Both defensive and nutritional hypotheses have been used to explain the improved performance of sucking insects on variegated plants. With portions of leaves unable to photosynthesize, variegated plants are likely to have less carbon assimilate to allocate to the production of defensive compounds than green plants. Alternatively, like plants grown in the shade, improved response of sucking insects on variegated plants may be caused by greater mobility of amino acid nitrogen and carbohydrates in the phloem to fuel compensatory production of leaves. We examined amino acid and sucrose content of stem exudate collected from red-variegated, yellow-variegated, and green-leafed coleus plants, Solenostemon scutellarioides (L.) Codd, and their relation to life history characteristics of citrus mealybug, Planococcus citri (Risso), to test these defensive and nutritional hypotheses. Although higher concentrations of both total amino acids and sucrose were found in green- and red-leaved plants, these quantitative measures were uncorrelated with life history parameters of mealybugs. In contrast, our analysis of qualitative measures of amino acid composition supported both defensive and nutritional hypotheses. Green-leafed plants had the greatest levels of shikimic acid precursors and lowest levels of other nonessential amino acids. Life history characteristics of citrus mealybugs were correlated negatively with increasing proportions of shikimic acid precursors and positively with increasing proportions of other nonessential amino acids. Thus, our studies of life history and stem chemistry indicate that improved suitability of variegated plants is consistent with both carbon- and nutrient-based theories of plant defense.
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