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Light and CO2 photosynthetic responses of 4 south American grasses grown under different levels of soil nitrogen availability.
Rosa, Luis1, Azzarini, Leticia1, Abreu, Clarissa1, Pilla, Patricia1, 1
ABSTRACT- The southern Brazilian grasslands have a high diversity of C3 and C4 grasses growing simultaneously. This coexistence, makes it a unique system to study their comparative ecophysiology of C3 and C4. In this study we examine the responses of C3 (Piptochaetium montevidense [PM] and Brisa subaristata [BS]) and C4 (Andropogon lateralis [AL] and Paspalum urvillei [PU]) species to different levels of soil N availability. Plants of each species were grown under each of three different nitrogen levels (0, 1.5 and 15 mM) in 2.5 L pots with a mixture of soil and sand (1 : 3, v : v). Nitrogen was supplied weekly through a solution of NH4 NO3. Light and CO2 response curves were measured with a Licor LI-6400 portable photosynthesis system on the youngest fully expanded leaf. Cuvette conditions were: Tleaf 25.5 °C, VPDL 1.6 kPa, CO2 concentration of 360 mol mol-1 (light curves), and light level 1500 mol m-2 s-1 (CO2 curves). AQY varied with species (0.04 ± 0.003 PM, 0.05 ± 0.003 AL, 0.05 ± 0.002 PU, and 0.07 ± 0.01 BS) but not with N level. Nitrogen treatment increased CO2 assimilation under either high or ambient Ci, in all but one species (AL), suggesting that N investment is directed to carboxylation instead of light capture. AL seems to direct N investment towards production of new leaves rather than an increase in leaf photosynthesis.
KEY WORDS: Apparent quantum yeild, Nitrogen, Photosynthesis