A field study was conducted with two wheat cultivars, viz., a relatively heat tolerant (HT) Kanchan and a heat sensitive (HS) Sonora to compare heat stress tolerance in terms of flag leaf senescence and mobilization of flag leaf nitrogen to developing kernel. The results clearly revealed that Sonora reached the highest reduction rate of flag leaf chlorophyll at the period of 16-20 Days After Anthesis (DAA), i.e., 5 days earlier than Kanchan. The early attainment of the peak of reduction rate of flag leaf chlorophyll in Sonora indicated early onset of flag leaf senescence under post-anthesis heat-stressed environment. Due to heat stress, Sonora showed higher and significant reduction in specific leaf mass (SLM) from 20 DAA and that continued until maturity which was absent in Kanchan. Faster loss of flag leaf nitrogen occurred simultaneously with higher gain of kernel nitrogen in Sonora under heat-stressed environment, compared to Kanchan. This might happen in response to higher demand of nitrogen in the developing kernel in Sonora, compared to Kanchan. Wider variation in flag leaf and kernel nitrogen between HT and HS cultivars suggest that under heat-stressed environment there was high demand of nitrogen in kernel in general, and higher demand in HS cultivars, in specific.

Temperature is one of the environmental variables that cannot be manipulated in the field, and crops are often selected for a region on the basis of their response to the temperature condition of that region. In Bangladesh, most of the wheat crops in rice-wheat cropping system are late-planted. Late-planted wheat plants face a period of high temperature stress during reproductive stages (Al-Khatib and Paulsen, 1990). High temperature is pacing wheat plant to early onset of flag leaf senescence (Mohi-Ud-Din et al., 2007). But the flag leaf being the main photosynthesizing organ of wheat (Araus et al., 1986), about 70% of total ¹⁴C-assimilates synthesized in flag leaf is exported to develop kernels (Duan et al., 2000). Early onset of flag leaf senescence adversely affects assimilate synthesis and its translocation to develop kernels, ultimately causing higher yield loss (Islam et al., 1993). In addition to late-planting heat stress, global warming will push the wheat farming further into heat-stressed environment in future and may cause further reduction of the present yield level.

It is very difficult to detect the exact start of leaf senescence with naked eye. But nowadays, SPAD value and accumulation of soluble carbohydrates in flag leaf have been well recognized as means of determining the onset of senescence process (Rajcan et al., 1999; and Mohi-Ud-Din et al., 2007). Also the use of extractable chlorophyll is an index of leaf senescence. In the present study, we have tried to explain the onset of leaf senescence on the basis of extractable chlorophyll, reduction rate of chlorophyll and specific leaf mass (SLM) between heat tolerant (HT) and heat sensitive (HS) wheat cultivars under heat-stressed environment. Harding et al. (1990) observed that heat stress injured the photosynthetic apparatus during reproductive growth of wheat and that diminished source activity and sink capacity. These may be equally important in reducing productivity. Fokar et al. (1998) observed higher and longer retention of chlorophyll in HT cultivars than HS cultivars under high temperature stress.

Life Sciences Journal, Leaf Mass, Wheat Flag Leaf, Global Warming, Double Beam Spectrophotometer, Reproductive Development, Flag Leaf Chlorophyll, Wheat Cultivars, Senescence Process, Heat-Stressed Environment, Rice-Wheat Cropping System.