Reduction In Leaf Water Potential Research Articles

Transcriptomic responses to drought stress in the Patagonian southern beech Nothofagus alpina

BackgroundDeciphering the genetic architecture of drought tolerance could allow the candidate genes identification responding to water stress. In the Andean Patagonian forest, the genus Nothofagus represents an ecologically relevant species to be included in different genomic studies. These studies are scarce in South American ecosystems however represent an important source of genomic data in order to interpret future climate-change environment scenarios of these emblematic forests. Here, we achieved the assemblage of the transcriptome of N. alpina while searching for key genes of activated or suppressed metabolic pathways in response to drought stress.ResultsDe novo transcriptome assembly resulted in 104,030 transcripts. Following confirmation of drought conditions, based on reduction of leaf water potential and stomatal conductance, a differential gene expression analysis resulted in 2720 significantly expressed genes (1601 up-regulated and 1119 down-regulated). Enrichment analysis (over-representation analysis and gene set enrichment analysis) resulted in more than one hundred stress-responsive term ontologies (i.e. biological processes) and pathways. Terms such as response to abscisic acid and pathways such as plant hormone signal transduction or starch and sucrose metabolism were over-represented. Protein–protein interaction assessment resulted in networks with significantly expressed top common hub gene clusters (e.g. plant-type cell wall biogenesis among down-regulated or ABA-signalling among up-regulated). These networks evidenced important regulators at gene expression such as transcriptional factors.ConclusionsResponses of N. alpina seedlings to drought stress were evidenced by the activation of several genes linked to GO biological processes and KEGG pathways, which were mainly based on over-expression of specific protein kinases, phosphatases, synthases and transcription factors. This suggests an up-regulation of signalling pathways, triggered through plant hormones such as abscisic acid or auxin, which could counteract the osmotic stress created as a probable immediate response to drought. On the other hand, groups of carbon fixation genes related to the galactose metabolism, photosynthesis, secondary wall biogenesis, and fatty acid biosynthesis degradation were down-regulated under drought. Overall, our results provide new genomic data for understanding how non-model long-lived trees of Patagonian forests would acclimate to environmental changes.

Diffusional and Biochemical Limitations to Photosynthesis Under Water Deficit for Field-Grown Cotton.

Water deficit stress limits net photosynthetic rate (AN), but the relative sensitivities of underlying processes such as thylakoid reactions, ATP production, carbon fixation reactions, and carbon loss processes to water deficit stress in field-grown upland cotton require further exploration. Therefore, the objective of the present study was to assess (1) the diffusional and biochemical mechanisms associated with water deficit-induced declines in AN and (2) associations between water deficit-induced variation in oxidative stress and energy dissipation for field-grown cotton. Water deficit stress was imposed for three weeks during the peak bloom stage of cotton development, causing significant reductions in leaf water potential and AN. Among diffusional limitations, mesophyll conductance was the major contributor to the AN decline. Several biochemical processes were adversely impacted by water deficit. Among these, electron transport rate and RuBP regeneration were most sensitive to AN-limiting water deficit. Carbon loss processes (photorespiration and dark respiration) were less sensitive than carbon assimilation, contributing to the water deficit-induced declines in AN. Increased energy dissipation via non-photochemical quenching or maintenance of electron flux to photorespiration prevented oxidative stress. Declines in AN were not associated with water deficit-induced variation in ATP production. It was concluded that diffusional limitations followed by biochemical limitations (ETR and RuBP regeneration) contributed to declines in AN, carbon loss processes partially contributed to the decline in AN, and increased energy dissipation prevented oxidative stress under water deficit in field-grown cotton.

Overexpression of OsACL5 triggers environmentally-dependent leaf rolling and reduces grain size in rice.

