Young daughter cladodes affect CO2 uptake by mother cladodes of Opuntia ficus-indica

BACKGROUND AND AIMS: Drought damages cultivated C3, C4 and CAM plants in the semi-arid lands of central Mexico. Drought damage to Opuntia is common when mother cladodes, planted during the dry spring season, develop young daughter cladodes that behave like C3 plants, with daytime stomatal opening and water loss. In contrast, wild Opuntia are less affected because daughter cladodes do not develop on them under extreme drought conditions. The main objective of this work is to evaluate the effects of the number of daughter cladodes on gas exchange parameters of mother cladodes of Opuntia ficus-indica exposed to varying soil water contents. METHODS: Rates of net CO2 uptake, stomatal conductance, intercellular CO2 concentration, chlorophyll content and relative water content were measured in mature mother cladodes with a variable number of daughter cladodes growing in spring under dry and wet conditions. KEY RESULTS: Daily carbon gain by mother cladodes was reduced as the number of daughter cladodes increased to eight, especially during drought. This was accompanied by decreased mother cladode relative water content, suggesting movement of water from mother to daughter cladodes. CO2 assimilation was most affected in phase IV of CAM (late afternoon net CO2 uptake) by the combined effects of daughter cladodes and drought. Rainfall raised the soil water content, decreasing the effects of daughter cladodes on net CO2 uptake by mother cladodes. CONCLUSIONS: Daughter cladodes significantly hasten the effects of drought on mother cladodes by competition for the water supply and thus decrease daily carbon gain by mother cladodes, mainly by inhibiting phase IV of CAM.     

Ecophysiology of a wild platyopuntia exposed to prolonged drought

The effects of reduced annual rainfall over a 7-year period on the daily patterns of gas exchange were analyzed for a wild platyopuntia (Opuntia robusta) growing in rocky soil. In spite of the prolonged water stress, stomata opened at night and net carbon gain was observed for eight 1-day sampling periods during 2000. Daily net CO₂ uptake increased more than five-fold from the end of the dry spring season to the middle of the summer wet season. Curtailment in the formation of both new cladodes and new roots is considered to be an avoidance strategy to water scarcity, which in combination with net CO₂ uptake responses to temperature and relative humidity at night, and perhaps association with arbuscular-mycorrhizal fungi, were the most important adaptations for O. robusta to the prolonged drought conditions that prevailed in its rocky environment. Such adaptations of O. robusta are in addition to the classical physiological and structural modifications of CAM plants to tolerate prolonged drought.     

CO2 Exchange and Growth of the Crassulacean Acid Metabolism Plant Opuntia ficus-indica under Elevated CO2 in Open-Top Chambers

CO2 uptake, water vapor conductance, and biomass production of Opuntia ficus-indica, a Crassulacean acid metabolism species, were studied at CO2 concentrations of 370, 520, and 720 [mu]L L-1 in open-top chambers during a 23-week period. Nine weeks after planting, daily net CO2 uptake for basal cladodes at 520 and 720 [mu]L L-1 of CO2 was 76 and 98% higher, respectively, than at 370 [mu]L L-1. Eight weeks after daughter cladodes emerged, their daily net CO2 uptake was 35 and 49% higher at 520 and 720 [mu]L L-1 of C02, respectively, than at 370 [mu]L L-1. Daily water-use efficiency was 88% higher under elevated CO2 for basal cladodes and 57% higher for daughter cladodes. The daily net CO2 uptake capacity for basal cladodes increased for 4 weeks after planting and then remained fairly constant, whereas for daughter cladodes, it increased with cladode age, became maximal at 8 to 14 weeks, and then declined. The percentage enhancement in daily net CO2 uptake caused by elevated CO2 was greatest initially for basal cladodes and at 8 to 14 weeks for daughter cladodes. The chlorophyll content per unit fresh weight of chlorenchyma for daughter cladodes at 8 weeks was 19 and 62% lower in 520 and 720 [mu]L L-1 of CO2, respectively, compared with 370 [mu]L L-1. Despite the reduced chlorophyll content, plant biomass production during 23 weeks in 520 and 720 [mu]L L-1 of CO2 was 21 and 55% higher, respectively, than at 370 [mu]L L-1. The root dry weight nearly tripled as the C02 concentration was doubled, causing the root/shoot ratio to increase with CO2 concentration. During the 23-week period, elevated CO2 significantly increased CO2 uptake and biomass production of O. ficus-indica.     

