Influence of Incremental Short Term Salt Stress at the Seedling Stage on Root Plasticity, Shoot Thermal Profile and Ion Homeostasis in Contrasting Wheat Genotypes

Abstract

Understanding the component traits determining salt stress tolerance is a major breeding target in wheat. The lack of genetic resources suited to salt-affected regions and the complexity of the traits involved impede progress in breeding salt-tolerant wheat varieties. This study was conducted with four bread wheats, namely (Triticum aestivum) Kharchia-65 (K-65), BT-Schomburgk (BTS), HD-2687, and HD-3298. Treatments were imposed on plants with varying electrical conductivity (control, 5 dS m−1, 10 dS m−1, and 15 dS m−1) with a combination of three different salts NaCl, CaCl2·2H2O, and Na2SO4. We evaluated variations in root system architecture, canopy temperature (depicted as a thermal image), reactive oxygen species (ROS) homeostasis, and leaf stomatal density in response to incremental doses of salt stress in a hydroponic experiment. As the plants were sampled after short-term exposure to stress (within 3 weeks of stress imposition), the plants were expected to be in a quiescent state. Due to the osmotic effect, the growth of the plants was compromised, and the associated decrease in stomatal conductance increased the canopy temperature. ROS accumulation and antioxidant enzyme activity did not follow a definite pattern. The antioxidant system’s tolerance to ROS comes into action much later in the tolerance mechanism. That could probably be the reason behind the varied response in ROS accumulation and antioxidant enzymes after short-term exposure to salt stress. Thermal images could effectively differentiate between salt-tolerant (K65) and sensitive (HD2687) genotypes. The variation in Na+/K+ ratio also suggested a genotypic variation in salt tolerance. The genotypes of K-65 maintained a better root system, while HD2687 showed severe reduction in root biomass and other root traits under salt stress. The PCA data also point out genotypic variation in lateral and main root traits in response to different salt stress levels. For salt tolerance in wheat, the main contributing root traits were total root length, total surface area, total root volume, tips, and other main, lateral root traits. The idea of differential control of RSA dynamics is novel and can be further explored to understand natural variation in salt stress tolerance.