COTTON RESPONSES TO SIMULATED INSECT DAMAGE - RADIATION-USE EFFICIENCY, CANOPY ARCHITECTURE AND LEAF NITROGEN-CONTENT AS AFFECTED BY LOSS OF REPRODUCTIVE-ORGANS

Key cotton pests feed preferentially on reproductive organs which are normally shed after injury. Loss of reproductive organs in cotton may decrease the rate of leaf nitrogen depletion associated with fruit gro wth and increase nitrogen uptake and reduction by extending the period of root and leaf growth compared with undamaged plants. Higher levels of leaf nitrogen resulting from more assimilation and less depletion could increase the photosynthetic capacity of damaged crops in relatio n to undamaged controls. To test this hypothesis, radiation-use effici ency (RUE = g dry matter per MJ of photosynthetically active radiation intercepted by the canopy) of crops in which flowerbuds and young fru its were manually removed was compared with that of undamaged controls . Removal of fruiting structures did not affect RUE when cotton was gr own at low nitrogen supply and high plant density. In contrast, under high nitrogen supply and low plant density, fruit removal increased se asonal RUE by 20-27% compared to controls. Whole canopy measurements, however, failed to detect the expected variations in foliar nitrogen d ue to damage. Differences in RUE between damaged and undamaged canopie s were in part associated with changes in plant and canopy structure ( viz. internode number and length, canopy height, branch angle) that mo dified light distribution within the canopy. These structural response s and their influence on canopy light penetration and photosynthesis a re synthetised in coefficients of light extinction (k) that were 10 to 30% smaller in damaged crops than in controls and in a positive corre lation between RUE(-1) and k for crops grown under favourable conditio ns (i.e. high nitrogen, low density). Changes in plant structure and t heir effects on canopy architecture and RUE should be considered in th e analysis of cotton growth after damage by insects that induce abscis sion of reproductive organs.