WATER-DEFICIT ENHANCED COTTON RESISTANCE TO SPIDER-MITE HERBIVORY

We investigated the responses of cotton (Gossypium hirsutum L.) to the combined effects of soil water deficit and twospotted spider mite (Te tranychus urticae Koch) infestation. Two mite treatments (-M: uninfest ed, +M: artificially infested 83 d after sowing), and two water regime s (+W: well watered, -W: water stressed) were combined factorially in four treatments. Mite colonies developed at similar rates in well-wate red and water-stressed crops. Despite the similar intensity of infesta tion, visual symptoms of mite injury were more marked in well-watered host plants (+M+W) than in their water-stressed counterparts (+M-W). L int yield of unstressed controls (-M+W) was 175 g m(-2). In uninfested crops, water deficit reduced yield by 30%, mites reduced the yield of well-watered crops by 92%, and the combination of mite infestation an d water deficit reduced yield by 72% (water effect: P < 0.01; mite and interaction effect: P < 0.0001). Differences in yield responses to mi tes between well-watered and water-stressed crops were mostly related to differences in reproductive partitioning. The interaction between m ites and water deficit was also significant for other crop variables i ncluding canopy temperature, leaf water potential, concentration of ni trogen in reproductive structures and seed oil concentration. The magn itude and consistency of the interaction between both stresses indicat es that, under our experimental conditions, mechanisms of adjustment t o water deficit may have enhanced cotton resistance to miles. This is further supported by (a) an increase in specific leaf weight and a par allel increase in leaf penetration resistance due to water deficit; (b ) a negative association between macroscopic symptoms of mite injury a nd leaf penetration resistance; and (c) a choice rest showing that adu lt female mites preferred to feed and oviposit on leaves from well-wat ered plants. (C) 1998 Annals of Botany Company.