Plant Breeding and Drought in C3 Cereals: What Should We Breed For?

Drought is the main abiotic constraint on cereal yield. Analysingphysiological determinants of yield responses to water may helpin breeding for higher yield and stability under drought conditions.The traits to select (either for stress escape, avoidance ortolerance) and the framework where breeding for drought stressis addressed will depend on the level and timing of stress inthe targeted area. If the stress is severe, breeding under stress-freeconditions may be unsuccessful and traits that confer survivalmay become a priority. However, selecting for yield itself understress-alleviated conditions appears to produce superior cultivars,not only for optimum environments, but also for those characterizedby frequent mild and moderate stress conditions. This impliesthat broad avoidance/tolerance to mild–moderate stressesis given by constitutive traits also expressed under stress-freeconditions. In this paper, we focus on physiological traitsthat contribute to improved productivity under mild–moderatedrought. Increased crop performance may be achieved throughimprovements in water use, water-use efficiency and harvestindex. The first factor is relevant when soil water remainsavailable at maturity or when deep-rooted genotypes access waterin the soil profile that is not normally available; the twolatter conditions become more important when all available wateris exhausted by the end of the crop cycle. Independent of themechanism operating, a canopy able to use more water than anotherwould have more open stomata and therefore higher canopy temperaturedepression, and ¹³C discrimination (¹³C) in plant matter. Thesame traits would also seem to be relevant when breeding forhot, irrigated environments. Where additional water is not availableto the crop, higher water-use efficiency (WUE) appears to bean alternative strategy to improve crop performance. In thiscontext ¹³C constitutes a simple but reliable measure of WUE.However, in contrast to lines performing better because of increasedaccess to water, lines producing greater biomass due to superiorWUE will have lower ¹³C values. WUE may be modified not onlythrough a decrease in stomatal conductance, but also throughan increase in photosynthetic capacity. Harvest index is stronglyreduced by terminal drought (i.e. drought during grain filling).Thus, phenological traits increasing the relative amount ofwater used during grain filling, or adjusting the crop cycleto the seasonal pattern of rainfall may be useful. Augmentingthe contribution of carbohydrate reserves accumulated duringvegetative growth to grain filling may also be worthwhile inharsh environments. Alternatively, extending the duration ofstem elongation without changing the timing of anthesis wouldincrease the number of grains per spike and the harvest indexwithout changing the amount of water utilized by the crop.     

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

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

Early growth,dry matter allocation and water use efficiency of two sympatric Populus species as affected by water stress

We exposed cuttings of two sympatric species of Sect. Tacamahaca Spach, Populus cathayana Rehder and Populus przewalskii Maximowicz, to two watering regimes in a greenhouse. In the semi-controlled environmental study, two watering treatments which were watered to 100 and 25% of field capacity were used, respectively. The effects of water deficit on early growth, biomass allocation and water use efficiency (WUE) were investigated. We found that there were significant interspecific differences in early growth, dry matter allocation and water use efficiency between two sympatric Populus species. Compared with P. cathayana, P. przewalskii showed higher shoot height, dry matter accumulation, number of leaves, total leaf area, fine root mass, fine root/total root ratio and water use efficiency under both well-watered and water-stressed treatments. On the other hand, P. przewalskii also showed higher root mass/foliage area ratio, root/shoot ratio and carbon isotope composition than P. cathayana under water-stressed treatment. The results suggested that there were different water-use strategies between two sympatric Populus species, P. przewalskii with higher drought tolerance may employ a conservative water-use strategy, whereas P. cathayana with lower drought tolerance may employ a prodigal water-use strategy. The findings confirm the existence of interspecific genetic differences in early growth, dry matter allocation and water use efficiency as affected by water stress, these variations in drought responses may be used as criteria for species selection and tree improvement.     

