Shoot Relative Water Content Research Articles

Drought priming reduces Calligonum mongolicum sensitivity to recurrent droughts via coordinated regulation of osmolytes, antioxidants, and hormones.

Pre-exposure of plants to abiotic stressors may induce stress memory and improve tolerance to subsequent stresses. Here, 3-month-old Calligonum mongolicum seedlings were exposed to drought (60 days) with (primed) or without (unprimed) early drought exposure of 50 days, to determine whether this enhances seedling resistance and investigate possible underlying mechanisms. Compared to unprimed, primed seedlings had higher biomass, shoot relative water content (15% and 22%), chlorophyll a, chlorophyll b, and carotenoids. They also had more superoxide anions (O2 -• ) and H2 O2 scavenging mechanisms through higher activity of SOD, CAT, APX, and dehydroascorbate reductase in assimilating shoots and roots, resulting in less ROS and oxidative stress damage. Plants also had higher ABA and JA but lower SA, likely reflecting an adaptive response to subsequent stress. Primed seedlings accumulated more IAA and brassinosteroids, which may account for their better growth. Accumulation of glycine betaine, pro, and total amino acids in assimilating shoots and roots of primed seedlings led to reduced osmotic stress. Drivers of responses of non-primed and primed seedlings to drought varied. Responses of primed seedlings were primarily characterized by more photosynthetic pigments, increased oxidative scavenging of O2 -• and H2 O2 , more phytohormones and osmolytes. Early drought priming of drought stress memory in C. mongolicum seedlings may provide a useful management approach to improve seedling establishment in vegetation restoration programs.

Seed priming with Beauveria bassiana improves growth and salt stress response in rice

One of the most important abiotic factors that hinder plant development, growth, and production is salt stress. In recent years, there has been a lot of interest in the biological treatment of salt stress in plants using beneficial microbes. The fungal endophyte Beauveria bassiana provides a wide variety of ecosystem services, like suppressing insect pests and pathogens and enhancing plant growth. However, the role of B. bassiana in reducing salt stress in plants has not yet been clarified. This study was undertaken to evaluate the performance of B. bassiana isolate BeauA1 primed rice under salt stress by estimating rice growth, stress parameters, and mitigator characteristics. Primarily, rice seeds were primed with BeauA1 and placed in an agar medium with 120 mM NaCl (≈12 dS m−1 salt solution) to observe the role of BeauA1 in the early establishment of rice seedlings in salt conditions. Seed priming with BeauA1 resulted in an enhancement of rice growth attributes under both control and NaCl stress conditions. In the pot experiment, the BeauA1 primed rice seedlings were planted in soil with different concentrations of salt, viz. 8, 10, and 12 dS m−1. The BeauA1 primed rice plants showed improvement in leaf succulence, leaf area, photosynthetic pigments, and shoot relative water content (RWC), leading to enhanced growth under both salt stress and control conditions. The biochemical study found that BeauA1 considerably increased proline content, total soluble sugars, total carbohydrates, and K+/Na+ in leaves. The antioxidant enzymes catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), glutathione S-transferase (GST), and nonenzymatic antioxidants phenol and flavonoid were upregulated in BeauA1-primed plants under both control and stressed conditions. Further significant reductions of the lipid peroxidation products malondialdehyde (MDA) and hydrogen peroxide (H2O2) by BeauA1 under salt stress were consistent with higher antioxidant activities in salt stress conditions. Principal component analysis (PCA) further validated BeauA1-primed plants' modulation of growth, antioxidant defense, and reduction of MDA and H2O2 in rice under salt-stress conditions. Our findings indicated that utilizing BeauA1 to reduce salt stress would be a useful strategy to increase rice yield in salt-affected regions.

Ethanol Treatment Enhances Physiological and Biochemical Responses to Mitigate Saline Toxicity in Soybean.