Major polyamines include putrescine, spermidine, spermine and thermospermine, which play vital roles in growth and adaptation against environmental changes in plants. Thermospermine (T-Spm) is synthetised by ACL5. The function of ACL5 in rice is still unknown. In this study, we used a reverse genetic strategy to investigate the biological function of OsACL5. We generated several knockout mutants by pYLCRISPR/Cas9 system and overexpressing (OE) lines of OsACL5. Interestingly, the OE plants exhibited environmentally-dependent leaf rolling, smaller grains, lighter 1000-grain weight and reduction in yield per plot. The area of metaxylem vessels of roots and leaves of OE plants were significantly smaller than those of WT, which possibly caused reduction in leaf water potential, resulting in leaf rolling with rise in the environmental temperature and light intensity and decrease in humidity. Additionally, the T-Spm contents were markedly increased by over ninefold whereas the ethylene evolution was reduced in OE plants, suggesting that T-Spm signalling pathway interacts with ethylene pathway to regulate multiple agronomic characters. Moreover, the osacl5 exhibited an increase in grain length, 1000-grain weight, and yield per plot. OsACL5 may affect grain size via mediating the expression of OsDEP1, OsGS3 and OsGW2. Furthermore, haplotypes analysis indicated that OsACL5 plays a conserved function on regulating T-Spm levels during the domestication of rice. Our data demonstrated that identification of OsACL5 provides a theoretical basis for understanding the physiological mechanism of T-Spm which may play roles in triggering environmentally dependent leaf rolling; OsACL5 will be an important gene resource for molecular breeding for higher yield.

Cadmium Accumulation in Cacao Plants (Theobroma cacao L.) under Drought Stress

The objective of this study was to determine Cd accumulation under water-deficit conditions by young cacao plants. The study was conducted under greenhouse conditions. Two full-sib families (IMC67 × PA121 and SCA6 × PA121), obtained through controlled crosses, and an open pollinated half-sib seedling family of IMC67, widely used as rootstock in Colombia, were employed. Plants were grown in Cd-contaminated soil (0.356 mg kg−1) without external sources of the metal. They were subjected to water deficit by suspending irrigation for consecutive periods of 19 and 27 days (D19 and D27), followed by rehydration. Water stress reduced leaf water potential (Ψleaf) with values from −1.51 to −2.09 MPa, with full-sib family SCA6 × PA121 being the most tolerant to water deficit. Cd concentration was influenced by biomass reduction (observed in IMC67 × PA121 and SCA6 × PA121) and transpiration rate (evident in IMC67) caused by water deficit. Full-sib progenies IMC67 × PA121 and SCA6 × PA121 accumulated more Cd in the plants than open pollinated IMC67, with higher Cd accumulation in leaves. The translocation factor (TF) revealed that the aboveground organs of the progenies were enriched with Cd (TF > 4). Water deficit increased Cd translocation from roots to leaves in IMC67 × PA121 and IMC 67, while there were no significant changes in SCA6 × PA121. Full-sib family SCA6 × PA121 stood out as the most promising progenies due to their water-stress tolerance and Cd accumulation stability. This study introduces a discussion about the influence of water stress on Cd accumulation in Theobroma cacao.

Toxicity and tolerance of nickel in sunflower (Helianthus annuus L.).

This study aimed at exploration of nickel (Ni) application (0, 10, 20, 30, and 40mg L-1) on physiological and biochemical attributes of sunflower cultivars (Hysun-33 and SF-187) grown in sand culture. Results revealed a significant decrease in vegetative parameters in both sunflower cultivars by increasing Ni concentration, although low levels of Ni (10mg L-1) improved growth attributes to some extent. Among photosynthetic attributes, 30 and 40mgL-1 Ni application severely reduced the photosynthetic rate (A), stomatal conductance (gs), water use efficiency (WUE), and Ci/Ca ratio but improved the transpiration rate (E) in both sunflower cultivars. The same level of Ni application also reduced leaf water potential, osmotic potentials, and relative water contents but increased leaf turgor potential and membrane permeability. At low level (10 and 20mg L-1), Ni improved the soluble proteins, while high Ni concentration decreased it. The opposite was true for total free amino acids and soluble sugars. To conclude, the high Ni concentration in various plant organs had a strong impact with the changes in vegetative growth, physiological and biochemical attributes. A positive correlation of growth, physiological, water relations, and gas exchange parameters at low levels of Ni and negative correlation at higher Ni level confirmed that the supplementation of low Ni levels greatly modulated studied attributes. Based on observed attributes, Hysun-33 showed high tolerance to Ni stress as compared to SF-187.

