Estimating Canopy Dark Respiration for Crop Models.

Crop production is obtained from accurate estimates of daily carbon gain. Canopy gross photosynthesis (Pgross) can be estimated from biochemical models of photosynthesis using sun and shaded leaf portions and the amount of intercepted photosynthetically active radiation (PAR). In turn, canopy daily net carbon gain can be estimated from canopy daily gross photosynthesis when canopy dark respiration (Rd) is known. Respiration is difficult to estimate and several methods have been developed; some models determine it from growth and maintenance, others estimate respiration from nitrogen content. Measurements of respiration using ¹³CO₂ indicate that dark respiration is proportional to gross photosynthesis. The ratio of Rd/Pgross was studied using canopy gas exchange data of pepper canopies. The Rd/Pgross ratio was found to remain constant when PAR was increased. Similar constant Rd/Pgross ratios were calculated from wheat and potato data measured in controlled environments. Using a constant Rd/Pgross ratio can simplify crop models estimating canopy-scale daily carbon gain.