Silverleaf whitefly stress impairs sugar export from cotton source leaves

Silverleaf whitefly (SLW), Bemisia argentifolii Bellows and Perring, is one of the most noxious pests of numerous field and vegetable crops, causing b illions of dollars worth of damage throughout the world. SLW is a phloem fe eder whose feeding is likely to interfere with phloem transport, The aim of this study was to test the hypothesis that SLW infestation impairs carbohy drate export from source leaves, and consequently increases their carbohydr ate content. The youngest fully expanded leaves of cotton (Gossypium hirsut um L., cv. Siv'on), grown under SLW-infested and noninfested conditions, we re characterized for their diurnal changes in carbohydrate content and phot oassimilate export, SLW infestation induced a considerable reduction in net photosynthetic rate (P-n), coupled with increased sucrose, glucose and fru ctose and decreased starch concentrations. Export rate was determined after (CO2)-C-14 pulse-labeling both by in situ monitoring of leaf radioactivity and by analyzing the content and radioactivity of the major carbon metabol ites. Radioactive counting indicated a lower rate of C-14 efflux for the in fested plants. A similar trend was found for the specific activities of suc rose and the three soluble sugars combined (sucrose, glucose and fructose). A single exponential decay function with asymptote was fitted to the above efflux curves. All the calculated exponential coefficients demonstrated lo wer export rates after SLW injury. These results indicate that SLW impairs photoassimilate export, suggesting possible down-regulation of P-n due to i ncreased foliar soluble sugar contents.