Acute and Rapid Response of Melissa officinalis and Mentha spicata to Saline Reclaimed Water in Terms of Water Relations, Hormones, Amino Acids and Plant Oxylipins.

The use of reclaimed water is considered an efficient tool for agricultural irrigation; however, the high salinity associated to this water could compromise plant quality and yields. Balm and spearmint plants were submitted for 15 days to three irrigation treatments in a controlled chamber: control with EC: 1.2 dS m-1 (control), reclaimed water from secondary effluent (EC: 1.6 dS m-1) (S) and water from secondary effluent with brine (EC: 4.4 dS m-1) (SB). The plant water status, stomatal and hormonal regulation, nutritional response, concentration of amino acids and plant oxidative stress-based markers, as well as growth were evaluated. Both species irrigated with saline reclaimed water reduced leaf water potential and gas exchange in comparison with control plants, following 2 days of exposure to irrigation treatments. Nevertheless, spearmint plants recovered photosynthetic activity from the seventh day onwards, maintaining growth. This was attributed to hormonal changes and a greater accumulation of some amino acids and some plant oxylipins (phytoprostanes) in comparison to balm plants, which contributed to the improvement in the organoleptic and health-promoting properties of spearmint. A longer irrigation period with saline reclaimed water would be necessary to assess whether the quality of both species, especially spearmint, could further improve without compromising their growth.

Tolerance and Recovery Capacity to Reclaimed Wastewater Irrigation of Salvia officinalis and Asteriscus maritimus Plants Inoculated with Arbuscular Mycorrhizae

This work attempts to identify which of two species with different levels of salinity tolerance, Salvia officinalis L. or Asteriscus maritimus L., is more suitable for irrigation with reclaimed wastewater, as well as the effect of the arbuscular mycorrhiza Glomus iranicum on the plant. The experiment was carried out in a growth chamber with a first phase, where both species were irrigated with good quality water, a second phase in which the plants were irrigated with reclaimed wastewater, and a third phase in which the plants were irrigated with good quality water again (recovery). Salinity caused a reduction in leaf water potential, stomatal conductance and net photosynthesis in both species. The percentage of mycorrhization was higher in Asteriscus than in Salvia, mitigating the decrease in leaf water potential. There was osmotic adjustment in Salvia, although the proline content increased in both species. The damages produced were clearer in Salvia, in which lipid peroxidation values were higher. Likewise, the visual appearance of the leaves showed symptoms of toxicity in this species, although the mycorrhizae diminished these effects. Irrigation with good quality water induced the recovery of lipid peroxidation in both species, as well as the appearance of new leaves in Salvia.

Effects of Severe Drought Stress on Some Physiological and Biochemical Parameters of AMF Inoculated C. arietinum

In this study, physiological and biochemical changes caused by mycorrhizal symbiosis in chickpea plants under drought conditions were investigated in both root and leaf. Drought stress reduced leaf water potential, but mycorrhizal symbiosis caused a significant increase in leaf water potential. However, the application of mycorrhiza under drought stress caused an increase in the amount of elements that are very important for the development of the plant in the root and leaf. In our study, drought increased the proline concentration and MDA content, while mycorrhiza application decreased them in both leaf and root. In addition, while mycorrhizal application increased the activity of catalase, it decreased the activity of superoxide dismutase. In general, enzyme activities were found to be higher in the leaf, but no distinct pattern was obtained between root and leaf in other analyzes. The study shows that the responses of mycorrhizal symbiosis in chickpea plants may change depending on the severity of the drought. Especially antioxidant enzyme activities and proline content patterns reveal that more comprehensive studies should be conducted on these issues. However, continuing studies until determining the effects of AMF symbiosis on grain yield under drought may provide more comprehensive results.

