Photosynthetic Photon Flux Density, Carbon Dioxide Concentration and Temperature Influence Photosynthesis in Crotalaria Species

Crotalarias are tropical legumes grown as cover crops or as green manure to improve soil fertility. As an understory plant in plantation systems, these cover crops receive low levels of irradiance and are subjected to elevated levels of CO2 and temperatures. A greenhouse experiment was conducted to evaluate the independent short-term effects of photosynthetic photon flux density (PPFD), external carbon dioxide concentration (Ca) and temperature (T) on net photosynthesis (PN), internal CO2 (Ci), stomatal conductance (gs) and transpiration (E) in four Crotalaria species (C. breviflora, C. mucronata, C. ochroleuca, C. spectabilis). These crotalarias responded differently to changing PPFD, Ca and T. In all the Crotalaria species, increasing PPFD from 50 to 1500 mol m -2 s -1 increased PN by 21 fold, increased gs by 2.3 fold (136%), decreased Ci by 3.9 times, and increased E by 2.1 times. Increasing the external Ca from 100 to 1000 cm 3 m -3 increased PN by 4.7 fold, decreased gs by 1.3 times, increased Ci by 28 fold, and decreased E by 1.2 times. Increasing the T from 25 to 35 °C increased PN of Crotalaria species by 11%, decreased gs by 33%, decreased Ci by 64%, and increased E by 56%. Shade management is critical to maintaining the productivity of these tropical legumes.