Measure Of Water-use Efficiency Research Articles

The slope of assimilation rate against stomatal conductance should not be used as a measure of water use efficiency or stomatal control over assimilation

Quantifying water use efficiency, and the impact of stomata on CO2 uptake are pivotal in physiology and efforts to improve crop yields. Although tempting, relying on regression slopes from assimilation-stomatal conductance plots to estimate water use efficiency or stomatal control over assimilation is erroneous. Through numerical simulations, I substantiate this assertion. I propose the term ‘instantaneous transpiration efficiency’ for the assimilation-to-transpiration ratio to avoid confusion with ‘intrinsic water use efficiency’ which refers to the assimilation-to-stomatal conductance ratio, and recommend to compute both metrics for each gas exchange data point.

Shared responsibility for global water stress from agri-food production and consumption and opportunities for mitigation

Understanding how agri-food consumption by individual countries contributes to their own water stress and that of other agri-food producer countries, and identifying opportunities to help reduce global water stress is important for ensuring responsible agri-food consumption and production. Assessing 26 agri-food commodities traded between 189 countries using a multi-regional input-output model (MRIO), the contribution of agri-food production to the water stress of producer countries was quantified. Then, the responsibility of consumer countries in aggravating the water stress of producer countries was assessed. Finally, an indicator of opportunity for consumer countries to target co-investment in improving the water-use efficiency of producer countries where they can achieve the highest marginal reduction in water stress was developed. The USA was the most significant contributor to global water stress, adding 42 percentage points to its own domestic water stress and 179 percentage points to the water stress of other producer countries via trade. The opportunity for reducing water stress was highest for the Dominican Republic, Qatar, and Ukraine. Production of irrigated Vegetables, fruits & nuts for export was the major contributor to the national water stress of these countries. This work can help nations reduce their contribution to global water stress through targeted investments in water-use efficiency measures across their global supply chain and help to progress towards Sustainable Development Goals 6 (Clean Water), 12 (Responsible Consumption and Production), and 17 (Global Partnerships for Sustainable Development).

The effects of an experimental drought on the ecophysiology and fruiting phenology of a tropical rainforest palm

AbstractAimsAnthropogenic climate change is predicted to increase mean temperatures and rainfall seasonality. How tropical rainforest species will respond to this climate change remains uncertain. Here, we analysed the effects of a 4-year experimental throughfall exclusion (TFE) on an Australian endemic palm (Normambya normanbyi) in the Daintree rainforest of North Queensland, Australia. We aimed to understand the impact of a simulated reduction in rainfall on the species’ physiological processes and fruiting phenology.MethodsWe examined the fruiting phenology and ecophysiology of this locally abundant palm to determine the ecological responses of the species to drought. Soil water availability was reduced overall by ~30% under a TFE experiment, established in May 2015. We monitored monthly fruiting activity for 8 years in total (2009–2018), including 4 years prior to the onset of the TFE. In the most recent year of the study, we measured physiological parameters including photosynthetic rate, stomatal conductance and carbon stable isotopes (δ 13C, an integrated measure of water use efficiency) from young and mature leaves in both the dry and wet seasons.Important FindingsWe determined that the monthly fruiting activity of all palms was primarily driven by photoperiod, mean solar radiation and mean temperature. However, individuals exposed to lower soil moisture in the TFE decreased significantly in fruiting activity, photosynthetic rate and stomatal conductance. We found that these measures of physiological performance were affected by the TFE, season and the interaction of the two. Recovery of fruiting activity in the TFE palms was observed in 2018, when there was an increase in shallow soil moisture compared with previous years in the treatment. Our findings suggest that palms, such as the N. normanbyi, will be sensitive to future climate change with long-term monitoring recommended to determine population-scale impacts.

