Discussion on appropriate positions for CO₂ enrichment within plant canopy by measurements of light intensity, CO₂ concentration, leaf area and photosynthetic rate in tomato grown by high-wire system

Abstract

　This study aimed to discuss an effective technique of CO₂ enrichment within a canopy of tomato grown by high-wire system in green house. Leaf area, photosynthetic photon flux density (PPFD), CO₂ concentration and photosynthetic rate were measured at different height above ground level in the tomato canopy. The leaf area of the middle (1.2 to 1.8 m) and lower layer (0.6 to 1.2 m) was larger than that of the upper layer (1.8 to 2.2 m). The PPFD from the upper to lower layer was decreased from 370 to 45 μmol m^-2 s^-1 in winter. However, CO₂ concentration varied little among the layers, and was below 400 μmol mol^-1 during the day. The maximum photosynthetic rate (P_max) of middle leaves was similar to that of upper leaves. The P_max of lower leaves were less than a half of the other leaves. The photosynthetic rates of middle and lower leaves were increased by 1.2 to 1.3 times with elevated CO₂ concentration from 400 to 800 μmol mol^-1 under low PPFD (e.g., 125 μmol m^-2 s^-1). Thus, it was considered that elevating CO₂ concentration at middle layer was appropriate for CO₂ enrichment within a canopy of tomato grown by high-wire system.