Chlorophyll fluorescence parameters,CO2 photosynthetic rate and regeneration capacity as a result of complete submergence and subsequent re-emergence in rice (Oryza sativa L.)

Regeneration capacity of submerged rice (Oryza sativa) seedlings in terms of CO₂ photosynthetic rate, chlorophyll a fluorescence and chlorophyll and carbohydrate content were investigated in three Indica rice cultivars namely FR 13A, Kalaputia and IR 42 that differed in submergence tolerance. Twenty-one day old plants were completely submerged under water for 8 days. Subsequently, plants were kept under normal conditions with 5–10 cm of stagnant water above soil surface for a further period of 15 days. After complete submergence, all genotypes showed inhibition of CO₂ photosynthetic rate. Submergence treatment resulted in a significant reduction of Rubisco activity. Maximal photochemical efficiency (F_v/F_m) of PS II and area above the fluorescence curve between F_o and F_m decreased more under submergence especially in susceptible cultivar IR 42. When re-aerated, the plants recovered to various degrees. The carbohydrate content of plants was found to be significantly and positively associated with submergence tolerance and regeneration growth. The tolerant cultivar (FR 13A) could survive submergence apparently because it possessed 1.9–2.0 and 3.2–3.7-fold more non-structural carbohydrate content before and after submergence compared to the susceptible cultivar (IR 42) and it had a better capability to restore its photosynthetic capacity during post-submergence periods.     

Submergence-tolerant rice withstands complete submergence even in saline water: Probing through chlorophyll a fluorescence induction O-J-I-P transients

Plants experience multiple abiotic stresses during the same growing season. The implications of submergence with and without saline water on growth and survival were investigated using four contrasting rice cultivars, FR13A (submergence-tolerant, salinity-susceptible), IR42 (susceptible to salinity and submergence), and Rashpanjor and AC39416 (salinity-tolerant, submergence-susceptible). Though both FR13A and IR42 showed sensitivity to salinity, FR13A exhibited higher initial biomass as well as maintained greater dry mass under saline condition. Greater reduction of chlorophyll (Chl) contents due to salinity was observed in the susceptible cultivars, including FR13A, compared to the salinity-tolerant cultivars. Exposure of plants to salinity before submergence decreased the survival chance under submergence. Yet, survival percentage under submergence was greater in FR13A compared to other cultivars. Generally, the reduction in the Chl content and damage to PSII were higher under the submergence compared to salinity conditions. The submergence-tolerant cultivar, FR13A, maintained greater quantities of Chl during submergence compared to other cultivars. Quantification of the Chl a fluorescence transients (JIP-test) revealed large cultivar differences in the response of PSII to submergence in saline and nonsaline water. The submergence-tolerant cultivar maintained greater chloroplast structural integrity and functional ability irrespective of the quality of flooding water.     

Molecular and Physiological Mechanisms of Flooding Avoidance and Tolerance in Rice

Submergence is one of the major constraints in rice production. The main factor limiting rice survival during submergence is oxygen deprivation. To cope with flooding conditions, rice has developed two survival strategies: either rapid elongation of the submerged tissues to keep up with the rising water level or no elongation to save carbohydrate resources for maintenance of energy production under submerged and concomitant hypoxic conditions. The survival strategies used by rice have been studied quite extensively and the role of several phytohormones in the elongation response has been established. The mechanisms of submergence tolerance include metabolic changes, for instance, the shift to an ethanolic fermentation pathway, reduced elongation growth to save carbohydrates and energy for maintenance processes, and protective antioxidant systems. Current molecular technology can provide tools for the understanding of mechanisms developed by rice to survive submergence. In addition, cloning of genes related to submergence tolerance might open new ways to genetic improvement of this crop.     

Flash flooding resistance of rice genotypes of Oryza sativa L., O. glaberrima Steud., and Interspecific hybridization progeny

In the costal area and inland valleys of Guinea, flash floods occur frequently following heavy rain during the rainy season. Young rice seedlings are particularly vulnerable to submergence stress due to insufficient height and carbohydrate reserves. In an attempt to characterize the physiological responses of young seedlings to flash flood, a wide genetic base including O. sativa, O. glaberrima, and interspecific hybridization progenies (IHP) was used.

