Measurement, modeling and seasonal changes of canopy photosynthesis in 'nyoho' strawberry

The photosynthetic rate of a strawberry crop on ground area basis was determined by measuring the changes in CO2 concentration in commercial greenhouses, using the estimated rate of soil respiration and number of exchanges during the dark period. To estimate the photosynthetic curves, four theoretical equations; (1) y=a+bc^x, (2) y=a+b/(c+x), (3) y=a+b\/x and (4) y=a+bx, were fitted to the photosynthetic responses of canopy to light at several ranges of CO₂ concentration by the least square method. When the observed light intensity was higher than 300 Wm² and the CO ₂ concentration was less than 500 mL m³, the asymptotic equation (1) was fitted better than the others and reasonable rates of saturated photosynthesis were obtained with equation (1). At higher CO₂ concentrations (>1100 mL m³), the asymptotic equations (1) and/or (2) could not be fitted, as such high values were observed only in the morning when the light intensity was less than 150 W m², considerably lower than the light saturation point. At various CO₂ concentrations, although the photosynthetic response to light should be estimated with the equation (1), it was possible to estimate with the equation (3), as there were few differences among the curves obtained with the equations (1), (2) or (3), when the light intensity was less than 200 W m². The estimated rate of canopy photosynthesis was higher in December than February or March. This should be due to the difference in the leaf area index (LAI) of the canopy. The estimated photosynthetic rate at 100 W m² was increased with the increase in the CO, concentration up to 1000 mL m³, and the rate was higher at 100 W m² and 600 mL m³ than at 200 W m ² and 350 mL m³. The CO₂ concentration in the strawberry greenhouses should be maintained at ca. 1000 mL m³ with supplemental enrichment, especially under low light conditions.