Drought-Adaptive Mechanisms of Young Sweet Cherry Trees in Response to Withholding and Resuming Irrigation Cycles

The present work evaluates the main adaptive mechanisms developed by young sweet cherry trees (Prunus avium L.) to cope with drought. For this purpose, the young trees were subjected to two drought cycles with different water stress intensities followed by a recovery period. Three irrigation treatments were applied: control treatment (CTL) irrigated to ensure non-limiting soil water conditions; moderate water stress (MS) subjected to two drying cycles whose duration was dependent on the time elapsed until the trees reached values of midday stem water potential (Ψstem) of −1.3 and −1.7 MPa for the first and second cycle, respectively; and severe water stress (SS) similar to MS, but with reference values of −1.6 and −2.5 MPa. In-between drought cycles, MS and SS trees were irrigated daily as the CTL trees until reaching Ψstem values similar to those of CTL trees. The MS and SS trees showed an important stomatal regulation and lower vegetative growth. The decreasing leaf turgor potential (Ψturgor) during the drought periods accounted for 40–100% of the reduction in leaf water potential at midday (Ψmd). The minimum osmotic potential for mature leaves was about 0.35 MPa lower than in well-irrigated trees. The occasional osmotic adjustment observed in MS and SS trees was not sufficient to maintain Ψturgor values similar to the CTL trees or to increase the specific leaf weight (SLW). The leaf insertion angle increased as the water stress level increased. Severe water stress (Ψstem < −2.0 MPa) resulted in clear early defoliation as a further step in water conservation.

High relative humidity improve chilling tolerance by maintaining leaf water potential in watermelon seedlings

Low temperature is a major environmental factor that severely impairs plant growth and productivity. Although the response to low temperature stress is well studied, the mechanisms of chilling tolerance are still not well understood. Here, we describe experiments that aimed to determine whether relative humidity (RH) contribute to chilling tolerance by regulating leaf water potential in watermelon seedlings. Plants exposed to chilling stress (10 °C/5 °C day/night) were severely wilted, and the water potential in their leaves was decreased. We found that maintaining high RH when plants were subjected to chilling-stress conditions attenuated the reduction in leaf water potential, reduced electrolyte leakage, improved the stability of photosynthesis, and alleviated chilling damage. Pretreatment with ABA increased chilling tolerance in low RH conditions but became ineffective in high RH conditions. Analysis of endogenous ABA content indicated that water potential mediated chilling tolerance was independent of ABA. Analysis of stomatal resistance indicated that the maintenance of water potential was related to stomatal resistance but that the balance between water absorption and loss is more important. In conclusion, high RH maintained leaf water potential and cell turgor, maintained better cell morphology, improved stomatal conductance and thus, ultimately improved the chilling tolerance of watermelon seedlings.

Recycled Wastewater and Reverse Osmosis Brine Use for Halophytes Irrigation: Differences in Physiological, Nutritional and Hormonal Responses of Crithmum maritimum and Atriplex halimus Plants

Halophytes are capable of coping with excessive NaCl in their tissues, although some species may differ in their degree of salt tolerance. In addition, it is not clear whether they can tolerate other confounding factors and impurities associated with non-conventional waters. The experiment was performed in a greenhouse with Crithmum maritimum and Atriplex halimus plants, growing on soil and irrigated with two different water types: reclaimed wastewater (RWW) (EC: 0.8–1.2 dS m−1) and reverse osmosis brine (ROB) (EC: 4.7–7.9 dS m−1). Both species showed different physiological and nutritional responses, when they were irrigated with ROB. Atriplex plants reduced leaf water potential and maintained leaf turgor as consequence of an osmotic adjustment process. Atriplex showed higher intrinsic water use efficiency than Crithmum, regardless of the type of water used. In Crithmum, the water status and photosynthetic efficiency were similar in both treatments. Crithmum presented a higher leaf accumulation of B and Ca ions, while Atriplex a higher amount of K, Mg, Na and Zn. Crithmum plants irrigated with ROB presented higher concentrations of 1-aminocyclopropane-1-carboxylic acid and trans-zeatin-glucoside, whereas abscisic acid concentration was lower. Atriplex showed a lower concentration of trans-zeatin-riboside and scopoletin. The characteristics associated to water irrigation did not influence negatively the development of any of these species, which confirms the use of brine as an alternative to irrigate them with conventional waters.