Soil salinity, a major environmental concern, significantly reduces plant growth and production all around the world. Finding solutions to reduce the salinity impacts on plants is critical for global food security. In recent years, the priming of plants with organic chemicals has shown to be a viable approach for the alleviation of salinity effects in plants. The current study examined the effects of exogenous ethanol in triggering salinity acclimatization responses in soybean by investigating growth responses, and numerous physiological and biochemical features. Foliar ethanol application to saline water-treated soybean plants resulted in an enhancement of biomass, leaf area, photosynthetic pigment contents, net photosynthetic rate, shoot relative water content, water use efficiency, and K+ and Mg2+ contents, leading to improved growth performance under salinity. Salt stress significantly enhanced the contents of reactive oxygen species (ROS), malondialdehyde, and electrolyte leakage in the leaves, suggesting salt-induced oxidative stress and membrane damage in soybean plants. In contrast, ethanol treatment of salt-treated soybean plants boosted ROS-detoxification mechanisms by enhancing the activities of antioxidant enzymes, including peroxidase, ascorbate peroxidase, catalase, and glutathione S-transferase. Ethanol application also augmented the levels of proline and total free amino acids in salt-exposed plants, implying a role of ethanol in maintaining osmotic adjustment in response to salt stress. Notably, exogenous ethanol decreased Na+ uptake while increasing K+ and Mg2+ uptake and their partitioning to leaves and roots in salt-stressed plants. Overall, our findings reveal the protective roles of ethanol against salinity in soybean and suggest that the use of this cost-effective and easily accessible ethanol in salinity mitigation could be an effective approach to increase soybean production in salt-affected areas.

Effects of rhizobia and arbuscular mycorrhizal fungi on yield, size distribution and fatty acid of soybean seeds grown under drought stress

Soybean (Glycine max L.) is among the most economically important legumes that provide more than 1/4 of food (for man) and animal feed. However, its yield is comparatively low, most especially under drought stress. The aim of this study therefore was to assess the ability of Rhizobium spp. and mycorrhizal fungi to enhance the yield, seed size and fatty acid content of soybean grown under semi-arid environment. Rhizobium sp. strain R1 was found to possess nitrogen-fixing gene coniferyl aldehyde dehydrogenase function while Rhizobium cellulosilyticum strain R3 was found to have nitrogen-fixing genes cysteine desulfurase SufS and cysteine desulfurase IscS activity. Soybean (Glycine max L) seeds inoculated with Rhizobium spp. and mycorrhizal fungi were cultivated in soil exposed to drought stress. Rhizobium spp. inoculation and mycorrhization alleviate drought stress and increase yield, size and fat content of soybean seeds. This increase in the aboveground parameters was accompanied with an increase in belowground mycorrhizal spore number, percentage root mycorrhization and aboveground shoot relative water content (RWC) in the dually inoculated (R1 + R3MY) soybean plants. In particular, the dually inoculated (R1 + R3MY) soybean plants revealed 34.3 g fresh weight, 15.1 g dry weight and soybean plants singly inoculated with Rhizobium sp. strain R1 (R1) produced more large seeds with 12.03 g dry weight. The non-inoculated (control) seeds contained a higher percentage of moisture content compared to the microbially amended seeds while seeds co-inoculated with Rhizobium cellulosilyticum strain R3 and mycorrhizal consortium revealed the highest percent (8.4 %) of fat. Several fatty acids that are of significant health benefits to humans were observed in the soybean seeds. In order to gain insights into the bacterial communities of rhizospheric soil collected at different stages of soybean growth, class-based Heat-map analysis was performed on the Miseq sequenced data. The core bacteria that were found in the rhizospheric soil were Verrumicrobia, Proteobacteria, Gemmatimonadetes, Firmicutes, Cyanobacteria, Chloroflexi, Bacteroidetes, Actinobacteria, Acidobacteria, Planctomycetes, Deinococcus thermus and Nitrospira suggesting that the rhizobia and fungi used in this study can also improve soil microbial diversity.