Ectomycorrhizas and water relations of trees: a review

There is plenty of evidence for improved nutrient acquisition by ectomycorrhizas in trees; however, their role in water uptake is much less clear. In addition to experiments showing improved performance during drought by mycorrhizal plants, there are several studies showing reduced root hydraulic conductivity and reduced water uptake in mycorrhizal roots. The clearest direct mechanism for increased water uptake is the increased extension growth and absorbing surface area, particularly in fungal species with external mycelium of the long-distance exploration type. Some studies have found increased aquaporin function and, consequently, increased root hydraulic conductivity in ectomycorrhizal plants while other studies showed no effect of ectomycorrhizal associations on root water flow properties. The aquaporin function of the fungal hyphae is also likely to be important for the uptake of water by the ectomycorrhizal plant, but more work needs to be done in this area. The best-known indirect mechanism for mycorrhizal effects on water relations is improved nutrient status of the host. Others include altered carbohydrate assimilation via stomatal function, possibly mediated by changes in growth regulator balance; increased sink strength in mycorrhizal roots; antioxidant metabolism; and changes in osmotic adjustment. None of these possibilities has been sufficiently explored. The mycorrhizal structure may also reduce water movement because of different fine root architecture (thickness), cell wall hydrophobicity or the larger number of membranes that water has to cross on the way from the soil to the xylem. In future studies, pot experiments comparing mycorrhizal and nonmycorrhizal plants will still be useful in studying well-defined physiological details. However, the quantitative importance of ectomycorrhizas for tree water uptake and water relations can only be assessed by field studies using innovative approaches. Hydraulic redistribution can support nutrient uptake during prolonged dry periods. In large trees with deep root systems, it may turn out that the most important function of mycorrhizas during drought is to facilitate nutrient acquisition.     

Nitrate uptake ability by maize roots during and after drought stress

The effects of different intensities and durations of soil drought and re-watering on the nitrate uptake ability of maize roots were studied. Plants were grown in split-root containers with one part of the root system subjected to different intensities and durations of soil drought and re-watering while the other part of the root system was continuously watered to 23% (w/w) soil water content (70% water capacity). Experiments were performed in split-root containers to maintain a high growth rate, thus ensuring high nutrient demand of the shoot irrespective of the soil water regime. To avoid limitation of nitrate uptake by transport processes in the dry soil, and to ensure a uniform ¹⁴N/¹⁵N ratio at the root surface, ¹⁵N was applied to the roots by placing them into an aerated nutrient solution with 0.5 mM Ca(¹⁵NO₃)₂. Shoot elongation and biomass were only slightly affected by drought in one root compartment when the soil in the other root compartment was kept wet. Therefore, the growth-related nutrient demand of the shoot remained at a high level. At moderate levels of soil drought (10% w/w water content) the ability of the roots for N-uptake was not affected even after 10 d of drought. N-uptake ability was reduced to about 20% of the well-watered control only when the soil water content was decreased to 5%. Total soluble sugar content of the roots increased with increasing soil drought, indicating that low N-uptake ability of roots subjected to severe soil drought was not caused by low assimilate supply from the shoot. Nitrate uptake ability of roots maintained in very dry soil (5% soil water content w/w) even for a prolonged period of 8 d, recovered within 3 d following re-watering. Root growth increased one day after re-watering. A short-term experiment with excised roots formerly subjected to severe soil drought showed that nitrate uptake ability recovered in old and young root segments after 2 d of re-watering. Obviously, the increase in N-uptake ability after re-watering was caused not only by new root growth but also by recovery of the uptake ability of formerly stressed roots.     