Mapping and confirmation of quantitative trait loci (QTLs) associated with carbon isotope ratio (δ13C) in soybean

AbstractInsufficient moisture availability often limits soybean [Glycine max (L.) Merr.] yield. Carbon isotope ratio (δ13C) provides an integrated measure of water use efficiency in C3 plants due to its substantial genetic variance, high heritability, and small genotype × environment interaction (G × E). The objective of this study was to identify quantitative trait loci (QTLs) associated with δ13C using a recombinant inbred line population derived from a cross between ‘KS4895’ and ‘Jackson’. The field experiment was conducted in five environments to evaluate δ13C under rainfed and irrigated conditions. Analysis of variance of δ13C averaged over environment and irrigation treatment showed significant effects of genotype (G), environment (E), and G × E interactions. Heritability of δ13C in different environments and irrigation treatments ranged from 66 to 79%. Averaged over environments and irrigation treatments, heritability was 83%. A total of 24 QTLs associated with δ13C were identified and clustered in nine genomic regions on seven chromosomes. The QTL clusters on Gm05 (1), Gm06 (2) and Gm20 (1) were detected across different environments and irrigation regimes. Collectively, these four QTL clusters accounted for 55% of the phenotypic variation in δ13C. The QTLs on Gm06 and Gm20 also showed additive × additive epistasis that contributed approximately 4.2% to the total phenotypic variation. Several identified δ13C QTLs overlapped with QTLs associated with other physiological traits related to plant water status, biological nitrogen fixation, and plant morphology. The identified genomic regions may be an important resource in genomic selection studies to improve drought tolerance in soybean.

Fog Water Is Important in Maintaining the Water Budgets of Vascular Epiphytes in an Asian Tropical Karst Forests during the Dry Season

Fog may be an important source of water for forest vascular epiphytes on trees, because they lack direct access to sources of soil water, but little is known about the water use proportions from various sources and potential water uptake pathways in epiphytes. Here, we analyzed leaf carbon isotope ratios as a measure of water use efficiency (WUE), proportions of fog, rain, and soil water use, and foliar water uptake (FWU) in species of epiphyte and their host trees in a tropical karst dwarf forest in China during the dry season. We found that the WUE, as represented by leaf δ13C, was generally enriched in the epiphyte species compared to their host trees. Epiphytes used substantial proportions of fog water, whereas water use in the host trees was dominated by soil water. The leaves of epiphytes and host trees absorbed water following immersion in water for 3 h and FWU possibly related to foliar epicuticular structures, such as fungal endophytes. Our results show a divergence of water use strategies between epiphytes and their hosts and highlight the importance of fog water for epiphytes during the dry season and under a climate change scenario with a reduced occurrence of fog events.

Component crop physiology and water use efficiency in response to intercropping

Abstract Interspecies specific interactions are generally regarded as drivers of plant productivity in multispecies agroecosystems. Complementary use of resource in diverse communities can enhance community productivity through optimal use of plant-available resources and positive interactions such as facilitation can ameliorate high abiotic stress conditions. We studied the effects on physiological response, leaf traits and water use efficiency of a multifunctional species intercropping system consisting of peanut (Arachis hypogaea L.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], okra [Abelmoschus esculentus (L.) Moench], cowpea [Vigna unguiculata (L.) Walp.], and pepper (Capsicum annuum L.) planted alone or in various intercropping combinations in a low fertilizer input system in the peak of summer heat in Texas. Differences in gas exchange measurements were detected only in watermelon in year 2 of the study when okra was the dominant crop. This same year watermelon specific leaf area (SLA) was significantly higher when okra was present in a treatment and particularly in the three and four species combinations, Wpwo and Wpwoc, 27.5 and 31.0 m2 kg−1, respectively, as compared to watermelon grown in monoculture, strip intercropped with peanut (Spw) and within row intercropped with peanut (Wpw), 20.4, 20.1, and 19.8 m2 kg−1, respectively. This corresponds with an increase watermelon leaf N concentration and a decrease in leaf C:N ratio in Wpwo and Wpwoc treatments. No differences in d13C composition, a measure of water use efficiency over the leaf lifespan, were detected across cropping system for each species. Water use efficiency based on per plant production (WUEyield) indicated an increase in water use efficiency in dominant crops such as watermelon in 2011 and okra in 2012, but a reduction in WUEyield subordinate crops such as cowpea and pepper both years of the study. Peanut grown in monoculture and strip intercropped with watermelon had significantly lower leaf water potential values in 2012, −2.2 and −2.1 MPa, respectively, as compared to intercropping systems increasing in level of integration (Wpw = − 1.1, Wpwo = − 0.6, Wpwoc = − 1.3, Wall = −1.1 MPa), indicating peanut benefited from alterations to microclimate and facilitative interactions with companion crops in some intercropping systems through a reduction in plant water stress. The results from this study suggest there may be a benefit to a multifunctional intercropping system in the form of increased food production per unit of water input in dominant crops and reduced water stress for some component species. This is important to producers; showing a method to increase overall crop production without increasing water inputs.

