Quantification of temperature, CO2, and light effects on crop photosynthesis as a basis for model-based greenhouse climate control

SummaryDetailed measurements of crop photosynthesis at supra-optimal temperatures and high CO2 levels, to validate models for use in model-based greenhouse climate control, are still lacking. We performed CO2 gas exchange measurements to estimate gross crop photosynthesis (Pgc) from measured net crop gas exchange in the daytime, and at night, with temperature and CO2 conditions higher than the normal temperature range, and with two crops of different architecture: tomato, and cut chrysanthemum. From these measurements, Pgc was predicted at photosynthetic photon flux densities (IPPFD) of 300, 600, 900 and 1,200 µmol m–2 s–1 for different temperatures (from 20°C to 33°C) and CO2 concentrations (400, 700, and 1,000 µmol mol–1). From these predictions, the optimum temperature that maximised Pgc was determined. CO2 concentration had a strong and similar effect on Pgc in both crops, and this effect decreased with increasing CO2 level. For example, at 32°C, there was a 55% or 49% increase in Pgc between 400 and 1,000 µmol mol–1 CO2 in chrysanthemum and tomato, respectively. A clear shift to higher optimum temperatures at elevated CO2 levels was observed, and was different for the two crops. Chrysanthemum had a lower temperature optimum than tomato for a maximum Pgc (e.g., at 1,000 µmol mol–1 CO2 and 600 µmol m–2 s–1 IPPFD, the difference was 3.1°C). Compared to leaf photosynthesis, crop photosynthesis had a lower temperature optimum (the difference could be several °C), and the shift in optimum temperature from a low to a high CO2 level was lower for a canopy compared to a leaf. Therefore, optimising the leaf photosynthetic rate in model-based greenhouse climate control would not result in optimum crop photosynthesis.