Physiological and molecular response of cotton (Gossypium hirsutum L.) to heat stress at the seedling stage

The ideal temperature range for the optimal growth and development of cotton is 25 °C–32 °C and high temperature adversely affects the metabolic activities of plant cells. This study was aimed to screen heat-tolerant cotton genotypes based on physiological and molecular parameters. Experiments were carried out during 2019–2020 at the MNS-University of Agriculture, Multan, Pakistan. The research comprised two parts. In the first experiment, 30 cotton genotypes were sown in a completely randomized design with three replications under laboratory conditions for the determination of cell membrane thermostability. Principal component analysis was performed, and four genotypes, i.e., two heat-tolerant (‘CRIS-5A’ and ‘VH-338’) and two heat-sensitive (‘FH-242’ and ‘VH-281’) genotypes, were selected. In the second experiment, the screened cotton genotypes were sown in pots in a factorial complete randomized design with three replications and two treatments (normal and heat treatment). Heat stress was applied at the seedling stage, and eight leaf samples (one from each experimental unit) were collected. Two genes were used for molecular analysis and were amplified in all eight cDNA samples. Molecular analysis indicated the presence of HSP70 and HSP26 genes in the cotton genotypes, and the expression of these genes was measured by using ImageJ software. The gene expression level of HSP70 was very high (16.41%) in ‘VH-281’, which is a heat-sensitive genotype under heat stress. The sensitive genotype ‘FH-242’ exhibited the highest gene expression level of HSP26 (20.32%) under normal conditions. A similar sequence of HSP70 gene of Agave sisalana was amplified for the first time in cotton. It is a good indicator for screening heat tolerant cotton genotypes at the molecular level.