Warming and water deficit impact the reproductive features of the tropical forage species Stylosanthes capitata

Increased temperature and water stress, alone and in combination, are among the factors that have the greatest influence on plant development and reproduction. However, little information is available about the effects of such stresses on tropical plant species. In this study, we investigated the effects of warming and two soil moisture conditions on the mating system of the tropical forage species Stylosanthes capitata, including the impact on seed anatomy and germination. Treatments consisted of two temperature levels, ambient (aT) and elevated (eT, +2 °C above ambient temperature), and two soil water conditions, regular irrigated (rI) and non-irrigated (nI, rainfed). We used a Temperature Free-Air Controlled Enhancement (T-FACE) facility to increase canopy temperature under field conditions. Under nI, regardless of temperature, plants exhibited a 39.5 % increase in the outcrossing rate which was attributed to limited floral resources. Consequently, there was a decreased rate of self-fertilization. We observed contrasting effects between irrigation and temperature for druse density; while nI induced a high druse density in the seed cotyledons, eT showed lower druse density, resulting in an intermediate condition under nIeT. Elevated temperature, regardless of the soil moisture levels, also led to an increase in the mass of one hundred seeds and caused a 36 % thinning of the seed coat cuticle, presumably resulting in greater water absorption. In the germination process, seeds derived from plants developed under eT and irrigation conditions showed longer roots with more seeds germinating in the first 120 hours. The effects of warming and soil water content on seed production and germination observed here could have important consequences for the reproduction of tropical forage species such as S. capitata in future climate scenarios.