Long-term estimation of the canopy photosynthesis of a leafy vegetable based on greenhouse climate conditions and nadir photographs

In horticultural crop production, accurately estimating the canopy photosynthetic rate (A(c)) based on greenhouse climate conditions is of practical value for predicting and controlling crop growth and yield. We developed a method for continuously estimating Ac in a greenhouse from readily obtainable information (i.e., climatic data and canopy photographs) by combining a canopy photosynthesis model with image analysis. The canopy photosynthesis model was based on a sun/shade representation of the crop canopy combined with models of single-leaf photosynthesis, stomatal conductance, mass transfer and leaf energy balance. This combination allowed 1) the incorporation of all major climatic variables (i.e., radiation, CO2 concentration, air temperature, humidity, and wind velocity) into the Ac estimation and 2) the simultaneous estimation of Ac, canopy transpiration rate (E-c), and leaf temperature (T-L). The leaf area index (L-c), which changes considerably throughout the growth period of a crop canopy, was evaluated nondestructively from the gap fractions of nadir digital photographs (i.e., the fractions of nonleaf area). The canopy photosynthesis model and image analysis results were combined and used to estimate the A(c), E-c, and TL of spinach canopies grown at ambient and elevated CO2 concentrations (c. 400 and 800 mu mol mol(-1), respectively). Regardless of the CO2 concentration, the estimates of A(c), E-c, and T-L were in good agreement with measurements obtained with the open chamber method throughout the growth period. Analyzing the sensitivity of A(c) to input variables revealed that the effects of climatic variables on A(c) can vary considerably depending on the L-c and incoming radiation. The proposed method enables not only the estimation of A(c) from readily obtainable information but also the quantitative evaluation of the effect of climate control on A(c) in horticultural greenhouses.

Automated summary

This study developed a method to continuously estimate the canopy photosynthetic rate (A(c)) in a greenhouse by combining a canopy photosynthesis model with image analysis, allowing for the estimation of A(c), E-c, and T-L. The method enables the estimation of A(c) from readily obtainable information and quantitative evaluation of the effect of climate control on A(c) in horticultural greenhouses.