Effects of Simulated Tobacco Hornworm (Lepidoptera: Sphingidae) Defoliation on Growth Dynamics and Physiology of Tobacco as Evidence of Plant Tolerance to Leaf Consumption

Simulated tobacco hornworm, Manduca sexta (L.), defoliation was conducted by hand at four growth stages of tobacco, Nicotiana tabacum (L.), to investigate potential compensatory mechanisms for buffering foliar losses. Plants defoliated at plant establishment and posttopping growth stages employed several compensatory processes including a) periods of increased efficiency in assimilating dry matter, occasionally correlated with increases in chlorophyll, soluble proteins, and ribulose 1,5-bisphosphate carboxylase oxygenase content; b) partitioning of assimilates to leaf tissue at expense of support structures; c) increased leaf area per unit of total plant dry weight; d) delayed plant senescence. At maturity, despite these compensatory mechanisms, growth and economic yield were significantly lower among plants defoliated before the topping growth stage. Our results suggest that reduction in growth and yield was related to summation of reduced leaf area and lower net assimilation rate among remaining foliage. Plants defoliated at the posttopping stage, however, showed no significant yield loss or impairment of growth. This study provides additional evidence that plants have evolved physiological mechanisms for minimizing adverse effects of defoliation by leaf-chewing insects