Acetic acid improves drought acclimation in soybean: an integrative response of photosynthesis, osmoregulation, mineral uptake and antioxidant defense.

Exposure to drought stress negatively affects plant productivity and consequently threatens global food security. As global climates change, identifying solutions to increase the resilience of plants to drought is increasingly important. Several chemical treatments have recently emerged as promising techniques for various individual and combined abiotic stresses. This study shows compelling evidence on how acetic acid application promotes drought acclimation responses in soybean by investigating several morphological, physiological and biochemical attributes. Foliar applications of acetic acid to drought-exposed soybean resulted in improvements in root biomass, leaf area, photosynthetic rate and water use efficiency; leading to improved growth performance. Drought-induced accumulation of reactive oxygen species, and the resultant increased levels of malondialdehyde and electrolyte leakage, were considerably reverted by acetic acid treatment. Acetic acid-sprayed plants suffered less oxidative stress due to the enhancement of antioxidant defense mechanisms, as evidenced by the increased activities of superoxide dismutase, ascorbate peroxidase, catalase, glutathione peroxidase and glutathione S-transferase. Improved shoot relative water content was also linked to the increased levels of soluble sugars and free amino acids, indicating a better osmotic adjustment following acetic acid treatment in drought-exposed plants. Acetic acid also increased stem/root, leaf/stem and leaf/root mineral ratios and improved overall mineral status in drought-stressed plants. Taken together, our results demonstrated that acetic acid treatment enabled soybean plants to positively regulate photosynthetic ability, water balance, mineral homeostasis and antioxidant responses; thereby suggesting acetic acid as a cost-effective and easily accessible chemical for the management of soybean growth and productivity in drought-prone areas.

Effect of sodium silicate on early growth stages of wheat under drought stress

Wheat yield is decreasing due to climate change, and a method to prevent decreasing yield during drought stress is desirable. In this study, wheat cultivars (Koso and Jokyung) were treated with 15% polyethylene glycol-6000 (PEG) and PEG + Si solution (6.5, 8.7, 13.1 and 26.1 mM). The effect of Si treatment on the alleviation of drought stress was measured using the germination test, shoot relative water content (RWC), seedling stage observation, and quantitative real time polymerase chain reaction (qRT-PCR). The results of root/shoot length ratio and shoot length ratio showed that Si treatment induced the alleviation of drought stress in Jokyung cultivar. The result of qRT-PCR showed the alleviation of drought stress in Koso cultivar. In addition, the results with shoot RWC and seedling stage observation showed that the alleviation effects of Si treatment was observed with both Koso and Jokyung cultivar at the high concentration of Si (26.1 mM). All these results suggest that Si treatment at a high concentration could be employed to alleviate drought stress in wheat.

Physiological traits determining tolerance to intermittent drought in the Capsicum annuum complex

The use of physiological traits directly-linked to mechanisms of drought tolerance (DT) as selection criteria could accelerate the development of tolerant varieties. To identify such physiological traits in chilli pepper (Capsicum annuum L.), we screened a range of diverse genotypes, a series of parental breeding lines produced from C. annuum and C. frutescens/C. chinense crosses, under well-watered and water-stressed (65% depletion of available soil water) conditions in the field with a movable rainshelter. Water regime x genotype interaction was significant (p < 0.05) in the majority of measured traits, which showed significant genotypic variation under the two water regimes. Stomatal conductance (gs) showed strong positive correlations with yield under both water regimes. Furthermore, relative reduction of gs in response to drought positively correlated with relative yield reduction. Canopy temperature (Tc) showed significant negative correlations with yield in both water regimes. Lower yield reductions due to drought were associated with lower increases of Tc across the genotypes. Similarly, lower reductions in shoot relative water content were strongly correlated to lower yield reductions. Yield under drought (Yd) showed significant positive correlation to maximum rooting depth and dry root weight. Greater flower numbers and conversion of flowers to pods, traits indicating the DT of reproductive processes positively correlated with Yd. Factor Analysis showed that the above physiological, growth and reproductive traits constituted composite traits which underpin DT in chilli. As such, they can be used as screening tools in identifying drought tolerant germplasm in breeding programmes of Capsicum annuum.

