EXOGENOUS APPLICATION OF CALCIUM AND POTASSIUM TO ALLEVIATE THE ADVERSE BIOCHEMICAL EFFECTS OF DROUGHT STRESS IN BRASSICA NAPUS L. SEEDLINGS

Abstract

Sustainable food production in the changing global environment is a major challenge to the world community. Depleting water resources, shrinking arable land and population explosion are further worsening the situation. The agricultural extension is under threat mainly due to the environmental stresses like drought, salinity, changing rainfall pattern and warmer temperature due to enhanced atmospheric carbon dioxide. The adverse effects of drought stress may be alleviated by mineral nutrition of crops. In this experiment, Brassica napus L. seedlings were subjected to 10 days period of drought stress (Relative Water Content: 58.98 %), one week after supplementation with 30, 60 and 90 mM solutions of Ca(NO 3 ) 2 .4H 2 O; 50, 100 and 150 mM solutions of KNO 3 and 30, 50 and 100 mM solutions of NH 4 NO 3 in three doses at alternate days. Certain physiological parameters like relative water content, rate of water loss from excised leaves and membrane leakage were used to quantify and estimate the intensity of drought stress. Samples were analyzed in triplicate and standard deviation was calculated. Imposition of drought stress was found to affect the physiological and biochemical characteristics of Brassica napus L. seedlings. Supplementation of the given minerals (Ca 2+ , K + and N) induced certain physiological and biochemical modifications in Brassica napus L. seedlings under irrigated and/or drought stress conditions. It was found that drought stress increased the rate of water loss and hence decreased the relative water content. Exogenous application of Ca 2+ and K + improved the water status of the seedlings by correcting these parameters, however, exogenous N showed a negative impact. The stressed seedlings suffered from damage as shown by higher level of membrane leakage (44.38 ± 3.07 %) and hydrogen peroxide accumulation (26.30 ± 3.21 μmol/g fresh weight). These effects of drought were however, mitigated in the seedlings subjected to exogenous supplementation of Ca 2+ and K + . An important compatible osmolyte, proline was produced in greater quantity (9.15 ± 0.74 μmol/g fresh weight) in stressed seedlings as compared to the control (02.00 ± 0.18 μmol/g fresh weight). In the stressed seedlings treated with all the given supplements, greater proline contents were observed, with the larger linear effect of Ca 2+ . The greatest proline content (23.93 ± 0.98 μmol/g fresh weight) was noted for the stressed seedlings supplemented with 90 mM Ca(NO 3 ) 2 .4H 2 O. Under irrigated conditions, the viii seedlings treated with all the given supplements gave almost similar proline concentration as that of the corresponding control seedlings. Activities of catalases (EC: 1.11.1.6) and ascorbate peroxidases (EC: 1.11.1.11) were about 45.00 and 82.2 % higher in stressed seedlings than the control, both of which were further increased with the increasing supplementations of Ca 2+ and K + under stressed conditions. The seedlings showed about 47.75 and 41.58 % loss in fresh and dry weight respectively, under stress condition relative to the control and irrigated plants. It was found that exogenous Ca 2+ and K + decreased the fresh weight of the irrigated seedlings and slightly increased the fresh weight of the stressed seedlings with the significant increase at 90 mM Ca 2+ (23.26 % increase) and 150 mM K + (KNO 3 ) (24.67 % increase) relative to the respective control seedlings. Under both conditions, the dry weights of plants increased with increasing concentration of exogenous Ca 2+ and K + . The total chlorophyll content exhibited about 41.92 % loss due to drought stress, which was protected to some extent by the exogenously supplied Ca 2+ and K + in stressed seedlings. However, in no case, the total chlorophyll content reached the control level in stressed seedlings. Under irrigation condition, the seedlings at all treatments showed almost similar chlorophyll content. Protein content was found to decrease by about 28.38 % due to the imposition of stress period, which was substantially increased after supplementations with all minerals (Ca 2+ , K + and N). The mineral composition of Brassica napus L. seedlings was affected under drought stress as well as under exogenously provided supplements. The Ca, K, N, C, S, P, Mg, Fe and Na contents were decreased by about 3.45, 58.82, 28.40, 4.11, 21.64, 26.90, 1.61, 52.56 and 30.13 % respectively under drought stress. Supplementations differently affected these mineral elements. Supplemental Ca 2+ and K + increased the shoot-Ca, K, N, C, S and P contents under irrigated and drought stress conditions. Drought stress affected the fatty acid contents. The palmitic acid, stearic acid and linoleic acid contents were decreased by about 6.26, 2.25 and 24.00 % respectively, whereas, the oleic acid and linolenic acid contents were increased by 99.66 and 4.74 % respectively. Exogenous supplements variously affected the fatty acid composition. Under irrigated condition, increasing supplemental Ca 2+ concentration increased the stearic acid, linoleic acid and linolenic acid contents and decreased the oleic acid content. Under drought stress condition, supplemental Ca 2+ was found to increase the ix palmitic acid, stearic acid and linoleic acid contents. The fatty acid composition was affected by K + -supplementation. The concentrations of palmitic acid, stearic acid and linoleic acid were increased under both irrigated as well as drought stress conditions, whereas, the oleic acid content was increased only under irrigated condition. Drought stress also affected the free amino acids composition. The free amino acids like aspartic acid, threonine, serine, proline, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, arginine and the total amino acids were increased by about 31.38, 19.47, 88.96, 507.09, 57.14, 34.09, 35.62, 6.04, 104.76, 4.42, 64.34, 2.68 and 57.68 % respectively, while, the glutamine, glycine, alanine and methionine were decreased by about 26.89, 21.18, 13.08 and 9.63 % respectively. Exogenous Ca 2+ mostly increased the concentration of aspartic acid, threonine, serine, glutamine, proline, glycine, alanine, valine, methionine, isoleucine, leucine, tyrosine and total amino acids under both irrigated as well as drought stress conditions. It was revealed that supplemental K + mostly increased the contents of aspartic acid, threonine, glutamine, proline, glycine, isoleucine, histidine, lysine, arginine and total amino acids under both irrigated as well as drought stress conditions.