Can CO2 enrichment modify the effect of water and high light stress on biomass allocation and relative growth rate of cork oak seedlings?

To test whether the impact of an enriched-CO₂ environment on the growth and biomass allocation of first-season Quercus suber L. seedlings can modify the drought response under shade or sun conditions, seedlings were grown in pots at two CO₂ concentrations × two watering regimes × two irradiances. Compared to CO₂, light and water treatment had greater effects on all morphological traits measured (height, stem diameter, number of leaves, leaf area, biomass fractions). Cork oak showed particularly large increases in biomass in response to elevated CO₂ under low-watered (W−) and high-illuminated conditions (L+). Allocation shifted from shoot to root under increasing irradiance (L+), but was not affected by CO₂. Changes in allocation related to water limitation were only modest, and changed over time. Relative growth rate (RGR) and net assimilation rate (NAR) were significantly greatest in the L+/W+ treatment for both CO₂ concentrations. Changes in RGR were mainly due to NAR. Growth responses to increased light, water or CO₂ were strongest with light, medium with water availability and smallest for CO₂, in terms of RGR. The rise in NAR for light and water treatments was counterbalanced by a decrease in SLA (specific leaf area) and LMF (leaf mass fraction). Results suggest that elevated CO₂ caused cork oak seedlings to improve their performance in dry and high light environments to a greater extent than in well-irrigated and low light ones, thus ameliorating the effects of soil water stress and high light loads on growth.     

Transpiration by an emergent macrophyte: source of water and implications for nutrient supply

The contribution of sediment interstitial water and the water column to the transpiration stream of Myriophyllum aquaticum (Vellozo) Verdcourt was determined to estimate the significance of mass flow in supply of sediment nutrients for plant growth. Sediment interstitial water accounted for about 2% of the water transpired over a 37 day period. Because of the small volume of water that originated in the sediment we concluded that mass flow did not significantly enhance nutrient supply to the roots of M. aquaticum. Relative growth rate (RGR) of adventitious, water roots was greater than whole plant RGR, and RGR of sediment roots was not significantly different from zero, indicating a shift in the biomass allocation after emergence of the apical meristem into the air. Water use, measured by the transpiration coefficient, averaged 260 ml H₂O mg DW^-1, which is similar to C-4 terrestrial plants. M. aquaticum has leaf characteristics commonly associated with xerophytic habitats. These characteristics may be necessary if a high transpiration rate and a mechanical requirement for high cell turgor pressure, required by a reliance upon hydrostatic pressure for support of the aerial stems, are mutually exclusive because of morphological constraints on hydraulic conductivity.     

Genetic Control of Water Use Efficiency and Leaf Carbon Isotope Discrimination in Sunflower (Helianthus annuus L.) Subjected to Two Drought Scenarios

High water use efficiency (WUE) can be achieved by coordination of biomass accumulation and water consumption. WUE is physiologically and genetically linked to carbon isotope discrimination (CID) in leaves of plants. A population of 148 recombinant inbred lines (RILs) of sunflower derived from a cross between XRQ and PSC8 lines was studied to identify quantitative trait loci (QTL) controlling WUE and CID, and to compare QTL associated with these traits in different drought scenarios. We conducted greenhouse experiments in 2011 and 2012 by using 100 balances which provided a daily measurement of water transpired, and we determined WUE, CID, biomass and cumulative water transpired by plants. Wide phenotypic variability, significant genotypic effects, and significant negative correlations between WUE and CID were observed in both experiments. A total of nine QTL controlling WUE and eight controlling CID were identified across the two experiments. A QTL for phenotypic response controlling WUE and CID was also significantly identified. The QTL for WUE were specific to the drought scenarios, whereas the QTL for CID were independent of the drought scenarios and could be found in all the experiments. Our results showed that the stable genomic regions controlling CID were located on the linkage groups 06 and 13 (LG06 and LG13). Three QTL for CID were co-localized with the QTL for WUE, biomass and cumulative water transpired. We found that CID and WUE are highly correlated and have common genetic control. Interestingly, the genetic control of these traits showed an interaction with the environment (between the two drought scenarios and control conditions). Our results open a way for breeding higher WUE by using CID and marker-assisted approaches and therefore help to maintain the stability of sunflower crop production.     

