Responses of Senna reticulata, a legume tree from the Amazonian floodplains, to elevated atmospheric CO2 concentration and waterlogging

Abstract

Tree species from the Amazonian floodplains have to cope with low oxygen availability due to annual pulses of inundation that can last up to 7 months. Species capable of adapting to flooding and/or waterlogged conditions usually partition their storage to favor starch and allocate it to roots, where carbohydrates are used to maintain respiration rates during waterlogging. In spite of climate change, virtually nothing is known about how elevated atmospheric CO2 concentration ([CO2]) will affect plants when combined with waterlogging. In this work, we used open top chambers to evaluate the effect of elevated [CO2] during a period of terrestrial phase and in subsequent combination with waterlogged conditions to determine if the surplus carbon provided by elevated [CO2] may improve the waterlogging tolerance of the fast-growing Amazonian legume tree Senna reticulata. During the terrestrial phase, photosynthesis was ca. 28 % higher after 30, 45 and 120 days of elevated [CO2], and starch content in the leaves was, on average, 49 % higher than with ambient [CO2]. Total biomass was inversely correlated to the starch content of leaves, indicating that starch might be the main carbohydrate source for biomass production during the terrestrial phase. This response was more pronounced under elevated [CO2], resulting in 30 % more biomass in comparison to ambient [CO2] plants. After 135 days at elevated [CO2] an inversion has been observed in total biomass accumulation, in which ambient [CO2] presented a greater increment in total biomass in comparison to elevated [CO2], indicating negative effects on growth after long-term CO2 exposure. However, plants with elevated [CO2]/waterlogged displayed a greater increment in biomass in comparison with ambient [CO2]/waterlogged that, unlike during the terrestrial phase, was unrelated to starch reserves. We conclude that S. reticulata displays mechanisms that make this species capable of responding positively to elevated [CO2] during the first pulse of growth. This response capacity is also associated with a “buffering effect” that prevents the plants from decreasing their biomass under waterlogged conditions.