Twelve day-old seedlings were submerged for 7 days, and plant height, dry matter accumulation (DMA), and lodging were measured. Upland rice (O. sativa) showed greater shoot elongation, larger reduction in DMA during submergence, and higher lodging, which lead to low flash flooding resistance (FFR). The physiological traits of most O. glaberrima and upland rice (O. sativa) to flash flood were opposite to those of submergence tolerant cultivars, as evidenced from the results of a principal component analysis. The physiological response of Saligbeli was different to other O. glaberrima genotypes in terms of FFR. Saligbeli exhibited enhanced shoot elongation with the increase in DMA during submergence. These features seemed to be a unique way to cope with submergence. Some IHP adapted to lowland cultivation showed traits similar to Saligbeli. The vigorous growth of Saligbeli and IHP underwater should be further investigated for improving FFR in rice.     

Root growth,aerenchyma development,and oxygen transport in rice genotypes subjected to drought and waterlogging

Soils under field conditions may experience fluctuating soil water regimes ranging from drought to waterlogging. The inability of roots to acclimate to such changes in soil water regimes may result in reduced growth and function thereby, dry matter production. This study compared the root and shoot growth, root aerenchyma development, and associated root oxygen transport of aerobic and irrigated lowland rice genotypes grown under well-watered (control), waterlogged, and droughted soil conditions for 30 days. The aerobic genotypes were as tolerant as the irrigated lowland genotypes under waterlogging because of their comparable abilities to enhance aerenchyma that effectively facilitated O₂ diffusion to the roots for maintaining root growth and dry matter production. Under drought, aerobic genotypes were more tolerant than the irrigated lowland genotypes due to their higher ability to maintain nodal root production, elongation, and branching, thus, less reduction in dry matter production. Aerenchyma was also formed in droughted roots regardless of genotypes, but was resistant to internal O₂ transport under O₂ deficiency. The ability of roots to resist temporal variations in drought and waterlogging stresses might have strong implications for the adaptation of rice growing in environments with fluctuating soil water regimes.     

Mechanism associated with nonstructural carbohydrate accumulation in submergence tolerant rice (Oryza sativa L.) cultivars

Nonstructural carbohydrate (NSC) accumulation in submergence tolerant rice cultivars (cv) was studied in six Indica rice [Oryza sativa (L.)] cv under control and simulated submerged conditions. Tolerant cultivars accumulated greater contents of NSC compared to the susceptible cultivars. Starch and total NSC content showed significant positive association with survival percentage. On the other hand, elongation due to submergence was significantly a negative association with survival. The CO₂ photosynthetic rate, chlorophyll content, maximum photochemical efficiency of PS II (Fv/Fm), and activities of Rubisco were not significantly different between tolerant and susceptible cv under control condition. The ADP glucose pyrophosphorylase (AGPPase) activity was significantly higher in the tolerant cv and was a positive association with starch/NSC, whereas Fructose 1,6-diphosphatase (FDPase) activity was significantly higher in susceptible cv compared to tolerant cv and was a negative association with starch/NSC. Greater activities of AGPPase along with lower activities of FDPase might facilitate greater accumulation of NSC in tolerant rice cultivars.     

Screening tolerance to phosphorus starvation and haplotype analysis using phosphorus uptake 1 (Pup1) QTL linked markers in rice genotypes

Phosphorus (P) deficiency tolerance is a pivotal trait for plant growth and development. Most of the commercial modern cultivars lack this trait and reported it as a very serious problem limiting crop productivity. This trait is advantageous if present in modern high yielding varieties as it increases the yield under the phosphorus-deficient soil conditions. With the importance of phosphorus deficiency tolerance, the present investigation was carried out with an objective to screen for tolerance to phosphorus deficiency using solution culture and phosphorus uptake 1 (Pup1) locus linked markers in 30 diverse rice genotypes. A wide range of varied responses to P deficiency in rice genotypes for all the traits were observed. Root length and enzyme activity showed increased mean performance under the − P condition when compared to + P condition. Medium to high heritability estimates were obtained for most of the traits. Correlation analysis showed that the traits: root P content, fresh shoot weight, dry shoot weight, and shoot length showed highly significant correlations with each other under − P conditions. Based on the hydroponics and molecular screening, three genotypes viz., ADT (R) 48, Improved Pusa Basmati 1 and UPLRI 5 were classified as tolerant for its response to P deficiency as they possessed significant increase in desirable root and shoot traits, increased acid phosphatase enzyme and these genotypes also possessed the Pup1 allele for all the five markers. The selected genotypes may be useful for the exploration of novel genes conferring phosphorus deficiency tolerance and used as donor parents in the breeding programs. Absence of this allele in the rice genotypes viz., drought tolerant (Anna (R) 4) and submergence tolerant (CR 1009 Sub 1) may warrant the development of multiple abiotic stress tolerance cultivars for upland and submergence cropping systems in future rice breeding program.Electronic supplementary materialThe online version of this article (10.1007/s12298-020-00903-1) contains supplementary material, which is available to authorized users.     