Effects of shade, drought and daughter cladodes on the CO2 uptake by cladodes of Opuntia ficus-indica

The effects of shade on the physiology of opuntias have received little attention, notwithstanding that shade regularly occurs in both wild stands and cultivated populations. This research evaluates the effects of shade on the physiology of cladodes of Opuntia ficus‐indica, with and without daughter cladodes, as they are exposed to progressive drought. The stress caused by shade, drought and daughter cladodes reduced photosynthesis by mother cladodes and was associated with decreases in relative water content, parenchyma thickness and chlorophyll content. Shade exacerbated the physiological drought of mother cladodes imposed by daughter cladodes and by reduced soil water content.     

Recovery of citrus surface roots following prolonged exposure to dry soil

The effects of prolonged exposure to dry surface soil on the capacity of roots to take up water and phosphorus were examined in mycorrhizal sour orange (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In the first experiment, upper portions of the pots were either irrigated every 2-3 d, droughted for 14 d, droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrigated and fertilized every 2-3 d. Phosphorus uptake capacity was estimated in excised roots using ³²P in aerated 50, 750, and 1500 M P solutions. Exposure to dry soil had no appreciable effect on P uptake capacity. In the second experiment, the ability of intact root to acquire water and P in the 8 d following rewatering after roots were exposed to localized drought for 14 and 43 d was examined. Roots were observed non-destructively using small transparent tubes (2 cm diameter) and a rigid borescope. Soil water depletion was monitored using time-domain reflectrometry. Phosphorus (³²P) was added at various depths in the soil in the upper compartment and uptake was assessed by non-destructively counting beta particle emissions from leaves using a scintillation probe. Similar to the first experiment, localized drought had no effect on P uptake and soil water depletion in citrus roots compared to continuously irrigated plants. Water and P uptake in the first few days apparently occurred from existing roots because of delayed production of new roots in the droughted treatment. Thus, citrus roots exposed to extended periods of dry soil apparently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an overall rooting strategy where essentially no surface roots are shed following prolonged exposure to dry soil.     

Gas exchange of Agropyron desertorum: diurnal patterns and responses to water vapor gradient and temperature

Summary The response of leaf gas exchange to environmental variables were measured at different levels of drought stress for Agropyron desertorum, a naturalized perennial bunchgrass of the semiarid shrub steppes of western North America. Leaf conductance (stomatal plus boundary layer) was more sensitive to changes in water vapor gradient than to changes in leaf temperature. Assimilation was sensitive to both temperature and vapor gradient, and also appeared to be affected by conductance and high transpiration rates. The magnitudes of both assimilation and conductance decreased with increased drought conditions. Diurnal patterns of gas exchange were measured during 3 growing seasons. For a typical spring day with moderate leaf temperature and vapor gradient, diurnal patterns were similar for plants at different levels of soil water availability. Assimilation was relatively constant during most of the day, but conductance decreased during the afternoon. Total daily carbon gain was decreased to a lesser extent than daily water loss as soil water was depleted. Consequently, the ratio of daily carbon gain to daily water loss, i.e. daily water use efficiency, increased with decreased soil water content for diurnals under spring conditions. Diurnal patterns of assimilation for a typical summer day with high leaf temperature and vapor gradient differend from those for a spring day. An afternoon decrease in assimilation was typical during a summer day. Daily carbon gain, water use, and water use efficiency for summer diurnals decreased only under severe drought conditions. Almost complete recovery of assimilation and conductance occurred if leaf microclimate was ameliorated during the afternoon of either spring or summer diurnals. Thus, conditions responsible for a midday depression in assimilation during a single day did not have persistent effects on leaf gas exchange. Daily carbon gain of a typical summer day was restricted by leaf microclimate during the afternoon, but daily water use efficiency was not relatively increased by the amelioration of leaf microclimate.     