Effects of drought stress and N supply on the growth, biomass partitioning and water-use efficiency of Sophora davidii seedlings

A greenhouse experiment was conducted in order to understand the adaptation responses to different water and N conditions, and further explore if additional N supply could improve the water-use efficiency (WUE) and adaptability of Sophora davidii seedlings under dry conditions. Two-month-old seedlings were subjected to a completely random design with three water (80, 40 and 20% water field capacity (FC)) and three N supply (N0: 0, Nl: 92 and Nh:184 mg N kg⁻¹ soil) regimes. Drought stress dramatically decreased seedlings height, basal diameter, leaf number, leaf area, root length, and biomass production. An increase in below-ground biomass was observed indicating a higher root/shoot ratio (R/S) under drought stress conditions, and drought further decreased relative water content (RWC) and WUE. On the other hand, S. davidii seedlings exhibited strong responses to N supply, but the responses were inconsistent with the various N supply levels. Low N supply (Nl) increased seedlings height, basal diameter, leaf number, leaf area, and biomass production, but decreased root length. In contrast, high N supply (Nh) decreased or had little effect on these growth characteristics. N supply increased leaf percentages, but decreased fine root percentages. In addition, Nl rather than the other two N treatments increased leaf area ratio (LAR), leaf/fine root mass ratio (L/FR), R/S and RWC under severe drought stress (20% FC), even though these parameters could increase with the Nh treatment under well-watered condition (80% FC). Moreover, Nl also increased WUE under three water conditions, but Nh had little effect on WUE under drought stress conditions (40% FC and 20% FC). The results suggested that water and N co-limited the growth of S. davidii seedlings, and the seedlings exhibited great positive responses to Nl in this study. Appropriate or low N supply, therefore, would be recommended to stimulate growth, enhance WUE, alleviate drought stress, and consequently contribute to S. davidii seedling establishment under dry condition, but excess N supply should be avoided.     

Photosynthetic responses of wheat (Triticum aestivum L.) to combined effects of drought and exogenous methyl jasmonate

Drought stress limits wheat growth and productivity. The response of wheat (Triticum aestivum L.) to different water supply conditions (well-watered and drought-stressed) and exogenous methyl jasmonate (MeJA; 0 and 0.25 μM) was studied. The application of MeJA enhanced wheat adaptability to drought stress by physiological and metabolic adjustments. Drought stress reduced net photosynthetic rate (P _N), stomatal conductance (g _s), transpiration rate (E), and water-use efficiency (WUE) in wheat. The application of exogenous MeJA decreased also g _s and E, but stimulated WUE. Meanwhile, MeJA mitigated the decline of P _N, g _s, and WUE induced by drought stress and midday depression by 6–183%. Both drought stress and exogenous MeJA induced stomatal closure, which improved water status and delayed plant senescence. MeJA enhanced the activities of superoxide dismutase, peroxidase, catalase, and reduced malondialdehyde content. P _N-PAR response curves showed that MeJA mitigated the decline of maximum P _N, apparent quantum yield, and saturation irradiance, and the increase of compensation irradiance. Drought stress and exogenous MeJA increased dark respiration rate and showed an additive effect. These results indicated that 0.25 μM MeJA enhanced the photosynthesis under drought stress mainly by improving the water status and antioxidant capacity of wheat.     

Biomass production and water use efficiency in perennial grasses during and after drought stress

Drought is a great challenge to agricultural production, and cultivation of drought‐tolerant or water use‐efficient cultivars is important to ensure high biomass yields for bio‐refining and bioenergy. Here, we evaluated drought tolerance of four C₃ species, Dactylis glomerata cvs. Sevenop and Amba, Festuca arundinacea cvs. Jordane and Kora, Phalaris arundinacea cvs. Bamse and Chieftain and Festulolium pabulare cv. Hykor, and two C₄ species Miscanthus × giganteus and M. lutarioriparius. Control (irrigated) and drought‐treated plants were grown on coarse and loamy sand in 1 m² lysimeter plots where rain was excluded. Drought periods started after harvest and lasted until 80% of available soil water had been used. Drought caused a decrease in dry matter yield (DM; P < 0.001) for all species and cultivars during the drought period. Cultivars Sevenop, Kora and Jordane produced DM at equal levels and higher than the other C₃ cultivars in control and drought‐treated plots both during and after the drought period. Negative correlations were observed between stomatal conductance (g_s) and leaf water potential (P < 0.01) and positive correlations between g_s and DM (P < 0.05) indicating that g_s might be suitable for assessment of drought stress. There were indications of positive associations between plants carbon isotope composition and water use efficiency (WUE) as well as DM under well‐watered conditions. Compared to control, drought‐treated plots showed increased growth in the period after drought stress. Thus, the drought events did not affect total biomass production (DM_total) of the whole growing season. During drought stress and the whole growing season, WUE was higher in drought‐treated compared to control plots, so it seems possible to save water without loss of biomass. Across soil types, M. lutarioriparius had the highest DM_total (15.0 t ha^?1), WUE_total (3.6 g L^?1) and radiation use efficiency (2.3 g MJ^?1) of the evaluated grasses.     

