GROWTH ANALYSIS OF COTTON CROPS INFESTED WITH SPIDER-MITES .1. LIGHT INTERCEPTION AND RADIATION-USE EFFICIENCY

Two-spotted spider mites (Tetranychus urticae Koch) are important pest s of cotton (Gossypium hirsutum L.). The effects of mites on cotton ph otosynthesis have been investigated at the leaf and cytological levels but not at the canopy level. Our objective was to quantify the effect s of timing and intensity of infestation by mites on cotton radiation- use efficiency (RUE). Leaf area, light interception, RUE, canopy tempe rature, and leaf nitrogen concentration (LNC) were assessed during two growing seasons in crops artificially infested with mites between 59 and 127 d after sowing. Normal and okra-leaf cultivars were compared. A mite index (MI = natural log of the area under the curve of mite num ber vs thermal time) was used to quantify the cumulative effects of mi tes on RUE, LNC, and canopy temperature. Crop growth reduction due to mites was greater in early- than in late-infested crops. Growth reduct ion was primarily due to reductions in RUE, but in the more severe tre atments accelerated leaf senescence and, hence, reduced light intercep tion also contributed to reductions in crop growth. At a given date, i nfested okra-leaf crops usually had greater RUE than their normal-leaf counterparts. Both plant types, however, responded similarly to a giv en level of mite infestation. The ability of the okra-leaf cultivar to maintain greater RUE levels can be attributed, therefore, to its rela tive inhospitality for the development of mite colonies rather than to an intrinsically greater capacity to maintain photosynthesis under mi te damage. Canopy temperature, LNC, and RUE showed a similar, biphasic pattern of response to MI. In the first phase, response variables wer e almost unaffected by mites. In the second phase, there was a marked decline in RUE and LNC and a marked increase in canopy temperature wit h increasing MI. These results suggest (i) some degree of compensatory photosynthesis at low to moderate levels of mite infestation, and (ii ) reductions in RUE of mite-infested cotton crops involved alterations in both canopy gas diffusion and metabolic activity. Quantitative rel ationships between RUE and MI were developed that could be used to lin k mite and crop growth models.