Physiological and molecular responses of stress sensitive and tolerant banana genotypes to drought, heat and their combination

Individual drought and heat stresses affect the growth, development and productivity of crops and their combination reduces productivity even further. In banana, combined stress caused 100% yield loss of ‘Grand Nain’ (GN, AAA genome) genotype, while the yield loss was only 46% in ‘Hill Banana’ (HB, AAB genome). We have compared physiological and molecular responses of these two contrasting banana genotypes under individual drought, heat and their combined stress under controlled as well as in open field conditions. Individual drought and combined stress caused higher reduction in leaf relative water content, increased ion leakage and H₂O₂ content in GN plants as compared to HB plants. However, individual heat stress resulted in minimum changes in physiological parameters. Furthermore, the expression of DREB (A-1 and A-2 group) and stress responsive NAC genes revealed higher background molecular responses in leaves of HB plants for individual stresses as compared to GN plants. However, combination of heat and drought stress suppressed their expressions in HB but activated them in GN. Interestingly, the expression of the DREB and stress responsive NAC genes was blocked in the drought tolerant HB even when subjected to combined drought and heat stress in field, unlike in GN. Most of these genes were strongly upregulated within 30-60 min upon application of exogenous abscisic acid (ABA) and the increase was prominent in GN. This response in HB was associated with better stomatal control over transpiration thus avoiding the need for stress pathway activation unlike in GN. The study suggests that the B genome in the stress-tolerant HB may be responsible for tolerating more drastic combined stresses without taking recourse to the expression of DREB (A-1 and A-2 group) and stress responsive NAC genes.