Arbuscular mycorrhizal symbiosis modifies the effects of a nitric oxide donor (sodium nitroprusside;SNP) and a nitric oxide synthesis inhibitor (Nω-nitro-L-arginine methyl ester;L-NAME) on lettuce plants under well watered and drought conditions

Arbuscular mycorrhizal (AM) symbiosis is known to help the host plant to overcome environmental stresses as drought by a combination of multiple mechanisms including enhancing of root water uptake capacity. On the other hand, Nitric oxide (NO) is involved in regulating the response of plants to environmental stresses and colonization process of AM fungi. The objective of this research was to study how AM and non-AM lettuce plants responded to a NO donor (sodium nitroprusside; SNP) or to a NO synthesis inhibitor (Nω-nitro-L-arginine methyl ester hydrochloride; L-NAME) under well watered and drought conditions. Most remarkable results were that L-NAME increased the percentage of AM colonized roots under both water regimes and AM plants modified the shoot:root ratio by both chemicals under well watered conditions. Also, the deleterious effects of SNP treatment were partially prevented by AM symbiosis. Moreover, NO could be involved in the diminution of leaf water content under drought conditions, and SNP treatment seems to favor apoplastic water path inside roots. Therefore, different outcomes of relative water content, stomatal conductance and root hydraulic conductivity observed between AM and non-AM plants could be mediated by NO.     

Water uptake and hydraulic redistribution across large woody root systems to 20 m depth

Deep water uptake and hydraulic redistribution (HR) are important processes in many forests, savannas and shrublands. We investigated HR in a semi‐arid woodland above a unique cave system in central Texas to understand how deep root systems facilitate HR. Sap flow was measured in 9 trunks, 47 shallow roots and 12 deep roots of Quercus, Bumelia and Prosopis trees over 12 months. HR was extensive and continuous, involving every tree and 83% of roots, with the total daily volume of HR over a 1 month period estimated to be approximately 22% of daily transpiration. During drought, deep roots at 20 m depth redistributed water to shallow roots (hydraulic lift), while after rain, shallow roots at 0–0.5 m depth redistributed water among other shallow roots (lateral HR). The main driver of HR appeared to be patchy, dry soil near the surface, although water may also have been redistributed to mid‐level depths via deeper lateral roots. Deep roots contributed up to five times more water to transpiration and HR than shallow roots during drought but dramatically reduced their contribution after rain. Our results suggest that deep‐rooted plants are important drivers of water cycling in dry ecosystems and that HR can significantly influence landscape hydrology.     

Arbuscular mycorrhizal fungi and biochar improves drought tolerance in chickpea

Arbuscular mycorrhizal fungi (AMF) inoculation and biochar amendment has been reported to improve growth of several crop plants however their role in stress amelioration individually as well as in combination has not been worked out. This experiment was conducted to evaluate the application of AMF and biochar on the performance of chickpea under drought stress. The treatments included the individual as well as combined treatment of AMF and biochar to drought stressed and normal chickpea plants. Plants inoculation improved growth in terms of shoot and root length, leaf area and number of branches which was observed to show a steep decline due to drought stress. Drought declined the AMF colonization potential though biochar amendment ameliorated the negative effects of drought significantly by improving the spore population, number of mycelium, vesicle and arbuscules and the percentage of colonization as well. Increased chlorophyll synthesis in biochar and AMF treated plants was obvious, which lead to significant enhancement in the net photosynthetic efficiency. Drought stress also declined the relative water content (RWC) and membrane stability index (MSI), while treatment of biochar and AMF either individually or in combination mitigated the deleterious effects to considerable extent and caused a significant enhancement in RWC and MSI under normal conditions. Amendments with biochar and AMF inoculation increased the nitrogen fixation attributes including the number and weight of nodules, leghemoglobin content and activity of nitrate reductase enzyme leading to greater uptake and assimilation of nitrogen in them when compared to drought stressed plants. Drought stressed chickpea plants exhibited considerable reduction in uptake of nitrogen and phosphorous which was ameliorated by biochar and AMF treatments. It could be suggested that increase in growth and physiological attributes in chickpea due to biochar amendments and AMF inoculation under drought stress were plausibly due to their involvement in nitrogen and phosphorous uptake, chlorophyll synthesis and photosynthesis.     

