Nitrogen accumulation and partitioning in shoots of cotton plants infested with two-spotted spider mites

In cotton (Gossypium hirsutum L.), leaves are the main site of nitrogen reduction and constitute a large reservoir of organic nitrogen. Foliar herbivores, therefore, are likely to have detrimental effects on the nitrogen economy of the plant. A field experiment was conducted to investigate the effects of two-spotted spider mites (Tetranychus urticae Koch) on the accumulation and partitioning of nitrogen in cotton shoots. Control plants and plants infested with mites 3 times in the growing season were compared. Once established, mite colonies grew exponentially. After peaking at 35-64 mites/leaf, mite numbers declined sharply. Mites markedly affected both shoot nitrogen accumulation and partitioning. The amount of nitrogen in shoots of mite-infested plants peaked earlier than in controls, and reached maximum values that were 50-69% of the controls. Early infestation (at the onset of reproductive growth) had a greater effect than infestations initiated during active reproductive growth. The concentration of nitrogen in leaves declined faster in mite-infested plants than in controls, mostly due to accelerated 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 reproductive organs was generally greater in mite-infested plants than in controls. Allometric analysis showed that this was related to both the small size of mite-infested plants, and true increases in nitrogen content probably associated with translocation from senescing leaves.