24-Epibrassinolide alleviates drought effects in young Carapa guianensis plants,improving the hydraulic safety margin,gas exchange and antioxidant defence

• Climate change is increasing the frequency of extreme events such as droughts, limiting plant growth and productivity. Exogenous application of plant growth regulators, such as 24-epibrassinolide (EBR), might be a solution as this molecule is organic, eco-friendly, and biodegradable. This is the first research to examine possible roles of EBR on the hydraulic safety margin, physiological behaviour, and metabolism in Carapa guianensis Aubl. (Meliaceae) exposed to drought. C. guianensis is a widely distributed tree in tropical forests of the Amazon.
• The objective was to determine whether EBR can improve tolerance to water deficit in young C. guianensis by measuring hydraulic traits, nutritional, biochemical and physiological responses, and biomass. The experiment had four randomized treatments: two water conditions (control and water deficit) and two concentrations of EBR (0 and 100 nM EBR).
• EBR increased the water potential and hydraulic safety margin, increased CO₂ fixation, and improved stomatal performance. EBR also stimulated antioxidant defences (SOD, CAT, APX, and POX).
• Overall, tretreatment with EBR improved drought tolerance of young C. guianensis plants.

     

Thermonastic leaf movements: a synthesis of research with Rhododendron

Thermonastic leaf movements in Rhododendron L. occur in response to freezing temperatures. These movements are composed of leaf curling and leaf angle changes that are distinct leaf movements with different responses to climatic factors. Leaf angle is controlled by the hydration of the petiole, as affected by soil water content, atmospheric vapour pressure, and air temperature. In contrast, leaf curling is a specific response to leaf temperature, and bulk leaf hydration has little effect. The physiological cause of leaf curling is not well understood, but the mechanism must lie in the physiology of the cell wall and/or regional changes in tissue hydration. Available evidence suggests that intercellular freezing is not a cause of leaf curling.
Manipulation experiments demonstrate that changes in leaf orientation in Rhododendron most likely serve to protect the leaves from membrane damage due to high irradiance and cold temperatures. In particular, the pendent leaves protect the chloroplast from photoinhibition. Leaf curling may serve to slow the rate of thaw following freezing, a common phenomenon in the Appalachian mountains of the U.S. The thermonastic leaf movements have a greater importance to plants in a dim environment because the potential impact to canopy carbon gain is greater than in high light environments.
These leaf movements have several implications for horticultural management. There seems to be a trade-off between water stress tolerance and freezing stress tolerance by leaf movements. Thermonastic leaf movements may be a major mechanism of cold stress tolerance in Rhododendron species. The actual physiological cause of leaf movement has not been elucidated and many more species need to be evaluated to verify the general importance of leaf movements to Rhododendron ecology and evolution.     

Interaction of drought and high temperature on photosynthesis and grain-filling of wheat

Drought and high temperature often occur simultaneously, but their effects on crops are usually investigated individually. Our objective was to compare effects of drought, high temperature, and their interactions on photosynthesis and grain-growth of wheat (Triticum aestivum L.). Plants (cv. Len) were grown uniformly in well-watered soil at 25/20 ± 2 °C day/night until anthesis, when they were subjected to regimes of no drought (soil at field capacity) and drought (plant water potential of –.0 to –2.4 MPa) at 15/10, 25/20, and 35/30 °C in controlled environments until physiological maturity. Drought decreased photosynthesis, stomatal conductance, viable leaf area, shoot and grain mass, and weight and soluble sugar content of kernels but increased plant water-use efficiency. High temperature hastened the decline in photosynthesis and leaf area, decreased shoot and grain mass as well as weight and sugar content of kernels, and reduced water-use efficiency. Interactions between the two stresses were pronounced, and consequences of drought on all physiological parameters were more severe at high temperature than low temperature. The synergistic interactions indicated that productivity of wheat is reduced considerably more by the combined stresses than by either stress alone, and that much of the effect is on photosynthetic processes.     

