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

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.

Keywords: maize (Zea mays L); pressure chamber; soil drying; stomatal closure; transpiration rates.

Figure 1

Vertical profiles of volumetric soil water content in each replication.

Figure 2

Effect of light intensity and pressurization on transpiration rates for varying soil water contents. (A–D) Effect of pressurization on transpiration. (E) Effect of light intensity and soil moisture on transpiration in pressurized and (F) unpressurized plants.

Figure 3

Measured xylem suction and transpiration rate for decreasing water contents (WC) and increasing light intensity (1–4) for replicate 1 (2 and 3 are shown as Supplementary Material). The solid lines are the model fits.

Figure 4

Effect of light intensity on normalized soil-plant conductance k* = k/k_max (where k_max is the soil-plant conductance in the wettest soil and lowest light intensity) at varying soil water contents (WC). Relative soil-plant conductance k* decreased with increasing light intensity due to higher transpiration rates and with decreasing soil water contents due to the decreasing soil hydraulic conductivity.

Figure 5

The relation between intercept (Po) and the soil matric potential. The points below (above) the 1:1 line indicate a more negative (positive) osmotic potential in the leaf xylem than in the soil.