A new indicator of ecosystem water use efficiency based on surface soil moisture retrieved from remote sensing

Ecosystem water use efficiency is an important indicator of carbon and water cycle coupling. This study presents a new measure of water use efficiency, soil water use efficiency (SWUE), based on gross primary production and surface soil moisture derived from remote sensing products (ECV-SM). Variation in SWUE among biomes, climate conditions, and latitudes from 2000 to 2014 was comprehensively assessed. Average global SWUE over this 15-year period was approximately 3.47 gC/kgH2O. SWUE was relatively high for ecosystems near the equator and decreased gradually with increasing latitude. At the biome level, high SWUE was measured in evergreen broadleaf forests, and lower values were found in shrublands. Compared with two other commonly used indicators of water use efficiency, EWUE (ratio of gross primary production to evapotranspiration) and RUE (ratio of gross primary production to precipitation), average SWUE from 2000 to 2014 was significantly higher and had the largest range of values. In addition, spatial distributions of these three indicators varied greatly. The new indicator SWUE will help promote understanding of soil water use in various ecosystems.

Xylem traits and water-use efficiency of woody species co-occurring in the Ti Tree Basin arid zone

Species with low density of intact branches are likely to have higher growth rates than species with high density of intact branches, but at the cost of a lower water-use efficiency and larger sensitivity to xylem embolism. The hydraulic niche separation theory proposes that species co-exist by having a range of traits to allow differential access to resources within heterogeneous environments. Here, we examined variation in branch xylem anatomy and foliar carbon stable isotopes (δ13C) as a measure of water-use efficiency (WUE) in seven co-occurring species, Acacia aneura, Acacia bivenosa, Corymbia opaca, Eucalyptus camaldulensis, Erythrina vespertilio, Hakea sp., and Psydrax latifolia, in an arid zone open Corymbia savanna on the Ti Tree Basin, Northern Territory, Australia. We test the following hypotheses: (1) Species with large conductive areas exhibit a low density of intact branches, while species with small conductive areas have a significantly higher density of intact branches. (2) Species with smaller conductive areas exhibit more enriched values of δ13C and therefore have larger WUE than those with larger conductive areas and (3) there is an inverse correlation between theoretical sapwood hydraulic conductivity and vessel implosion resistance. The results of this study demonstrated significant variation in density of intact branches, ranging from 0.38 to 0.80 g cm−3 and this variation was largely explained by variation in sapwood conductive area. Species with low conductive areas (P. latifolia, Hakea sp. and Acacia species) exhibited large values of WUE (r 2 = 0.62, p < 0.05). These species are likely to be less vulnerable to cavitation by having small conductive areas and thicker fibre walls. We demonstrated a significant (r 2 = 0.83, p = 0.004) negative correlation between theoretical sapwood hydraulic conductivity and vessel implosion resistance. These results are discussed in relation to hydraulic niche separation.

Influence of within year treatments and between year environmental differences on peach leaf ash and carbon isotopic discrimination responses

Plant ash content is related to water use efficiency (WUE) (dry matter accumulation per unit of transpiration) and Δ13C in a range of C3 species. In breeding programs, the genetic×environment interaction is measured with multiple locations and multiple years of evaluation but the genetic×environment interaction of ash or Δ13C has not been studied. The purpose of this study was to assess how within year cultural variation and yearly environmental variation affect peach leaf ash and carbon discrimination as measures of water use efficiency using cultural practices evaluated over a 13 year period of time. ‘Loring’ peach (Prunus persica) on seedling ‘Halford’ rootstock were planted in 1983 in a 4.5m×6m spacing in square vegetation-free-areas (VFA) around each tree (0.36 and 13.0m2). Three irrigation treatments (none, drip irrigated during stage 3 of fruit development, and fertigation during stage 3 of fruit development) were imposed on each VFA. Leaf ash and carbon isotope discrimination (Δ) were measured in leaf tissue collected prior to harvest. Analysis of variance indicated that there were no differences in VFA or irrigation for either ash or Δ13C, however there were yearly interactions. Δ13C was significantly related to the growing season pan evaporation but the slope was low indicating a minimal response while leaf ash was not significantly related. In contrast, leaf ash was significantly related to the growing season rainfall amount while Δ13C was unrelated. Both leaf ash and Δ13C were unrelated to the growing season water deficit. The coefficient of variation for leaf ash and Δ13C are 0.16 and 0.04, respectively, over a 13 year sampling period. These data suggest that breeding programs screening for increased water use efficiency using leaf ash or Δ13C can anticipate a minimal amount of within location variation and thus place more emphasis on environmental or between-location variation.