华山新麦草光合特性对干旱胁迫的响应

以濒危植物华山新麦草为材料进行盆栽试验,设置3个水分梯度,研究生长指标、气体交换、叶绿素荧光参数、光合色素含量、光响应特征及丙二醛(MDA)含量.结果表明:随干旱胁迫的加剧,株高、叶宽和光合色素含量降低,根长和丙二醛(MDA)含量增加;水分胁迫导致净光合速率(PN)、气孔导度(gs)、蒸腾速率(E)、最大荧光(Fm)、光合电子传递速率(ETR)和光化学猝灭系数(qP)降低,胞间CO2浓度(Ci)、瞬时水分利用效率(WUE)、初始荧光(Fo)和非光化学淬灭系数(NPQ)升高;光系统Ⅱ最大光化学效率(Fv/Fm)不变;干旱胁迫下光响应曲线拟合结果显示,干旱胁迫造成最大光合速率和光能利用效率下降.综上表明,干旱对光系统Ⅱ的伤害是制约华山新麦草光合作用的主要原因.     

Cold hardiness and water relations parameters in Rhododendron cv. Catawbiense Boursault subjected to drought episodes

We determined whether increase in cold hardiness of Rhododendron cv. Catawbiense Boursault induced by water stress was correlated with changes in tissue water relations. Water content of the growing medium was either maintained near field capacity for the duration of the study or plants were subjected to drought episodes at different times between 15 July and 19 February. Watering during a drought episode was delayed until soil water content decreased below 0.4 m³ m⁻³ then watering was resumed at a level to maintain soil water content between 0.3 and 0.4 m³ m⁻³. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves. Water relations parameters were determined using pressure-volume analysis. Exposure to drought episodes increased cold hardiness during the cold acclimation stage in late summer and fall but not during the winter. When water-stressed plants were re-watered to field capacity, the previous gain in cold hardiness gradually disappeared. Water relations parameters correlating with seasonal changes of cold hardiness included dry matter content (r =−0.67). apoplastic water content (r =−0.60), and water potential at the turgor loss point (r = 0.40). Changes of cold hardiness in water-stressed plants in reference to well-watered plants were correlated with changes of all water relations parameters, except for osmotic potential at full turgor (r = 0.13). It is proposed that water stress reduced the hydration of cell walls, thereby increasing their rigidity. Increased rigidity of cell walls could result in a development of greater negative turgor pressures at subfreezing temperatures and therefore increased resistance to freeze dehydration.     

Seasonal evolution of diffusional limitations and photosynthetic capacity in olive under drought

This study tests the hypothesis that diffusional limitation of photosynthesis, rather than light, determines the distribution of photosynthetic capacity in olive leaves under drought conditions. The crowns of four olive trees growing in an orchard were divided into two sectors: one sector absorbed most of the radiation early in the morning (MS) while the other absorbed most in the afternoon (AS). When the peak of radiation absorption was higher in MS, air vapour pressure deficit (VPD) was not high enough to provoke stomatal closure. In contrast, peak radiation absorption in AS coincided with the daily peak in VPD. In addition, two soil water treatments were evaluated: irrigated trees (I) and non-irrigated trees (nI). The seasonal evolution of leaf water potential, leaf gas exchange and photosynthetic capacity were measured throughout the tree crowns in spring and summer. Results showed that stomatal conductance was reduced in nI trees in summer as a consequence of soil water stress, which limited their net assimilation rate. Olive leaves displayed isohydric behaviour and no important differences in the diurnal course of leaf water potentials among treatments and sectors were found. Seasonal diffusional limitation of photosynthesis was mainly increased in nI trees, especially as a result of stomatal limitation, although mesophyll conductance (g(m)) was found to decrease in summer in both treatments and sectors. A positive relationship between leaf nitrogen content with both leaf photosynthetic capacity and the daily integrated quantum flux density was found in spring, but not in summer. The relationship between photosynthetic capacity and g(m) was curvilinear. Leaf temperature also affected to g(m) with an optimum temperature at 29 degrees C. AS showed larger biochemical limitation than MS in August in both treatments. All these suggest that both diffusional limitation and the effect of leaf temperature could be involved in the seasonal reduction of photosynthetic capacity of olive leaves. This work highlights the need for models of plant growth and ecosystem function to incorporate new parameters affecting the distribution of photosynthetic capacity in canopies.     