Genotypic differences in red clover (Trifolium pratense L.) response under severe water deficit

AimsIn Ontario, Canada, acreage of red clover (Trifolium pratense L.) intercropped with winter wheat (Triticum aestivum L. em. Thell) has declined, despite well-documented soil and yield benefits. The decline has resulted from increasing prevalence of stand non-uniformity, which has been attributed in part to soil moisture deficits. We examined whether there are genotypic differences in drought response between red clover varieties.MethodsA double-cut (Belle) and a single-cut variety (Altaswede) were grown under four different durations of drought (4, 8, 12 and 16 days below 15% relative soil water content, RSWC). Shoot dry weight, shoot relative water content (RWC), leaf area and crown water content were measured in control, drought and drought + recovery treatments.ResultsBelle used significantly more water during soil moisture deficit and had greater leaf area, shoot dry weight and RWC compared to Altaswede. In contrast, Altaswede had significantly higher survival rates than Belle, attributed to maintenance of meristematic tissue viability in the crown where re-growth, after shoot tissue desiccation, can occur.ConclusionsBy demonstrating genotypic variation in survival strategies of red clover, traits can be identified for the development of improved varieties. Varieties with higher survival rates during drought will result in more uniform stands and increased utilization of red clover for environmental and yield benefits.

Impact of drought stress induced by polyethylene glycol on growth, water relations and cell viability of Norway spruce seedlings.

We investigated physiological responses of 7-week-old Norway spruce seedlings to water deficits of different intensities. Hydroponically grown seedlings were subjected to mild (-0.15MPa), strong (-0.5 and -1.0MPa) and extreme (-1.5MPa) water deficit induced by polyethylene glycol 6000, and their growth parameters, water status and physiological activity were analyzed. Seedlings effectively restricted water loss under drought, and even under extreme water deficit, shoot relative water content did not fall below 85%. Water stress induced substantial decreases in the osmotic potentials of root and needle cell sap, up to 0.3-0.4MPa under extreme water deficit, though this did not result from water loss or accumulation of K+ and Na+ ions. Seedling growth was very susceptible to water stress because of poor capacity for cell wall adjustment. Water stress injured seedling roots, as evidenced by the loss of root cell physiological activity estimated by the ability to hydrolyse fluorescein diacetate and by increased root calcium content up to 8-10-fold under extreme water stress. At the same time, root hair growth was enhanced, especially under mild water deficit, which increased the root water-absorbing capacity. In summary, seedlings of Norway spruce were characterized by high susceptibility to water stress and concurrently by pronounced ability to maintain water status. These characteristics are fully consistent with spruce confinement to moist habitats.

Tolerance to soil water stress by Oryza sativa cv. IR20 was improved by expression of Wsi18 gene locus from Oryza nivara

Wild rice genotypes are rich in genetic diversity. This has potential to improve agronomic rice by allele mining for superior traits. Late embryogenesis abundant (LEA) proteins are often associated with desiccation tolerance and stress signalling. In the present study, a group 3 LEA gene, Wsi18 from the wild rice Oryza nivara was expressed under its own inducible promoter element in stress susceptible cultivated indica rice (cv. IR20). The resulting transgenic plants cultivated in a greenhouse showed enhanced tolerance to soil water deficit. Transgenic plants had higher grain yield, plant survival rate, and shoot relative water content compared to wild type (WT) IR20. Cell membrane stability index, proline and soluble sugar content were also greater in transgenic than WT plants under water stress. These results demonstrate the potential for improving SWS tolerance in agronomically important rice cultivar by incorporating Wsi18 gene from a wild rice O. nivara.

