Ionic combating mechanisms and their comparative effects on seed hardening under simulated supra-optimal environmental conditions

Abstract

Heat extremes and limited moisture are two of the most dominant environmental factors impacting stand establishment of rainfed sorghum {Sorghum bicolor L. Moench). Hardening is the process of exposing plants to gradual levels of stress and acclimation to foster better response to post-treatment stress factors. Three experiments were carried out under phytotron and germination cabinet conditions to test the effects of osmotic soaking of sorghum seeds with sodium chloride (NaCl) on germination and growth under simulated heat stress. The hypothesis was that the NaCl treatment forms an acclimation to stress by inducing hormonal responses to ionic toxicity caused by salt. This acclimation would lead to a lowered degree of response when the seed is exposed to future stress; namely, heat and/or drought. Independent variables included NaCl concentration, treatment duration, storage and genotype, whereas germination and growth were dependent variables. Further experiments tested various methods of achieving the initial acclimation "signahng" whilst reducing ionic toxicity through combating ions. Longer soaking treatment durations (2-3 days) and higher NaCl concentrations (16g NaCl P^) were detrimental to germination in comparison to lower concentrations (8g NaCl 1~^) and shorter durations (1 day). An interaction between concentration and duration of treatment existed where high concentrations performed better at lower treatment durations and vice versa. Combining 10 g NaCl 1~^ with 5g calcium sulphate to combat ionic toxicity produced a greater advancement of germination than NaCl alone. Drying duration of seeds did not affect subsequent germination nor did storage for 10 days to 1 month. The effects of osmotic conditioning are discussed and could have potential for improving sorghum success rates in harsh arid environments.