Long-term and Continuous Measurement of Canopy Photosynthesis and Growth of Spinach

Photosynthesis is one of the most important determinants in crop growth and yield because photosynthesis is virtually the only means for crops to obtain carbon substrates required for the growth and maintenance of their bodies (Amthor, 2000). A number of models for crop growth and yield, therefore, have been constructed based on carbon balance (i.e., net photosynthesis: gross photosynthesis minus respiration) (e.g., Spitters et al., 1989; Jones et al., 1991; Bouman et al., 1996; Marcelis et al., 2006; Thornley, 2011). To construct models of crop growth and yield, photosynthesis at the canopy scale, rather than at the singleleaf scale, needs to be evaluated, because crops usually constitute a canopy in an agricultural field. Canopy photosynthesis differs from single-leaf photosynthesis: canopy photosynthesis depends not only on environmental elements (e.g., photosynthetically active radiation, CO2 concentration, temperature, humidity, wind, etc.) but also on the structure of the canopy (e.g., leaf area index (LAI), which is the total area of one side of the leaf per unit ground area; Chen and Black, 1992) (Medlyn et al., 2003; Monsi and Saeki, 2005). Variation in LAI in a crop canopy is indeed a much more important determinant of variation in the growth rate of the canopy than is variation in the photosynthetic rate per unit leaf area (Gifford and Evans, 1981; Lawlor, 1995). Canopy photosynthesis can be assessed by micrometeorological methods (e.g., eddy covariance and aerodynamic methods) or chamber methods. Micro-meteorological methods have the advantage of not disturbing the microclimate around a crop canopy (Müller et al., 2009). However, these methods are not applicable to greenhouse studies, because prerequisites of the methods, such as enough fetch length and homogeneous vegetation, are not met in a typical greenhouse environment (Baldocchi, 2003; Jones, 2014). In contrast, the use of chamber methods, where crops are enclosed by a small transparent chamber to measure a change in CO2 concentration in the chamber, is the only way to estimate canopy photosynthesis in a greenhouse environment. Chamber methods are classified into two types: open and closed chamber methods. In the closed chamber method, complete closure of a chamber is temporarily needed to estimate the photosynthetic rate of enclosed crops; photosynthetic rate is estimated by multiplying the rate of change in CO2 concentration by the chamber volume. However, this temporary closure of the chamber disturbs the microclimate around the enclosed crops (e.g., increase in temperature by solar radiation, increase in humidity by transpiration, decrease in CO2 con-