use efficiency was measured by gas exchange at four different degrees of water stress. Under well watered conditions, differences in leaf intrinsic water use efficiency (A N/g s) among growth forms were limited to woody evergreen semi-shrubs, which presented the highest values. Under water stress conditions, differences became more evident, with a trend for an increase in A N/g s from woody evergreen shrubs, through semi-deciduous shrubs and herbaceous to woody evergreen semi-shrubs. The observed variation in A N/g s correlated with several physiological (leaf water potential, soil to leaf hydraulic conductance and stomatal conductance) and morphological (stomatal density) parameters, displaying a general relationship for all growth forms. This suggests that the capacity for withstanding water limitation is adaptive for all Mediterranean species. However, when A N/g s was related to leaf mass area, this relationship was not generally applicable, and depended on growth forms, suggesting that different growth forms display specific morphological adjustments in response to water shortage.

Physiological Responses to Different Substrate Water Contents: Screening for High Water-use Efficiency in Bedding Plants

Efficient use of irrigation water is increasingly important in the production of bedding plants. Two approaches to efficient water use include reducing irrigation water wastage during production by growing plants at the optimal substrate water content (θ) and growing species with high water-use efficiency (WUE). However, there is little information on the effects of different θ levels on leaf physiology of bedding plants and variation in WUE among different species of bedding plants. The objectives of this study were to determine the effects of θ on leaf water relations, gas exchange, chlorophyll fluorescence, and WUE of bedding plants and to identify the physiological basis for differences in WUE between two bedding plant species. We grew salvia ‘Bonfire Red’ (Salvia splendens Sellow ex Roemer & J.A. Schultes), vinca ‘Cooler Peppermint’ [Catharanthus roseus (L.) G. Don.], petunia ‘Lavender White’ (Petunia × hybrida Hort ex. Vilm.), and impatiens ‘Cherry’ (Impatiens walleriana Hook F.) at four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m−3) using an automated irrigation controller. Regardless of species, leaf water potential (Ψw) and leaf photosynthesis (A) of all four species were lower at a θ of 0.09 m3·m−3 and were not different among the other θ levels, but stomatal conductance to H2O (g S) was lower at 0.09 than at 0.15 and 0.22 m3·m−3 and highest at 0.32 m3·m−3. WUE of bedding plants at different θ levels depended on species. The WUE of petunia was unaffected by θ, whereas for the other three species, WUE was higher at a θ of 0.09 m3·m−3 than at 0.32 m3·m−3. Differences in WUE between petunia and salvia were partly from differences in photosynthetic capacity between the two species. Based on the response of A to leaf internal CO2 concentration (Ci), mesophyll conductance to CO2 [gm (a measure of photosynthetic capacity)] was higher in petunia than salvia, whereas gas phase conductance to CO2 (gCO2 ) was similar for these two species, which resulted in higher WUE in petunia than salvia.

Water‐use efficiency of forest ecosystems in eastern China and its relations to climatic variables

Carbon (C) and water cycles of terrestrial ecosystems are two coupled ecological processes controlled partly by stomatal behavior. Water-use efficiency (WUE) reflects the coupling relationship to some extent. At stand and ecosystem levels, the variability of WUE results from the trade-off between water loss and C gain in the process of plant photosynthetic C assimilation. Continuous observations of C, water, and energy fluxes were made at three selected forest sites of ChinaFLUX with eddy covariance systems from 2003 to 2005. WUE at different temporal scales were defined and calculated with different C and water flux components. Variations in WUE were found among three sites. Average annual WUE was 9.43 mg CO(2) g(-1) H(2)O at Changbaishan temperate broad-leaved Korean pine mixed forest, 9.27 mg CO(2) g(-1) H(2)O at Qianyanzhou subtropical coniferous plantation, and 6.90 mg CO(2) g(-1) H(2)O at Dinghushan subtropical evergreen broad-leaved forest. It was also found that temperate and subtropical forest ecosystems had different relationships between gross primary productivity (GPP) and evapotranspiration (ET). Variations in WUE indicated the difference in the coupling between C and water cycles. The asynchronous response of GPP and ET to climatic variables determined the coupling and decoupling between C and water cycles for the two regional forest ecosystems.

