Water extraction by grain sorghum in a sub-humid environment. II. Extraction in relation to root growth

Abstract

An experiment was conducted under continuous drying in the field to examine the relationship between soil water extraction and root growth of grain sorghum. Extraction was analysed in terms of two components: (1) the time when the extraction front reached a depth, defined as the moment when soil water content (θ) begins to decline exponentially with time and, (2) the decline of θ with time at each depth after the extraction front arrives. A second aim was to test if severe assimilate limitation, caused by shading the crop during late vegetative growth, would influence the rate of root front penetration and root length accumulation.

The root front penetrated from sowing at a constant downward rate of 2.72 cm day−1. Down to 100 cm depth, the extraction front lagged behind the root front; thereafter the extraction and root fronts descended together, to reach the same maximum depth of 190 cm shortly after anthesis. Total root length also reached a maximum at that time. Shading had no effect on the rate of root front penetration and length accumulation, despite reducing above-ground growth by 70%.

Once the root front arrived at a given depth, root proliferation continued until about 40-20% of the extractable water in the layer remained. As a consequence of the roots proliferating ahead of the extraction front, a small amount of water was taken up before the extraction front arrived and the extraction rate at each depth reached a maxium when half of the extractable water remained. As a consequence, the decline in θ was better described by a sigmoidal, than an exponential curve, particularly in the middle layers of the profile. The applicability of the sigmoidal curve to describe the decline in θ with time is limited, however, to the special situation where root length increases during water extraction.