Antitranspirant-induced Heat Stress in the Parasitic PlantStriga hermonthica—A Novel Method of Control

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Abstract
Leaf temperatures (T1) of the parasitic plant Striga hermonthica are substantially below those of the air (Ta), [Ta−T1] reaching 7 °C at Ta = 40 °C. This results from high rates of transpiration and the consequent evaporative cooling of the leaf. Application of an antitranspirant, which mechanically impedes foliar loss of water vapour, reduced transpiration and stomatal conductance by 40% and 57%, respectively, and reduced [Ta− T1] to 2 °C at Ta = 40 °C. The temperature sensitivity of photosynthesis in the host-parasite association differed, the optima (Topt) being 37.2 and 40.1 °C for S. hermonthica and sorghum, respectively. Once Topt had been exceeded in S. hermonthica net photosynthesis declined rapidly, reaching the lethal limit (Tmax) at 42.6 °C. S. hermonthica is particularly sensitive to high temperatures and antitranspirant-induced overheating leads to blackening and shrivelling of the leaf after as little as 4 h at Ta = 40 °C. Application of an antitranspirant under field conditions in the Sudan at Ta = 40 °C resulted in 28% and 67% reductions in transpiration and stomatal conductance, together with a 5 °C increase in T1, and subsequent leaf death. In addition to these short-term physiological responses, antitranspirant spraying of the arasite increased the grain and straw yield of the crop by factors of 3.4 and 2.6, respectively. Antitranspirants may have potential use as a method of controlling Striga in the field.