Environmental factors influencing growth, freezing tolerance and photosynthesis in Thellungiella and Arabidopsis

Thellungiella salsuginea is an Arabidopsis related, promising model plant for stress tolerance studies. This study aimed to characterize environmental responses of Yukon and Shandong ecotypes of T. salsuginea in comparison with Arabidopsis thaliana – Columbia ecotype. The study comprised an analysis of plant growth, freezing tolerance, photosynthesis and respiration under various growth regimes. The experimental tools and techniques included visual observation, physical measurements, electrical conductivity, spectrophotometry, chlorophyll a fluorometry, P700 spectroscopy and oxygen-electrode polarography. The taxa showed several commonality and divergence in physiological responses. From the similarity of constitutive freezing tolerance (lethal temperature for 50% plants, LT50 = -7.5 ºC) between the taxa, cold acclimation induced highest level of cold tolerance in Yukon (LT50 =-21ºC) followed by Shandong (LT50 = -17ºC) and Arabidopsis (LT50 = -14ºC). A drought for four days followed by cold acclimation for one week furthered LT50 to -25oC in Yukon and Shandong, but not in the Arabidopsis. Yukon and Shandong required either vernalization or ecotype-specific irradiances for the reproductive transition, while Arabidopsis underwent flowering across varied growth regimes. The Thellungiella ecotypes also contrasted from Arabidopsis in photosynthetic properties with their higher chlorophyll and carotenoid contents, higher constitutive resistance to photoinhibition and higher amount of oxidizable photosystem I reaction-center chlorophyll (P700+) than that of Arabidopsis. Yukon distinguished itself from other taxa with its higher growth sensitivity to daily photon irradiance, greater acclimative gain in freezing tolerance, higher rates of dark respiration and lower net assimilation, while Shandong exhibited higher electron transport through photosystem II, greater fraction of excitation energy partitioning towards photosystem II photochemistry and contrasting trend of P700+ due to cold acclimation. In Shandong, the P700+ seemed to decrease due to cold acclimation at higher irradiances, while it appeared to increase in Yukon and Arabidopsis. The findings revealed that Yukon and Shandong possessed wider phenotypic plasticity than that of Arabidopsis. Contrasting manifestation of environmental responses including the inducible freezing tolerance, reproductive transitioning and carbon economy promise Thellungiella to be the subject of great scientific curiosity to unravel the extremophilic adaptation of plants. On the other hand, the physiological versatility of Arabidopsis in the moderate environment substantiated its preferential use as a model system representing mesophytic adaptation