Agradecimentos: We are grateful to Prof. Dr. Nadja Cristhina de Souza Pinto (Instituto de Química, Universidade de São Paulo) for the helpful comments and essential help in the RuBisCO essay calculations. This work was partly funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES), Finance Code 001 to SL, by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, n° 2011/50637-0 and 2018/12667-3) to HM, n° 2016/09699-5 to AZG, and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, n° 309504/2014-7 and 303497/2018-1) to HM Ver menos

Abstract: Plants that perform the Crassulacean acid metabolism (CAM), which obtain CO2 overnight and convert it mainly in malic acid, successfully grow in environments with water and nutrient shortages, that is partly associated with their higher water- and nitrogen-use efficiencies. Water and nutrient limitations can impair photosynthesis through the reduction of RuBisCO and increment of photorespiration, disturbing the plant carbon balance. In this context, we conducted a controlled experiment with the epiphytic C3-CAM bromeliad Guzmania monostachia to investigate how the combined water and nutritional deficits affect the activity of RuBisCO and its activation state (RAS), and to evaluate the efficiency of photosynthesis during the transition from C3 to CAM. Apart from an increase in CAM activity, bromeliads submitted to both water and nutritional deficits showed higher RAS values and unaltered RuBisCO activity compared to C3 bromeliads and, surprisingly, the maximum quantum efficiency of photosynthesis increased. Glucose, fructose and starch levels were maintained, while sucrose concentrations increased over time. These results, combined with the high RAS values, suggest an increased efficiency of RuBisCO functioning. Our results reinforce the ability of epiphytic bromeliads to deal with stressful habitats by a higher efficiency of RuBisCO during the transition to CAM, another feature that may allow their evolution in the epiphytic environment Ver menos