Submergence tolerance in relation to variable floodwater conditions in rice

Flash floods adversely affect rice productivity in vast areas of rainfed lowlands in South and Southeast Asia and tropical Africa. Tolerant landraces that withstand submergence for 1–2 weeks were identified; however, incorporation of tolerance into modern high-yielding varieties through conventional breeding methods has been slow because of the complexity of both the tolerance phenotype and floodwater conditions, and the ensuing discrepancies encountered upon phenotyping in different environments. Designing an effective phenotyping strategy requires a thorough understanding of the specific floodwater characteristics that most likely affect survival during flooding. We investigated the implications of floodwater temperature and light penetration, caused by artificial shading, seasonal variation, or water turbidity, for seedling survival after submergence. Three field experiments were conducted using rice genotypes contrasting in their tolerance of submergence: FR13A and Kusuma (tolerant); Gangasiuli (intermediate); Sabita, CRK-2-6 and Raghukunwar (elongating/avoiding types); and IR42 (sensitive). We tested the hypotheses that warmer floodwater decreases plant survival and that turbid water augment plant mortality by causing effects similar to those caused by shading, by reducing light penetration. Plants survive better when water is cooler, and survival decreased at about 8% per unit increase in water temperature above 26 °C. Lower intensity of light and warmer temperatures seem to reduce biomass and increase mortality under flooding. An increase in the concentrations of O₂ and CO₂ and a decrease in water pH did not improve survival in clear unshaded water. Turbid floodwater was more damaging to rice as plant mortality increased as the percentage of silt increased, and the effects of water turbidity cannot be explained by the reduction in light penetration alone. Even the most tolerant rice cultivar, FR13A, experienced higher mortality when flooded with turbid floodwater. Correlation studies revealed that cultivars with the capacity to maintain higher biomass, higher chlorophyll, and non-structural carbohydrate concentrations after submergence had higher survival. These findings help to understand the variation observed in submergence tolerance when screening is done under different environments. The study could have implications for designing proper screening strategies and assessing the damage submergence causes across different rice-growing regions.     

Metabolic changes in rice seedlings with different submergence tolerance after desubmergence

When flash flood intolerant rice cultivars are submerged, they show greater morphological changes such as elongation and chlorosis than tolerant cultivars. These morphological responses are caused by ethylene produced during submergence, however, a visible damage of intolerant cultivars is markedly developing after desubmergence rather than during submergence, which is probably due to oxidative damage. We studied the effect of ethylene produced during submergence on antioxidant content and oxidative damage after desubmergence. When rice (Oryza sativa) was submerged for 8 days, both tolerant cultivar (BKNFR) and intolerant cultivars (Mashuri and IR42) showed a decrease in ascorbate concentration during submergence. After 3 days of desubmergence, the tolerant cultivar showed a rapid recovery of total ascorbate and ascorbic acid, whereas intolerant cultivars showed a slow recovery of them, an increase in malondialdehyde formation, and low survival rate (about 30%). However, applying 200 mg l⁻¹ of AgNO₃ as an ethylene antagonist to intolerant cultivars suppressed the decrease in ascorbate and the increase in malondialdehyde formation after desubmergence, and improved survival rate to about 60%. Ascorbic acid supply to leaf discs from submerged IR42 suppressed increase in malondialdehyde formation by incubation under the light for 24 h. In addition, strong negative correlations were observed between malondialdehyde formation with ascorbate concentration (r=−0.93) and with percentage of survival (r=−0.98). Our results indicate that the accumulated ethylene during submergence adversely affected antioxidant mechanism in intolerant rice cultivars after desubmergence, and ascorbic acid was an important antioxidant in vivo for the recovery of submerged rice seedlings.     