干旱胁迫下AMF对云南蓝果树叶片解剖结构的影响

苯菌灵为杀真菌剂,在土壤含水量为32.32％、29.63％、25.86％、19.39％、12.93％和6.46％的条件下,分别添加苯菌灵和不添加苯菌灵,形成“低AMF”和“高AMF”处理。该研究以云南蓝果树幼苗叶片为材料,利用盆栽试验研究了干旱胁迫下丛枝菌根真菌( AMF)对云南蓝果树幼苗叶片解剖结构及抗旱性的影响。结果表明：添加苯菌灵处理显著降低了不同水分处理条件下AMF侵染率,随着干旱胁迫程度加剧,云南蓝果树幼苗根部的AMF侵染率显著降低。轻度胁迫条件下(土壤含水量为29.63％),叶片解剖结构参数未发生显著变化;土壤含水量低于25.86％,云南蓝果树幼苗表现出较高的抗旱性,苯菌灵处理可以显著影响叶片角质层厚度、栅栏组织厚度和上表皮厚度等7个叶片结构指标,证明了高AMF可以增强代表云南蓝果树幼苗叶片抗旱性的结构性状。土壤含水量为25.86％、19.39％和12.93％时苯菌灵处理的效果较土壤含水量为6.46％时更显著,这是因为6.46％的土壤含水量严重抑制AMF的侵染,说明AMF侵染程度会影响云南蓝果树幼苗的抗旱性。进一步用隶属函数值法对10个叶片性状进行综合评价,发现高AMF处理可增强云南蓝果树幼苗的抗旱性。该研究结果为AMF在濒危物种云南蓝果树保护过程中的合理利用提供了理论依据。     

Water uptake and structural plasticity along roots of a desert succulent during prolonged drought

Desert succulents resume substantial water uptake within 1–2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the monocotyledonous leaf succulent Agave deserti Engelm. substantial root mortality occurred only for lateral roots near the soil surface; nearly all main roots were alive at 180 d of drought. New main roots were initiated and grew up to 320 mm at soil water potentials lower than – 5·0 MPa, utilizing water from the shoot. The hydraulic conductivity of distal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored following 45 d of drought but not after 90 and 180 d. The production of new lateral roots and the renewed apical elongation of main roots occurred 7–11 d after rewetting following 180 d of drought. Hydraulic conductivity was higher in the distal region than at midroot and often increased again near the root base, where many endodermal cells lacked suberin lamellae. Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity along a root allows A. deserti to capitalize on intermittent or heterogeneous supplies of water.     

Water Relations, Diurnal Acidity Changes, and Productivity of a Cultivated Cactus, Opuntia ficus-indica

Physiological responses of the Crassulacean acid metabolism (CAM) plant Opuntia ficus-indica (Cactaceae) were studied on a commercial plantation in central Chile. Young cladodes (flattened stems) and flower buds exhibited daytime stomatal opening, whereas mature cladodes and fruit exhibited the nocturnal stomatal opening characteristic of CAM plants. Severe water stress suppressed the nocturnal stomatal opening by mature cladodes, but their high water vapor conductance occurring near dawn was not affected. Nocturnal acidity increases were not as sensitive to water stress as was the nocturnal stomatal opening. The magnitude of the nocturnal acidity increases depended on the total daily photosynthetically active radiation (PAR), being 90% PAR-saturated at 27 moles per square meter per day for a mean nighttime air temperature of 5°C and at 20 moles per square meter per day for 18°C. Inasmuch as the PAR received on unshaded vertical surfaces averaged about 21 moles per square meter per day, nocturnal acidity increases by the cladodes were on the verge of being PAR-limited in the field. The net assimilation rate, which was positive throughout the year, annually averaged 3.4 grams per square meter per day for 1.0- and 2.0-year-old plants. Plants that were 5.4 years old had 7.2 square meters of cladode surface area (both sides) and an annual dry weight productivity of 13 megagrams (metric tons) per hectare per year when their ground cover was 32%. This substantial productivity for a CAM plant was accompanied by the highest nocturnal acidity increase so far observed in the field, 0.78 mole H⁺ per square meter.     

