Testing the carbon starvation hypothesis using a late-successional neotropical tree species with green photosynthetic stems.

Carbon starvation is one of the proposed mechanisms of drought-induced plant mortality. However, it has not been implicated in drought mortality as much as hydraulic failure. We tested the role of carbon on responses to drought by limiting stem photosynthesis and increasing tissue non-structural carbohydrates (NSC) in saplings of the tropical tree Calophyllum longifolium Willd. (Calophyllaceae). We first artificially increased [NSC] by exposing half of the saplings to 2,000 µmol mol-1 of CO2, while the other half remained at ambient [CO2] for 6 wk. Following CO2 treatments, there were no significant differences in predawn leaf water potential (Ψpd), leaf photosynthetic rate, and stem re-assimilation rate between elevated and ambient [CO2] treatments, whereas stem re-assimilation percentage (percentage of dark respiration rate that is re-assimilated) was greater in ambient [CO2]. Exposure to elevated [CO2] produced higher starch and total [NSC] in the tissues of those plants. We then covered the stems of half of the plants from each CO2 treatment to block stem photosynthesis and applied a drought treatment to all plants. Light exclusion reduced stem photosynthesis and most traits responded similarly to drought across the 4 treatment levels. Drought decreased soluble sugar concentration and increased starch concentration with minimal effects of prior [CO2] treatment. Despite initial differences in starch and total [NSC] between the 2 [CO2] treatment levels, and physiological responses to light exclusion, leaf and plant mortality occurred at the same pace. Our results demonstrate that stem photosynthesis does not contribute to drought survival in saplings of C. longifolium.