Comparative Analysis of the Combined Effects of Different Water and Phosphate Levels on Growth and Biological Nitrogen Fixation of Nine Cowpea Varieties.

Water deficit and phosphate (Pi) deficiency adversely affect growth and biological nitrogen fixation (BNF) of legume crops. In this study, we examined the impact of interaction between soil water conditions and available soil-Pi levels on growth, nodule development and BNF potential of nine cowpea varieties grown on dry savanna soils. In our experimental design, soils with different available soil-Pi levels, i.e., low, moderate, and high soil-Pi levels, collected from various farming fields were used to grow nine cowpea varieties under well-watered and water-deficit conditions. Significant and severe water deficit-damaging effects on BNF, nodulation, growth, levels of plant-nitrogen (N) and -phosphorus (P), as well as shoot relative water content and chlorophyll content of cowpea plants were observed. Under well-watered and high available soil-Pi conditions, cowpea varieties IT07K-304-9 and Dan'Ila exhibited significantly higher BNF potential and dry biomass, as well as plant-N and -P contents compared with other tested ones. Significant genotypic variations among the cowpeas were recorded under low available soil-Pi and water-deficit conditions in terms of the BNF potential. Principal component (PC) analysis revealed that varieties IT04K-339-1, IT07K-188-49, IT07K-304-9, and IT04K-405-5 were associated with PC1, which was better explained by performance for nodulation, plant biomass, plant-N, plant-P, and BNF potential under the combined stress of water deficit and Pi deficiency, thereby offering prospects for development of varieties with high growth and BNF traits that are adaptive to such stress conditions in the region. On another hand, variety Dan'Ila was significantly related to PC2 that was highly explained by the plant shoot/root ratio and chlorophyll content, suggesting the existence of physiological and morphological adjustments to cope with water deficit and Pi deficiency for this particular variety. Additionally, increases in soil-Pi availability led to significant reductions of water-deficit damage on dry biomass, plant-N and -P contents, and BNF potential of cowpea varieties. This finding suggests that integrated nutrient management strategies that allow farmers to access to Pi-based fertilizers may help reduce the damage of adverse water deficit and Pi deficiency caused to cowpea crop in the regions, where soils are predominantly Pi-deficient and drought-prone.

Investigative approaches associated with plausible chemical and biochemical markers for screening wheat genotypes under salinity stress

ABSTRACTTwenty genotypes of wheat resulting from different crossings between some wheat parental lines were compared for salt stress (control and gradually increasing salinity). Ion content in root, shoot, and flag leaves and also the root and shoot dry weights were measured. Based on these results, eight genotypes among the twenty were selected as susceptible, semi-tolerant, and tolerant genotypes for evaluating their biochemical characteristics. Results indicated that concentration of sodium (Na+) and potassium (K+) in shoot, root, and flag leaves of stressed plants were, respectively, higher and lower than that in the non-stressed plants. Overall, salinity stress caused reductions in root and shoot dry weights and relative water content (RWC), but enhancement in pigments content. Concentrations of the total carbohydrate, total protein, and soluble proline were higher in plants under salt stress condition. Salinity stress induced higher production in hydrogen peroxide (H2O2) and malondialdehyde (MDA) and also higher activity of catalase (CAT) and ascorbic peroxidase (APX) as antioxidant enzymes, but lower activity of peroxidase (POD). Genotypes 4s, Arg, and 386dh had generally higher enzymatic activity and other tolerant indices, and hence they can be introduced as tolerant genotypes for more study by the plant breeders. On the other hand, genotype 278s was most susceptible based on the most results.

Cellular Biochemical Changes in Selaginella tamariscina (Beauv.) Spring and Sellaginella plana (Desv. ex Poir.) Heiron. as Induced by Desiccation.

