Inducing Terminal Heat Stress Tolerance

Abstract

Cotton is considered as backbone of Pakistan’s economy. It is a summer season crop. However, severely high temperature (> 45°C) often prevails at its reproductive stages which is a great threat to its productivity. The aim of present study was to mitigate the bad impacts of heat stress on cotton through foliar application of boron, zinc or chitosan. Initially in (2016) a pot trial was carried out in glass house to compare the heat tolerance of available cotton cultivars. Experiment was conducted using completely randomized design (CRD) with split treatment structure and replicated four times. Heat stress was imposed by placing pots in glass house. The experimental material consisted of fifteen cultivars of cotton viz. FH-Lalazar, FH-142, FH-114, CIM-598, CIM-599, CIM-602, VH-282, VH-326, VH-341, MNH-886, MNH-888, MNH-992, IUB-13, IUB-212 and IUB-222. One set of pots was placed in ambient environment (H0; control) while other set was placed in glasshouse for 15 days after germination (H1; heat imposition). On the basis of relative leaf water contents, cell membrane thermo-stability, antioxidant enzymes and biomass accumulating parameters (root, shoot length, fresh and dry weight) these genotypes were grouped in to three classes. Genotypes CIM-598, CIM-599, CIM-602, VH-282, VH-326, VH-341, MNH-888, MNH-992 and IUB-13 expressed heat tolerance; FH-Lalazar, FH-142, MNH-886, IUB-212 and IUB-222 represented medium susceptibility while FH-114 depicted highest susceptibility to terminal heat. Out of these a popular medium heat tolerant variety FH-142 was selected for further experimentation. Thereafter, two independent field experiments were conducted for two consecutive years 2016 and 2017 at Students’ Farm, Department of Agronomy, University of Agriculture Faisalabad, Pakistan. Both experiments were arranged in randomized complete block design (RCBD). Each treatment was repeated thrice and randomized in split structure. Heat stress (for 4 days at flower beginning; for 8 days at flower beginning) was the main factor in both experiments. A control (H0; ambient temperature) was also maintained. First experiment consisted of combinations of (0, 1 g L-1) and zinc (0, 2 g L-1) foliar spray in subplots. In second trial, foliar application of chitosan in different concentrations (0, 0.2, 0.4, 0.6 and 0.8 g L-1) were made in subplots. Negative allegations of heat stress were more pronouncing under ‘heat stress at flower initiation for 8 days’ than ‘heat stress at flower initiation for 4 days’. In the first field experiment foliar applied zinc alone (zinc 2 g L-1) and in combination with boron (1 g boron L-1 + 2 zinc g L-1) showed statistically alike and comparatively more growth, yield, physiological, biochemical and quality related attributes compared to control and foliar applied boron alone under heat stress environments. While, statistically more antioxidants enzymes, osmo-protectants, chlorophyll contents and water relations attributes and significantly lesser malondialdehyde contents were perceived with combined application of two nutrients (1 g boron L-1 + 2 g zinc L-1) than other treatments under ‘no heat stress’. In second field experiment, chitosan at 0.6 and 0.8 g L-1 performed equally well, but better than its lower doses, in improving the performance of heat-stressed cotton. Moreover, biochemical attributes variations were found statistically significant under varying temperature regimes. And foliar applied micronutrients (boron and zinc) in combination and growth regulator (chitosan) proved more advantageous under heat compared to control