Salinity stress and drought stress differentially affect biomass allocation, leaf orientation and performance of two sesame (Sesamum indicum L.) cultivars.

Abstract

Background: Drought and salinity stresses are major and interrelated threats to crop productivity in arid lands. Understanding the mechanisms of tolerance to salinity and drought stresses is critical for selection of productive crop cultivars.

Methods: This work investigates the differential impact of salinity and drought stresses on growth and performance of Sohag 1 (Sohg) and Shandaweel 3 (Shnd) cultivars of sesame at the same water potential (0, -100, -250 and - 450 kPa) and nutrient supply in a hydroponic sand culture. Plant response to stress was evaluated in terms of growth and biomass fractionation, leaf orientation and gas exchange as well as the contents and fractionation of leaf photosynthetic pigments and carbohydrates.

Results: The horizontal leaf orientation of Shnd was marginally affected by stress; meanwhile, the vertical leaves of Sohg became more vertical under salinity stress but less vertical under drought stress. Foliage of the two sesame cultivars was more tolerant to salt stress than to drought stress, with better tolerance of Shnd than Sohg. This was evident from the progressive drought-induced inhibition of foliage growth versus a mild retardation or even benefit by low salinity, along with slightly less severe effect of high salinity. Salinity targeted particularly root growth with favored biomass allocation to the foliage. By contrast, the enhancement of root growth and retardation of foliage growth under drought implies favored biomass allocation to root. Salinity reduced leaf pigment content in the two cultivars, particularly Sohg but drought either increased it or was without effect. Stress, post a threshold of -100 kPa, reduced stomatal conductance, the rates of photosynthesis and transpiration and water use efficiency, with stronger impact of salinity than drought. The lowering in leaf carbohydrate content was more aggressive under salinity than under drought, and was associated with increasing proportion of pectin at the expense of starch but variable changes in soluble sugars.

Conclusion: Salinity and drought stresses, at equivalent water potentials, differentially affect sesame growth, gas exchange, leaf orientation, photosynthetic pigments and carbohydrate fractions in a genotype-dependent pattern. Among the two investigated sesame cultivars, Shnd seems more stress-tolerant than Sohg. The present findings, obtained from a hydroponic sand culture under partially protected conditions, needs further evaluation under field conditions employing wider collection of genotypes.