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Within-individual physiology constrains carnivorous investment in the rainbow plant Byblis guehoi more than does environmental light intensity

Abstract The cost–benefit model of plant carnivory posits that benefits of carnivory exceed its costs only in environments where light and moisture levels are high, while soil nutrient levels are low. Carnivorous investment is a plastic trait that can be regulated by individual plants. The literature provides strong support for nutrient-dependent plasticity in carnivorous investment, but is less conclusive regarding light-dependent responses, while nothing is known of the effect of within-individual physiological constraints on carnivorous investments in carnivorous plants. We examined the relative importance of light availability and within-individual factors on the carnivorous investment of Byblis guehoi Lowrie & Conran, a recently described flypaper-type carnivorous plant from Western Australia. Twelve pots of B. guehoi plants were cultivated under four different fluorescent light setups with 12-h, light–dark cycles under controlled laboratory conditions. Mucilage production after six weeks was quantified and modelled against light intensity and other within-individual factors. Total leaves borne per stem had a strong, hump-shaped relationship with mucilage production. Other within-individual factors were weak predictors as well, but light intensity was not a good predictor. Our results suggest that, contrary to the expectation of the cost–benefit model of plant carnivory, individual plant investment in carnivory may be more dependent upon within-individual physiological constraints than environmental light availability, although the latter is likely to affect carnivorous investment in more indirect ways through individual plants’ physiological conditions. These results thus do not contradict the cost–benefit model, but identify an important caveat to it, which may have been overlooked previously.