Isotopic carbon composition and related characters of dominant species along an environmental gradient in Inner Mongolia, China

Carbon isotope composition ( δ 13C value) provides a useful measure of integrated water-use efficiency (WUE) of plants and can be used to rank the drought tolerance of different species. We determined the δ 13C values, leaf water content (LWC) and free proline content of 52 plant species selected from eight plant communities along a moisture gradient in the Xilin River Basin, Inner Mongolia. The δ 13C values and WUE of C 3 plant species tended to increase as the occurrence frequency and relative biomass of the species increased in the entire basin. The mean foliage proline concentration of all species in a community was higher when the habitat was drier and the mean LWC of plants decreased with increasing soil aridity. Significantly positive relationships existed between the foliar proline concentration and occurrence frequency and relative biomass of the species, respectively. Our results indicated that inter- and intra-species variations in WUE might significantly influence the outcome of competitive interactions and played a large role in determining species composition of plant communities. In addition, proline accumulation in leaves might strengthen competitive ability of plants growing in drought prone habitats.

Natural selection on ecophysiological traits of a fern species in a temperate rainforest

Unlike other species of the genus Blechnum, the fern Blechnum chilense occurs in a wide range of habitats in Chilean temperate rainforest, from shaded forest understories to abandoned clearings and large gaps. We asked if contrasting light environments can exert differential selection on ecophysiological traits of B. chilense. We measured phenotypic selection on functional traits related to carbon gain: photosynthetic capacity (Amax), dark respiration rate (Rd), water use efficiency (WUE), leaf size and leaf thickness in populations growing in gaps and understorey environments. We assessed survival until reproductive stage and fecundity (spo- rangia production) as fitness components. In order to determine the potential evo- lutionary response of traits under selection, we estimated the genetic variation of these traits from clonally propagated individuals in common garden experiments. In gaps, survival of B. chilense was positively correlated with WUE and negatively correlated with leaf size. In contrast, survival in shaded understories was positively correlated with leaf size. We found positive directional fecundity selection on WUE in gaps population. In understories, ferns of lower Rd and greater leaf size showed greater fecundity. Thus, whereas control of water loss was optimized in gaps, light capture and net carbon balance were optimized in shaded understories. We found a significant genetic component of variation in WUE, Rd and leaf size. This study shows the potential for evolutionary responses to heterogeneous light environments in functional traits of B. chilense, a unique fern species able to occupy a broad successional niche in Chilean temperate rainforest.

Seasonal variations and environmental control of water use efficiency in subtropical plantation

To understand the seasonal variations of water use efficiency (WUE) of coniferous plantation in the subtropical monsoon area, the experiment was conducted in 2003 and 2004 which presented two distinguished climatic conditions (severe summer drought in 2003 and normal climatic condition in 2004). The water stress influenced WUE greatly, which caused a special seasonal WUE pattern. WUE reached the minimum in summer drought and the maximum in winter, which was contrary to the variation of gross primary production (GPP) and canopy evaporation (Fw). In winter, GPP and Fw increased along with the increasing of air temperature and vapor pressure deficit (VPD), with the similar increasing rate. However, in drought summer, there was an adverse trend among GPP/Fw and air temperature and VPD, and the decreasing rate of GPP was far larger than that of Fw. In summer, the conservation of WUE was changed because of the environmental factors, resulting in the decreasing WUE. The photosynthesis and transpiration of vegetation were mainly controlled by the environmental factors in winter, and the impact of stomatal regulation was relatively weak. In summer, Fw was mainly controlled by the stomatal closure and GPP by both environmental factors and stomatal closure.

Adaptive genetic variation in water-use efficiency and gum yield in Acacia senegal provenances grown on clay soil in the Blue Nile region, Sudan

The genetic variation in shoot growth, water-use efficiency (as assessed by δ 13C) and gum production was compared among eight Acacia senegal provenances originating from contrasting conditions of the gum arabic belt in central Sudan. The experimental site was in the clay plain region. Carbon isotope ratios were obtained from leaves and branch wood samples of the dissimilar A. senegal provenances. The clay provenances were distinctly superior to the sand provenances in all traits studied but especially in basal diameter and crown width, reflecting their adaptation to the environment. Values of δ 13C were less negative in the sand group compared to the clay both in leaves and in branch wood. This suggests that sand provenances displayed conservative water use and high drought tolerance. Of the clay provenances, the local one displayed a high negative δ 13C value, which indicates less conservative water use that results in high productivity at this particular clay-soil site. Water use appears to correspond to the environmental conditions prevailing at collection sites for these provenances. Results suggest that A. senegal provenances in the clay part of the gum belt are adapted for fast growth rate and high biomass and gum productivity as compared with the provenances from the sand regions: a negative correlation was found between tree gum yield and δ 13C. The variation in water use and gum production was greater among provenance groups than within them, suggesting that selection among rather than within provenances would result in distinct genetic gain in gum yield.