The biochemical changes in two Selaginella species namely, S. tamariscina (Beauv.) Spring and S. plana (Desv. ex Poir.) Heiron., as induced by desiccation and subsequent rehydration were explored. Plants were allowed to dehydrate naturally by withholding irrigation until shoot's relative water content (RWC) reached <10%. After which, dehydrated plants were watered until fully rehydrated states were obtained which was about 90% RWC or more. Desiccation-tolerance characteristics were observed in S. tamariscina while desiccation-sensitivity features were seen in S. plana. Membrane integrity was maintained in S. tamariscina but not in S. plana as evidenced in the relative electrolyte leakage measurements during desiccation phase and the subsequent rehydration stage. Pigment analyses revealed conservation of some chlorophylls and carotenoids during desiccation and reaching control levels following rehydration in S. tamariscina. Very low pigment contents were found in S. plana during desiccation phase and the pigments were not recovered during rehydration attempt. Meanwhile, compatible solute determination showed rise in total sugar and proline contents of desiccated S. tamariscina only, indicating presence of biochemical protection machineries in this species and absence of such in S. plana during dehydrating conditions. These data indicate that one key element for desiccation-tolerance in lower vascular plants is the ability to protect tissues from severe damages caused by intense desiccation.

Comparative Drought Response in Eleven Diverse Alfalfa Accessions

AbstractAlfalfa (Medicago sativa L.) production is negatively affected by drought stress. This is particularly true for alfalfa grown on non‐irrigated rangelands. Thus, the development of drought‐tolerant alfalfa cultivars is of great significance. A greenhouse study was conducted to evaluate 11 alfalfa accessions including several that are adapted to rangeland conditions and two commercial accessions, for their performance under drought condition. Water supply was adjusted based on the transpiration rate of individual plants to compensate for 100, 75, 50 or 25 % of transpirational water loss. We found that RS, a naturalized alfalfa collected from the Grand River National Grassland in South Dakota, showed the best resistance to drought condition. It showed the smallest reduction in stem elongation (36 %), relative growth rate (14 %), and shoot dry mass (40 %) production under the severest drought tested in this study relative to the non‐drought treatment. While RS showed less biomass production under well‐watered conditions, it produced similar or more shoot biomass under drought conditions compared to other accessions. Associated with the drought resistance or less sensitivity to drought, RS showed greater capability to maintain root growth, shoot relative water content, and leaf chlorophyll content compared to other accessions. Different from other accessions, RS showed increasing water use efficiency (WUE) as water deficit became severe, reaching the greatest WUE among 11 accessions. Our results suggest that RS is a valuable genetic resource that can be used to elucidate physiological and molecular mechanisms that determine drought resistance in alfalfa and to develop alfalfa with improved WUE.

Rapid adjustment of leaf angle explains how the desert moss, Syntrichia caninervis, copes with multiple resource limitations during rehydration.

Although the desert moss Syntrichia caninervis Mitt. is extremely desiccation tolerant, it still requires water and photosynthates for growth. The ecological significance of the leaf angle in maintaining a balance between water and light availability is critical to its survival. Active leaf repositioning balances water and light availability following rehydration. S. caninervis can adjust leaf angles from a steep (84-69°) to a stable level at 30° within 7s after rehydration, obtaining maximum net photosynthetic gain at a shoot relative water content of ~60%. Leaf morphological characters, (leaf hair points, surface papillae and costal anatomy) and ultrastructural changes (chloroplast reordering and loss of lipid reserves as shown by changes in osmiophilic globules) were linked to rapid leaf spreading, water gain and sunlight reflectivity of leaves during rehydration. The high 377.20±91.69 (cm2g-1) surface area to mass ratio was a major factor in facilitating the rapid response to rewetting. Hyaline cells of the leaf base absorbed water, swelled and forced the leaf away from the stem as soon as rehydration commenced. Loss of leaf hair points retards leaf angle adjustment during rehydration.

Water redistribution determines photosynthetic responses to warming and drying in two polar mosses.

