Root depth: a trait to increase water use and yield of wheat

Abstract

Crops with deeper roots could potentially capture more soil resources and as a consequence yield more. However, as sampling roots by soil coring is challenging, there are few examples of genetic diversity determined under field conditions. Canopy temperature, an indicator of transpiration, could be used instead of direct coring to screen for wheat varieties with increased access to deep water and hence deep roots in the field. In this thesis we aimed (i) to seek genetic diversity in rooting depth, root length density and relate these traits to yield in a wide range of triticale and wheat germplasm, and (ii) to test the usefulness of continuously-monitored canopy temperature and soil water status for phenotyping two commercial wheat varieties that differ in rooting depth. In the first set of field experiments, rooting depth, root length density and yield were measured in 34 wheat and 2 triticale varieties. Roots were sampled by soil-coring with a tractor-mounted hydraulic press and were later counted by the 'core break' method. Root length density was predicted from root count density. In the second set of experiments, canopy temperature was measured with fixed infra-red thermometers, and soil water suction was determined with gypsum blocks buried at 20 cm intervals, from 20 to 160 cm depth. A crop water-stress index (CWSI) was calculated to normalise for the effects of vapour pressure deficit over canopy temperature. Soil water retention curves fitted to the soil of the site were used to convert soil water suction into soil water content. Shoot biomass and grain yield were estimated from 0.7 m^2 samples per plot in all experiments. In the experiments seeking genetic variability, we found that triticale produced deeper roots than commercial spring-wheat (p < 0.10), and shorter varieties produced deeper roots than taller varieties (p < 0.10). Moreover, rooting depth was related to shoot biomass (R^2 = 0.66, p < 0.001) and grain yield (R^2 = 0.56, p < 0.001) across experiments and genotypes but not between genotypes within the same experiment. In the experiments analysing canopy temperature and water-use continuously, differences in deep-root length were not statistically significant between the two varieties. The variety Gregory had greater root length at depths beneath 1 m, was cooler, used more water and that water was withdrawn from deeper soil layers than the other variety, Derrimut. Using CWSI gave better predictions of soil water status than canopy temperature per se. By taking up more water during grain filling, Gregory produced more yield at a rate of 54 kg ha^-1 mm^-1 . CWSI did not correlate with day-to-day changes in water use. We conclude that (i) there is genetic diversity in rooting depth within triticale and wheat germplasm; (ii) by enabling the calculation of a CWSI, continuously measured canopy temperature allows phenotyping of root systems with superior deep water access.