Efficient Water Use for Sustainable Irrigation Industry

The aim of this paper is to evaluate the economic efficiency of irrigated agricultural enterprises using a non-radial data envelopment analysis approach. While a number of studies have used radial measures based on data envelopment analysis to obtain efficiency scores for a given production technology, we calculate non-radial measures to understand the efficiency of using individual inputs employed in the production process. In particular, the measures of economic efficiency are decomposed into water use efficiency and managerial efficiency. This decomposition enables us to obtain an efficiency score for the use of water as an environmentally sensitive input in irrigated crop production systems. Treating water input in this way goes beyond traditional measurement of water use efficiency, as the calculated efficiency scores can be used as indicators of sustainability in terms of water withdrawals for irrigation purposes. The results show that the overall efficiency for the considered irrigated enterprises is quite high. This is in contrast to the findings on the water use efficiency scores, which are fairly low. This indicates that while Australian irrigated farms are comparatively more efficient in overall farm activity management, they are not very efficient in managing water resources. In turn, this threatens the sustainability of this industry. There is a substantial variation of water use efficiency scores across irrigated enterprises and across regions. Analysing these variations can provide important insights for current policy and for future efforts to improve water use efficiency that will lead towards more sustainable irrigation industry.

Productivity, evapotranspiration, and water use efficiency of corn and tomato crops simulated by AquaCrop under contrasting water stress conditions in the Mediterranean region

The FAO AquaCrop model has been conceived as a tool for simulating, on a daily scale, the canopy cover (CC), biomass and the actual evapotranspiration and for simulating, on a seasonal scale, the final biomass, the harvested yield, the cumulate actual evapotranspiration, and the crop water use efficiency. This performance was analysed after a series of tests carried on 12 crop cycles, concerning corn and tomato grown in the Mediterranean region, and having three levels of plant water stress: absence of plant water stress (control), moderately stressed and severely stressed. The results highlight the effects of three factors affecting the AquaCrop performances: the species, the level of plant water stress during the crop cycle and the output variable to simulate.The AquaCrop adequately simulates the daily canopy cover (CC) in control treatments of tomato and corn, and in moderate stress treatment of corn. In the severe stressed treatment of corn, the simulated values of CC were close to the measured values only from sowing to 60 days after sowing, after that the simulated values do not fit the measurements.The AquaCrop model adequately simulates the daily biomass accumulation under all treatments in tomato and under non-stressed and moderate stressed treatments in corn. However, the simulated biomass outputs were generally overestimated during the late stages of the crop cycles and, consequently, the yield also exhibited a tendency to be overestimated. Nevertheless, the yield overestimation can be retained as acceptable because the normalised differences (D) between the simulations and measured values were less than 15% on average. An exception was the tomato yield simulated in the severely stressed treatment, for which D was greater than 30%. In contrast, in the case of the severely stressed treatment in corn, AquaCrop did not exhibit any aptitude for simulating the biomass or the grain yield. In fact, the model predicts the absence of any yield production, while 5 t ha−1 of grain were actually measured in the severely stressed treatment.The daily actual evapotranspiration simulated by AquaCrop was consistent with the observations only in the case of the control treatments of tomato, in all the three seasons. In contrast, for the other treatments (all treatments in corn and all stressed treatments in tomato), the quality of the evapotranspiration simulation was poor.In general, AquaCrop underestimated the seasonal values of evapotranspiration. The normalised differences between the seasonal values of the observations and simulations are acceptable in the case of the tomato evapotranspiration (D=−7%). However, in the case of corn, the differences are related to the level of plant water stress, and they become unacceptable (D=−36%) in the severely stressed treatments.The overestimation of the yield and the underestimation of the seasonal evapotranspiration cause the simulations of the water use efficiency to be overestimated. In the specific case of corn, due to the unacceptable performance of the model under severely stressed treatments, the linear regression between the observations and measurements of water use efficiency is unsatisfactory.The potential uses of the AquaCrop model as a tool for research purposes aimed to enhance the water efficiency and as a tool for managing irrigation have been deeply discussed. The paths that should be followed in the future to improve the model simulations have also been suggested