Predicting impacts of climate change requires an understanding of the sensitivity of species to temperature, including conflated changes in humidity. Physiological responses to temperature and clump-to-air vapour pressure difference (VPD) were compared in two Antarctic moss species, Ceratodon purpureus (Hedw.) Brid. and Schistidium antarctici (Cardot) L.I. Savicz & Smirnova. Temperatures from 8 to 24°C had no significant effects on photosynthesis or recovery from drying, whereas high VPD accelerated drying. In Schistidium, which lacks internal conduction structures, shoots dried more slowly than the clump, and photosynthesis ceased at high shoot relative water content (RWC), behaviour consistent with a strategy of drought avoidance although desiccation tolerant. In contrast, shoots of Ceratodon have a central vascular core, but dried more rapidly than the clump. These results imply that cavitation of the hydroid strand enables hydraulic isolation of extremities during rapid drying, effectively slowing water loss from the clump. Ceratodon maintained photosynthetic activity during drying to lower shoot RWC than Schistidium, consistent with a strategy of drought tolerance. These ecophysiological characteristics may provide a functional explanation for the differential distribution of Schistidium and Ceratodon along moisture gradients in Antarctica. Thus, predicting responses of non-vascular vegetation to climate change at high latitudes requires greater focus on VPD and hydraulics than temperature.

Identification of Sri Lankan rice varieties having osmotic and ionic stress tolerance during the first phase of salinity stress

Salt tolerance of plants during the first phase of salt stress requires a combination of physiological traits conferring tolerance to both osmotic stress and ionic stress. Accordingly, the objectives of the present study were: (a) to screen a substantial number of varieties (102) from the Sri Lankan rice ( Oryza sativa ) germplasm and thereby identify varieties with osmotic and ionic tolerance during Phase I of salt stress development; and (b) to identify the physiological mechanisms that are responsible for osmotic and ionic tolerance. The 102 varieties tested included 51 traditional, 4 old-improved and 47 new-improved varieties, which were screened in nutrient solutions at 100 mM NaCl. Whole plant salt tolerance was quantified using relative leaf area (RL) and relative total dry weight (RW), i.e. leaf area and total dry weight at 100 mM Na+ as ratios of their respective values at 1 mM Na+, the unstressed control. Cluster analysis based on RL and RW enabled classification of Phase I salt tolerance of the screened varieties into five groups as, ‘highly tolerant’ (6 varieties), ‘tolerant’ (25), ‘moderately tolerant’ (32), ‘susceptible’ (30) and ‘highly susceptible’ (9). Capacity to maintain relatively high tissue water content, capacity for Na+ exclusion at the root surface and root xylem and tissue tolerance to excess shoot Na+ were identified as tolerance mechanisms of rice for salinity at Phase I. High shoot tolerance to Na+ contributed to tolerance in 40 out of the 63 varieties identified as Phase I-tolerant to varying degrees. In comparison, high or medium capacity to maintain either whole plant or shoot relative water content contributed to tolerance in 48 varieties, while Na+ exclusion at either of the two control points contributed to tolerance in 33 varieties. Based on the varietal phenotyping of the present study, suitable varieties can be selected: (a) for direct recommendation to areas of mild to transient salinity; (b) as parents in conventional breeding programmes to develop salt tolerant rice varieties; (c) for molecular biological studies to identify genes responsible for Phase I salt tolerance. DOI: http://dx.doi.org/10.4038/jnsfsr.v40i3.4699 J.Natn.Sci.Foundation Sri Lanka 2012 40 (3):251-280

Root development, shoot growth and yields of maize as affected by irrigation schedules in a minor season in tropical Asia

