Habib-ur-Rehman Athar

Strategies for Crop Improvement Against Salinity and Drought Stress: An Overview

Abiotic stresses such as salinity, drought, nutrient defi ciency or toxicity, and fl ooding limit crop productivity world-wide. However, this situation becomes more problematic in developing countries, where they cause food insecurity for large populations and poverty, particularly in rural areas. For example, drought stress has affected more than 70 million hectares of rice-growing land world-wide. While salt stress and nutrient stress render more than 100 million hectares of agricultural land uncultivable thereby resulting in low outputs, poor human nutrition and reduced educational and employment opportunities. Thus, abiotic stresses are the major factors of poverty for millions of people. In this scenario, it is widely urged that strategies should be adopted which may be used to get maximum crop stand and economic returns from stressful environments. Major strategies include breeding of new crop varieties, screening and selection of the existing germ-plasm of potential crops, production of genetically modifi ed (GM) crops, exogenous use of osmoprotec-tants etc. In the last century, conventional selection and breeding program proved to be highly effective in improving crops against abiotic stresses. Therefore, breeding for abiotic stress tolerance in crop plants (for food supply) should be given high research priority. However, extent and rate of progress in improving stress tolerance in crops through conventional breeding program is limited. This is due to complex mechanism of abiotic stress tolerance, which is controlled by the expression of several minor genes. Furthermore, techniques employed for selecting tolerant plants are time consumable and consequently expensive. During the last decade, using advanced molecular biology techniques different researchers showed some promising results in understanding molecular mechanisms of abiotic stress tolerance as well as in inducing stress tolerance in some potential crops. These fi ndings emphasized that future research should focus on molecular, physiological and metabolic aspects of stress tolerance to facilitate the development of crops with an inherent capacity to withstand abiotic stresses. This would help stabilize the crop production, and signifi cantly contribute to food and nutritional security in developing countries and semi-arid tropical regions.

Influence of nickel stress on growth and some important physiological/biochemical attributes in some diverse canola (Brassica napus L.) cultivars

To assess the effect of nickel on six canola cultivars a series of experiments were conducted. On the basis of shoot dry weight cvs. Shiralee and Range found to be nickel tolerant, Dunkeld and Ester as nickel sensitive, while the remaining cultivars intermediate. Nickel accumulation in shoots was lower in the nickel sensitive cultivars followed by that in the tolerant ones. Leaf water and osmotic potentials decreased significantly due to high concentration of Ni(2+). Decrease in osmotic potential was positively associated with accumulation of total free amino acids. By comparing accumulation of individual amino acids, pattern of accumulation of the amino acids was different in different cultivars. However, only histidine, serine and cysteine increased in appreciable amount in the xylem sap of different canola cultivars. Overall, nickel tolerant cultivars Shiralee and Range showed higher levels of histidine, serine and cysteine under varying levels of nickel than the others. This higher accumulation of histidine, serine and cysteine was positively related to nickel tolerance in all canola cultivars. Thus, differential nickel tolerance in canola cultivars proposed to be associated with relative detoxification of Ni by developing complexes with histidine, serine and cysteine and can be used as potential indicators of nickel tolerance in canola.

Photosynthetic capacity of canola ( Brassica napus L.) plants as affected by glycinebetaine under the salt stress

Salinity causes reduction in growth and severe losses of crop productivity by affecting various biochemical and physiological processes including photosynthesis. Plants sense and adapt to salt stress by modulating different physiological processes including accumulation of osmoprotectants. Glycinebetaine (GB) is an important osmoprotectant and found to have ameliorative effects on the growth of plants by altering ion homeostasis, photosynthetic and antioxidant capacity. In this study, it was attempted to reveal how GB improves photosynthetic activity and salt tolerance of two canola cultivars which differ in degree of salt tolerance. Two cultivars (Dunkled and Cyclone) of canola ( Brassica napus L.) were grown under non-saline or saline (150 mM NaCl) conditions. Glycinebetaine (100 mM) was foliarly applied to both non-stressed and salt stressed plants of both canola cultivars at the vegetative growth stage. Salt stress reduced growth of both canola cultivars, however, cv. Dunkled was superior to cv. Cyclone. Foliar application of GB improved leaf relative water contents (RWC), osmotic potential and proline accumulation in salt stressed plants. The chlorophyll fluorescence transient (CFT) remained unchanged at O and J phase while at I and P phase it was affected by salt stress in both cultivars that was ameliorated by GB application. The positive values of K band after 1000 µs under salt stress revealed the reduced efficiency of oxygen evolving complex (OEC). The GB application enhanced electron transport chain and decreased heat dissipation under salt stress. This effect was more in cultivar Cyclone as compared to Dunkled. Furthermore, a considerable proteomic variation was noted in canola cultivars after application of GB under both saline and non-saline condition. The results suggested that exogenous foliar application of GB ameliorated the adverse effects of salt stress on both cultivars of canola by osmotic adjustment, growth improvement, increased light absorption by reaction centers, efficient energy trapping and enhanced electron transport chain in both cultivars of canola.