Carbon Isotope Discrimination Accurately Reflects Variability in WUE Measured at a Whole Plant Level in Rice

Water use efficiency (WUE) is physiologically linked to discrimination of the stable isotope of carbon (Δ13C) in leaves of plant species. We determined the genetic variability in WUE by gravimetric approach and Δ13C among 34 diverse germplasm accessions of rice (Oryza sativa L.). The leaf Δ13C ranged between 18.7 and 21.6‰, representing a significant variability and showed a strong inverse relationship with WUE. The gravimetrically determined WUE represents time integrated values, and hence its regression with Δ13C strongly proves the relevance of Δ13C as a surrogate for WUE in rice. For a trait to be successfully exploited for crop improvement, it should have low genotype × environment (G × E) interaction. Six contrasting genotypes selected and examined in a separate experiment showed good correspondence in both WUE and Δ13C between the experiments indicating that WUE is genetically controlled in rice and hence can be exploited through breeding. A prior knowledge on the constituent physiological factors controlling WUE is an important prerequisite for exploiting this trait in crop improvement programs. An inverse relationship between WUE and mean transpiration rate (MTR) indicates a stomatal control of WUE among rice genotypes. Although total biomass normally decreases while selecting for high WUE from among conductance types, a few promising genotypes with high WUE coupled with moderately high total biomass can still be identified for further crop improvement.

Water, heat fluxes and water use efficiency measurement and modeling above a farmland in the North China Plain

Net radiation ( R n ), water vapor flux ( LE ), sensible heat flux ( H s ) and soil heat flux (G) were measured above a summer maize field with the eddy-covariance technique, simulation and analysis of water, heat fluxes and crop water use efficiency were made with the RZ-SHAW model at the same time in this study. The results revealed significant diurnal and seasonal variability of water vapor flux for summer maize. Most part of R n was consumed by the evapotranspiration of the summer maize. The proportion of water vapor flux to net radiation (( LE/R n ) increased with the crop development and peaked around milk-filling stage with a value of 60%, a slightly lower than that obtained by the RZ-SHAW model. Daily evapotranspiration estimated by the model agreed with the results measured with the eddy-covariance technique, indices of agreement (IA) for hourly water vapor fluxes simulated and measured were above 0.75, root mean square errors (RMSE) were no more than 1.0. Diurnal patterns of H s showed the shape of inverted ”U” shifted to the forenoon with a maximum value around 11:30 (Beijing time), while LE exhibited an inverted “V” with a maximum value at around 13:00, about an hour later than H s . Diurnal change of CO 2 showed an asymmetrical “V” curve and its maximal rates occurred at about 11:30. Variations of water use efficiency during the phonological stages of the summer maize showed a rapid increase with the photosynthetic photon flux density (PPFD) after sunrise, a state of equilibrium around 10:00 followed a decrease. Maximum values of water use efficiency were 24.3, and its average value ranged from 7.6 to 10.3 g kg -1 .

The Effect of Irrigation and Genotype on Carbon and Nitrogen Isotope Composition in Peanut (Arachis hypogaea L.) Leaf Tissue

Water scarcity is a significant problem faced by producers worldwide and is becoming an increasing problem to growers in the U.S. peanut (Arachis hypogaea L.) producing areas due to years of drought and increasing urban demands on water resources. Because of this, high wateruse efficiency (WUE) has now become a priority in many peanut breeding programs. To support this effort, the variation in WUE, as measured by carbon isotope composition (d 13 C), of three commonly grown peanut cultivars was evaluated under differing irrigation environments during 2001 and 2002 at a research farm in Shellman, GA, U.S.A. The specific experimental objectives were: 1) to determine if genetic variability existed in d 13 C, d 15 N SLA, and SPAD among three commonly grown U.S. peanut genotypes; 2) to determine if differing irrigation levels affected the pattern of variability; and 3) to quantify the relationship between d 13 C and the measured leaf phenotypic characteristics. During both 2001 and 2002, cv. Georgia Green had significantly higher yields and lower d 13 C and SPAD chlorophyll content than the other two genotypes, and lower %N than the cultivar C99R. Specific leaf area and %C for Georgia Green were significantly greater than for the cultivar AT201. In addition, irrigation treatment significantly affected yield such that the NI (nonirrigated) treatment yields were significantly lower than any of the irrigated treatments (33%, 66%, or 100%). However, the irrigation effects on leaf phenotypic characteristics were less apparent with differences existing only for d 15 N, SPAD chlorophyll, and SLA. The correlation of d 13 C and yield was significant for C99R in 2001 and AT201 in 2002, while correlations with d 13 C and the other leaf phenotypic characteristics were scarce. This makes the utility of the traits as easily measured surrogates for WUE very limited.

