Responses of Root Hydraulic Properties and Transpirational Factors to a Top Soil Drying in &amp;lt;i&amp;gt;Cajanus cajan&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;Sesbania sesban&amp;lt;/i&amp;gt;

Nobuhito Sekiya,Hideki Araki

doi:10.4236/ajps.2013.412a3005

Abstract

Responses of leaf area (LA), stomatal conductance (gs), root length (RL) and root hydraulic conductance per unit of root length (Lpunit) to top soil dryness were investigated. Pigeon pea (Cajanus cajan) and sesbania (Sesbania sesban) were grown in a vertical split-root system. From sixty-six days after sowing, the top soil was dried while the bottom soil was kept wet. Pigeon pea increased LA while maintaining leaf water potential (ΨL) by reducing gs. Increased transpirational demand through canopy development was compensated for by increasing water extraction in the bottom soil. This was achieved by increasing not only RL but also Lpunit. Sesbania kept constant levels of gs, causing a transient reduction of ΨL. ΨL of sesbania was, then, recovered by increasing only RL, but not Lpunit, in the bottom soil while suspending LA extension, suggesting that sesbania regulated only the root area to LA ratio. This study demonstrated a species-specific significance of Lpunit and coordination among Lpunit, RL, gs and LA in exploitation of wet-deeper soils in response to top soil dryness.

Highlights

Reduction of water supply induces top soil dryness, causing a difference in soil water availability between soil layers
It was reported that defoliation of leaves, which decreases the ratio of leaf area to root length, rapidly raised stomatal conductance of sugarcane [13]
Both pigeon pea and sesbania continued to extract water from the top soil after withholding water to the top soil, and the soil water content declined to 8% (v·v−1) at Day 1 in pigeon pea and Day 3 in sesbania

Summary

Introduction

Reduction of water supply induces top soil dryness, causing a difference in soil water availability between soil layers. To exploit such a heterogeneous resource in soils, plants are known to exhibit various root responses. To examine the plant capacity in exploitation of such water sources, the balance between transpiration and water supply must be taken into account rather than focusing only on root factors. Transpiration is a function of two factors; stomatal conductance and leaf area so that either or both of the two factors should determine apparent root water uptake. The potential of root function for water supply is primarily determined by two factors: total root length (or root area) and hydraulic conductance of the roots. It was reported that defoliation of leaves, which decreases the ratio of leaf area to root length, rapidly raised stomatal conductance of sugarcane [13]

Methods

Results

Discussion

Conclusion