Investigating soil tipping suction in Axonopus compressus grown in poorly graded sand using a novel framework.

Critical suction ranges for plants upon drought stress are essential for devising precision irrigation schemes in de-desertification measures in arid urban regions. The objective of the current study is to develop a novel framework for drought stress evaluation, considering measured changes in photosynthetic leaf parameters with root zone soil suction. A native grass species Axonopus compressus was grown in three different growth media—bare sand, sand with periodic organic nutrient treatment and sand amended with 5% biochar for 56 days, followed by continued drought. The corresponding grass leaf response of stomatal conductance (SC), quantum photosynthetic efficiency (QPE), effective photochemical quantum yield, i.e. Y(II), and maximum photochemical quantum yield, i.e. (Fv/Fm), were measured. The threshold suction at around 10 kPa for all treatment types indicates the state at which an initial drop in transpiration occurred, marked by significant change in SC, QPE and Y(II). A new terminology termed as "tipping suction" was coined to describe the condition wherein the grass reaches a "life or death state", a stage at which the grass could progress to permanent wilting or return to sustenance. This stage is marked by a rapid decrease of Y(II), and the tipping suction ranging between 55 and 200 kPa is dependent on soil water retention capacity and the plant root biomass. The tipping suction differs from the permanent wilting point wherein complete suppression of Fv/Fm occurred. This study suggests that it is ideal to calculate plant-available water content by considering threshold suction and tipping point for maintaining optimum growth and resist plant mortality, respectively. [ABSTRACT FROM AUTHOR]