Nutrient allocation of Macaranga triloba ant plants to growth, photosynthesis and indirect defence

1. Allocation patterns are an important aspect of plant life strategies. In particular, many hypotheses on plant antiherbivore defence - such as the growth differentiation balance hypothesis and the resource availability hypothesis - concern constraints on resource allocation to defence, and assume defence to be limited by nutrient supply under most, yet not all, conditions. 2. Field-grown Macaranga triloba saplings were fertilized, and temporal and spatial patterns in the response of growth, photosynthesis and investment in biotic defence against herbivores and pathogens were measured simultaneously. Macaranga triloba produces food bodies to nourish mutualistic ants which protect their hosts against herbivores and pathogens. Food body production rate is quantitatively related to ant colony structure, and the latter is correlated with defensive efficacy. Food body production can therefore serve as a measure of the plant's investment in defence. 3. Food body production responded quickly, and on all food body-producing stipules, to increased nutrient supply. In contrast, photosynthetic capacity responded much more slowly, to a smaller degree, and only in leaves that emerged after the onset of fertilization. No significant effects on plant growth were seen. 4. Our results show that food body production by M. triloba is regulated directly by nutrient availability. There was no evidence that increased nutrient supply was preferably allocated to growth and/or photosynthesis, which forms a central assumption of most hypotheses on plant antiherbivore defence. 5. Phenotypic plasticity and the ability to make use of short pulses in nutrient supply are of particular importance in pioneer plants inhabiting rapidly changing habitats. New theoretical frameworks are required for the inclusion of phenotypically plastic defence traits in the theory of plant antiherbivore defence.