Modulation of relative growth rate and its components by water stress in Mediterranean species with different growth forms

Effects of water availability on seedling growth were analysed in eight Mediterranean species naturally occurring in the Balearic Islands. Seedlings were grown outdoors during summer under two irrigation treatments: field capacity and 35% of field capacity. The relative growth rate (RGR) strongly depended on the growth form, from highest values in herbs to lowest in woody perennials. The main component associated with interspecific variation in RGR was the specific leaf area (SLA), and a quantitative grouping of the different growth forms appeared along the regression line between both parameters. The slow-growing species, i.e. woody perennial shrubs, had the lowest SLA and the fast-growing perennial herbs, the highest, while woody semi-deciduous shrubs appeared intermediate. Decreases in RGR due to water stress were analysed in terms of the relative contribution of the leaf mass ratio (LMR), SLA and the net assimilation rate (NAR). Pooling all species, the decrease in RGR caused by water deficit was mainly explained by decreases in SLA. However, this general pattern was strongly dependent of growth form. Thus, in the woody perennial plants, the decrease in RGR was accompanied by a three-fold decrease in NAR which, however, increased in perennial herbs. SLA increased with decreasing water supply in woody perennial plants, and decreased in woody semi-deciduous shrubs and perennial herbs. Finally, decreases in LMR partly explained decreases in RGR in perennial herbs and woody perennial shrubs. This different response of the different growth forms may reflect differences in seedling adaptation and surviving strategies to drought periods.     

Differential responses in water use efficiency in two varieties of Catharanthus roseus under drought stress

Two varieties, rosea and alba, of Catharanthus roseus (L.) G. Don. were screened for their water use efficiency under two watering regimes, viz. 60 and 100% filed capacity in the present study. Drought stress was imposed at 60% filed capacity from 30 to 70 days after sowing, while the control pots were maintained at 100% filed capacity throughout the entire growth period. Leaf area duration, cumulative water transpired, water use efficiency, net assimilation rate, mean transpiration rate, harvest index, biomass and yield under the water deficit level were measured from both stressed and well-watered control plants. Water use efficiency significantly increased in both varieties under water stress. Drought stress decreased leaf area duration, cumulative water transpired, net assimilation rate, mean transpiration rate, harvest index, and biomass yield in both varieties studied. Among the varieties, rosea variety showed the best results.     

Phosphorus concentrations in the leaves of defoliated white clover affect abscisic acid formation and transpiration in drying soil

Increased leaf phosphorus (P) concentration improved the water-use efficiency (WUE) and drought tolerance of regularly defoliated white clover plants by decreasing the rate of daily transpiration per unit leaf area in dry soil. Night transpiration was around 17% of the total daily transpiration. The improved control of transpiration in the high-P plants was associated with an increased individual leaf area and WUE that apparently resulted from net photosynthetic assimilation rate being reduced less than the reductions in the transpiration (27% vs 58%). On the other hand, greater transpiration from low-P plants was associated with poor stomatal control of transpirational loss of water, less ABA in the leaves when exposed to dry soil, and thicker and smaller leaf size compared with high-P leaves. The leaf P concentration was positively related with leaf ABA, and negatively with transpiration rates, under dry conditions ( P < 0.001). However, leaf ABA was not closely related to the transpiration rate, suggesting that leaf P concentration has a greater influence than ABA on the transpiration rates.     

Effects of soil water deficit and nitrogen nutrition on water relations and photosynthesis of pot-grown Coffea canephora Pierre

Coffea canephora plants (clone INCAPER-99) were submitted to low N (LN) or high N (HN) applications and two watering regimes (daily irrigation and irrigation every 5 days for a month). Although water potential was not altered significantly by N, HN plants showed higher relative water content than did LN plants under water deficit. Only HN plants exhibited some ability for osmotic adjustment. Plants from both N treatments increased their cell wall rigidity under drought, with a more pronounced augmentation in HN plants. In well-watered plants, carbon assimilation rate increased with increasing N while stomatal conductance did not respond to N supply. Under drought conditions, carbon assimilation decreased by 68-80% compared to well-watered plants, whereas stomatal conductance and transpiration rate declined by 35% irrespective of the N applications. Stable carbon isotope analysis, combined with leaf gas exchange measurements, indicated that regardless of the watering treatments, N increased the long-term water use efficiency through changes in carbon assimilation with little or no effect on stomatal behaviour.     

