Transpiration Reduction in Maize (Zea mays L) in Response to Soil Drying.

Mutez Ali Ahmed,Mohsen Zarebanadkouki,Andrea Carminati,Mathieu Javaux,Gaochao Cai,Faisal Hayat

doi:10.3389/fpls.2019.01695

Abstract

The relationship between leaf water potential, soil water potential, and transpiration depends on soil and plant hydraulics and stomata regulation. Recent concepts of stomatal response to soil drying relate stomatal regulation to plant hydraulics, neglecting the loss of soil hydraulic conductance around the roots. Our objective was to measure the effect of soil drying on the soil-plant hydraulic conductance of maize and to test whether stomatal regulation avoids a loss of soil-plant hydraulic conductance in drying soils. We combined a root pressure chamber, in which the soil-root system is pressurized to maintain the leaf xylem at atmospheric pressure, with sap flow sensors to measure transpiration rate. The method provides accurate and high temporal resolution measurements of the relationship between transpiration rate and xylem leaf water potential. A simple soil-plant hydraulic model describing the flow of water across the soil, root, and xylem was used to simulate the relationship between leaf water potential and transpiration rate. The experiments were carried out with 5-week-old maize grown in cylinders of 9 cm diameter and 30 cm height filled with silty soil. The measurements were performed at four different soil water contents (WC). The results showed that the relationship between transpiration and leaf water potential was linear in wet soils, but as the soil dried, the xylem tension increased, and nonlinearities were observed at high transpiration rates. Nonlinearity in the relationship between transpiration and leaf water potential indicated a decrease in the soil-plant hydraulic conductance, which was explained by the loss of hydraulic conductivity around the roots. The hydraulic model well reproduced the observed leaf water potential. Parallel experiments performed with plants not being pressurized showed that plants closed stomata when the soil-plant hydraulic conductance decreased, maintaining the linearity between leaf water potential and transpiration rate. We conclude that stomata closure during soil drying is caused by the loss of soil hydraulic conductivity in a predictable way.

Highlights

Drought is a primary constraint to plant growth and crop production worldwide
We measured the relationship between leaf water potential and transpiration rates in maize at various soil water contents and light intensity
We have found that reductions in transpiration occurred in correspondence to reductions in soil-plant hydraulic conductance, which were caused by the loss of soil hydraulic conductivity around roots

Summary

Introduction

Drought is a primary constraint to plant growth and crop production worldwide. Mechanisms by which drought impacts plant growth are complex and involve feedbacks between stomata regulation, plant hydraulics and soil drying. Each element is characterized by hydraulic conductances (which can be variable) and capacitances. Water flows from soil to the roots, and along the xylem till the leaf tissues and stomata, where it evaporates into the atmosphere following the cohesiontension theory (Pickard, 1981; Sperry et al, 1998). The hydraulic conductivities of the xylem, of the roots and of the soil are extremely variable. Xylem vessels tend to cavitate at high tension, causing a large drop in the axial conductance of the xylem (Sperry et al, 1998). When plants are exposed to severe drying, their roots shrink and lose part of their contact to the soil (Carminati et al, 2013), which further decreases the conductance between rhizosphere and root. Plants can close this gap and attenuate the drop in conductivity by secreting mucilage (Carminati et al, 2010) or by growing root hairs (Carminati et al, 2017)

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