Response of Contrasting Rice Genotypes to Zinc Sources under Saline Conditions

Abiotic stresses are among the major limiting factors for plant growth and crop productivity. Among these, salinity is one of the major risk factors for plant growth and development in arid to semi-arid regions. Cultivation of salt tolerant crop genotypes is one of the imperative approaches to meet the food demand for increasing population. The current experiment was carried out to access the performance of different rice genotypes under salinity stress and Zinc (Zn) sources. Four rice genotypes were grown in a pot experiment and were exposed to salinity stress (7 dS m⁻¹), and Zn (15 mg kg⁻¹ soil) was applied from two sources, ZnSO₄ and Zn-EDTA. A control of both salinity and Zn was kept for comparison. Results showed that based on the biomass accumulation and K⁺/Na⁺ ratio, KSK-133 and BAS-198 emerged as salt tolerant and salt sensitive, respectively. Similarly, based on the Zn concentration, BAS-2000 was reported as Zn-in-efficient while IR-6 was a Zn-efficient genotype. Our results also revealed that plant growth, relative water content (RWC), physiological attributes including chlorophyll contents, ionic concentrations in straw and grains of all rice genotypes were decreased under salinity stress. However, salt tolerant and Zn-in-efficient rice genotypes showed significantly higher shoot K⁺ and Zn concentrations under saline conditions. Zinc application significantly alleviates the harmful effects of salinity by improving morpho-physiological attributes and enhancing antioxidant enzyme activities, and the uptake of K and Zn. The beneficial effect of Zn was more pronounced in salt-tolerant and Zn in-efficient rice genotypes as compared with salt-sensitive and Zn-efficient genotypes. In sum, our results confirmed that Zn application increased overall plant’s performance under saline conditions, particularly in Zn in-efficient and tolerant genotypes as compared with salt-sensitive and Zn efficient rice genotypes.     

The beneficial effect of reduced elongation growth on submergence tolerance of rice

Adverse effects of elongation growth on tolerance to completesubmergence for up to 14 d were evaluated in rice seedlingsof cultivars which differed in submergence tolerance. Thereis a good negative correlation between per cent survival andelongation growth of genotypes during complete submergence (r= – 0.81). When elongation growth underwater is minimizedby application of a gibberellin biosynthesis inhibitor, percent survival increases by as much as 50 times for one cultivar.These effects are likely related to elongation growth since(i) addition of gibberellin had the opposite effect by reducingsurvival, and (ii) when the elongation inhibitor and gibberellinwere added together, there was no effect on elongation growthand the per cent survival did not change. A GA-deficient mutantof rice which had little elongation ability during submergenceshowed a high level of submergence tolerance when plants weresubmerged at equal initial dry weights and carbohydrate levelsrelative to a submergence-tolerant cultivar. These results areconsistent with the hypothesis that elongation growth competeswith maintenance processes for energy and hence reduces survivalduring submergence. The impact of these findings is that inenvironments where elongation ability is not required, thereis a potential to increase submergence tolerance of agriculturallyimportant cultivars by selecting for least elongation, at leastduring periods of complete submergence. Furthermore, this trade-offbetween stimulated elongation growth and submergence tolerancewill have important ecological consequences for the distributionof plant species in different flood-prone environments. Key words: Gibberellin, growth, Oryza sativa, rice, submergence     

Comprehensive physiological analyses and reactive oxygen species profiling in drought tolerant rice genotypes under salinity stress

Rice being a staple cereal is extremely susceptible towards abiotic stresses. Drought and salinity are two vital factors limiting rice cultivation in Eastern Indo-Gangetic Plains (EIGP). Present study has intended to evaluate the consequences of salinity stress on selected drought tolerant rice genotypes at the most susceptible seedling stage with an aim to identify the potential multi-stress (drought and salt) tolerant rice genotype of this region. Genotypic variation was obvious in all traits related to drought and salt susceptibility. IR84895-B-127-CRA-5-1-1, one of the rice genotypes studied, exhibited exceptional drought and salinity tolerance. IR83373-B-B-25-3-B-B-25-3 also displayed enhanced drought and salt tolerance following IR84895-B-127-CRA-5-1-1. Variations were perceptible in different factors involving photosynthetic performance, proline content, lipid peroxidation, K⁺/Na⁺ ratio. Accumulation of reactive oxygen species (ROS) disintegrated cellular and sub-cellular membrane leading to decreased photosynthetic activities. Therefore, accumulation and detoxification of reactive oxygen species was also considered as a major determinant of salt tolerance. IR84895-B-127-CRA-5-1-1 showed improved ROS detoxification mediated by antioxidant enzymes. IR84895-B-127-CRA-5-1-1 seedlings also displayed significant recovery after removal of salt stress. The results established a direct association of ROS scavenging with improved physiological activities and salt tolerance. The study also recommended IR84895-B-127-CRA-5-1-1 for improved crop performance in both drought and saline environments of EIGP. These contrasting rice genotypes may assist in understanding the multiple stress associated factors in concurrent drought and salt tolerant rice genotypes.     