Effect of drought and rewatering on photosynthetic physioecological characteristics of soybean

Field experiments were conducted on soybean Glycine max, yudou29, a major cultivated variety in the Henan Province of China to study the relationship between photosynthetic characteristics and other physioecological parameters of its leaves under soil drying and rewatering treatments. The study showed that the dawn water potential of soybean leaves under the drying treatment was very close to that of soybean leaves under well-watered treatments (CK) when soil water content was higher than 47% of field water capacity (FWC). But when soil water content dropped below 47% of FWC, the leaf water potential decreased rapidly, indicating a significant threshold reaction. The dawn water potential threshold of soybean leaves was about ?1.02 MPa. Below this, the leaf water potential and net photosynthesis ratio dropped rapidly. When the soil water content was 47%, the leaf water potential and net photosynthesis ratio were nearly as high as those in CK, but the transpiration ratio was 67% lower, indicating that transpiration was more sensitive to drought than photosynthesis. After rewatering, the water status of soybean leaves improved, the net photosynthesis ratio and transpiration ratio increased linearly, and leaf stomata conductance (Gs) also recovered quickly. These results showed that after stress removal, soybean had fast-growing characteristics.     

前期水淹对牛鞭草后期干旱胁迫光合生理响应的影响

水淹和干旱是限制植物生长的两种主要环境因子。三峡库区消落带由于其特殊的地形条件和人工水文节律,呈现以年度为周期的“水淹-落干”交替变化的水文变动特征,在消落带生长的植物因此受到水淹和干旱交替胁迫的双重影响。为了探究库区蓄水对消落带植被干旱耐受性的影响,以当年生牛鞭草扦插苗为试验对象,设置对照组(CK)、表土水淹组(SF)、全淹组(TF)、对照-干旱组(CD)、表土水淹-干旱组(SFD)、全淹-干旱组(TFD)6个处理组,研究不同水分处理对牛鞭草光合特性的影响。结果表明:(1)水淹和干旱胁迫均对牛鞭草光合特性造成显著影响;(2)水淹胁迫阶段,与CK组相比,牛鞭草SF和TF组净光合速率、气孔限制值和水分利用效率显著下降,胞间CO_2浓度显著上升;(3)干旱胁迫阶段,牛鞭草CD和SFD组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等光合参数显著低于CK组,TFD组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等指标与CK组无显著差异;(4)复水阶段,各处理组净光合速率、气孔导度、胞间CO_2浓度和蒸腾速率等指标均与CK组无显著差异。研究表明,前期水淹并未增加牛鞭草对后期干旱胁迫的敏感性,牛鞭草对水淹和干旱胁迫均具有较好的耐受性,有助于牛鞭草对库区消落带生境变化的适应性。     

Photochemical response to drought acclimation in two sunflower genotypes

The effects of drought acclimation on CO₂ assimilation and light utilization were investigated in two sunflower genotypes ( Helianthus annuus L., T32 and Viki) in relation to water deficit and/or high light conditions. Drought interaction with PSII efficiency was observed in the genotype T32 with a sustained decrease in the potential photochemical efficiency of PSII, F_n/F_m. In response to drought acclimation, T32 displayed some tendency to accumulate closed PSII traps (higher value of 1-qp) without an enhancement of thermal deactivation (Stem-Volmer non-photochemical quenching, NPQ). Irrespective of the growth conditions (growth chamber or greenhouse), only Viki was responsive to drought acclimation, with (1) increased net photosynthesis in well-watered plants, (2) higher maintenance of photochemical electron transfer under water deficit and/or high light, (3) limited PSII inactivation (lower value of 1-qp) through increased non-photochemical energy dissipation (Stern-Volmer NPQ) which was readily reversible even at low leaf water potentials, and (4) higher F_v/F_m recovery after high light treatment. Additionally, drought acclimation delayed turgor loss during subsequent water stress in Viki. Thus, the response to drought acclimation, with an adjustment of water relations and of energy utilization by PSII, was observed under both growth conditions and was mainly genotype dependent.     