Maize is the most important upland cereal in tropical Asia, grown in both major and minor seasons under rainfed conditions. Due to the inadequate rainfall in the minor season, the crop is subjected to water stress, and irrigation helps to produce high yields. Smallholders who grow maize on flat beds in their allotments often use surface flood irrigation whenever irrigation water is available, which leads in most instances to inefficient use of this valuable resource. A field study was carried out over two minor seasons in Sri Lanka to determine the impact of different schedules of irrigation, developed on the basis of time intervals (3-, 7-, 14- or 21-day intervals or no irrigation as a control), which can easily be practised by smallholders, on the root development, shoot growth, seed yield and water use efficiency of maize. Irrigation at 3-day intervals produced fine roots in the top layers of the soil. Increasing the time interval between irrigation schedules to 7, 14 or 21 days reduced the percentage of fine roots, but developed more, heavier roots in the lower soil layers, as determined by root length densities (RLD) and root weight densities (RWD). Longer irrigation intervals or lack of irrigation resulted in a smaller number of heavier roots in the soil profile. The leaf water potential was affected to a greater degree than shoot water content or relative water content. The seed yield and harvest index were highest when maize was irrigated at 7-day intervals. In contrast, irrigation water use efficiency (IWUE) was highest at an irrigation interval of 14 days. The potential for optimizing water use in surface irrigation in flat beds while obtaining high yields in a tropical Asian minor season, when maize is subjected to moisture stress under smallholding conditions, is presented on the basis of this study.

Salt-induced stress responses of Brassica (Brassica juncea L.) genotypes

The effects of salt stress were evaluated in two Brassica juncea cultivars, Varuna and TM2, in relation to growth, osmolyte accumulation and antioxidant enzymes at 50, 100 and 200 mmol l−1 NaCl concentration. In general, significant reduction in shoot and root growth and relative water content was observed under salt stress.Among the varieties, extent of reduction for these parameters was more pronounced and significant in the case of Varuna than TM2. Significant accumulation of glycine betaine was observed in Varuna and TM2 in 100 and 200 mmol l−1 salt stress treatments than control but the magnitude of increase was significantly higher in TM2. In general, salt stress led to increased activities of Superoxid dismutase, Catalase and Guaiacol peroxidase. The SOD activity was significantly higher in Varuna than TM2. Catalase activity showed an increase up to 100 mmol l−1 NaCl concentration whereas it decreased at 200 mmol l−1 in both the cultivars. Based on the results, TM2 appears to have higher salt tolerance than Varuna. It is also suggestive that osmotic adjustment and antioxidant enzymes play an important role in contributing to salinity tolerance in Brassica juncea L.

Effects of Salinity on Seedling Biomass Production and Relative Water Content of Some Haricot Bean (Phaseolus vulgaris) Varieties

The aim of this research was to study the effects of salinity on fourteen haricot bean (Phaseolus vulgaris) varieties at 0, 2, 4, 8 and 16 dS/m. Data analysis was carried out using jmp 5 (version 5.0) and SPSS (version 12) statistical softwares. Seedling Shoot Fresh Weight (SSFW), Seedling Shoot Dry Weight (SSDW), Seedling Root Fresh Weight (SRFW) and Seedling Root Dry Weight (SRDW) were measured; and seedling Shoot Relative Water Content (SRWC) and seedling Root Relative Water Content (RRWC) were calculated. The two ways ANOVA for varieties found statistically insignificant with respect to Seedling Root Fresh Weight (SRFW), seedling Shoot Relative Water Content (SRWC) and seedling root relative water content (RRWC) (p>0.05) but it was significant with respect to Seedling Shoot Fresh Weight (SSFW), Seedling Shoot Dry Weight (SSDW) and Seedling Root Dry Weight (SRDW) (p 0.05) with respect to the entire parameters. Varieties such as Awash-1, Argene, Chore, Mexican 142 and Awash Melka were found to be salt tolerant during seedling biomass production and in Relative Water Content (RWC). On the other hand, varieties Red Kidney (DRK), Dimtu, Gofta, Cranscope and Sinkinesh were found to be salt sensitive during seedling biomass production and in RWC. The rest haricot bean varieties were intermediate in their salt tolerance. The study affirmed the presence of broad intraspecific genetic variation in haricot bean varieties for salt stress with respect to their early biomass production and Relative Water Content (RWC).