Morphological responses of sorghum seedlings to drought, heat, and combined stresses

Summary Drought and heat stress are a global concern affecting crop productivity. The current study investigated the daily shoot and root length growth of 48 elite African sorghum genotypes and two commonly grown check varieties in response to heat and drought stresses applied individually and in combination at the early (7-day-old) and late (21-day-old) seedling stages. Genotype, stress, and their interaction significantly affected root and shoot length at both stages. Our findings indicated that the combined stresses suppressed daily shoot length growth at both stages. Drought, heat, and combined stresses equally suppressed daily root length growth during the early seedling growth stage, and drought applied separately showed the highest negative effect on root length at the late seedling stage. In general, the stress treatments showed the utmost negative effects in daily shoot and root length growth at early seedling stage than the late stage. Heat stress induced the highest relative growth reduction of 78% in hypocotyl length followed by combined stresses at 77.6% and 70.8% for drought stress. The average hypocotyl length changes ranged between 0.37 and 2.31 mm per day at early seedling stage. Root length daily growth was reduced by 69.1% under combined stress, 67.6% under heat stress, and 63.3% under drought stress at early seedling stage. Root length changes ranged between 0.35 and 2.96 mm per day at early seedling stage. At late seedling stage, the highest relative daily growth reduction was observed in shoot length (36.6%) under combined stresses while drought stress induced the highest relative daily root length reduction of 10.8%. The average shoot length changes ranged between 5.1 and 8.1 mm per day at late seedling stage while root length changes ranged between 2.7 and 3.5 mm. In reference to the independent genotypic effects, genotype IS13904 displayed the highest performance in hypocotyl and root length growth at the early seedling stage. At the late seedling stage, genotypes IS6994 and NPGRC1478 showed tolerance to at least two of the assessed stress conditions in terms of daily root length growth. Genotypes IS30164, IS30015, and IS9567 showed similar resistance in shoot growth. The overall analysis of both shoot and root daily length growth at both seedling stages revealed the resistance of genotypes NPGRC1478 and IS30164 to drought stress applied separately and combined stressors. The identified sorghum genotypes can be used as potential donors towards tolerance to combined stresses at both seedling stages and are recommended for utilization in hot and dry agroecologies of sub-Saharan Africa due to their potential vigour in early establishment.