Measures of leaf-level water-use efficiency in drought stressed endophyte infected and non-infected tall fescue grasses

Neotyphodium coenophialum [Morgan-Jones and Gams], grows in the above-ground parts of tall fescue [Lolium arundinaceum (Schreb.) Darbysh.]. It is an asexual fungus that is transmitted through seed of its host plant. This grass/endophyte association is enhanced by the protection of the host from herbivory and improved drought stress. We investigated how a decline in leaf-level stomatal conductance impacts the instantaneous water-use efficiency (WUE), in endophyte-infected (E+) versus non-infected (E?) Kentucky-31 tall fescue grasses grown in a controlled environmental chamber over a 10-week period. Grasses were cut at 6 weeks after germination and allowed to regrow under high and low soil moisture availability. One week after cutting, soil moisture was allowed to decline in the low water treatment for 2 weeks until severe stress was demonstrated through a decline in stomatal conductance to less than 100 mmol m^?2 s^?1. We found no differences in WUE between E+ and E? plants when water was not limiting while higher WUE was exhibited in E+ plants relative to E? plants under severe drought stress. The E? plants showed an 18-fold reduction in mean WUE and a 70-fold reduction in photosynthesis under drought stress, while there was no change in WUE and only a fourfold decline in photosynthesis between well-watered and drought stressed E+ plants at 21 days. While there were no differences in the rates of transpiration between E+ and E? plants under severe drought stress, differences in WUE can be attributed mainly to higher photosynthetic rates of E+ than E? plants. The difference in photosynthetic rates between E+ and E? plants under drought conditions could not be explained by differences in stomatal conductance and Rubisco (EC 4.1.1.39) activities.     

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

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

Genome-wide association mapping of yield components and drought tolerance-related traits in cotton

Drought causes serious yield losses in cotton production throughout the world. Association mapping allows identification and localization of the genes controlling drought-related traits which will be helpful in cotton breeding. In the present study, genetic diversity analysis and association mapping of yield and drought traits were performed on a panel of 99 upland cotton genotypes using 177 SSR (simple sequence repeat) markers. Yield parameters and drought tolerance-related traits were evaluated for two seasons under two watering regimes: water-stressed and well-watered. The traits included seed cotton yield (SCY), lint yield (LY), lint percentage (LP), water-use efficiency (WUE), yield potential (YP), yield reduction (YR), yield index (YI), drought sensitivity index (DSI), stress tolerance index (STI), harmonic mean (HM), and geometric mean productivity (GMP). The genotypes with the least change in seed cotton yield under drought stress were Zeta 2, Delcerro, Nazilli 87, and DAK 66/3 which were also the most water-use efficient cultivars. The average genetic diversity of the panel was 0.38. The linkage disequilibrium decayed relatively rapidly at 20–30 cM (r²?≥?0.5). We identified 30 different SSR markers associated with the traits. Fifteen and 23 SSR markers were linked to the traits under well-watered and water-stress conditions, respectively. To our knowledge, most of these quantitative yield and drought tolerance-associated loci were newly identified. The genetic diversity and association mapping results should facilitate the development of drought-tolerant cotton lines with high yield in molecular breeding programs.     