Partial Substitution of K by Na Alleviates Drought Stress and Increases Water Use Efficiency in Eucalyptus Species Seedlings.

Eucalyptus, the most widely planted tree genus worldwide, is frequently cultivated in soils with low water and nutrient availability. Sodium (Na) can substitute some physiological functions of potassium (K), directly influencing plants’ water status. However, the extent to which K can be replaced by Na in drought conditions remains poorly understood. A greenhouse experiment was conducted with three Eucalyptus genotypes under two water conditions (well-watered and water-stressed) and five combination rates of K and Na, representing substitutions of 0/100, 25/75, 50/50, 75/25, and 100/0 (percentage of Na/percentage of K), to investigate growth and photosynthesis-related parameters. This study focused on the positive effects of Na supply since, depending on the levels applied, the Na supply may induce plants to salinity stress (>100 mM of NaCl). Plants supplied with low to intermediate K replacement by Na reduced the critical level of K without showing symptoms of K deficiency and provided higher total dry matter (TDM) than those Eucalyptus seedlings supplied only with K in both water conditions. Those plants supplied with low to intermediate K replacement by Na had improved CO2 assimilation (A), stomatal density (Std), K use efficiency (UEK), and water use efficiency (WUE), in addition to reduced leaf water potential (Ψw) and maintenance of leaf turgidity, with the stomata partially closed, indicated by the higher values of leaf carbon isotope composition (δ13C‰). Meanwhile, combination rates higher than 50% of K replacement by Na led to K-deficient plants, characterized by the lower values of TDM, δ13C‰, WUE, and leaf K concentration and higher leaf Na concentration. There was positive evidence of partial replacement of K by Na in Eucalyptus seedlings; meanwhile, the ideal percentage of substitution increased according to the drought tolerance of the species (Eucalyptus saligna < Eucalyptus urophylla < Eucalyptus camaldulensis).

Rootstock Genotypes Shape the Response of cv. Pinot gris to Water Deficit

Understanding the physiological basis underlying the water stress responses in grapevine is becoming increasingly topical owing to the challenges that climate change will impose to grapevine agriculture. Here we used cv. Pinot gris (clone H1), grafted on a series of tolerant (1103Paulsen; P), sensitive (SO4) and recently selected (Georgikon28; G28, Georgikon121; G121, Zamor17; Z17) rootstocks. Plants were either subjected to reduced water availability (WS) or maintained at pot capacity (WW). Photosynthetic (light response curves), stomatal and in vivo gas exchange analysis were carried out as well as dynamics of daily water use (WU), leaf area accumulation with affordable RGB imaging pipelines and leaf water potential. Significant genotypic variation was recorded between rootstocks for most of the traits analyzed under optimal conditions with P and SO4 showing a more vigorous growth, higher CO2 assimilation rate, stomatal conductance and stomatal density per unit of leaf area than G28, G121, Z17 (p &lt; 0.001). Under WS, rootstocks induced different water stress response in Pinot gris, with G28 and G121 showing a higher sensitivity of water use to reduced water availability (WS) (p = 0.021) and no variation for midday leaf water potential until severe WS. P, Z17 and to some extent SO4 induced a pronounced near-anisohydric response with a general WU maintenance followed by reduction in leaf water potential even at high levels of soil water content. In addition, G28 and G121 showed a less marked slope in the linear relationship between daily water use and VPD (p = 0.008) suggesting elevated sensitivity of transpiration to evaporative demand. This led to an insensitivity for total dry weight biomass of G28 and G121 under WS conditions (p &lt; 0.001). This work provides: (i) an in-depth analysis for a series of preferable traits under WS in Pinot gris; (ii) a characterization of Pinot gris × rootstock interaction and a series of desirable traits under WS induced by several rootstocks; (iii) the potential benefit for the use a series of affordable methods (e.g., RGB imaging) to easily detect dynamic changes in biomass in grapevine and quickly phenotype genotypes with superior responses under WS. In conclusion, the near-isohydric and conservative behavior observed for G28 and G121 coupled with their low vigor suggest them as potential Pinot gris rootstock candidates for sustaining grapevine productivity in shallow soils likely to develop terminal stress conditions.

