Estimation of the optimal leaf area index (LAI) of an eggplant canopy based on the relationship between the nighttime respiration and daytime photosynthesis of the lowermost leaves

Abstract

• Optimal leaf area index (LAI opt ) of eggplant was estimated based on carbon balance. • Night respiration (Σ R n ) and day photosynthesis (Σ A ) of lower leaves were correlated. • LAI opt can change considerably depending on solar radiation. • LAI should be maintained lower in winter and higher in summer. In year-round horticultural fruit production, proper management of the leaf area index (LAI) is important to improve fruit yields. This study investigated the optimal LAI (LAI opt ) of an eggplant canopy based on the daily incoming solar radiation (Σ S out ) and the carbon balance of the lowermost leaves. A simple analytical LAI opt model was developed by combining the Beer–Lambert law and a linear relationship between nighttime respiration (Σ R n ) and daytime net photosynthesis (Σ A ). Model parameters were derived from measurements (i.e., measurements of Σ R n and Σ A , the light-photosynthesis curve, and light extinction within the canopy). Σ R n and Σ A were linearly correlated ( R 2 = 0.67) for the lowermost leaves but not for the uppermost leaves ( R 2 = 0.13); the light-photosynthesis curves of the uppermost and lowermost leaves did not differ much under low-light conditions but differed considerably under high-light conditions; and the extinction coefficient of light was estimated to be 0.93. Using the obtained model parameters and publicly available Σ S out data, LAI opt was simulated. The simulation revealed that the LAI opt of an eggplant canopy can vary considerably depending on Σ S out ; a tenfold increase in Σ S out (3.0 to 30 MJ m −2 ) corresponded to a threefold increase in LAI opt (1.2 to 3.7 m 2 m −2 ). The developed model will be useful for obtaining estimates of LAI opt in year-round horticultural fruit production.