杠柳幼苗对不同强度干旱胁迫的生长与生理响应

以黄土丘陵区常见灌木杠柳(Periploca sepium Bunge.)的两年生苗木为试验材料,模拟不同程度的土壤干旱环境,研究了土壤干旱对杠柳生长和生理生化特性的影响。结果表明,在3种土壤水分条件下杠柳的耗水和快速生长期均集中在6-8月,与黄土丘陵区雨热期重叠。在干旱胁迫下杠柳生长减慢,生物量累积减小,生物量优先向根系分配,根冠比显著增大。与适宜水分下相比,干旱胁迫下杠柳的水分利用效率随生物量与耗水量减小而显著升高,表明杠柳具有节约型水分利用对策。杠柳在干旱前期和中期丙二醛(MDA)含量下降、膜透性略有增加,干旱末期MDA含量和细胞膜透性与适宜水分相比显著升高。在中度干旱与严重干旱下,杠柳的超氧化物歧化酶与过氧化物保护酶活性持续上升直到试验末期才稍有下降。干旱胁迫对杠柳叶片渗透调节物质含量的影响显著,脯氨酸、可溶性糖和可溶性蛋白含量均随着干旱胁迫程度的加大而显著升高,且随着胁迫时间的延长,脯氨酸和可溶性糖含量一直保持上升趋势;可溶性蛋白含量在前期急剧上升,中、后期下降,但含量一直高于适宜水分处理。本研究表明,杠柳能够充分利用黄土丘陵区局部雨热资源优势、具备减少地上蒸发面积、增加地下生长、吸收深层土壤水源等御旱策略以及保持较高的抗氧化酶活性和渗透调节能力等生理学耐旱机制,本研究从生长、生理特征上揭示了杠柳在黄土高原植被自然恢复中普遍存在的原因。     

Salicylic Acid Application Mitigates Oxidative Damage and Improves the Growth Performance of Barley under Drought Stress

Drought is a severe environmental constraint, causing a significant reduction in crop productivity across the world. Salicylic acid (SA) is an important plant growth regulator that helps plants cope with the adverse effects induced by various abiotic stresses. The current study investigated the potential effects of SA on drought tolerance efficacy in two barley (Hordeum vulgare) genotypes, namely BARI barley 5 and BARI barley 7. Ten-day-old barley seedlings were exposed to drought stress by maintaining 7.5% soil moisture content in the absence or presence of 0.5, 1.0 and 1.5 mM SA. Drought exposure led to severe damage to both genotypes, as indicated by phenotypic aberrations and reduction of dry biomass. On the other hand, the application of SA to drought-stressed plants protected both barley genotypes from the adverse effects of drought, which was reflected in the improvement of phenotypes and biomass production. SA supplementation improved relative water content and proline levels in drought-stressed barley genotypes, indicating the osmotic adjustment functions of SA under water-deficit conditions. Drought stress induced the accumulation of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and superoxide (O₂ ^•− ), and the lipid peroxidation product malondialdehyde (MDA) in the leaves of barley plants. Exogenous supply of SA reduced oxidative damage by restricting the accumulation of ROS through the stimulation of the activities of key antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX). Among the three-applied concentrations of SA, 0.5 mM SA exhibited better mitigating effects against drought stress considering the phenotypic performance and biochemical data. Furthermore, BARI barley 5 showed better performance under drought stress than BARI barley 7 in the presence of SA application. Collectively, our results suggest that SA played a crucial role in improving water status and antioxidant defense strategy to protect barley plants from the deleterious effects of water deficiency.     

Population differences in water-use efficiency of Eucalyptus microtheca seedlings under different watering regimes

Gas exchange, water-use efficiency (WUE), carbon isotope composition ( Δ ¹³C) and growth traits were compared among 5 populations of Eucalyptus microtheca F. Muell. Seedlings grown from seed collected across the natural distribution of the species were maintained under water-stressed and well-watered conditions. Gas exchange was measured in terms of net photosynthesis (A) and transpiration (E); WUE was measured in terms of instantaneous water-use efficiency (WUE_i) and transpiration efficiency (WUE_T); growth traits were measured in terms of total biomass (TB), root/shoot ratio (RS), and specific leaf area density (DEN). Significant differences in all traits were detected among the populations. Overall population variation was 1.68–2.50 and 1.48–2.26 μmol CO₂ uptake per mmol H₂O transpired (WUE_i), 1.97–3.04 and 1.64–2.36 g dry matter accumulation per kg water transpired (WUE_T), and Δ ¹³C was −28.81 to −26.75‰ and −30.56 to −30.04‰ under the water-stressed and well-watered conditions, respectively. In addition, WUE_i, WUE_T and Δ ¹³C were significantly correlated with A, E, RS, DEN and TB. The study indicated that measurement of WUE may be a useful trait for selecting genotypes with improved drought adaptation and biomass productivity under different environmental conditions.     

