Elevated CO2 and warming affect pollen development in a tropical legume forage species.

• Tropical plant reproductive processes are vulnerable to climate change. • Warming and eCO 2 induced early degeneration of tapetum cells of pollen sacs. • Warming and eCO 2 induced a less differentiated aperture of pollen and compaction of pollen exine. • Warming and eCO 2 induced abnormalities which may hinder the viability of pollen grain. Global climate change is expected to have impacts on the physiological, phenological, and morphological traits of plants. However, the vulnerability of tropical plant reproductive processes in response to climate change events has been poorly studied. Here, we assess if warming and elevated CO 2 compromise the pollen characteristics of Stylosanthes capitata Vogel, a tropical legume forage species. This work was conducted in a Trop-T-FACE (combined Free-Air Temperature Controlled Enhancement and Free-Air Carbon Dioxide Enrichment) facility, where we exposed the plants to four treatments: C (Control -ambient atmospheric carbon dioxide concentration [CO 2 ] and ambient temperature); eCO 2 (increase in [CO 2 ] to 600 ppm and ambient temperature); eT (canopy temperature increase by 2°C and ambient [CO 2 ]); and eCO 2 + eT, a combination of both treatments. We analyzed pollen morphology of samples taken from the different treatments through light microscopy (LM) and scanning electron microscopy (SEM). In addition, pollen viability was analyzed through colorimetry. Based on the histological LM analysis, the tapetum cells of pollen sacs showed early degeneration under eT (72%) added to hypertrophy under eCO 2 (67%) and eCO 2 + eT (90%) treatments. SEM images showed compaction of pollen exine and less distinguishable pollen apertures in the treated plots (eCO 2 , eT , and eCO 2 + eT), possibly, by the early death of the tapetum cells. These morphological abnormalities may hinder the viability of pollen grains, as observed especially in the eCO 2 + eT treatment (%V=64%) that was the lowest in comparison with the Control (71%) and the other treatments (eCO 2 = 69%, eT = 67%). These results indicate that during the reproductive cycle of S. capitata pollen sacs and pollen grains are vulnerable to warming, elevated CO 2 , and their combined effects. [ABSTRACT FROM AUTHOR]