干旱和复水对大豆光合生理生态特性的影响

选用大豆作为实验材料,研究干旱和复水对大豆光合生理的影响,以期为大豆抗旱栽培和高效利用水分提供理论依据.通过研究发现,在土壤相对含水量高于47%时,处理组大豆凌晨叶片水势和对照组相比基本没有下降,但当土壤相对含水量低于47%时,处理组叶片水势急剧下降,表现为一定的阈值反应,存在明显的凌晨叶水势临界值.大豆开花前期叶片的凌晨叶水势阈值约为-1.02MPa,低于此临界值,叶片水势急剧下降,叶片净光合速率也明显降低.研究发现,在实验的第3天,处理组土壤相对含水量为47%,叶片水势与对照组相比下降了7%,蒸腾速率为对照组的67%,净光合速率为对照组的90%,水分利用效率比对照组高35%,这说明大豆的蒸腾比光合对干旱更敏感.因此,可利用这一结果采取适度干旱等措施达到节水增产的目的.复水后大豆叶片水分状况得到改善,大豆叶片的净光合速率和蒸腾速率都表现为接近于直线的上升,气孔导度的恢复也很快,这表明大豆存在着胁迫解除后快速生长的特征.但是,干旱对大豆的生长等生理过程是否存在滞后效应,滞后效应的大小等问题还需要进一步的研究.     

Sequence of drought response of maize seedlings in drying soil

Leaf elongation in monocotyledonous plants is sensitive to drought. To better understand the sequence of events in plants subjected to soil drying, leaf elongation and transpiration of maize seedlings ( Zea mays L.) of 4 cultivars were monitored continuously and the diurnal courses of the root and leaf water relations were determined. Results from this study indicate the following sequence of drought response: Leaf elongation decreased before changes in the leaf water relations of non‐growing zones of leaf blades were detected and before transpiration decreased. Reductions in leaf elongation preceded changes in the root water potential (ψ_w). Root ψ_w was not a very sensitive indicator of soil dryness, whereas the root osmotic potential (ψ_s) and root turgor (ψ_p) were more sensitive indicators. The earliest events observed in drying soil were a significant increase in the largest root diameter class (1 720 to 1 960 gm) and a decrease in leaf elongation ( P = 0.08) 2 days after withholding water. Significant increases in root length were observed 2 days later. Soil drying increased the number of fine roots with diameters of <240 µm. Slight increases in soil strength did not affect leaf elongation in the drying soil.     

The contribution of drought-related decreases in foliar nitrogen concentration to decreases in photosynthetic capacity during and after drought in prairie grasses

While stomatal closure usually limits photosynthesis during drought, our previous results suggest that drought-related decreases in foliar nitrogen concentration (N_L) limit photosynthesis during recovery from drought in prairie grasses. Here we estimate the importance of decreases in N_L to decreased photosynthetic capacity (PS_cap) during drought and a subsequent recovery period in three perennial C₄ prairie grasses. PS_cap (O₂ evolution at light and CO₂ saturation) decreased 69 to 78% during drought in these grasses, and full recovery of PS_cap required 8 to 12 days, until younger leaves were expanded or older leaves were repaired, depending on species. Decreases in N_L explained 38 to 51% of the loss of PS_cap during drought and accounted for 51 to 69% of the total loss of PS_cap integrated over the post-drought recovery period. N-related loss of PS_cap appeared to result more from decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (4.1.1.31), and other soluble photosynthetic enzymes, than from decreases in thylakoid N-containing compounds. Decreases in quantum yield of O₂ evolution and F_v/F_m (variable-to-maximum fluorescence of dark-adapted leaves) during drought were small, so we assumed that little damage to photsystem II (PSII) and thylakoid membrane function occurred. Further, F₀ (minimum F) decreased or remained unchanged, dark F₀ was greater than light F₀, and decreases in photochemical quenching (the fraction of oxidized PSII) were reversed within 1–3 days after drought. Therefore, prolonged increases in non-photochemical quenching (q_n; thermal dissipation of excess light energy) during and after drought were indicative of protective downregulation and were likely associated with disproportionate loss of soluble photosynthetic proteins during drought. In support of this, post-drought recovery of q_n paralleled recovery of N_L and PS_cap. Thus, in C₄ prairie grasses, loss of PS_cap during drought is largely the result of decreases in shoot N_L and of associated protective downregulation, decreasing carbon assimilation for 1–2 weeks after drought.     