Carbon isotope ratios of plants and occurrences of C4 species under different soil moisture regimes in arid region of Northwest China

Carbon isotope ratio of leaf dry matter, δ ¹³C, was measured on species occurring within Baiyin desert community, consisting of valley, slope and ridge microhabitats, and within Shandan desert community, consisting of Gobi desert and seasonal flooded creek microhabitats, in Northwest China. δ ¹³C of C₃ species increased with a decrease in soil water availability, suggesting that water-use efficiency (WUE) increased with decreasing soil moisture, whereas for all C₄ species, δ ¹³C tended to decrease with decreasing soil water availability, suggesting that WUE also increased with decreasing soil moisture. Above results indicated that water-use pattern was conservative under drought for C₄ and C₃ plants. In this present study, C₄ species' occurrences within different microhabitats were investigated and C₄ plants were observed to be absent and/or scarce within relatively lower soil moisture microhabitats, whereas they occurred and/or even had a high abundance within relatively higher soil moisture microhabitats, suggesting limited moisture available was a key factor of limiting C₄ distribution in arid region of Northwest China.     

Water Use Efficiency by Switchgrass Compared to a Native Grass or a Native Grass Alfalfa Mixture

Perennial grass systems are being evaluated as a bioenergy feedstock in the northern Great Plains. Inter-annual and inter-seasonal precipitation variation in this region will require efficient water use to maintain sufficient yield production to support a mature bioenergy industry. Objectives were to evaluate the impact of a May–June (early season) and a July–August (late season) drought on the water use efficiency (WUE), amount of water used, and biomass production in monocultures of switchgrass (Panicum virgatum L.), western wheatgrass (Pascopyrum smithii (Rydb.) Á. Löve), and a western wheatgrass–alfalfa (Medicago sativa L.) mixture using an automated rainout shelter. WUE was strongly driven by biomass accumulation and ranged from 5.6 to 7.4 g biomass mm^?1 water for switchgrass to 1.06 to 2.07 g biomass mm^?1 water used with western wheatgrass. Timing of water stress affected WUE more in western wheatgrass and the western wheatgrass–alfalfa mixture than switchgrass. Water deficit for the western wheatgrass–alfalfa mixture was 23 % lower than western wheatgrass (P?=?0.0045) and 31 % lower than switchgrass (P?<?0.0001) under the May–June stress water treatment, while switchgrass had a 37 and 38 % greater water deficit than did western wheatgrass or western wheatgrass–alfalfa mixture, respectively (P?<?0.001) under the July–August water stress treatment. Water depletion was always greatest in the upper 30 cm. Switchgrass had greater WUE but resulted in greater soil water depletion at the end of the growing season compared to western wheatgrass and a western wheatgrass–alfalfa mixture which may be a concern under multi-year drought conditions.     

Morphological and physiological responses of two coffee progenies to soil water availability

Drought is a major environmental constraint affecting growth and production of coffee. The effects of water supply on growth, biomass allocation, water relations, and gas exchange in two coffee progenies representing drought-tolerant (Siriema) and drought-sensitive (Catucaí) genotypes were compared. They were grown in 12-L pots until 4-months old, when they were submitted to two watering treatments for 60 d: plants receiving either 100% transpired water (control plants) or a fraction (about 40%) of the amount of water transpired by control plants (drought-stressed plants). Under control conditions, Siriema grew faster than Catucaí. Regardless of the watering regimes and progenies, relative growth rate (RGR) was positively correlated both with net assimilation rate (NAR) and long-term water-use efficiency (WUE), but not with differences in biomass allocation. Both progenies responded to drought stress through (i) similar decreases in both RGR and NAR with marginal, if any, changes in allocation; (ii) decreases in leaf water potential, which occurred to a greater extent in Catucaí than in Siriema, even though they have showed similar abilities to adjust osmotically and elastically; (iii) similar reductions in net photosynthesis due mainly to nonstomatal factors; and (iv) decreases in transpiration rate coupled with increased long-term WUE. However, the lower transpiration rate and the higher long-term WUE as found in Siriema relative to Catucaí under control conditions persisted under drought conditions. Overall, the major differences between these progenies were largely associated with differences in plant water use, which was likely related to the improved water status of Siriema. The possible implications of selecting coffee genotypes for high WUE are discussed.     