Growth enhancement in two potential cereal crops, maize and wheat, by exogenous applicationation of glycinebetaine

Ameliorative effect of exogenously applied glycinebetaine (GB) on growth, photosynthetic and antioxidant capacities of two potential cereals wheat (cv. S-24) and maize (cv. Golden) grown under salt stress was assessed in two different independent experiments. Plants of maize were grown at 0 or 10 dS/m NaCl, while those of wheat were subjected to 2.17 or 14.67 dS/m NaCl salinity. Different levels of GB, i.e., 0 (unsprayed), 50 and 100 mM (in 0.10% Tween-20 solution) were applied as a foliar spray to both wheat and maize plants at the vegetative growth stage. Salt stress reduced the growth and yield of both maize and wheat plants. However, salt-induced reduction in growth and yield of both maize and wheat was ameliorated by exogenous application of GB, but this enhancement effect was more in wheat than that in maize. Furthermore, this GB-induced growth and yield enhancement was positively associated with increased endogenous GB, photosynthetic capacity, and superoxide dismutase (SOD) activity. Although exogenous application of GB improved photosynthetic capacity of both maize and wheat by increasing stomatal conductance, and thus favoring higher CO2 fixation rate, this effect seemed to be partial in maize. In addition, the GB-induced reduction in transpiration rate in wheat compared with that in maize was found to be an additional factor that might have contributed to a better growth and yield of wheat under salt stress. The activity of only SOD was enhanced by GB application in both maize and wheat under saline conditions. Thus, it is likely that both applied GB and intrinsic SOD scavenged reactive oxygen species in these potential cereals under saline conditions. In view of all these findings, it can be concluded that the adverse effects of salt stress on cereals such as maize and wheat can be alleviated by the exogenous application of GB, which in turn enhances photosynthetic capacity and modulates activities of antioxidant enzymes. Furthermore, effectiveness of GB application on regulation of photosynthetic and antioxidant capacities was found to be species specific.

Influence of urea application on growth, yield and mineral uptake in two corn (Zea mays L.) cultivars

A pot experiment was conducted in a wire netting green house in order to assess the beneficial effect of urea on corn cultivars (C-20 and C-79) differing in yield production. Corn plants were grown in loam soil with alkaline in reaction. Application of varying urea levels did not change the physico-chemical properties of soil. Four weeks-old corn plants were subjected to varying levels of urea (0, 50, 100, 175 and 225 kg/ha urea) for 8 (mid-season harvest) and 15 weeks (final harvest). After which, plants growth, yield and mineral nutrient status of the two corn cultivars were assessed for both mid-season and final harvest. It was observed that cv. C-20 showed higher (P<0.1) growth and yield. Addition of varying levels of urea enhanced the growth and yield of both the corn cultivars. However, 175 kg urea /ha was found to be more effective (P<0.1) in enhancing growth and yield of both the cultivars. The rate of urea application at a rate of 175 kg/ha had greater beneficial effect on cv. C-20 due to improved mineral nutrient (N, P, K, Cu, Fe, Mn and Zn) status of the cultivar. Keywords: Corn cultivars, fertilizer treatment, urea, mineral nutrition, plant growth

Foliar Application of Micronutrients in Mitigating Abiotic Stress in Crop Plants

Balanced and precise crop nutrients application is a pre-requisite tool for meeting the second Sustainable Development Goals for achieving food security and improved human nutrition and to promote agriculture under stressful environments. Large increase in productivity cannot be attained without ensuring that plants are supplied with adequate and balanced mineral nutrients. Mineral fertilizers are an important basic resource for nearly half of the world’s crop production. The productivity of food, feed, fiber, and biofuel is ought to be paralleled with global population that is expected to reach nine billion before the middle of twenty-first century. Among the nutrient management system, the foliar feeding provides a potential prospective option to meet the diversity of site- and crop-specific conditions, irrigation water supplies, eco-edaphic factors and farm management, and also concurrent economic and environmental prospective. Taking trade-off into account, the foliar feeding of essential nutrients will accrue in mitigating the negative effects of abiotic stresses, with concurrent maximizing productivity. This technique is cost-effective and beneficial under stressful conditions. The plants well-fed with essential nutrients have greater tolerance capacity in response to abiotic stresses. Thereby foliar feeding of nutrients is also referred as climate-smart agriculture. The response of foliar feeding varies greatly due to crop species, growth stages, concentration of added nutrient solution, and the relative water content in the plant parts. The plant nutrients, their importance in crop production, and some insights into how to best manage them by foliar feeding method in response to abiotic stresses are discussed.