Do photosynthetic metabolism and habitat influence foliar water uptake in orchids?

• Epiphytic and rupicolous plants inhabit environments with limited water resources. Such plants commonly use Crassulacean Acid Metabolism (CAM), a photosynthetic pathway that accumulates organic acids in cell vacuoles at night, so reducing their leaf water potential and favouring water absorption. Foliar water uptake (FWU) aids plant survival during drought events in environments with high water deficits. We hypothesized that FWU represents a strategy employed by epiphytic and rupicolous orchids for water acquisition and that CAM will favour increased water absorption.
• We examined 6 epiphyte, 4 terrestrial and 6 rupicolous orchids that use C3 (n = 9) or CAM (n = 7) pathways. Five individuals per species were used to evaluate FWU, structural characteristics and leaf water balance.
• Rupicolous species with C3 metabolism had higher FWU than other species. FWU (C_max and k) could be related to succulence, SLM and leaf RWC. The results indicated that high orchid leaf densities favoured FWU, as area available for water storage increases with leaf density. Structural characteristics linked to water storage (e.g. high RWC, succulence), on the other hand, could limit leaf water absorption by favouring high internal leaf water potentials.
• Epiphytic, rupicolous and terrestrial orchids showed FWU. Rupicolous species had high levels of FWU, probably through absorption from mist. However, succulence in plants with CAM appears to mitigate FWU.

     

Ameliorating effects of biochar on photosynthetic efficiency and antioxidant defence of Phragmites karka under drought stress

• Biochar (BC) has been reported to improve growth and drought resistance in many plants. However, adequate information on the drought resistance mechanism mediated of BC on Phragmites karka, a bioenergy plant, is not available.
• The impact of BC addition (0%, 0.75% and 2.5%) on plant growth and physiology of P. karka under drought was assessed.
• Soil water‐holding capacity and soil water content were significantly improved with 0.75% BC as compared with the un‐amended controls.
• This resulted in improved plant performance under drought conditions. An increase of parameters, such as plant fresh and dry biomass, root to shoot ratio and root mass fraction, was paralleled by an increase of chlorophyll content, net photosynthesis rate and water use efficiency of plants. Plants treated with 0.75% BC experienced less oxidative stress due to higher photosystem II efficiency and stimulated activity of antioxidant defense systems.
• Our results demonstrate that soil amendment with 0.75% BC allow the potential energy plant P. karka to grow in an arid habitat.

     

Cold storage to overcome dormancy affects the carbohydrate status and photosynthetic capacity of Rhododendron simsii

Global warming leads to increasing irregular and unexpected warm spells during autumn, and therefore natural chilling requirements to break dormancy are at risk. Controlled cold treatment can provide an answer to this problem. Nevertheless, artificial cold treatment will have consequences for carbon reserves and photosynthesis. In this paper, the effect of dark cold storage at 7 °C to break flower bud dormancy in the evergreen Rhododendron simsii was quantified. Carbohydrate and starch content in leaves and flower buds of an early (‘Nordlicht’), semi‐early (‘M. Marie’) and late (‘Mw. G. Kint’) flowering cultivar showed that carbon loss due to respiration was lowest in ‘M. Marie’, while ‘Mw. G. Kint’ was completely depleted of starch reserves at the end of cold treatment. Gene isolation resulted in a candidate gene for sucrose synthase (SUS) RsSus, which appears to be homologous to AtSus3 and had a clear increase in expression in leaves during cold treatment. Photosynthesis measurements on ‘Nordlicht’ and the late‐flowering cultivar ‘Thesla’ showed that during cold treatment, dark respiration decreased 58% and 63%, respectively. Immediately after cold treatment, dark respiration increased and stabilised after 3 days. The light compensation point followed the same trend as dark respiration. Quantum efficiency showed no significant changes during the first days after cold treatment, but was significantly higher than in plants with dormant flower buds at the start of cold treatment. In conclusion, photosynthesis stabilised 3 days after cold treatment and was improved compared to the level before cold treatment.     

