Physiological and growth tolerance in wheat to Russian wheat aphid (Homoptera : Aphididae) injury

Plant tolerance to injury from insect herbivores has several advantages as a pest management approach. However, its use is limited because mechanisms conferring plant tolerance are not well understood. We hypothesize that pla nt physiological responses, specifically photosynthesis, substantially cont ribute to plant tolerance to arthropod injury. This hypothesis was tested o n 3 wheat (Triticum eastivum L.) lines that differed in their mode of resis tance to the Russian wheat aphid, Diuraphis noxia (Mordvilko). The lines we re 'Arapahoe' (a susceptible line), PI 137739 (an antibiotic line),and PI 2 62660 (a tolerant line). These lines were grown in a greenhouse, aphids wer e maintained on plants for 1 wk, and physiological responses of these lines were determined. Light curve and fluorescence data indicated that the prim ary mechanism for photosynthetic rate reduction in aphid-injured leaves is via interference of the photochemical efficiency at the initial stage of ph otosynthesis. Aphid-injured seedlings had lower light-saturation points, wh ich suggested less efficient use of light energy compared with control seed lings. Immediately after aphid removal, aphid injury reduced chlorophyll fl uorescence and photosynthetic rates in all lines, but PI 137739 (with antib iosis) had significantly greater photosynthetic rate reduction. Photosynthe tic rates of the tolerant line, PI 262660, began recovering 3 d after aphid removal with complete photosynthetic recovery 7 d after aphid removal. Thi s gradual photosynthetic compensation did not occur in the other 2 lines. P I 262660 also had greater leaf area and more dry matter when compared with the other cultivars. This study demonstrates that photosynthetic adjustment s can significantly contribute to plant tolerance resulting from arthropod injury. Moreover, evidence here indicates that an active plant defense thro ugh antibiosis comes at the cost of reduced capacity for physiological tole rance and compensation.