Proteomics reveals the adaptability mechanism of Brassica napus to short-term boron deprivation

The application of pyrogenic carbon, biochar, to agricultural soils is currently discussed as a win-win strategy to sequester carbon in soil, thus improving soil fertility and mitigate global warming. Our aim was to investigate if biochar may improve plant eco-physiological responses under sufficient water supply as well as moderate drought stress. A fully randomized greenhouse study was conducted with the pseudo-cereal Chenopodium quinoa Willd, using three levels of biochar addition (0, 100 and 200?t ha^?1) to a sandy soil and two water treatments (60% and 20% of the water holding capacity of the control), investigating growth, water use efficiency, eco-physiological parameters and greenhouse gas (GHG) fluxes. Biochar application increased growth, drought tolerance and leaf-N- and water-use efficiency of quinoa despite larger plant?Cleaf areas. The plants growing in biochar-amended soil accumulated exactly the same amount of nitrogen in their larger leaf biomass than the control plants, causing significantly decreased leaf N-, proline- and chlorophyll-concentrations. In this regard, plant responses to biochar closely resembled those to elevated CO₂. However, neither soil- nor plant?Csoil-respiration was higher in the larger plants, indicating less respiratory C losses per unit of biomass produced. Soil-N₂O emissions were significantly reduced with biochar. The large application rate of 200?t ha^?1 biochar did not improve plant growth compared to 100?t ha^?1; hence an upper beneficial level exists. For quinoa grown in a sandy soil, biochar application might hence provide a win-win strategy for increased crop production, GHG emission mitigation and soil C sequestration.     

Water deficit and plant competition effects on growth and water-use efficiency of white clover ( Trifolium repens,L.) and ryegrass (Lolium perenne,L.)

The combined effects of soil water deficit and above and below ground interspecific plant competition on the growth, water-use efficiency (WUE), and measured carbon isotopic composition (δ¹³C) values of white clover and ryegrass were studied. White clover and ryegrass were grown in specially designed crates 1) individually; 2) in shoot competition; or 3) in shoot + root competition and either well-watered or at a moderate or severe soil water deficit. The effects of shoot + root competition on shoot dry matter growth were substantial and benefited both white clover and ryegrass when well-watered or at a moderate soil water deficit, while severely reducing white clover shoot dry matter growth at severe soil water deficit. Plant competition did not affect the WUE of white clover or ryegrass. As soil water deficit increased, the WUE of white clover did not change whereas the WUE of ryegrass increased and was greater than that of white clover. This was attributed to the lower leaf water conductance of ryegrass which conserved water and maintained growth longer compared to white clover. A stronger correlation existed between soil water deficit and measured δ¹³C values for ryegrass at each plant competition level (P<0.001) than existed for white clover (individual: P<0.01; shoot + root: P<0.001; shoot: P<0.10). Unlike white clover, the relationship between measured δ¹³C values and shoot dry matter growth indicated that C assimilation for ryegrass was dependent on type of plant competition. That WUE remained constant for white clover while measured δ¹³C values increased as soil water deficit increased, suggests that the role below ground respiration rate played in determining δ¹³C values increased. The WUE of white clover appears to be independent of the nature of the competition between plants and the soil water deficit level at which it is grown, whereas for ryegrass, the addition of root competition to shoot competition should lead to increases in its WUE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of stem sap flow and leaf photosynthesis in <Emphasis Type="Italic">Tamarix chinensis</Emphasis> to soil moisture in the Yellow River Delta,China

