Photosynthesis Influenced by Irradiance, External Carbon Dioxide Concentration and Temperature in Crotalaria Species.

Abstract

In tropical plantation crops perennial legumes are grown as understory cover crops or as green manure crops to improve soil fertility and to reduce soil degradation.  These understory plants receive very limited irradiance and encounter elevated levels of CO₂ and temperature.  A greenhouse experiment was conducted to evaluate the independent short-term effects of photosynthetic photon flux density (PPFD), external carbon dioxide concentration [CO₂] and temperature on net photosynthesis (P_N), internal CO₂ (Ci), stomatal conductance (Gs) and transpiration (E) in four Crotalaria species (C. breviflora, C. mucronata, C. ochroleuca, C. spectabilis).  In all the Crotalaria species, increasing PPFD from 50 to 1500 µmol m^-2 s^-1 increased P_N by 21 fold.  Increasing the external [CO₂] from 100 to 1000 cm³ m^-3 increased P_N  by about 5 fold.  Increasing the temperature from 25 to 35 °C increased P_N of Crotalaria species by 11%.  Shade management is critical to maintaining the productivity of these tropical perennial legumes.