Soil-water thresholds for the responses of leaf expansion and gas exchange: A review

Typical responses of leaf expansion and gas exchange rate to plant available soil water (PAW) can be described with two straight Lines that intersect at PAW,, i.e., the PAW threshold for which the rate of the process in stressed plants starts to diverge from a reference value. PAW, is a parameter widely used in modelling plant responses to water deficits. It also reflects some apparent physiological mechanism because plants appear to be able to sense soil water status or related variable(s). in this paper comparisons are made between PAW(t) for various species (monocots and dicots) and plant processes (leaf expansion and gas exchange) in order to: (i) point out methodological sources of variation in published values of PAW(t); and (ii) analyse variations in PAW, in relation to plant and environmental factors. Reported values of PAW(t) vary over almost the whole possible range of PAW (i.e., 0 to 1). Average thresholds reflect the greater responsiveness to water deficits of tissue expansion (average PAW(t) = 0.56) relative to gas exchange (0.40). Average PAW(t) for leaf water potential (0.61) and stomatal conductance (0.37) are very close to the average thresholds for expansion and gas exchange, respectively. Soil water thresholds for leaf expansion are also shown to discriminate between plant types (0.50 for monocots vs. 0.66 for dicots) and soils (0.72 for coarse- vs. 0.43 for fine-textured soils). The simplicity of characterising plant responses to water stress in terms of PAW is attractive. In agreement with known physiological relationships, however, our analysis highlights how, for given processes and species, the measured value of PAW, can be affected by evaporative demand, root distribution, soil texture and soil bulk density, among other factors, thus making explicit some of the assumptions underlying the use of fixed soil-water thresholds in simulation models.