Ectomycorrhiza affect architecture and nitrogen partitioning of beech (Fagus sylvatica L.) seedlings under shade and drought

The aim of this study was to investigate the influence of ectomycorrhizal fungi (EMF) on the architecture of and nitrogen (N) partitioning in young beech (Fagus sylvatica) plants in response to different light regimes and water deprivation. We hypothesized that EMF modify biomass partitioning and architecture of young beech plants by increased N uptake in comparison with non-mycorrhizal (NM) plants and that therefore, the drought responses of EM and NM plants diverge. We anticipated that full light-exposed plants were more drought tolerant due to improved water status and nutrition, whereas shade-acclimated EM plants were more drought susceptible because of decreased mycorrhizal colonization. To test these hypotheses seedlings were grown in native or sterilized forest soil. To avoid effects of soil pretreatment NM and EM plants were transplanted into sand-peat culture systems and exposed to shade, drought or the combination of both factors. Shade resulted in reduced root biomass production decreasing the root-to-shoot ratio. Mild drought stress (pre-dawn water potential [Ψ_pd] = −1.3 MPa) did not affect biomass partitioning. EMF colonization did not increase plant biomass, but had strong effects on root architecture: the numbers of root tips as well as the absolute and specific root lengths were increased because of formation of thin roots, especially in the diameter classes from 0.2 to 0.8 mm. In contrast to our expectation N uptake of well irrigated EM plants was not increased despite their larger potential for soil exploitation. Overall, EM plants exhibited higher amounts of carbon fixed per unit of N taken up than NM plants and shifted N partitioning towards the roots. Beneficial effects of EMFs were apparent under mild drought but the responses differed depending on the light availability: shaded EM plants showed a delay in the decrease of Ψ_pd; light exposed EM plants showed increased N uptake compared with NM beeches. These results indicate that EMFs are involved in mediating divergent responses of beech to drought depending on the light availability.     

地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响

干旱区因降水稀少,地下水成为荒漠植被重要且稳定的水源。选取古尔班通古特沙漠南缘建群种植物梭梭（Haloxylon ammodendron）为研究对象,通过测量不同地下水埋深（3.45、9.08、10.47、13.27 m和15.91 m）下生长季前期和后期同化枝生理生化指标（黎明水势、正午水势、含水量、氯离子、钠离子、脯氨酸和非结构性碳水化合物）和生长与形态特征（生长速率和胡伯尔值）,旨在认识荒漠植物对地下水埋深增加和季节性干旱的响应特征和调节适应机制。结果表明：（1）梭梭应对地下水埋深变化的生理调节对策,是采取先降低后升高黎明前同化枝水势、降低新枝形成期同化枝生长速率、增大胡伯尔值和积累非结构性碳水化合物的策略;（2）梭梭应对生长季大气干旱的生理调节对策,是通过降低黎明前同化枝水势、维持较高胡伯尔值、积累钠离子和消耗淀粉抵御季节性干旱;（3）在大气干旱与地下水水文干旱交互作用下,梭梭是采取降低正午同化枝水势、维持较高的同化枝含水量和积累可溶性糖的生态策略。综上所述,梭梭在响应地下水水文干旱和季节性大气干旱的生理特征间存在差异。研究结果丰富了水文和大气干旱对梭梭生理和生长影响的认知,可以为基于地下水资源管理的干旱区荒漠植被保育提供参考。     

