Effect of root temperature on plant response functions for tomato: comparison of static and dynamic salinity stress indices

Abstract

The agronomic study of plant response to salinity uses root zone salinity as a primary treatment variable. Root zone salinity is a static index in that its value does not depend on any variable factors in the soil-plant-air continuum (SPAC) that simultaneously affect growth. In order to take into account the effect of variable parameters in the SPAC, the concept of a dynamic index was introduced and defined as the salt flux to the shoot relative to growth rate. The objective of this study was to compare the variability of these two indices as measures of salt tolerance in different environments and to ascertain which correlative variable is more directly linked to yield reduction; root zone salinity, a static index, or ion flux to the shoot, a dynamic index. This was accomplished by comparing the root temperature dependence of threshold values for each index respectively. Yield response to salinity was obtained for tomato (Lycopersicon esculentum Mill., cv. Heinz 1350-vf) at two root temperatures, 18 °C and 25 °C, using 14 levels of salinity, 0 to 140 mM Cl, with a 2:1 Na-Ca ratio. Based on parametric analysis, the root zone chloride concentration threshold was increased 92% over the root temperature studied. In contrast, the threshold values of the dynamic indices were not statistically different at the two root temperatures. Plant shoot chloride concentration factor, expressed as the ratio of salt concentration in the shoot to salt concentration in the nutrient solution, decreased exponentially from 8 to 2 with increasing root zone salinity and was independent of temperature. Water use exhibited a strong temperature dependence and showed a sharp decline at root zone salinities corresponding to root zone salinity threshold values. Root/shoot ratio was independent of root zone salinity at least up to 80 mM Cl, and was significantly higher at the lower root temperature. It was concluded that root zone osmotic potential is not the fundamental causative factor determining the onset of yield reduction for the two temperatures studied, and that other elements related to growth and ion transport combine to determine a threshold value. These results indicate that it is a critical value of the salt flux to the shoot in relation to shoot growth rate that determines the onset of yield reduction and therefore is related more to the average salinity in the shoot than the average root zone salinity. This is evidence that specific biochemical mechanisms responsible for yield reduction reside in the shoot.