Thehypothesis that root rot caused by Pythium irregulare
reduces the water use efficiency of wheat was tested in a system which
simulated field conditions with late season water stress. Inoculation with Pythium significantly
reduced transpiration during vegetative growth, so that plants entered
post-anthesis drought with more available water. Although weekly transpiration
rates were higher in inoculated plants than controls during the later stages of
drought, infected plants were unable to make use of all of the extra water.
There were no significant effects of inoculation on shoot biomass or grain
yield, while total transpiration was reduced by 14%. Infected plants therefore
had significantly higher integrated water use efficiency (grain yield relative
to transpiration) than controls. Infected plants were significantly more
stressed than controls during the drought, despite higher soil moisture, and
showed reduced ability to use stomatal conductance to regulate leaf water
potential. Pythium infection
caused adverse changes to plant water use and water relations, but these did
not translate into reductions in growth or yield. This, and the unexpected
increase in water use efficiency, highlights the need to consider interactions
with other environmental stresses when making assumptions about the effects of
root diseases on crop productivity.
Wheat
(Triticumaestivum) is one of the most important food crops in the world. In
rainfed agriculture, the potential yield of wheat is limited by availability of
water, with an attainable Water Use Efficiency (WUE) of 22 kg grainha-1mm-1
having been calculated for dry areas in Australia, North America, China and the
Mediterranean basin (Sadras and Angus 2006). However, the average WUE in each
of these areas is less than half of this (Sadras and Angus 2006). Among the
constraints that potentially limit the ability of wheat to make effective use
of available water is root disease. While the effect of various root diseases
on the yield of wheat is well established, there has been very little
examination of their effect on water relations and WUE.
showed
that cereal cyst nematode greatly reduced the transpiration of wheat plants,
and that this effect could be mimicked by root pruning, suggesting that disease
reduced the effective volume of the root system. On the other hand found a
relatively small effect of inoculation with Gaeumannomyces graminis var. tritici (cause
of take-all) on transpiration in wheat. However, there was a larger effect on
carbon assimilation rates, leading to a reduction in instantaneous WUE (ratio
of carbon assimilation to transpiration measured at the individual leaf level. In previous work we have studied the effects of Pythium irregulare
on water relations of wheat in a hydroponic system. Root infection with Pythium reduced
rates of transpiration and carbon assimilation, and shoot biomass, but did not
significantly affect WUE.
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