Carbon and water balances for young fruits of platyopuntias

Questions relating to transpired versus retained water for fruits, the xylem versus the phloem as water supplier to the fruits, and the importance of fruit photosynthesis for fruit dry mass gain were examined in the field for 6 species of platyopuntias ( Nopalea cochenillifera , Opuntia ficus-indica , O. megacantha , O. robusta , O. streptacantha and O. undulata ), cacti with flattened stem segments (cladodes). For plants with fruits midway between floral bud appearance and fruit maturation, transpiration was greater at night for the cladodes, as expected for Crassulacean acid metabolism (CAM) plants, but greater during the daytime for the fruits of all 6 species. Nevertheless, net CO₂ uptake by fruits of these platyopuntias occurred predominantly at night, as expected for CAM plants. The water potential of the young fruits (average of −0.41 MPa) was higher than that of the cladodes (average of −0.60 MPa), indicating that water entered the fruits via the phloem rather than via the xylem. Solution entry into the fruits via the phloem supplied the water lost by transpiration and allowed for increases in fruit fresh mass (daily transpiration averaged 3.2-fold higher than daily water content increases), while the accumulating solutes were apparently polymerized to account for the higher water potentials of the fruits compared with the cladodes. The phloem thus acts as the sole supplier of water and the main supplier of dry mass (90%) to such young fruits of platyopuntias.     

Effects of homobrassinolide application on plant metabolism and grain yield under irrigated and moisture-stress conditions of two wheat varieties

Homobrassinolide (BR) was applied either as a seed treatment or foliar spray to two contrasting wheat varieties, viz. C306 (drought tolerant) and HD2329 (drought susceptible), to examine its effects on plant metabolism and grain yield under irrigated and moisture-stress/rainfed conditions. BR application resulted in increased relative water content, nitrate reductase activity, chlorophyll content and photosynthesis under both conditions. BR application also improved membrane stability (lower injury). These beneficial effects resulted in higher leaf area, biomass production, grain yield and yield related parameters in the treated plants. All the treatments were significantly better than the untreated control. Generally, 0.05 ppm either as a seed treatment or foliar spray was more effective than the 0.01 ppm treatment. The drought-tolerant genotype C306 showed more response to BR application under moisture-stress/rainfed condition than HD 2329. Increased water uptake, membrane stability and higher carbon dioxide and nitrogen assimilation rates under stress seemed to be related to homobrassinolide-induced drought tolerance.     

Gas exchange and growth responses to elevated CO2 and light levels in the CAM species Opuntia ficus-indica

Gas exchange and dry-weight production in Opuntia ficus-indica, a CAM species cultivated worldwide for its fruit and cladodes, were studied in 370 and 750 μmol mol⁻¹ CO₂ at three photosynthetic photon flux densities (PPFD: 5, 13 and 20 mol m⁻² d⁻¹). Elevated CO₂ and PPFD enhanced the growth of basal cladodes and roots during the 12-week study. A rise in the PPFD increased the growth of daughter cladodes; elevated CO₂ enhanced the growth of first-daughter cladodes but decreased the growth of the second-daughter cladodes produced on them. CO₂ enrichment enhanced daily net CO₂ uptake during the initial 8 weeks after planting for both basal and first-daughter cladodes. Water vapour conductance was 9 to 15% lower in 750 than in 370 μmol mol⁻¹ CO₂. Cladode chlorophyll content was lower in elevated CO₂ and at higher PPFD. Soluble sugar and starch contents increased with time and were higher in elevated CO₂ and at higher PPFD. The total plant nitrogen content was lower in elevated CO₂. The effect of elevated CO₂ on net CO₂ uptake disappeared at 12 weeks after planting, possibly due to acclimation or feedback inhibition, which in turn could reflect decreases in the sink strength of roots. Despite this decreased effect on net CO₂ uptake, the total plant dry weight at 12 weeks averaged 32% higher in 750 than in 370 μmol mol⁻¹ CO₂. Averaged for the two CO₂ treatments, the total plant dry weight increased by 66% from low to medium PPFD and by 37% from medium to high PPFD.     

Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited

There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.