Drought resistance, water-use efficiency, and yield potential—are they compatible, dissonant, or mutually exclusive?

This presentation is a concept review paper dealing with a central dilemma in understanding, designing, and acting upon crop plant improvement programs for drought conditions. The association among yield potential (YP), drought resistance (DR), and water-use efficiency (WUE) is often misunderstood, which in turn can lead to conceptual oversight and wrong decisions in implementing breeding programs for drought-prone environments. Although high YP is the target of most crop breeding programs, it might not be compatible with superior DR. On the other hand, high YP can contribute to yield in moderate stress environments. Plant production in water-limited environments is very often affected by constitutive plant traits that allow maintenance of a high plant water status (dehydration avoidance). Osmotic adjustment (OA) is a major cellular stress adaptive response in certain crop plants that enhances dehydration avoidance and supports yield under stress. Despite past voiced speculations, there is no proof that OA entails a cost in terms of reduced YP. WUE for yield is often equated in a simplistic manner with DR. The large accumulation of knowledge on crop WUE as derived from research on carbon isotope discrimination allows some conclusions on the relations between WUE on the one hand, and DR and YP on the other, to be made. Briefly, apparent genotypic variations in WUE are normally expressed mainly due to variations in water use (WU; the denominator). Reduced WU, which is reflected in higher WUE, is generally achieved by plant traits and environmental responses that reduce YP. Improved WUE on the basis of reduced WU is expressed in improved yield under water-limited conditions only when there is need to balance crop water use against a limited and known soil moisture reserve. However, under most dryland situations where crops depend on unpredictable seasonal rainfall, the maximisation of soil moisture use is a crucial component of drought resistance (avoidance), which is generally expressed in lower WUE. It is concluded that the effect of a single ‘drought adaptive’ gene on crop performance in water-limited environments can be assessed only when the whole system is considered in terms of YP, DR, and WUE.

Water-use efficiency, leaf area and leaf gas exchange of cowpeas under mid-season drought

Ten cowpea ( Vigna unguiculata L. “Walp.”) genotypes were grown in a growth chamber under well-watered conditions up to early flowering and were then either subjected to water deficit or were continually well-watered. Water deficit was induced by withholding irrigation until the soil water potential was −75 kPa, which was then maintained for 10 days. Variation in water use efficiency (WUE), leaf area, specific leaf area (SLA), leaf area ratio (LAR) and leaf gas exchange (i.e. assimilation, transpiration, stomata conductance and internal CO 2 concentration) in response to water deficit were investigated. Water deficit treatment reduced mean water use by 21%. This caused between 11 and more than 40% reduction of biomass across the genotypes. Reductions in biomass were due to decline in leaf gas exchange and leaf area during water deficit. Water deficit improved the WUE of two genotypes (IFH 27-8 and Lobia) by approximately 20%, but caused moderate to huge reductions in most genotypes. High relative water content (RWC) of leaves was maintained in some of the genotypes by stomata closure and a reduction of leaf area. Drought avoidance by maintaining high leaf water content was negatively associated with leaf area as well as SLA. High assimilation rate under water deficit was associated with high RWC. Decline in assimilation rate were due mainly to stomata closure, however, some evidence of non-stomatal regulation were also found. WUE and instantaneous water use efficiency (IWUE, a molar ratio of assimilation to transpiration) were not directly associated, but IWUE and leaf internal CO 2 concentration (c i) were negatively related while c i was also moderately related with SLA. Overall, significant genotypic variations in leaf gas exchange parameters were found, which can give some indications of superiority when comparing cowpea genotypes for agronomic fitness under drought. The lack of relationship between WUE and IWUE was due to the instantaneous measurement of leaf gas exchange, which can be corrected by calculations to reflect the entire season.

Productivity and water use of rainfed pearlmillet (Pennisetum glaucum) as influenced by summer ploughing and in situ moisture conservation practices under semi arid conditions of north west India .

