Vol. 84 No: 2

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

Authors:
O. KÖRNER, E. HEUVELINK and Q. NIU

pp: 233-239

Abstract:
Detailed 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 (P_gc) 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, P_gc was predicted at photosynthetic photon flux densities (I_PPFD) 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 P_gc was determined. CO₂ concentration had a strong and similar effect on P_gc in both crops, and this effect decreased with increasing CO₂ level. For example, at 32°C, there was a 55% or 49% increase in P_gc 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 P_gc (e.g., at 1,000 μmol mol^-1 CO₂ and 600 μmol m^-2 s^-1 I_PPFD, 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.

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