Effect of soil type on tipping point hydrological requirements for Axonopus compressus grass under extreme drought stress

Critical soil suctions (threshold, tipping point, and permanent wilting) corresponding to initial drought response, near-death stage, and complete mortality, respectively; is essential for formulating irrigation schemes of vegetation grown in compacted soil under drought conditions. The effect of soil types on these critical soil suctions are unexplored and is crucial in understanding the soil-specific plant water functions. This study aims to establish the drought response of Axonopus compressus (grass), based on stomatal conductance (g_s) and chlorophyll fluorescence parameters (CI) grown in different soil types. A. compressus were grown in six soil types (2 coarse-grained and 4 fine-grained soils) for 8 weeks, followed by continued drought condition. The g_s and CI were monitored along with soil suction and moisture content. Both leaf and root growth were observed to be higher in coarse-grained soils than fine-grained soils, even though the water retention of the coarse-grained soils were comparatively less. Drought stress initiation in plants was captured by ψ_threshold from the CI (especially in fine-grained soils) before the g_s response. The three critical soil suctions estimated from the correlation between CI and ψ were found to be increasing with higher soil clay fraction. Corresponding plant available water contents (based on v/v volumetric water content) with each of three critical soil suctions were found to be dependent on the relative growth of canopy to root growth that occurred in different soil medias. Especially, plant available water in ‘tipping suction’ was dependent on the soil clay fraction (i.e., higher fraction could restrict root water uptake) and is presented with a simple empirical correlation for A. compressus.