The impact of subcanopy light environment on the hydraulic vulnerability of Rhododendron maximum to freeze-thaw cycles and drought

The vulnerability of xylem to embolism development in Rhododendron maximum L., an evergreen diffuse-porous shrub, was investigated in relation to the frequency of winter freeze–thaw cycles in high and low light sites of the Eastern US. Though the frequency of freeze–thaw cycles during the winter was lower in North Carolina than in Virginia, the hydraulic conductivity of 3-year-old branches was reduced by up to 60% by winter embolism development in North Carolina compared to less than 30% in Virginia. Generally, small vessel diameters and volumes were associated with a significant resistance to embolism formation resulting from repeated freeze–thaws of xylem sap. In stems grown in high light sites (gaps), larger vessel volumes, and greater diameter growth of stems were associated with a significantly higher degree of freeze–thaw embolism development than in those grown in the low light sites. Thus, the growth patterns of R. maximum stems, under conditions of higher light availability, rendered them more susceptible to freeze–thaw-induced embolisms. Vulnerability to drought-induced embolism in stems was not affected by light environment. Rhododendron maximum was relatively sensitive to drought-induced embolism because 50% loss of hydraulic conductivity occurred at a water potential of -2.2 MPa. The distribution and gas exchange of R. maximum are constrained by the dual effects of freeze-thaw cycles and drought on vascular function.     

Differences in the response of sunflower (Helianthus annuus) subjected to flooding and drought stress

Wample, R. L. and Thornton, R. K. 1984. Differences in the response of sunflower ( Helianthus annuus ) subjected to flooding and drought stress.
Comparison of drought- and flood-stressed sunflower plants ( Helianthus annuus L. hybrid 894) showed some similarities in response but differences in the mechanisms responsible for the responses to stress. Drought–stressed plants showed typical reductions in leaf water potential with increasing stress accompanied by increased leaf resistance. Photosynthesis declined while photorespiration increased after 48 and 96 h of drought stress. A primary reason for reduced photosynthesis in drought-stressed plants was increased stomatal resistance. No significant 0change in leaf water potential or in leaf resistance in flooded plants was found in this study. However, photosynthesis declined in a manner similar to that in drought-stressed plants and photorespiration showed only a transient increase at 48 h. Dark respiration was significantly higher at 48 and 96 h but the magnitude of the increase cannot account for the reduction in photosynthesis. Since the photosynthetic rate of flooded plants declined while stomata remained open, an effect at a more fundamental level is suggested and is thought to be related to disruption of carbohydrate transport.     

Rose leaf elasticity changes in response to mycorrhizal colonization and drought acclimation

Tissue elasticity can affect plant response to drought, in terms of turgor maintenance and water uptake from drying soils. The purpose of this study was to determine the effect of mycorrhizal colonization and drought acclimation on rose ( Rosa hybrida L. cv. Samantha) leaf elasticity. Bulk elasticity was characterized by the pressurevolume method using plots of the elastic modulus as a function of leaf turgor pressure, total water potential and relative water content. The treatments, arranged in a 2 × 3 factorial design, included acclimated and unacclimated plants, and either Glomus irararadices Schenck and Smith, Glomus deserticola Trappe, Bloss and Menge, or a non-mycorrhizal control. Plants with root mycorrhizal colonization showed reduced leaf elasticity (i.e. higher elastic moduli) over a broad range of leaf waler potential and water content. Both mycorrbizal colonization and acclimation facilitated the maintenance of positive values of turgor and elasticity at lower leaf water potential and water content than in controls. Mycorrhizal infections may aid plants in acclimating to water deficits through effects on leaf tissue elasticity.     

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Despite mounting evidence showing that C₄ plants can accumulate more biomass at elevated CO₂ partial pressure (p(CO₂)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C₄ plants to elevated p(CO₂) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO₂) can stimulate the growth of both well-watered and water-stressed C₄ plants. First, through enhanced leaf CO₂ assimilation rates due to increased intercellular p(CO₂). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO₂ assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO₂ fixation in the bundle sheath or the presence of C₃-like photosynthesis in young C₄ leaves are unlikely explanations for the high CO₂-responsiveness of C₄ photosynthesis. The interactions between elevated p(CO₂), leaf temperature and shoot water relations on the growth and photosynthesis of C₄ plants are identified as key areas needing urgent research.