Water relations and non-structural carbohydrate responses to the combined effects of defoliation and progressive drought in a dioecious tree

Increases in drought frequency and insect defoliation are emerging threats that typically affect the plantation production of Populus yunnanensis, which is an endemic dioecious tree species in China. Nevertheless, it is difficult to predict which sex of P. yunnanensis will be more affected by the combined effects of water limitations and defoliation. In this study, we examined the combined effects of progressive drought and defoliation on water relations, growth, gas exchange and leaf carbohydrate reserves in P. yunnanensis females and males to test whether defoliation alters the effects of drought and and how this response varies between genders. The experimental design included two watering regimes (well-watered and drought-stressed) and two levels of defoliation (leaves untouched and 60% defoliated). Defoliation exacerbated drought stress in females by decreasing photosynthesis, dry mass accumulation and leaf non-structural carbohydrate (NSC) concentration. In contrast, the total biomass of males grown under drought combined with defoliation was not significantly altered when compared to those grown under drought alone. These results suggest that defoliation does not aggravate the drought effects on males. The males presented a high capacity to maintain foliar water status through stomatal control under drought conditions irrespective of defoliation treatment. The conservative water use strategy in males resulted in a gradual reduction in leaf water potential, gas exchange rates and NSC. Our data showed that the different responses of the females and males to drought and defoliation were influenced by plant water status and consequent effects on changes in carbon assimilation and leaf non-structural carbohydrate reserves.

Comparison of soil and hydroponic cultivation systems for spinach irrigated with brackish water

The sustainable use of brackish water in agriculture demands the use of compatible cultivation systems. The objective of this study was measure differences in the water status of spinach plants (Spinacia oleracea L.), cv. Viroflay, cultivated in covered and uncovered soil and in a hydroponic floating system, using brackish water (EC = 0.8, 1.5, 3.0, 4.5, 6.0 and 7.5 dS m−1) in the irrigation of plants in soil and replacement of nutrient solution in the floating system. At 38 days after transplanting, leaf water potential and osmotic potentials of the crop and of the systems were determined, as well as the osmotic adjustment, water consumption, leaf succulence, water use efficiency, leaf sodium content and yield. The hydroponic system showed a linear increase in the fresh weight of the leaves due to the salinity increase. The use of plastic cover attenuated the effects of salinity by 16 % in comparison to the uncovered soil, but the hydroponic system was the least affected by salinity, despite having the highest water consumption, which was 58 % higher than that of the uncovered soil. The main adaptive strategies observed in Spinacia oleracea L. cv. Viroflay in response to salinity were: increase in osmotic adjustment and leaf succulence and reduction in leaf water potential. The responses, positive in general, are corroborated by the values of sodium concentration in the leaves and fresh weight, which showed the highest results in the hydroponic cultivation for both variables. It is recommended to use the plastic cover or hydroponic methods during spinach cultivation, especially when using brackish water.

