Effects of CO2 concentration on photosynthesis, transpiration and production of greenhouse fruit vegetable crops

The effect of the C0 ₂ concentration of the greenhouse air (C) in the range 200 to 1100 μmol mol ^-1was investigated in tomato ( Lycopersicon esculentum Mill.), cucumber ( Cucumis sativus L.), sweet pepper ( Capsicum annuum L.) and eggplant ( Solanum melongena L.), grown in greenhouses.

The effect of C on canopy net photosynthetic C0 ₂ assimilation rate (or photosynthesis, P) was expressed by a set of regression equations, relating P to PAR, C and LAI. A rule of thumb ('CO ₂ -rule') was derived, approximating the relative increase of P caused by additional C0 ₂ at a certain C. This C0 ₂ -rule is: X = (1000/C) ²* 1.5 (X in % per 100 μmol mol ^-1, and C in μmol mol ^-1). Two models for canopy photosynthesis were examined by comparing them with the experimental photosynthesis data. No 'midday depression' in P was observed.

The effects of C on leaf conductance ( g ) and on rate of crop transpiration ( E ) were investigated. An increase of 100 μmol mol ^-1in C reduced g by about 3-4% in sweet pepper, tomato and cucumber and by about 11 % in eggplant. The effect of C on E was analyzed by combining the regression equation for g with the Penman-Monteith equation for E . C had only a relatively small effect on E , owing to thermal and hydrological feedback effects. The decoupling of g and E was quantified. No timedependent variation or 'midday depression' in E was observed, and no significant effect of C on average leaf temperature was established.

In five experiments, the effect of C on growth and production and on specific features were analyzed: light use efficiency was increased by about 10 to 15% per 100 μmol mol ^-1increase in C; fruit set of sweet pepper was greatly increased by high C; allocation of biomass to fruits was increased by high C in sweet pepper and cucumber; specific leaf area (SLA) was reduced by 15 to 20% at 150 to 250 μmol mol ^-1increase in C (except in cucumber); dry matter content (DMC) of vegetative organs slightly increased at high C (also not in cucumber); fruit production (dry weight) was most affected by C in sweet pepper; fresh weight fruit production per unit CO ₂ was highest in cucumber; fruit quality was not influenced by C. High C promoted the 'short leaves syndrome' in tomato and 'leaf tip chlorosis' in eggplant, probably related to calcium and boron translocation, respectively. The observed effect of C on production was larger than expected on the basis of the CO ₂ -rule. Intermittent CO ₂ supply (ICS) could under normal ventilation accomplish only a limited increase in average C, and hence a limited increase in production. No physiological advantages of ICS were revealed.