Oral Paper

         Ecophysiology

Role of stem photosynthesis and elevated CO2 on the drought response of a late-successional Neotropical tree species

Presenting Author
Eleinis Avila-Lovera
Description
It is widely known that stem photosynthesis plays important roles in the growth and survival of plant species from deserts and Mediterranean-type ecosystems. However, it is less clear how these advantages extend to species from wetter ecosystems, such as the wet tropical forests of Panama. We asked whether preventing stem photosynthesis had a negative effect on saplings of Calophyllum longifolium (Calophyllaceae), and whether this effect could be reverted by previously exposing plants to elevated [CO2]. We first exposed half of the saplings to 2000 ppm of [CO2], while the other half remained at ambient [CO2], for one month. We then placed the plants under a rain shelter and covered the stems of half of the plants in each of the [CO2] treatments to block stem photosynthesis and allowed the plants to dehydrate by withholding watering. We monitored pre-dawn leaf water potential (Ψpd), specific leaf area, leaf and stem temperature, leaf and stem gas exchange in light and dark every three days for two months, and also collected leaf and stem samples five times to analyze non-structural carbohydrates (NSC) as the drought progressed. There was no significant difference in Ψpd between elevated and ambient [CO2] treatments before or after the onset of drought, and there was no effect of light exclusion. However, Ψpd decreased from -0.20 MPa to almost -4 MPa as the drought treatment progressed. Before the drought treatment, leaf photosynthetic rate (A) was greater in the ambient [CO2] than in the elevated [CO2], despite lower stomatal conductance in the former. Stem photosynthetic re-assimilation rate (Ares) was also greater in the ambient [CO2] than in the elevated [CO2], although plants in both treatments had similar percent re-assimilation indicating differences in dark respiration in the stems with [CO2]. After the onset of drought, values of A in the ambient [CO2] treatment decreased more than those in plants subjected to elevated [CO2]. As expected, stem Ares was lower in the light-excluded stems, but light exclusion did not influence leaf mortality. Surprisingly, plants in the elevated [CO2] suffered greater leaf mortality. Overall, neither previous exposure to elevated [CO2], which did lead to greater NSC in plant tissues, nor allowing stem photosynthesis to occur had an important role in explaining plant responses during drought in Calophyllum longifolium saplings.