Shade induces contrasting light photosynthetic performance between Signal and Guinea Grasses

Signal grass (Urochloa decumbens) and guinea grass (Megathyrsus maximus) are African grasses that are well established in the Brazilian Savannah and we tested their adaptation to different light intensity. Plants were grown for 45 days under 0% shade (full sun) and 25, 40, and 80% induced shade to evaluate their photosynthetic performance. Light curves showed higher values of electron transport rate, photochemical quenching, and effective quantum yield in plants subjected to 0 and 25% shade for signal grass and in 25 and 40% shade for guinea grass. The potential quantum yield evaluations revealed that signal grass felt the effects of excessive light around 11:30 am on plants subjected to 0 and 25% shade. Conversely, guinea grass showed these photoinhibition effects at the same shade level but in a longer time range (9:30 am to 1:30 pm). As shade increased, there was a reduction tendency of the pigment content in signal grass and the opposite was observed for guinea grass. Stomatal conductance showed different values during the day and among different shade levels and there were no differences in relative water content between treatments and species. Results indicated better photosynthetic performance for signal grass under high intensity and better photosynthetic performance for guinea grass subjected to intermediate and higher levels of shade. Altogether, the results indicate that guinea grass seems to be a more appropriate choice for silvopasture systems.