PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

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: T_leaf 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 C_i, 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