Effects of sub-low temperature and drought stress on water transport and morphological anatomy of tomato plant

We explored the effects of sub-low temperature and drought on water transport in tomato seedlings under normal temperature (25 ℃ day/18 ℃ night) and sub-low temperature (15 ℃ day/8 ℃ night) within the artificial climate chamber, and under normal irrigation (75%-85% field water holding capacity) and drought treatment (55%-65% field water holding capacity). We analyzed the effects of temperature and soil moisture on water transport, stomata and xylem vessel morpholo-gical and anatomical structure of tomato plants. The results showed that compared with condition of normal temperature + normal irrigation, drought treatment significantly reduced leaf water potential, transpiration rate, stomatal conductance, hydraulic conductance, sap flow rate, stomatal length, and diameter of leaf, stem and root conduit, and thus thickened the cell wall and enhanced the anti-embolism ability of conduit in leaf, stem and root. Leaf water potential, transpiration rate, stomatal conductance, hydraulic conductance, and conduit diameter in leaf, stem and root were significantly reduced by sub-low temperature treatment, but the stomata became larger, cell wall was thickened and the anti-embolism ability was enhanced in leaf, stem and root conduit. Under sub-low temperature condition, soil moisture did not affect leaf water potential, transpiration rate, stomatal conductance, hydraulic conductance, stomatal morphology, conduit structure of leaf and root. In conclusion, under drought treatment, the homeostasis in water relationship was obtained by the coordination of leaf, stem and root structure. Under sub-low temperature treatment, the regulation of water relationship was mainly dependent on the changes of conduit structure in leaf and root, which was less affected by soil moisture.

Water potential, biochemical indicators and yield of sugarcane irrigated with brackish water and leaching

ABSTRACT Knowledge on the water relations and biochemical responses of plants has been pointed out as important for selecting genotypes that are tolerant to abiotic stresses, such as drought and salinity. Thus, the objective of this study was to evaluate the leaf water potential, enzymatic activity and yield of sugarcane (RB 92579) irrigated with brackish waters and leaching fractions. The study was conducted from November 2016 to November 2017 in Recife, PE, Brazil. The experimental design was completely randomized, in a 5 x 2 factorial scheme, with four repetitions. Treatments consisted of five irrigation water salinity (0.5; 2.0; 4.0; 6.0 and 8.0 dS m-1) and two leaching fractions (0 and 0.17), corresponding to 100 and 120% of crop evapotranspiration. The values of irrigation water salinity were obtained by adding NaCl and CaCl2 at molar proportion of 1:1 (Ca:Na) to water from the local supply (ECw = 0.5 dS m-1). Readings of leaf water potential (Ψw) were taken at predawn, and +3 leaves were collected for the determination of catalase and ascorbate peroxidase activity at 90, 150, 210 and 270 days after planting. Yield was determined at 365 days after planting. Increase in water salinity reduced leaf water potential and yield, and increased the activity of catalase and ascorbate peroxidase, and the leaching fraction of 0.17 was able to reduce the deleterious effects of salts on plants.

Genotypic variations in leaf and whole-plant water use efficiencies are closely related in bread wheat genotypes under well-watered and water-limited conditions during grain filling

Wheat plants growing under Mediterranean rain-fed conditions are exposed to water deficit, particularly during the grain filling period, and this can lead to a strong reduction in grain yield (GY). This study examines the effects of water deficit after during the grain filling period on photosynthetic and water-use efficiencies at the leaf and whole-plant level for 14 bread wheat genotypes grown in pots under glasshouse conditions. Two glasshouse experiments were conducted, one in a conventional glasshouse at the Universidad de Talca, Chile (Experiment 1), and another at the National Plant Phenomics Centre (NPPC), Aberystwyth, UK (Experiment 2), in 2015. Plants were grown under well-watered (WW) and water-limited (WL) conditions during grain filling. The reductions in leaf water potential (Ψ), net CO2 assimilation (An) and stomatal conductance (gs) due to water deficit were 79, 35 and 55%, respectively, during grain filling but no significant differences were found among genotypes. However, chlorophyll fluorescence parameters (as determined on dark-adapted and illuminated leaves) and chlorophyll content (Chl) were significantly different among genotypes, but not between water conditions. Under both water conditions, An presented a positive and linear relationship with the effective photochemical quantum yield of Photosystem II (Y(II)) and the maximum rate of electron transport (ETRmax), and negative with the quantum yield of non-photochemical energy conversion in Photosystem II (Y(NPQ)). The relationship between An and Chl was positive and linear for both water conditions, but under WL conditions An tended to be lower at any Chl value. Both, instantaneous (An/E) and intrinsic (An/gs) water-use efficiencies at the leaf level exhibited a positive and linear relationship with plant water-use efficiency (WUEp = plant dry weight/water use). Carbon discrimination (Δ13C) in kernels presented a negative relationship with WUEp, at both WW and WL conditions, and a positive relationship with GY. Our results indicate that during grain filling wheat plants face limitations to the assimilation process due to natural senesce and water stress. The reduction in An and gs after anthesis in both water conditions was mainly due a decline in the chlorophyll content (non-stomatal limitation), whereas the observed differences between water conditions were mainly due to a stomatal limitation.

