Irradiance, external carbon dioxide concentration and temperature influence photosynthesis in tropical cover crop legumes

In plantation crops, perennial tropical legumes are grown as understory plants, where they receive limited irradiance and can be subjected to elevated levels of CO2. Independent short-term effects of photosynthetic photon flux density (PPFD), external carbon dioxide concentration [CO2] and temperature on net photosynthesis (Pn), internal CO2 (Ci), stomatal conductance (Gs) and transpiration (E) were assessed in 5 tropical perennial legume cover crops (calopo, jack bean, mucuna, white lead tree and perennial peanut). These legumes responded differently to changing PPFD, external [CO2] and temperature. In all legume species studied, reducing PPFD from 1000 to 50 mu mol/m(2)/s reduced Pn to less than 10% of that at the higher light level. Increasing the external [CO2] from 250 to 700 cm(3)/m(3) doubled Pn. Increasing temperature from 30 degrees C to 35 degrees C reduced Pn in calopo, jack bean and mucuna and increased Pn in white lead tree and perennial peanut, but magnitude of the differences was small. Gs increased with PPFD in all species except perennial peanut, but decreased substantially as external [CO2] increased only in white lead tree. Early in the establishment of plantations, soils receive nearly full sunlight. However, with increasing age, the amount of light reaching ground level is decreased drastically, to as low as 80-200 mu mol/m(2)/s. Cover crops that can grow in low light might survive longer and better protect the soil from erosion. Responses of these perennial legumes under the conditions imposed have shown that white lead tree and calopo would perform better than the other species as cover crops in tree plantations, where they would be subjected to high levels of shading. However, management of the tree canopy would be important to allow sufficient light through to achieve acceptable growth rates in the cover crops.