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short‐rotation bio‐energy culture

The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E_c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E_c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.     

半干旱黄土丘陵区五种植物的生理生态特征比较

通过测定陕北黄土丘陵区5种植物在旱季的光合速率、蒸腾速率和叶水势日变化,将植物划分为不同的水分生态适应类型.结果表明:柳枝稷的光合生理特征属于高光合、低蒸腾和高水分利用效率类型,其抗旱适应性特征属于高水势延迟脱水类型;苜蓿属于高光合高蒸腾低水分利用效率类型,耐旱性为高水势延迟脱水型;达乌里胡枝子属于低光合、低蒸腾、低水分利用效率类型,耐旱性为高水势延迟脱水型;白羊草属于高光合、蒸腾较高的水分利用效率中等型,耐旱性属于能忍耐脱水造成的低水势的一类.沙打旺属于高光合中等蒸腾速率中等水分利用效率类型,耐旱性为低水势延迟脱水型.     

Stable carbon isotopes as indicators of increased water use efficiency and productivity in white spruce (Picea glauca (Moench) Voss) seedlings

The relationship among water use efficiency (WUE), productivity and carbon isotopic composition (δ¹³C) in white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings representing 10 controlled crosses were planted in the field in individual buried sand-filled cylinders. The soil water content in the cylinders was measured using time domain reflectometry over two growing seasons and seedling water use determined by water balance. Two watering treatments were imposed: irrigation and dry land. There was significant (1.6–2.0%c) genetic variation in needle δ¹³C. Ranking of crosses in terms of δ¹³C was generally maintained over watering treatments and there was not a significant genetic versus environmental interaction. There was a positive correlation between δ¹³C and both intrinsic and long-term WUE (more positive δ¹³C with increased WUE) and between δ¹³C and productivity, suggesting a correlation due to variation in photosynthetic capacity. Root to shoot ratios did not increase in water-stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rather than through morphological changes. Our results indicate that it should be possible to use δ¹³C as a surrogate for WUE and to select white spruce genotypes for high WUE without compromising yield.     

水氮处理对岛礁植物假茉莉生理适应性的影响

为了解假茉莉(Clerodendrum inerme)对热带珊瑚岛礁的生理适应性,采用不同浇水频率和氮素处理,对假茉莉的光合特性及抗逆生理生化特性进行了研究。结果表明,不同浇水频率对于假茉莉生理特性影响较大,而不同氮素处理间的差异不大,说明假茉莉对水分胁迫更加敏感。在低浇水频率下,假茉莉叶绿素a含量和叶绿素a/b最高,且与其他浇水频率间的差异显著,净光合速率、气孔导度、水分利用率均显著高于高浇水频率,而蒸腾速率显著低于高浇水频率,表现出较好的光合同化效率和水分利用效率;中等浇水频率的脯氨酸含量最高,为(5.04±1.21)mg/g,可降低植物体的渗透压,保证干旱胁迫下对水分的吸收。低浇水频率的相对电导率最低,为0.17±0.03,可减少干旱胁迫对细胞膜系统的损伤。因此,假茉莉对干旱、贫瘠的热带珊瑚岛礁生境具有很好的适应能力。     

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.     

Phenotypic plasticity and growth temperature: understanding interspecific variability