Influence of Substrate Water Content and Daily Light Integral on Photosynthesis, Water Use Efficiency, and Morphology of Heuchera americana

Two investigations were conducted to determine the morphological and physiological impacts of varying light and substrate water levels on Heuchera americana ‘Dale's Strain’ (american alumroot). Both investigations used a capacitance sensor automated irrigation system to maintain constant substrate volumetric water contents (θ = volume of water/volume of substrate). In the first study, the substrate was maintained at one of eight θ ranging from 0.15 to 0.50 L·L−1. Leaf area of plants grown at the highest θ was more than twice that of plants grown at the lowest θ. Shoot dry weight also responded positively to θ increasing from 0.15 to 0.35 L·L−1, but plants did not have greater dry weights when maintained at θ higher than 0.35 L·L−1. The second experiment assessed american alumroot's performance under four daily light integrals (DLIs) (7.5, 10.8, 14.9, and 21.8 mol·m−2·d−1) with θ maintained at 0.35 L·L−1. Increasing DLI from 7.5 to 21.8 mol·m−2·d−1 caused shoot dry weight, leaf area, maximum width, and leaf count to change quadratically. Dry weight and leaf area reached their maximum at 10.8 mol·m−2·d−1, whereas leaf count was greatest at 14.9 mol·m−2·d−1. Increasing DLI to 21.8 mol·m−2·d−1 negatively impacted leaf area and leaf count but did not lower shoot dry weight. Leaf area ratio and petiole length of the uppermost fully expanded leaf decreased with increasing DLI. Measures of leaf-level net photosynthesis, light response curves, and CO2 response curves indicated no physiological differences among plants grown under different water or light levels. In both studies, long-term, whole crop measures of water use efficiency based on shoot dry weight and water applied (WUEc) did not reflect the same water use trends as instantaneous, leaf-level measures of WUE based on leaf gas exchange (WUEl). WUEc decreased with increasing θ and DLI, whereas WUEl was not influenced by θ and increased with increasing DLI. WUEl is often used to provide insight as to how various abiotic and biotic factors influence how efficiently water is used to produce biomass. However, these findings demonstrate that there are limitations associated with making such extrapolations.

Genotypic variation in photosynthesis in cacao is correlated with stomatal conductance and leaf nitrogen

Variation in photosynthetic parameters was observed between eight contrasting cacao (Theobroma cacao) genotypes. Net photosynthetic rate (PN) ranged from 3.4 to 5.7 μmol(CO2) m−2 s−1 for the genotypes IMC 47 and SCA 6, respectively. Furthermore, genotypic differences were detected in quantum efficiency ranging from 0.020 to 0.043 μmol(CO2) μmol−1(photon) for UF 676 and AMAZ 15/15, respectively. Differences in PN were correlated with both stomatal conductance (gs) and leaf nitrogen per unit area. Some variation in water use efficiency was observed between genotypes, both intrinsic (PN/gs) and instantaneous (PN/transpiration rate). Both measures of water use efficiency were a negative function of specific leaf area. Evidence was found for a trade-off mechanism between cacao genotypes in photosynthesis and leaf structure. High photosynthetic rate, expressed on a mass basis was associated with smaller leaves. Furthermore, thinner leaves were compensated for by a higher nitrogen content per unit mass.

The Physiology and Stability of Leaf Carbon Isotope Discrimination as a Measure of Water-Use Efficiency in Barley on the Canadian Prairies