Parthenium avoids drought: Understanding the morphological and physiological responses of the invasive herb Parthenium hysterophorus to progressive water stress

Parthenium hysterophorus L. is a noxious annual invasive herb which threatens biodiversity, food security and human and animal health in various tropical and sub-tropical regions globally. Given that these regions, including South Africa, experience frequent drought events, it is important that the invasive potential of this weed be better understood in the context of water stress. This study aimed to assess the physiological and morphological responses of P. hysterophorus to water stress. To elicit these responses, juvenile and adult plants were progressively water stressed over a six-week period. Despite significant reductions in leaf water potential, relative leaf water content and stomatal conductance from severe soil water stress, plants maintained positive net photosynthesis, albeit at a severely reduced rate. Concomitantly, water-use efficiency of P. hysterophorus increased by more than 300 %. Morphologically, water-stressed plants exhibited reductions in leaf area and leaf number, with adults accelerating flower production when compared to well-watered plants. Linked to this, were alterations in biomass, with juveniles and adults reducing above-ground biomass by 75 % and 35 %, respectively. Overall, many of the physiological and morphological responses are indicative of a largely drought avoidance strategy, allowing P. hysterophorus to persist during periods of water-stress as leafy basal rosettes (juveniles) or as reproductive herbs (adults). This research yielded valuable insights into the underlying water relations of P. hysterophorus, and how these may relate to the potential spread, detriment and management of the weed in various sub-tropical environments.

Impact of Combined Heat and Drought Stress on the Potential Growth Responses of the Desert Grass Artemisia sieberi alba: Relation to Biochemical and Molecular Adaptation.

Artemisia sieberi alba is one of the important plants frequently encountered by the combined effect of drought and heat stress. In the present study, we investigated the individual and combined effect of drought and heat stress on growth, photosynthesis, oxidative damage, and gene expression in A. sieberi alba. Drought and heat stress triggered oxidative damage by increasing the accumulation of hydrogen peroxide, and therefore electrolyte leakage. The accumulation of secondary metabolites, such as phenol and flavonoids, and proline, mannitol, inositol, and sorbitol, was increased due to drought and heat stress exposure. Photosynthetic attributes including chlorophyll synthesis, stomatal conductance, transpiration rate, photosynthetic efficiency, and chlorophyll fluorescence parameters were drastically reduced due to drought and heat stress exposure. Relative water content declined significantly in stressed plants, which was evident by the reduced leaf water potential and the water use efficiency, therefore, affecting the overall growth performance. Relative expression of aquaporin (AQP), dehydrin (DHN1), late embryogenesis abundant (LEA), osmotin (OSM-34), and heat shock proteins (HSP70) were significantly higher in stressed plants. Drought triggered the expression of AQP, DHN1, LEA, and OSM-34 more than heat, which improved the HSP70 transcript levels. A. sieberi alba responded to drought and heat stress by initiating key physio-biochemical and molecular responses, which were distinct in plants exposed to a combination of drought and heat stress.