NITROGEN ACCUMULATION AND PARTITIONING IN SHOOTS OF COTTON PLANTS INFESTED WITH 2-SPOTTED SPIDER-MITES

In cotton (Gossypium hirsutum L.), leaves are the main site of nitroge n reduction and constitute a large reservoir of organic nitrogen. Foli ar herbivores, therefore, are likely to have detrimental effects on th e nitrogen economy of the plant. A field experiment was conducted to i nvestigate the effects of two-spotted spider mites (Tetranychus urtica e Koch) on the accumulation and partitioning of nitrogen in cotton sho ots. Control plants and plants infested with mites 3 times in the grow ing season were compared. Once established, mite colonies grew exponen tially. After peaking at 35-64 mites/leaf, mite numbers declined sharp ly. Mites markedly affected both shoot nitrogen accumulation and parti tioning. The amount of nitrogen in shoots of mite-infested plants peak ed earlier than in controls, and reached maximum values that were 50-6 9% of the controls. Early infestation (at the onset of reproductive gr owth) had a greater effect than infestations initiated during active r eproductive growth. The concentration of nitrogen in leaves declined f aster in mite-infested plants than in controls, mostly due to accelera ted leaf senescence. This rapid decline in leaf nitrogen concentration could be one of the factors involved in the collapse of mite colonies in heavily infested plants. Nitrogen concentration of stems and repro ductive organs was generally greater in mite-infested plants than in c ontrols. Allometric analysis showed that this was related to both the small size of mite-infested plants, and true increases in nitrogen con tent probably associated with translocation from senescing leaves.