Temporal and seasonal water deficit is one of the major factors limiting crop yield on the Canadian prairie. Selection for low carbon isotope discrimination (Δ 13 C) or high water-use efficiency (WUE) can lead to improved yield in some environments. To understand better the physiology and WUE of barley under drought conditions on the Canadian prairie, 12 barley (Hordeum vulgare L.) genotypes with contrasting levels of leaf Δ 13 C were investigated for performance stability across locations and years in Alberta, Canada. Four of those genotypes ('CDC Cowboy', 'Niobe', '170011' and 'Kasota') were also grown in the greenhouse under well-watered and water-deficit conditions to examine genotypic variations in leaf Δ 13 C, WUE, gas exchange parameters and specific leaf area (SLA). The water-deficit treatment was imposed at the jointing stage for 10 days followed by re-watering to pre-deficit level. Genotypic ranking in leaf Δ 13 C was highly consistent, with '170011', 'CDC Cowboy' and 'W89001002003' being the lowest and 'Kasota' '160049' and 'H93174006' being the highest leaf Δ 13 C. Under field and greenhouse (well-watered) conditions, leaf Δ 13 C was significantly correlated with stomatal conductance (g s ). Water deficit significantly increased WUE, with 'CDC Cowboy' - a low leaf Δ 13 C genotype with significantly higher WUE and lower percentage decline in assimilation rate (A) and g s than the other three genotypes ('Niobe', '170011' and 'Kasota'). We conclude that leaf Δ 13 C is a stable trait in the genotypes evaluated. Low leaf Δ 13 C of 'CDC Cowboy' was achieved by maintaining a high A and a low g s , with comparable biomass and grain yield to genotypes showing a high g s under field conditions; hence, selection for a low leaf Δ 13 C genotype such as 'CDC Cowboy' maybe important for maintaining productivity and yield stability under water-limited conditions on the Canadian prairie.

The effects of water, nutrient availability and their interaction on the growth, morphology and physiology of two poplar species

We exposed cuttings of two poplar species, Populus cathayana Rehder and Populus przewalskii Maximowicz, from Sect. Tacamahaca Spach to two watering regimes (well-watered and water-stressed conditions) and to two nutrient regimes (with or without fertilization) in a greenhouse to determine how fertilization affects the growth, morphology and physiology of poplars under different water conditions. Under stress conditions, changes in early growth and dry matter allocation, and decrease in gas exchange and the related functions are usually observed. Moreover, the measurement of carbon isotope composition (δ 13C) provides an integrated measurement of water use efficiency. And abscisic acid (ABA) is a phytohormone which plays a prominent role in various physiological and biochemical processes related to environmental stresses. So we determine these characteristics and related parameters, and our results showed the following: (1) Fertilization promoted the growth of poplars under well-watered conditions, while under water-stressed conditions its effect on growth was negative. (2) Fertilization increased δ 13C, total N concentration, chlorophyll a/b and intrinsic efficiency of photosystem II (Fv/Fm) but decreased relative water content of leaves, stomatal conductance, transpiration rate and C/N ratio under both well-watered and water-stressed conditions. (3) Fertilization appeared to increase net photosynthesis rate and decrease ABA content under well-watered conditions, while it decreased net photosynthesis rate and increased ABA content under water-stressed conditions. Moreover, compared to P. cathayana, collected from a lower altitude region, P. przewalskii, collected from a high-altitude region, has a slower growth rate and stronger adaptability to drought stress, which perhaps resulted from its chronic adaptability to the low water availability of high-altitude region; but to the nutrient stress, there was no difference between the two species.

Morphological and physiological variation in western redcedar (Thuja plicata) populations under contrasting soil water conditions

Adaptation to precipitation conditions may induce genetic diversity that changes morphological and physiological traits. This hypothesis was investigated in the seedlings of seven western redcedar (Thuja plicata Donn ex D. Don) populations, which were collected along a precipitation transect from the Pacific coast to the southern interior of British Columbia, Canada. The experimental seedlings were either well-watered or soil-droughted and measured for growth, gas exchange rates, transpiration efficiency, and carbon isotope discrimination during or at the end of the third growing season. Significant variation was found in most of these morphological and physiological traits among the populations. Much of this variation occurred under well-watered, but not so much under droughted conditions. Mean height increments and transpiration efficiency showed a significant linear relationship, but biomass increments exhibited a quadratic relationship with precipitation on the origin site of these populations. Measurements of water use efficiency obtained from instantaneous gas exchange measurements, carbon isotope discrimination, and transpiration efficiency were intercorrelated in the seedlings. However, neither did any of these measurements consistently rank the populations, nor were they indicative of adaptation to climatic precipitation conditions in these western redcedar populations.

