High temperature stress responses of Salvia splendens and Viola X wittrockiana

Abstract

One of the greatest impediments affecting growth and development of bedding plants is high temperature stress. The deleterious effects of high temperature stress are the most pronounced during plug transplant and/or during the transition period from the nursery to the landscape. High temperature stress responses were investigated in Salvia splendens and Viola x wittrockiana to determine the morphological and physiological mechanism associated with the heat tolerance. Two genotypes for each species were used; a heat tolerant Vista and a heat sensitive Sizzler cultivar of salvia and Crystal Bowl Purple (CBP) a heat tolerant and Majestic Giant Red (MGR) a heat sensitive cultivar of pansy. Morphological traits such as short stature, greater total leaf area/plant, extensive root system; physiological traits like stomatal conductance, greater transpiration, and net photosynthesis were found to be characteristic traits of heat tolerant cultivars. Greater cell membrane thermostability (CMT) and expression of a heat shock protein of low molecular weight approximately 27kD may be responsible for heat tolerance in Vista and CBP. Primary soluble sugars sucrose and raffinose found in both the salvia cultivars, and greater concentrations in Vista at high temperatures, may be involved in stabilization of membrane lipid bilayer and/or acting as osmoprotectants under stress conditions. Short duration (3 h) heat preconditioning at 35 °C and 30 °C for salvia and pansy respectively during initial stages of growth helped to develop acquired thermotolerance. Heat sensitive cultivars acquired heat tolerance with heat preconditioning and acclimated to subsequent challenging temperatures. Preconditioning enhanced already existing traits of heat tolerant cultivars. Increased stomatal frequency and leaf thickness in salvia acquired during preconditioning. Cell membrane thermostability measured at a single temperature with increasing time of exposure was closely associated with increased gas exchange rates, leaf relative water content and overall marketable quality in salvia. These simple laboratory techniques to test heat tolerance have a greater advantage over time and space consuming field trials and may be an accurate and more efficient measure of heat tolerance.