Differential responses of antioxidant system and photosynthetic characteristics in four rice cultivars differing in sensitivity to sodium chloride stress

To keep pace with ever growing global population, progressive and sustained increase in rice production is necessary, especially in areas with extremely variable climatic conditions, where rice crop suffers from numerous abiotic stresses including salinity. Designing an effective phenotyping strategy requires thorough understanding of plant survival under stress. The investigation was carried out with four rice cultivars namely FR13A, IR42, Rashpanjor, and Pokkali that differed in salinity tolerance. The study showed that a genotype with initial vigour had some advantage in preserving shoot biomass under salt stress. Though both FR13A and IR42 showed sensitivity to salinity, FR13A with higher initial biomass maintained greater dry weight under saline condition. Increase of Na⁺:K⁺ ratio under salinity, due to accelerated absorption of Na⁺ and lesser absorption of K⁺ compared to control, was considerably higher in susceptible (118–200 %) than in tolerant (33–48 %) genotypes. While Na⁺ concentration in shoot increased significantly in both tolerant and susceptible genotypes, decrease in shoot K⁺ content was noticed only in susceptible genotypes. The imbalance of Na⁺ and K⁺ contents led to increased H₂O₂ production, causing greater peroxidation of membrane lipids and reduction in chlorophyll content and CO₂ photosynthetic rate. Certain chlorophyll fluorescence parameters could distinguish between salinity tolerant and sensitive genotypes. To protect the plant from oxidative damage, several enzymatic and nonenzymatic antioxidants such as ascorbate were involved. The genotypes with capacity to assemble antioxidant enzymes in time could detoxify the reactive oxygen species more efficiently, leading to greater protection and reduced impact of salt stress.     

Role of antioxidant and anaerobic metabolism enzymes in providing tolerance to maize (Zea mays L.) seedlings against waterlogging

The present investigation was undertaken to identify the possible mode of mechanism that could provide tolerance to maize (Zea mays L.) seedlings under waterlogging. Using cup method, a number of maize genotypes were screened on the basis of survival of the seedlings kept under waterlogging. Two tolerant (LM5 and Parkash) and three susceptible (PMH2, JH3459 and LM14) genotypes were selected for the present study. Activities of antioxidant and ethanolic fermentation enzymes and content of hydrogen peroxide (H2O2), glutathione and ascorbic acid were determined in roots of these genotypes after 72 h of waterlogging. Waterlogging treatment caused decline in activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) in all the genotypes. However, only susceptible genotypes showed slight increase in glutathione reductase (GR) activity. Significant reduction in APX/GR ratio in susceptible genotypes might be the cause of their susceptibility to waterlogging. The tolerant seedlings had higher GR activity than susceptible genotypes under unstressed conditions. Stress led to decrease in H202 and increase in glutathione content of both tolerant and susceptible genotypes, but only tolerant genotypes exhibited increase in ascorbic acid under waterlogging conditions. In the tolerant genotypes, all the enzymes of anaerobic metabolism viz. alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH) and pyruvate decarboxylase (PDC) were upregulated under waterlogging, whereas in susceptible genotypes, only ADH was upregulated, suggesting that efficient upregulation of entire anaerobic metabolic machinery is essential for providing tolerance against waterlogging. The study provides a possible mechanism for waterlogging tolerance in maize.     

Mechanisms associated with tolerance to flooding during germination and early seedling growth in rice (Oryza sativa)

Background and Aims

Flooding slows seed germination, imposes fatalities and delays seedling establishment in direct-seeded rice. This study describes responses of contrasting rice genotypes subjected to flooding or low oxygen stress during germination and discusses the basis of tolerance shown by certain cultivars.

Methods

In one set of experiments, dry seeds were sown in soil and either watered normally or flooded with 10 cm of water. Seedling survival and shoot and root growth were assessed and seed portions of germinating seedlings were assayed for soluble sugars and starch concentrations. The whole germinating seedlings were assayed for amylase and peroxidase activities and for ethylene production. Activities of enzymes associated with anaerobic respiration were examined and gene expression was analysed separately with seeds germinating under different amounts of dissolved oxygen in dilute agar.

