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 plant physiological responses, specifically photosynthesis, substantially contribute to plant tolerance to arthropod injury. This hypothesis was tested on 3 wheat ( Triticum eastivum L.) lines that differed in their mode of resistance to the Russian wheat aphid, Diuraphis noxia (Mordvilko). The lines were ‘Arapahoe’ (a susceptible line), PI 137739 (an antibiotic line), and PI 262660 (a tolerant line). These lines were grown in a greenhouse, aphids were maintained on plants for 1 wk, and physiological responses of these lines were determined. Light curve and fluorescence data indicated that the primary mechanism for photosynthetic rate reduction in aphid-injured leaves is via interference of the photochemical efficiency at the initial stage of photosynthesis. Aphid-injured seedlings had lower light-saturation points, which suggested less efficient use of light energy compared with control seedlings. Immediately after aphid removal, aphid injury reduced chlorophyll fluorescence and photosynthetic rates in all lines, but PI 137739 (with antibiosis) had significantly greater photosynthetic rate reduction. Photosynthetic rates of the tolerant line, PI 262660, began recovering 3 d after aphid removal with complete photosynthetic recovery 7 d after aphid removal. This gradual photosynthetic compensation did not occur in the other 2 lines. PI 262660 also had greater leaf area and more dry matter when compared with the other cultivars. This study demonstrates that photosynthetic adjustments can significantly contribute to plant tolerance resulting from arthropod injury. Moreover, evidence here indicates that an active plant defense through antibiosis comes at the cost of reduced capacity for physiological tolerance and compensation.