Carbon Assimilation Estimate in Canopies of Palisadegrass and Bermudagrass Genotypes.

Leaf photosynthesis rate is highly variable and often not well correlated with canopy assimilation and biomass accumulation in crops. Mathematical models have been developed that take into account characteristics such as leaf area index (LAI), light extinction coefficient (k), radiation flux density and maximum with leaf photosynthesis rate to estimate canopy photosynthesis. The objective of this experiment was to quantify leaf photosynthesis and estimate canopy photosynthesis of three forage grass genotypes (Mulato II brachiariagrass, Marandu palisadegrass, and Tifton 85 bermudagrass) based on sward architecture and light environment, in spring and summer. A randomized complete block design was employed with four replications. The experimental period was from 22 Sept. 2011 to 23 Feb. 2012. Plots were irrigated and fertilized with 185 kg N ha^-1 in split applications after each harvest. Carbon assimilation in leaves and canopy, LAI, light interception (LI) and k were evaluated. Canopy photosynthesis was estimated using the Boote and Jones model that uses LAI,  LI and k. From spring to summer there was an increase in leaf photosynthesis for Mulato II (29.5 to 36.1 mmol CO₂ m^-2 s^-1) and Marandu (29.1 to 39.4 mmol CO₂ m^-2 s^-1) but Tifton 85 did not change (30 to 30.3 mmol CO₂ m^-2 s^-1). The same pattern was recorded for sward photosynthesis, but  Mulato II had higher increase (33%) than Marandu (16%) in the summer. Genotypes had similar LAI, LI and k, which ranged  from 3.48 to 3.68, from 91 to 93% and from 0.70 to 0.74 respectively, resulting in small influence on sward photosynthesis.