A 2 year field experiment was conducted during the rainy season of 1997 and 1998 to study the effect of 3 summer ploughing (no summer ploughing, summer ploughing and summer ploughing FYM insecticide herbicide) and 6 moisture conservation practices (uniform row sowing, paired row planting, ridge furrow, straw mulch, kaolin and straw mulch kaolin) on the productivity and water use efficiency of rainfed pearlmillet [Pennisetum glaucum (L.) R. Br. .emend. Stuntz]. Summer ploughing FYM insecticide herbicide gave highest grain yield (22.70 and 25.09 qlha in 1997 and 1998 respectively), which was at par with summer ploughing alone but significantly superior to no summer ploughing. Marked variation in water use efficiency, consumptive use and moisture extraction pattern by pearlmillet was observed due to summer ploughing during both the years. Straw mulch kaolin recorded highest grain yield (21.78 and 26.97 qlha), consumptive use (266.33 and 298.72 mm) and water use efficiency (9.46 and 9.03 kg grainlhalmm) in 1997 and 1998, respectively, and the lowest being by uniform row sowing. Straw mulch kaolin and straw mulch alone were at par with each other but were superior to all other treatments in terms of yield, consumptive use and water use efficiency. Key words : Summer ploughing, Moisture conservation practices, Water use efficiency, Moisture extraction pattern, Consumptive use

Effects of Water and Nitrogen Supply on Water Use Efficiency and Carbon Isotope Discrimination in Edible Canna (Canna edulis Ker‐Gawler)

Abstract: Transpirational water use efficiency (WUE) is affected by nutrient and water supply, but relatively little is known about the response of Andean root and tuber crops, such as edible canna (Canna edulis Ker‐Gawler). Two Canna edulis genotypes were studied in semi‐controlled greenhouse experiments at the International Potato Centre (CIP) near Quito, Ecuador. Under conditions of low water supply, shoot dry matter (DM), leaf area and specific leaf nitrogen (SLN) all decreased, while WUE was higher and carbon isotope discrimination (Δ) decreased. The large water‐storing hypodermal cells characteristic of this plant's leaf morphology shrank, concertina‐like, the leaves became thinner, and the specific leaf area increased significantly (from 26 m‐2 kg‐1 to 43 m‐2 kg‐1). In a second experiment, plants were grown with three different levels of nitrogen supply: from low to high N supply, shoot DM increased significantly (from 16 to 37 g), along with leaf area and SLN. At the lowest level of nitrogen supply (N0), WUE was significantly lower and Δ increased. As expected, the linear correlation between Δ and WUE was negative, but variation in Δ could only explain 49 % of the variation in WUE. If, in addition to Δ, measured root fraction (RF) and estimated values of leaf‐to‐air vapour pressure deficit (Δe) and respiratory and unproductive carbon loss were used to calculate WUE, the correlation between measured and calculated WUE was substantially improved, except for the water stress treatment. It is considered that, for root and tuber crops, RF and Δe are the major variables when Δ of single leaves is to be used for up‐scaling to plant WUE.

Population differences in stable carbon isotope ratio of Pinus contorta Dougl. ex Loud.: relationship to environment, climate of origin, and growth potential

Stable carbon isotope composition (δ13C) was used to examine genetic and environmental variation in water-use efficiency of 11 Pinus contorta Dougl. ex Loud. populations from western Canada. Sapwood cores from 20-year-old saplings established at three sites in British Columbia, and shoots of greenhouse-grown, first-year seedlings, were analyzed. δ13C values of whole sapwood and isolated cellulose were correlated at r = 0.990 (p << 0.0001, n = 10). There were genetic differences in δ13C among populations. A population from the wet Pacific coast (P. contorta var. contorta) stood out from the others, which were all from the drier continental interior (P. contorta var. latifolia Engelm). This population had the highest indicated water-use efficiency. δ13C values of most populations increased from the wettest to the driest site with no significant change in ranking. Mean yield (stem volume) of the P. contorta var. latifolia populations was positively correlated with δ13C. Population differences at the seedling stage were not as pronounced, but δ13C values of seedling shoots and sapling wood cores were correlated. Among the 10 P. contorta var. latifolia sapling populations, δ13C decreased with an index of summer dryness and, less so, with increased elevation. It appears, therefore, that the most water-use efficient and most productive populations originate from relatively moderate, low-elevation sites with reduced likelihood of water stress.Key words: water-use efficiency, isotope discrimination, lodgepole pine, phenotypic plasticity, provenance trials.