Soil moisture is the main limiting factor for vegetation growth at shell ridges in the Yellow River Delta of China. The objective of this study was to explore the soil moisture response of photosynthetic parameters and transpiration in Tamarix chinensis Lour., a dominant species of shell ridges. Leaf photosynthetic light-response parameters and sap flow were measured across a gradient of relative soil water content (RWC), from drought (23%) to waterlogging (92%) conditions. Leaf photosynthetic efficiency and stem sap flow of T. chinensis showed a clear threshold response to soil moisture changes. Leaf net photosynthetic rate, water-use efficiency (WUE), light-saturation point, apparent quantum yield, maximum net photosynthetic rate, and dark respiration rate peaked at moderately high RWC, decreasing towards high and low values of RWC. However, peak or bottom RWC values substantially differed for various parameters. Excessively high or low RWC caused a significant reduction in the leaf photosynthetic capacity and WUE, while the high photosynthetic capacity and high WUE was obtained at RWC of 73%. With increasing waterlogging or drought stress, T. chinensis delayed the starting time for stem sap flow in the early morning and ended sap flow activity earlier during the day time in order to shorten a daily transpiration period and reduce the daily water consumption. The leaf photosynthetic capacity and WUE of T. chinensis were higher under drought stress than under waterlogging stress. Nevertheless, drought stress caused a larger reduction of daily water consumption compared to waterlogging, which was consistent with a higher drought tolerance and a poor tolerance to waterlogging in this species. This species was characterized by the low photosynthetic capacity and low WUE in the range of RWC between 44 and 92%. The RWC of 49–63% was the appropriate range of soil moisture for plant growth and efficient physiological water use of T. chinensis seedlings.     

Physiological responses of different olive genotypes to drought conditions

Gas exchange rates, chlorophyll fluorescence, pressure–volume relationships, photosynthetic pigments, total soluble sugars, starch, soluble proteins and proline concentrations were investigated in five Olea europaea L. cultivars with different geographical origins (Arbequina, Blanqueta, Cobran?osa, Manzanilla and Negrinha) grown under Mediterranean field conditions. We found considerable genotypic differences among the cultivars. Comparing the diurnal gas exchange rates, we observed that Cobran?osa, Manzanilla and Negrinha had high photosynthetic rate than Arbequina and Blanqueta. The first group reveals to be better acclimated to drought conditions, and appears to employ a prodigal water-use strategy, whereas Blanqueta and Arbequina, with high water-use efficiency, appear to employ a conservative water-use strategy. The degree of midday depression in photosynthesis was genotype dependent, with a maximum in Arbequina and a minimum in Negrinha. The reductions in the photosynthetic rate were dependent from both stomatal and non-stomatal limitations. Elastic adjustment plays an important role as drought tolerance mechanism. The group of cultivars that employ a prodigal water-use strategy revealed high tissue elasticity, whereas Arbequina and Blanqueta revealed high tissue rigidity. We also identified the existence of drought tolerance mechanisms associated with soluble proteins accumulation in the foliage. The high levels of soluble proteins in Arbequina may represent an increased activity of oxidative stress defence enzymes and may also represent a reserve for post stress recovery. In all cultivars, especially in Manzanilla, free proline was accumulated in the foliage. The discussed aspects of drought stress metabolism may have an adaptative meaning, supporting the hypothesis that olive cultivars native to dry regions, such as Cobran?osa, Manzanilla and Negrinha, have more capability to acclimate to drought conditions than cultivars originated in regions with a more temperate climate, like Arbequina and Blanqueta.     

Enhanced evapotranspiration was observed during extreme drought from Miscanthus,opposite of other crops

The impact of extreme drought and heat stress that occurred in the Midwestern U.S. in 2012 on evapotranspiration (ET), net ecosystem productivity (NEP), and water‐use efficiency (WUE) of three perennial ecosystems (switchgrass, miscanthus, prairie) and a maize/soybean agroecosystem was studied as part of a long‐term experiment. Miscanthus had a slower initial response but an eventually drastic ET as drought intensified, which resulted in the largest water deficit among the crops. The substantially higher ET at peak drought was likely supplied by access to deep soil water, but suggests that stomatal conductance of miscanthus during the drought may respond differently than the other ecosystems, consistent with an anisohydric strategy. While there was a discrepancy in the water consumption of maize and switchgrass/prairie in the early time of drought, all these ecosystems followed a water‐saving strategy when drought intensified. The gross primary production (GPP) of miscanthus dropped, but was reversible, when temperature reached 40 °C and still provided the largest total GPP among the ecosystems. Increased ET for miscanthus during 2012 resulted a large decline in ecosystem WUE compared to what was observed in other years. The biophysical responses of miscanthus measured during an extreme, historic drought suggest that this species can maintain high productivity longer than other ecosystems during a drought at the expense of water use. While miscanthus maintained productivity during drought, recovery lagged associated with depleted soil moisture. The enhanced ET of miscanthus may intensify droughts through increase supply of deep soil moisture to the atmosphere.