The subject of this review is the impact of long-term changes in temperature on plant growth and its underlying components. The discussion highlights the extent to which thermal acclimation of metabolism is intrinsically linked to the plasticity of a range of biochemical and morphological traits. The fact that there is often a trade-off between temperature-mediated changes in net assimilation rates (NAR) and biomass allocation [in particular the specific leaf area (SLA)] when plants are grown at different temperatures is also highlighted. Also discussed is the role of temperature-mediated changes in photosynthesis and respiration in determining NAR values. It is shown that in comparisons that do not take phylogeny into account, fast-growing species exhibit greater temperature-dependent changes in RGR, SLA, and NAR than slow-growing plants. For RGR and NAR, such trends are maintained within phylogenetically independent contrasts (i.e. species adapted to more-favourable habitats consistently exhibit greater temperature-mediated changes than their congeneric counterparts adapted to less-favourable habitats). By contrast, SLA was not consistently more thermally plastic in species from favourable habitats. Interestingly, biomass allocation between leaves and roots was consistently more plastic in slow-growing species within individual phylogenetically independent contrasts, when plants were grown under contrasting temperatures. Finally, how interspecific variations in NAR account for an increasing proportion of variability in RGR as growth temperatures decrease is highlighted. Conversely, SLA played a more dominant role in determining interspecific variability in RGR at higher growth temperatures; thus, the importance of SLA in determining interspecific variation in RGR could potentially increase if annual mean temperatures increase in the future.     

Drought tolerance in three hybrid poplar clones submitted to different watering regimes

Aims Poplars grown in North China may experience water-deficient periods in their life cycle. The aim of the present paper was to quantify the response of three clones to different watering regimes and to determine which clone among the three is the best adapted to drought conditions.Methods Three hybrid poplar clones (clone DN-34, R-247 and OP-367) were used in the present experiment. The seedlings of the three clones were grown under four watering regimes: control (well watered, 100% field water capacity (FC)) and three drought treatments (drought stress I, 50% FC; drought stress II, 40% FC; drought stress III, 30% FC). Changes in morphological, physical and biochemical indicators of the three hybrid poplar clones were investigated.Important findings Drought treatment (50%, 40% and 30% FC) decreased net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (gs), shoot height, total biomass and chlorophyll (Chl) content in all the three clones and it increased activities of antioxidant enzymes and free proline content. The highest values of the above-mentioned morphological and physiological parameters were recorded in clone OP-367 under 30% FC, followed by clone DN-34 and R-247. Relative leaf water content (RWC) and stem diameter (sd) markedly declined in clone R-247 and DN-34 under drought stress I, II and III, whereas RWC and sd declined in clone OP-367 only under drought stress II and III. Clone OP-367 had more RWC and sd than DN-34 and R-247. Only the 30% FC induced an increase in the root-to-shoot ratio (rs) and water use efficiency (WUE) in all the three clones. OP-367 was the most efficient clone in water absorption and use, for plants of the clone had the highest values of rs and WUE. Our data demonstrate that among the three clones, OP-367 was better able to maintain photosynthesis and growth and lower the damage caused by drought.     

Variation in relative growth rate of 20 Aegilops species (Poaceae) in the field: The importance of net assimilation rate or specific leaf area depends on the time scale

Field experiments reporting the relative growth rate (RGR) patterns in plants are scarce. In this study, 22 herbaceous species (20 Aegilops species, Amblyopyrum muticum and Triticum aestivum) were grown under field conditions to assess their RGR, and to find out if the differences in RGR amongst species were explained by morphological or physiological traits. Plants were cultivated during two months, and five harvests (every 13–19 days) were carried out. Factors explaining between-species differences in RGR varied, depending on whether short (13–19 days) or longer periods (62 days) were considered. RGR for short periods (4 growth periods of 13–19 days each) showed a positive correlation with net assimilation rate (NAR), but there was no significant correlation with leaf area ratio (LAR) (with the exception of the first growth period). In contrast, when growth was investigated over two months, RGR was positively correlated with morphological traits (LAR, and specific leaf area, SLA), but not with physiological traits (NAR). A possible explanation for these contrasting results is that during short growth periods, NAR exhibited strong variations possibly caused by the variable field conditions, and, consequently NAR mainly determined RGR. In contrast, during a longer growth period (62 days) the importance of NAR was not apparent (there was no significant correlation between RGR and NAR), while allocation traits, such as LAR and SLA, became most relevant.     

