PHYSIOLOGY OF SUSTAINING WHEAT YIELD UNDER LATE PLANTING HEAT STRESSED ENVIRONMENT

Abstract

Development of heat tolerant wheat genotypes and/or amelioration of the effect of heat stress through agronomic strategies are the possible options to sustain late planting yield reduction of wheat in Bangladesh. In the present investigation, three laboratory and two field experiments were carried out during July 2006 to October 2008. Twenty wheat genotypes were tested in laboratory experiment to investigate seed reserve utilization efficiency (SRUE) during germination and proline level at seedling stage as screening criteria against heat stress. Based on membrane thermostability test, the wheat genotypes showing <50% membrane injury were grouped as heat tolerant (HT) and the wheat genotypes showing >50% membrane injury were classified as heat sensitive (HS) genotypes. The HT wheat genotypes maintained higher (1.042 g/g) seed reserve utilization efficiency than that of HS genotypes (0.682 g/g) at high germination temperature but none of them showed such variation under optimum germination temperature. At high growing temperature (35°C) the HT genotypes produced more than double (>200%) proline than that at 25°C but the HS genotypes produced less quantity of proline at 35°C compared to that in HT genotypes. A significant negative correlation was found across the wheat genotypes between membrane injury (%) and SRUE at 35°C (r = - 0.78**) and also between seedling proline content at 35°C and percent membrane injury (r = -0.619**), Out of twenty genotypes, four heat tolerant (Kanchan, Fang 60, BAW 1059 and BL 1022) and two heat sensitive (Pavon 76 and Sonora) wheat genotypes were included for the field evaluation of physiological sensitivity and grain yield in heat stressed environment. Unlike HS genotypes a remarkable increase in kernel proline level due to heat stress was found in HT genotypes during early stages of rapid grain growth period which was found in flag leaf during later stages of rapid grain growth period. Flag leaf longevity reduced in all wheat genotypes due to late planting heat stress but the reduction was inconsistent between HT and HS wheat genotypes. The HT wheat genotypes showed longer grain growth duration, longer duration of rapid grain growth rate, higher stem reserve utilization and consequently maintained lesser reduction in grain dry weight per ear. Thus the relative grain yield of HT genotypes was higher than that showed by HS wheat genotypes under heat stress condition. Flag leaf and kernel soluble sugar level revealed that irrespective of heat tolerance Kanchan, Fang 60, Pavon 76 & Sonora were found to be sink limiting for kernel development under heat stress condition. Among these four genotypes sink limitation appeared 4 to 8 days later in HT genotypes than HS ones. In other two HT genotypes (BAW 1059 and BL 1022), sink limitation did not appear clearly under heat stress condition. Application of additional nitrogen and/or splitting of recommended dose of nitrogen fertilizer (RDNF) after anthesis was evaluated in the field in late planted wheat. Three HT (Kanchan, Fang 60 and BAW 1059) and one heat sensitive (Sonora) wheat genotypes were tested in this experiment. Three splits application of 50% extra N fertilizer + RDNF showed increased grain yield compared to that obtained from RDNF under heat stress conditions only in sink non-limiting genotype, BAW 1059. Longer flag leaf activity and greater stem reserve utilization mainly contributed to this yield increase in sink non-limiting genotype (BAW 1059). The effect of parent plant growth temperature was evaluated in terms of physiological attributes contributing to seed vigour. Seed vigour (as indicated by seed density, seed conductivity, seedling dry weight, production of normal seedling, seed reserve utilization efficiency and % seedling emergence) was more adversely affected in heat sensitive than the heat tolerant wheat genotypes due to higher parent plant growth temperature. The overall results indicate that determination of heat tolerance in wheat genotypes either by increased proline level or by increased seed reserve utilization efficiency, as found in the present study, were as effective as the widely used cell membrane thermostability test. However, for sustaining wheat yield under late. planting heat stress condition additional nitrogen fertilizer was found to be effective only in sink non-limiting wheat genotype.