Key Results

Flooding during germination reduced survival but to a lesser extent in tolerant genotypes. Starch concentration in germinating seeds decreased while sugar concentration increased under flooding, but more so in tolerant genotypes. Amylase activity correlated positively with elongation (r = 0·85 for shoot and 0·83 for root length) and with plant survival (r = 0·92). Tolerant genotypes had higher amylase activity and higher RAmy3D gene expression. Ethylene was not detected in seeds within 2 d after sowing, but increased thereafter, with a greater increase in tolerant genotypes starting 3 d after sowing. Peroxidase activity was higher in germinating seeds of sensitive genotypes and correlated negatively with survival.

Conclusions

Under low oxygen stress, tolerant genotypes germinate, grow faster and more seedlings survive. They maintain their ability to use stored starch reserves through higher amylase activity and anaerobic respiration, have higher rates of ethylene production and lower peroxidase activity as germinating seeds and as seedlings. Relevance of these traits to tolerance of flooding during germination and early growth is discussed.Key words: Amylase, anoxia, crop establishment, direct-seeded rice, ethylene, flooding, germination, hypoxia, Oryza sativa     

Morphological and physiological responses of rice seedlings to complete submergence (flash flooding)

Background and Aims

Reducing damage to rice seedlings caused by flash flooding will improve the productivity of rainfed lowland rice in West Africa. Accordingly, the morphological and physiological responses of different forms of rice to complete submergence were examined in field and pot experiments to identify primary causes of damage.

Methods

To characterize the physiological responses, seedlings from a wide genetic base including Oryza sativa, O. glaberrima and interspecific hybrids were compared using principle component analysis.

Key Results

Important factors linked to flash-flood tolerance included minimal shoot elongation underwater, increase in dry matter weight during submergence and post-submergence resistance to lodging. In particular, fast shoot elongation during submergence negatively affected plant growth after de-submergence. Also shoot-elongating cultivars showed a strong negative correlation between dry matter weight of the leaves that developed before submergence and leaves developing during submergence.

Conclusions

Enhancement of shoot elongation during submergence in water that is too deep to permit re-emergence by small seedlings represents a futile escape strategy that takes place at the expense of existing dry matter in circumstances where underwater photosynthetic carbon fixation is negligible. Consequently, it compromises survival or recovery growth once flood water levels recede and plants are re-exposed to the aerial environment. Tolerance is greater in cultivars where acceleration of elongation caused by submergence is minimal.Key words: Africa, flash floods, Oryza glaberrima, rainfed lowland, rice, shoot elongation, stress tolerance, submergence     

Responses of the antioxidative enzymes in Malaysian rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars under submergence condition

The potential involvement of activated oxygen species by submergence stress was studied in two Malaysian rice cultivars, MR219-4 and MR219-9, and cultivar FR13A that is known to be tolerant to submergence. Seedlings of these three rice cultivars were subjected to different submergence periods (4, 8, and 12 days). Under 8 days of complete submergence, FR13A cultivar showed higher lipid peroxidation in terms of malondialdehyde level and activities of antioxidative enzymes, superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) when compared to the MR219-4 and MR219-9 cultivars. MR219-9 showed higher SOD, APX, and GR activities after 12 days of submergence. The levels of SOD activity indicated that detoxification of O₂^·− to H₂O₂ was maintained at a stable level throughout the submergence stress until up to 8 days and increased rapidly at 12 days of submergence. The results indicated that tolerance to submergence in rice is associated until 8 days submergence for MR219-4 and FR13A cultivars. These findings suggested that tolerance to submergence stress in rice might be proven by increased the capacity of antioxidative system. In addition, CAT activity has much higher affinity for scavenges H₂O₂ than APX. Therefore, ascorbate glutathione cycle might be more efficient to scavenge H₂O₂.     

