Antitranspirant-induced Heat Stress in the Parasitic Plant Striga hermonthica--A Novel Method of Control

Leaf temperatures (T₁) of the parasitic plant Striga hermonthicaare substantially below those of the air (T_a), T_a–T₁]reaching 7 ?C at T_a = 40 ?C. This results from high rates oftranspiration and the consequent evaporative cooling of theleaf. Application of an antitranspirant, which mechanicallyimpedes foliar loss of water vapour, reduced transpiration andstomatal conductance by 40% and 57%, respectively, and reducedT_a– T₁] to 2 ?C at T_a = 40 ?C. The temperature sensitivityof photosynthesis in the host-parasite association differed,the optima (T_opt) being 37.2 and 40.1 ?C for S. hermonthicaand sorghum, respectively. Once Topt had been exceeded in S.hermonthica net photosynthesis declined rapidly, reaching thelethal limit (Tmax) at 42.6 ?C. S. hermonthica is particularlysensitive to high temperatures and antitranspirant-induced overheatingleads to blackening and shrivelling of the leaf after as littleas 4 h at T_a = 40 ?C. Application of an antitranspirant underfield conditions in the Sudan at T_a = 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, antitranspirantspraying of the arasite increased the grain and straw yieldof the crop by factors of 3.4 and 2.6, respectively. Antitranspirantsmay have potential use as a method of controlling Striga inthe field. Key words: Striga hermonthica, sorghum, photosynthesis, transpiration, high temperature stress, anti-transpirant