Physiological Basis of Heat Tolerance in Wheat (Triticum Aestivum L.)

Abstract

The present investigation was undertaken to study the physiological basis of heat tolerance in wheat. Six wheat cultivars viz., Gourab, Sourav, Kanchan, Shatabdi, Sonora and Kalyansona were used as study materials. Three experiments were conducted during 2005-06 to 2006-07. In the first experiment, heat tolerance of six wheat cultivars was evaluated through membrane thermostability (MTS) test. In the second experiment, seeds of wheat cultivars were germinated to study seed metabolic efficiency and seed reserves mobilization at I 6, 26 and 36° C temperature. Finally, in the third experiment, morpho-physiological changes of wheat cultivars were evaluated in relation to heat tolerance in field condition by seeding them November 30 and December 30 over two successive years. November 30 sowing was considered as normal growing condition, whereas December 30 sowing was regarded as late growing heat stress condition. Based on membrane thermostability (MTS) value, cultivars Shatabdi, Sourav, Kanchan and Gourab were grouped as heat tolerant (MTS>50%), whereas cultivars Sonora and Kalyansona were considered as heat sensitive (MTS<50%). Temperature had a profound effect on speed of germination and seed reserves mobilization. At moderate temperature (26°C), all the cultivars showed the highest seed metabolic efficiency (SME) than those of the lowest (I 6°C) and highest (36°C) temperatures. Heat tolerant (HT) cultivars Gourab, Sourav, Kanchan and Shatabdi attained higher SME than those of heat sensitive (HS) cultivars Sonora and Kalyansona in all the temperature regimes. Total dry matter (TOM), leaf area index (LAI), crop growth rate (CGR), leaf area duration (LAD) and net assimilation rate (NAR) were lower at the initial stage of growth and increased with plant age and all the cultivars showed higher values at normal growing condition compared to late growing condition except NAR in both the years. Higher NAR was observed at normal growing condition upto 60-70 DAS compared to late growing condition. Growth attributes such as RGR, LAR, RLGR, SLA and LWR for all the cultivars declined throughout the advancement of growth stages in both the growing conditions of every year. The HT cultivars showed higher values of TOM, LAI, CGR, LAD, SLA, RGR and NAR compared to HS cultivars in both the years. The number of days for attaining different phenological stages, the requirement of heat unit (GOD) and helio-thermal unit (HTU) were higher for normal growing condition compared to late growing condition. In heat tolerant cultivars, the number of days required for attaining different phenological stages was higher than the heat sensitive cultivars in both the sowings of every year and HT cultivars obtained higher GOD and HTU than the HS cultivars. At normal growing condition, all the cultivars showed higher heat use efficiency than those of late growing condition in both the years. But HT cultivars used heat more efficiently than the HS cultivars. Irrespective of growing conditions heat sensitive cultivars showed higher pre-anthesis stem reserves (PSR) mobilization to the final grain weight compared to heat tolerant cultivars. Both the HT and HS cultivars showed a common tendency of higher PSR mobilization to grain under late growing heat stress condition. But the increased values of PSR were lower in heat tolerant cultivars than those of heat sensitive cultivars. HT cultivars showed higher canopy temperature depression (CTD) in both the growing conditions than the HS cultivars. A sigmoid pattern of grain growth was found in all the cultivars for both the growing conditions. At normal growing condition, both the HT and HS cultivars had similar grain growth duration (35 DAA). But in late growing heat stress condition, HT cultivars had longer grain filling duration (30 DAA) compared to HS cultivars (25 DAA). In both the growing conditions, HT cultivars had higher grain growth rate than the HS cultivars. At late growing heat stress condition, HS cultivars exhibited larger increase in flag leaf proline along with larger decrease in grain proline at 8 days after anthesis (DAA) than the HT cultivars. At normal growing condition, the variation of total chlorophyll content in flag leaf between the HT and HS cultivars were lower compared to late growing condition. The chlorophyll reduction at late growing heat stress condition was clearly distinguishable between the HT and HS cultivars. Heat sensitive cultivars had higher chlorophyll reduction rate and attained 5 days earlier in highest reduction than the heat tolerant cultivars. Higher plant height, spike length, spike number per plant, floret number per spike and grain number per spike were observed at normal growing condition. At late growing heat stress condition, all the cul ti vars showed reduced performance in those parameters at different magnitude. Heat tolerant cultivars had higher relative values in those morphological attributes than the heat sensitive group at late growing condition. Late growing heat stress condition increased the sterility in all the cultivars. But the degrees of increments were higher in HS cultivars than the HT cultivars in both the years.The HT cultivars exhibited distinctly higher relative individual seed size and grain dry weight per spike. Under late growing heat stress condition, every cultivar showed statistically reduced seed size compared to normal growing condition. Finally, in yield parameter, the HS cultivars showed lower performances in both the growing conditions. At late growing condition, the grain yield and biological yield of all the cultivars were significantly reduced at different magnitude compared to normal growing condition. The HT cultivars attained higher relative yield compared to HS cultivars in both the years. From heat susceptibility index it was observed that cultivars Gourab, Sourav, Kanchan and Shatabdi were clearly lower heat susceptible (S<l .0) in grain yield and important yield components compared to cultivars Sonora and Kalyansona. The results of present sn1dy suggested that greater membrane thermostability in HT cultivars reflected on minimum changes in flag leaf chlorophyll, proline content in flag leaf and grain, less dependence on stem reserves mobiIization, higher grain growth duration with high filling rate, ability to maintain a cooler canopy environment and high relative performances in yield and important yield determinants and those can be considered as selective criteria of heat tolerance in wheat.