Morphological and physiological characterization of different genotypes of faba bean under heat stress

Heat stress (HS) is the major constraint to crop productivity worldwide. The objective of the present experiment was to select the tolerant and sensitive genotype(s) on the basis of morpho-physiological and biochemical characteristics of ten Vicia faba genotypes. These genotypes were as follows: Zafar 1, Zafar 2, Shebam 1, Makamora, Espan, Giza Blanka, Giza 3, C4, C5 and G853. The experimental work was undertaken to study the effects of different levels of temperature (control, mild, and modest) on plant height (PH) plant⁻¹, fresh weight (FW) and dry weight (DW) plant⁻¹, area leaf⁻¹, content of leaf relative water (RWC), proline content (Pro) and total chlorophyll (Total Chl), electrolyte leakage (EL), malondialdehyde level (MDA), hydrogen peroxide (H₂O₂), and activities of catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) enzymes. HS significantly affected growth performance of all genotypes. However, the magnitude of reduction in genotypes ‘C5’ was relatively low, possibly due to its better antioxidant activities (CAT, POD and SOD), and accumulation of Pro and Total Chl, and leaf RWC. In the study, ‘C5’ was noted to be the most HS tolerant and ‘Espan’ most HS sensitive genotypes. It was concluded that the heat-tolerant genotypes may have better osmotic adjustment and protection from free radicals by increasing the accumulation of Pro content with increased activities of antioxidant enzyme.     

Responses of growth and antioxidant systems in <Emphasis Type="Italic">Carthamus</Emphasis><Emphasis Type="Italic">tinctorius</Emphasis> L. under water deficit stress

An investigation was carried out to find out the extent of changes occurred in two safflower (Carthamus tinctorius L.) cultivars in response to water deficit stress. Two safflower cultivars namely IL.111 and Isfahan were used for the study. Thirty days after sowing, plants were grown under soil moisture corresponding to 100, 85, 70 and 55% field capacity for next 30 days. Water deficit treatments significantly decreased the shoot length, shoot dry matter, root dry matter, relative growth rate, leaf relative water content (LRWC) and leaf water potential (Ψ_W), whereas root length, root-to-shoot ratio, lipid peroxidation and antioxidant compounds such as ascorbic acid (AA), α-tocopherol (α-Toc) and reduced glutathione (GSH) and superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), and peroxidase (POX, EC 1.11.1.7) activities were increased. Water deficit stressed plants maintained higher levels of compounds and scavenging enzymes. Significant differences were observed between cultivars and irrigation levels treatments. The cv. IL.111 could be considered more tolerant to water stress than cv. Isfahan, registering greater biomass, LRWC and leaf water potential (Ψ_W), associated with high antioxidant activity.     

Discerning of Rice Landraces (Oryza sativa L.) for Morpho-physiological,Antioxidant Enzyme Activity,and Molecular Markers’ Responses to Induced Salt Stress at the Seedling Stage

Salinity has been identified as key abiotic stress factor limiting rice production in many countries around the globe, including Bangladesh. In the present study, we examined the effects of salt-induced toxicity on growth of rice landraces for screening salt-tolerant genotypes by assessing morpho-physiological, biochemical, and molecular responses. Screening of 28 rice genotypes at seedling stage was performed at 12 dS m⁻¹ salinity level in hydroponic media. Most of the rice genotypes showed an apparent reduction in growth traits, while a fewer showed less reduction under salinity stress. Euclidean clustering and heatmap based on morpho-physiological parameters dissected rice genotypes into three major clusters, viz., susceptible, moderately tolerant, and tolerant. Results of cluster analysis revealed Nonabokra, Hogla, Ghunsi, Holdegotal, Nonabokra, and Kanchon as salt-tolerant rice genotypes. These genotypes also were grouped using three microsatellite markers, viz., RM493, RM3412b, and RM140 that were closely linked to saltol QTL showed Hogla, Ghunsi, Holdegotal, Nonabokra, Kanchon, BINA dhan-8, and BINA dhan-10 as salt-tolerant genotypes considering genetic similarity in dendrogram. The positive relationships of Na⁺/K⁺ ratio with hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and antioxidant enzymes’ activity in the tolerant rice genotypes indicated their importance for improving salinity tolerance. The salt-tolerant landraces showed lower Na⁺/K⁺ ratio, high proline accumulation, lower H₂O₂ accumulation and MDA content, and higher catalase and ascorbate peroxidase activities. Higher antioxidant enzymes’ activity and lower H₂O₂ accumulation in tolerant genotypes indicate their abilities to fight against oxidative stress. Based on all morpho-physiological clustering, biochemical response, and molecular dendrogram, Nonabokra, Hogla, Ghunsi, Holdegotal, and Kanchon were identified as the salt-tolerant landraces. Therefore, these identified salt-tolerant landraces could be useful to improve breeding program for the development of salt-tolerant high-yielding rice cultivars in future.