Variable responses of stomatal conductance, growth, and yield to fulvic acid applications to wheat

Abstract

Studies in China have shown that fulvic acid (FA), applied after maximum leaf area is achieved, substantially increases the yield of wheat over a wide range of yield levels. The effects were greatest when there was a sustained drought and hot, dry winds. Yield increases appeared to result from the effect that FA had on stomatal conductance and leaf senescence. We report here the responses of predominantly Australian wheat genotypes to FA applied in both glasshouse and field. FA, extracted from coal in Australia or China, was applied to the leaves of wheat plants as a 0.05 or 0.1% spray. Although it reduced stomatal conductance by as much as 55% in particular studies and stimulated the growth of seedlings in other studies, the results were neither consistent nor predictable. We were unable to define a set of conditions under which the maximum effect could always be obtained. In the field studies, three slightly droughted crops were sprayed between late-boot stage and ear emergence. The effects of FA on stomatal conductance were variable amongst treatments 4 h after application, and had virtually disappeared one week later. Grain yield and harvest index were unchanged by FA. Similarly, in glasshouse studies, FA applied to plants at the onset of drought at four stages of development commencing at flag-leaf-emergence had no effect on grain yield or water use. It is concluded that, while FA does appear to have some effects in wheat, we were unable to identify the conditions required for it to influence grain yield and water use. We therefore recommend caution in its use as a foliar spray until these conditions can be defined.