Engineering Rubisco activase from thermophilic cyanobacteria into high-temperature sensitive plants

Abstract In the past decade, various strategies to improve photosynthesis and crop yield, such as leaf morphology, light interception and use efficiency, biochemistry of light reactions, stomatal conductance, carboxylation efficiency, and source to sink regulation, have been discussed at length. Leaf morphology and physiology are tightly coupled to light capturing efficiency, gas exchange capacity, and temperature regulation. However, apart from the photoprotective mechanism of photosystem-II (PSII), i.e. non-photochemical quenching, very low genetic variation in the components of light reactions has been observed in plants. In the last decade, biochemistry-based enhancement of carboxylation efficiency that improves photosynthesis in plants was one of the potential strategies for improving plant biomass production. Enhancement of activation of the ubiquitous enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) by Rubisco activase may be another potential strategy for improving a photosynthesis-driven increase in crop yield. Rubisco activase modifies the conformation of the active center in Rubisco by removing tightly bound inhibitors, thereby contributing to enzyme activation and rapid carboxylation. Thermophilic cyanobacteria are oxygenic photosynthetic bacteria that thrive in high-temperature environments. This critical review discusses the prospects for and the potential of engineering Rubisco activase from thermophilic cyanobacteria into temperature-sensitive plants, to increase the threshold temperature and survival of these plants in arid regions.

Improving drought tolerance in maize by foliar application of boron: water status, antioxidative defense and photosynthetic capacity

ABSTRACT Boron (B) plays a vital role in cell division and elongation in apical meristem. Drought stress (DS) severely reduced the B-uptake and thus growth and crop-productivity of plants. The aims of this study were to evaluate whether foliar application of B corrects physiological-disorders under DS in two contrasting maize genotypes i.e. Dekalb-6525 (drought-tolerant) and Yousafwala-hybrid (drought-sensitive). Initially, foliar rate of B (0, 2, 4 and 6 mg L−1) was optimized in terms of improved plant growth under drought-stress. Then, optimized rate of B (4 mg L−1) was applied to assess the physiological and biochemical basis of B-induced improved growth of maize under deficit-moisture supply. Drought-stress reduced the growth of plants by lowering in water-status (leaf water-relations), photosynthetic capacity (gas-exchange aspects, photosynthetic-pigments), membrane permeability, imbalance in redox potential (oxidative-stress, antioxidant-defense system) and tissue-B concentration. Application of B considerably improved maize growth by improving water-status, photosynthetic capacity, tissue-B concentration as well as up-regulation of antioxidative defense-system. Moreover, ameliorative effects of B on maize was also evident from stress relieving indicators such as slight decrease in accumulation of proline, total free amino-acids, total soluble-sugars and MDA content under water-deficit conditions. In addition, cultivar Dekalb-6525 showed considerable improving response to B application over Yousafwala-hybrid.

Inheritance Pattern of Fiber Related Traits under Normal and Hypoxia Conditions in Cotton (Gossypium Hirsutum L.)

ABSTRACT An 8 × 8 diallel crossing system was employed in F1 generation of cotton to study the inheritance pattern of fiber related traits like micronaire, strength and maturity under normal and hypoxia conditions. Analysis of variance revealed significant variation (P < 0.01) for all the traits under both environments. The regression coefficient b deviated significantly from zero but not from unity for all studies traits allowed further analysis of the data. The additive component (D) was significant for all the traits and more than the values of dominance components H1 and H2 in case of fiber strength under both conditions and for fiber uniformity under normal conditions thus showing the preponderance of additive genetic effects. For micronaire non-additive effects were predominant under both environments. Estimates of narrow sense heritability for micronaire were 38.6% under normal and 49.5% for hypoxia, 79 and 80% for fiber strength and for fiber uniformity the estimates were 79% under normal conditions and 63% under hypoxia conditions. Predominance of additive genetic effects, high heritability along with partial dominance for fiber strength under both conditions and for fiber uniformity under normal conditions is indicative of early generation selection but for micronaire under both conditions selection may be practiced in later generations.

Protein profiling analysis of Gossypium hirsutum (Malvales: Malvaceae) leaves infested by cotton whitefly Bemisia tabaci (Homoptera: Aleyrodidae)

The whitefly Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is an important pest of cotton in many countries including Pakistan. Even though it has gained resistance for various registered insecticides such as the neonicotinoid insecticide thiamethoxam, the cellular changes occurring in plants during whitefly infestation and resistance are poorly understood. In particular, the proteomic analysis of interaction between cotton plant and devastating whiteflies is poorly understood. To reveal the physiological response as well as the molecular interaction during the incursion of B. tabaci, one-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (1-D SDS-PAGE) following liquid chromatography coupled with tandem mass spectrometry (LC–MS/MS) was conducted, combined with in silico protein characterizations and protein–protein interaction following validation of LC–MS/MS results by quantitative real-time PCR. The shotgun proteomic analysis of cotton leaves infested with whitefly resulted in the identification of 38 proteins, and 24 under control (non-infested) conditions. Of them, 11 proteins were identified as shared proteins, 13 were differentially induced in non-infested conditions, and 27 were induced only under whitefly-infested conditions. The major proteins like MAP kinases, COBRA-like protein family, and NBS disease resistance protein were expressed under infested conditions; while the heat shock protein 90 and cyclic nucleotide-gated ion channel 5-like major proteins were expressed under whitefly non-infested conditions. The current manuscript presents new insights into the mechanism of whitefly infestation and our understanding of the roles of COB and COBLs in host-pest interaction. To the best of our knowledge, this is the first ever report to reveal the role of several proteins, including COB and COBLs in host-pest leaf interaction.