CONTRASTING PLANT PHYSIOLOGICAL ADAPTATION TO CLIMATE IN THE NATIVE AND INTRODUCED RANGE OFHYPERICUM PERFORATUM

How introduced plants, which may be locally adapted to specific climatic conditions in their native range, cope with the new abiotic conditions that they encounter as exotics is not well understood. In particular, it is unclear what role plasticity versus adaptive evolution plays in enabling exotics to persist under new environmental circumstances in the introduced range. We determined the extent to which native and introduced populations of St. John's Wort (Hypericum perforatum) are genetically differentiated with respect to leaf-level morphological and physiological traits that allow plants to tolerate different climatic conditions. In common gardens in Washington and Spain, and in a greenhouse, we examined clinal variation in percent leaf nitrogen and carbon, leaf delta(13)C values (as an integrative measure of water use efficiency), specific leaf area (SLA), root and shoot biomass, root/shoot ratio, total leaf area, and leaf area ratio (LAR). As well, we determined whether native European H. perforatum experienced directional selection on leaf-level traits in the introduced range and we compared, across gardens, levels of plasticity in these traits. In field gardens in both Washington and Spain, native populations formed latitudinal clines in percent leaf N. In the greenhouse, native populations formed latitudinal clines in root and shoot biomass and total leaf area, and in the Washington garden only, native populations also exhibited latitudinal clines in percent leaf C and leaf delta(13)C. Traits that failed to show consistent latitudinal clines instead exhibited significant phenotypic plasticity. Introduced St. John's Wort populations also formed significant or marginally significant latitudinal clines in percent leaf N in Washington and Spain, percent leaf C in Washington, and in root biomass and total leaf area in the greenhouse. In the Washington common garden, there was strong directional selection among European populations for higher percent leaf N and leaf delta(13)C, but no selection on any other measured trait. The presence of convergent, genetically based latitudinal clines between native and introduced H. perforatum, together with previously published molecular data, suggest that native and exotic genotypes have independently adapted to a broad-scale variation in climate that varies with latitude.

Promising eco-physiological traits for genetic improvement of cereal yields in Mediterranean environments

Summary Breeding for improved productivity has been tremendously successful in the last half-century, but needs to be even more efficient in the future. Hope based on contributions from molecular biology for improved yield potential seems to depend upon an improved knowledge of yield physiology. This knowledge may assist breeding either directly, recommending selection criteria, or indirectly identifying simpler traits that could be reliably mapped and selected for through marker-assisted selection. Physiological traits associated with improved performance under water-limited conditions, include phenology (that allows the crop to escape stresses) and those associated with improved water use, water use efficiency and partitioning. Undoubtedly, earliness has been the predominant trait improved for under Mediterranean conditions, and may not be a prospective trait for future breeding. Different traits that may confer the ability to the crop for capturing more water, such as deeper root systems or osmotic adjustment, may be unworkable in terms of their direct use in selection and surrogates would be needed. For instance, canopy temperature depression and discrimination against 13 C may be used to assess improved ability to capture water (in these cases yield is positively related to discrimination against 13 C in grains). Early vigour, which allows faster ground coverage, also increases the amount of water actually transpired by the canopy by reducing direct evaporation and presents substantial intraspecific variation, and selection for this trait may be successfully carried out either directly or through the use of vegetation indexes. Improved water use efficiency based on transpiration efficiency is largely restricted to conditions where additional water is not available. A constitutively low stomatal conductance or a high stomatal sensitivity may optimise the transpiration efficiency. In this context, discrimination against 13 C is also a simple and reliable measure of water use efficiency, and in cases in which no major differences in capturing water is possible discrimination against 13 C correlates negatively with yield. Substantial further improvements in partitioning may be limited in most cereals.

Perspective: Evolution of flower color in the desert annual Linanthus parryae: Wright revisited.

Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a long-standing debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long-term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue-flowered plants produced more seeds than white-flowered plants in years of relatively low seed production, whereas white-flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the 11 years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue-flowered plants typically had a fitness advantage in years of low spring precipitation, whereas white-flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower-color polymorphism at Pearblossom site 1, whereas gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water-use efficiency. Although the mechanism of selection remains elusive, our results refute Wright's conclusion that the flower color polymorphism in L. parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.

PERSPECTIVE: EVOLUTION OF FLOWER COLOR IN THE DESERT ANNUAL LINANTHUS PARRYAE: WRIGHT REVISITED

Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a long-standing debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long-term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue-flowered plants produced more seeds than white-flowered plants in years of relatively low seed production, whereas white-flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the 11 years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue-flowered plants typically had a fitness advantage in years of low spring precipitation, whereas white-flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower-color polymorphism at Pearblossom site 1, whereas gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water-use efficiency. Although the mechanism of selection remains elusive, our results refute Wright's conclusion that the flower color polymorphism in L. parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.Corresponding Editor: C. Eckert