CARBON-DIOXIDE EXCHANGE-RATES IN WHEAT CANOPIES .2. PHOTOSYNTHETIC AND PHYTOMASS PRODUCTION EFFICIENCIES

This field study explored the influence of canopy geometry on the photosynthetic and phytomass production efficiencies of wheat. Winter wheat (Triticum aestivum L. cv. Newton) was planted at the Evapotranspiration Research Site (Unit 2) located 14 km south of Manhattan, KS (39°09′N, 96°37′W) in north-south and east-west row orientations with 0.18-, 0.35-, 0.53-, and 0.71-m row spacings during the 1982-1983 and 1983-1984 growing seasons. Seeding rates were 67, 34, 22, and 17 kg ha-1 for the 0.18-, 0.35-, 0.53-, and 0.71-m row spacings, respectively. The soil type was a Muir silt loam (fine-silty, Mesic, Pachic Haplustoll). All plots were fertilized with 109 kg N ha-1 incorporated prior to planting. Row orientation and spacing treatments modified the wheat canopy structure and significantly influenced leaf area index, absorption of photosynthetically active radiation (PAR), and above-ground phytomass production values throughout the growing season. Relationships between the daily net and gross canopy CO2 exchange rates and daily absorption of PAR were linear. A common slope model described the net (0.54 g CO2 E-1) and gross (0.79 g CO2 E-1) photosynthetic efficiency for all canopies studied. Lineality was also observed between seasonal accumulation of daily absorbed PAR and phytomass production for most row orientation and spacing treatments (0.40-0.58 g E-1). Higher daily net and gross CO2 exchange rates and phytomass values in the narrow-spaced canopies were attributed to a greater quantity of daily absorbed PAR throughout the season, rather than differences in the efficiency of the various canopy structures. Good agreement existed between the photosynthetic and phytochemical efficiency derived over the 2-year study. Grain yields were highest in the narrow-spaced canopies and decreased with increasing row width. Significant linear relationships were observed between grain yield and accumulation of daily absorbed PAR from the initiation of the spring regrowth period (0.16 and 0.23 g E-1) and from anthesis (0.80 and 0.82 g E-1), until physiological maturity for the 1982-1983 and 1983-1984 seasons, respectively. © 1990.