Water‐Use Efficiency and Carbon Isotopic Composition in Reduced Tillage Systems

AbstractCropping systems that influence soil water availability are expected to influence the C isotopic composition (δ13C) of crop residues and consequently, the δ13C of soil organic matter. We hypothesized that the δ13C of plant residues and soil organic matter in three tillage systems (zero [ZT], minimum [MT], and conventional tillage [CT]) and three, 4‐yr crop rotations would vary and would relate to water‐use efficiency (WUE). The study was conducted on an Indian Head heavy clay (Udic Haploboroll) in Saskatchewan, Canada. The three crop rotations were fallow‐spring wheat (Triticum aestivum L.)‐spring wheat‐winter wheat, spring wheat‐spring wheat‐flax (Linum usitatissimum L.)‐winter wheat, and spring wheat‐flax‐winter wheat‐field pea (Pisum sativum L.). Water use was estimated yearly using mass balance. The δ13C of the standing crop residue, roots, and soil organic matter was determined 9 yr after the study was initiated. Although crop water use was higher in ZT (31.2 cm yr‐1) and MT (30.9 cm yr‐1) than in CT (28.8 cm yr‐1), corresponding differences in WUE and δ13C of plant tissue and soil organic matter were not detected. In one instance, within the most diversified rotation, WUE was reduced in ZT compared with CT; however, observed variations in WUE did not conform to theoretical expectations of the δ13C of plant residues and soil organic matter. Factors other than WUE, including soil fertility and timing of moisture deficits, may have influenced the degree to which C isotope discrimination was expressed in the plant residues and soil organic matter.

An Additional QTL for Water Use Efficiency in Soybean

Water use efficiency (WUE) is one of the physiological traits that has been associated with drought tolerance in soybean [Glycine max (L.) Merr]. In a previous publication, we reported the identification of four quantitative trait loci (QTL) that conditioned WUE in a F4-derived soybean population obtained from a cross of ‘Young’ × PI416937. The purposes of this study were to identify WUE QTL in a F2-derived soybean population from a cross of ‘S100’ × ‘Tokyo’ and to determine the consistency of WUE QTL across the two soybean populations. A restriction fragment length polymorphism (RFLP) linkage map was created from 142 markers and 116 F2-derived lines of the population. The map covered about 1100 cM of the soybean genome with 120 markers linked on 25 linkage groups. Twenty-two of the markers remained genetically unlinked. The WUE of the parents and the F2-derived lines was evaluated in a greenhouse at Athens, GA, in 1996. S100 produced 9% more dry matter per unit of water used than did Tokyo. Progeny lines with greater WUE than S100 were identified in this population. Two independent markers were associated with WUE in this soybean population. There was no interaction between the two markers. One (A063E) of the two independent markers was also associated with WUE in the Young × P1416937 population. Marker A489H, on Linkage Group (LG) L, was unique to the S100 × Tokyo population and explained 14% of the variation in WUE in this soybean population. Interval mapping with MAPMAKER/ QTL indicated that the most probable location of the QTL on LG L was at marker locus A489H. The likelihood of odds (LOD) score for the presence of a QTL at this location was 3.4. Thus, we have identified a previously unreported QTL for WUE on LG L.

Molecular Markers Associated with Water Use Efficiency and Leaf Ash in Soybean

Water use efficiency (WUE) is an important trait that has been associated with drought tolerance of crop plants, and leaf ash (LASH) generally related to WUE. A restriction fragment length polymorphism (RFLP) map was constructed from a soybean [Glycine max (L.) Merr.] population of 120 F4‐derived lines from a cross of ‘Youngș × PI416937. The purpose of this research was to identify quantitative trait loci (QTL) associated with WUE and LASH in 36‐d‐old, greenhouse‐grown plants. The experimental design was a randomized complete block with six replications. Significant (P < 0.01) phenotypic differences were detected among the lines for both traits. A total of four and six independent RFLP markers were associated with WUE and LASH and if combined each group of markers would explain 38 and 53% of the variability in the respective traits. One marker locus (cr497‐1), on USDA Linkage Group J, explained 13.2% of the variation in WUE indicating the presence of a major QTL. The LASH was negatively correlated with WUE (r = −0.40**), and two QTL were associated with both WUE and LASH. For each of these QTL, the allele for increased WUE was associated with reduced LASH.