Regulation of Transpiration to Improve Crop Water Use

Decreasing fresh water supplies and increasing agricultural drought threaten sustainable worldwide crop production. Consequently, there is a global priority to develop crops with higher water use efficiency (WUE): biomass production or yield per unit of water used. Water use efficiency varies substantially among species and genotypes within a species, and a major effort is now underway to identify the genetic determinants of WUE. Today, it is known that genotypes in primary gene pools exhibit allelic variation for WUE through mechanisms that regulate transpiration, which is the conductance of water through stomata, the cuticle, and the boundary layer. Because of the differential diffusion properties of water and carbon dioxide (CO₂) through these pathways, it is feasible that WUE could be improved by decreasing transpiration without a concomitant reduction in CO₂ uptake. Since CO₂ uptake and transpirational water loss occur predominantly through stomatal pores, it is not surprising that genes involved in stomatal development and stomatal opening/closing impact WUE. Furthermore, loss- and gain-of-function genetic screens have identified genes that regulate transpiration and WUE by yet undetermined mechanisms. This review will discuss the genetic determinants that regulate transpiration and WUE in the context of the modern agricultural goal of improving WUE while sustaining biomass and yield.     

氮对苹果幼树水分利用效率的影响

以２年生盆栽新红星／平邑甜茶苹果树为试材,初步探讨了土壤不同水分状况下氮肥对植株水分利用效率（ＷＵＥ）及有关参数的影响,结果表明,充足供水时,随施Ｎ量的增加,植株ＷＵＥ降低,施Ｎ导致气孔导率（Ｇｓ）增大,对蒸腾（Ｔｒ）的提高幅度大于光合（Ｐｎ）;供水不足时,施Ｎ植株的ＷＵＥ明显高于对照,不同施Ｎ水平的ＷＵＥ表现为：高Ｎ〉中Ｎ〉低Ｎ,ＷＵＥ改善是由于叶肉羧化能力提高,导致光合增强。     

Drought tolerance in the alpine dandelion, Taraxacum ceratophorum (Asteraceae), its exotic congener T. officinale, and interspecific hybrids under natural and experimental conditions

We compared water relations and adaptations to drought stress in native and invasive exotic dandelions, Taraxacum ceratophorum and T. officinale. Photosynthesis (A), transpiration (E), and water use efficiency (WUE; carbon gained/water lost) were measured for the two species under extreme drought in the alpine tundra of Colorado, USA. We also subjected both species and F(1) hybrids to a dry-down experiment to determine how relative physiological performance varied with water availability. Photosynthesis and transpiration in the field were low and did not differ between Taraxacum congeners; however, native T. ceratophorum had higher WUE than T. officinale. After 6 days of greenhouse drought, photosynthesis and transpiration were reduced in T. officinale compared to T. ceratophorum. Taraxacum ceratophorum maintained high WUE under control and drought treatments. Conversely, WUE in T. officinale was highly plastic between watered (low WUE) and dry-down (high WUE) treatments. Hybrids did not exhibit heterosis; instead, they were similar to T. officinale in A and E and intermediate to the parental species in WUE. Overall, results suggest that native dandelions are more drought tolerant than invasive congeners or their hybrids, but have less plasticity in WUE. Arid habitats and occasional drought in mesic sites may provide native dandelions with refugia from negative interactions with invasives.     

Recovery responses of photosynthesis,transpiration, and stomatal conductance in kidney bean following drought stress

Photosynthesis, transpiration, stomatal conductance and chlorophyll fluorescence characteristics were examined in kidney bean plants, with developing gradually water stress for several days after watering and then permitted to recover by re-watering. The photosynthetic rate, transpiration rate, and stomatal conductance decreased rapidly by withholding water for 2 days. The F_v/F_m of chlorophyll fluorescence characteristics slightly decreased when the water was withheld for 7 days. After re-watering the rate of recovery of photosynthesis, transpiration, and stomatal conductance decreased gradually as the days without watering became longer. The differences existed in rates of recovery of photosynthesis, transpiration, and stomatal conductance following drought stress. Among the fractional recoveries the highest was photosynthesis, and the lowest was stomatal conductance. Photosynthesis rate following drought stress was rapidly recovered until 2 days after re-watering, then recovered slowly. The critical time for the recovery of photosynthesis was recognized. The results show clearly a close correlation between the leaf water potential and the recovery level and speed of photosynthesis, transpiration, and stomatal conductance.