Remote-monitoring of the physiological-ecological status of crops. IV. Quantitative relationship between photosynthetic rate and transpiration rate per vapor pressure deficit for corn and soybean under field conditions.

Abstract

The objective of the present study was to obtain fundamental knowledge to monitor remotely the physiological and ecological status of crops in fields. In this paper the relationships between photosynthetic rate (Pn), transpiration rate (Tr), vapor pressure deficit (VPD), and other meteorological data such as photosynthetically active radiation (PAR) were investigated under field conditions. A model relating Pn and Tr was presented. The influence of chlorophyll concentration (Chl) of leaves on Pn was also examined. 1. A theoretical model to interrelate Pn and Tr was presented, which was based on the biophysical processes for gas and vapor transfer via stomata and boundary layer (Eq.1-6). Pn = a[Ca-Ci][Tr/VPD1] (6) where, a is a physical constant, Ca, Ci are ambient and substomatal CO2 concentration, respectively. The Pn is a product of the difference of those two CO2 concentrations and Tr/VPD as indicated in the eq. (6). 2. Close positive correlations between Pn and Tr/VPD were obtained for both corn and soybean as the results of experiments under field conditions. The correlation coefficients were around 0.9 and consistently higher than those coefficients between Pn and Tr in all cases. This relationship held under a wide range of environmental and crop conditions such as air temperature, PAR, VPD, soil water content, crop variety, chloropyll concentration of leaves, developmental stage, leaf position, rolling or wilting of leaves and time of measurements (Tables 1, 2). 3. Linear regression equations were obtained on the above relationships for both corn and soybean. According to those equations the difference of ambient and substomatal CO2 concentrations remains constant, because Pn is proportional to Tr/VPD. The difference of external and internal CO2 concentration was estimated as 159ppm for corn and 51ppm for soybean from the regression coefficients of those equations (Figs. 1∼2). Since the possibility of remote estimation of transpiration rate Tr had been already shown by INOUE10), the photosynthetic activity could also be estimated remotely by means of combination of remotely sensed data and meteorological data connecting above relationships. 4. As a result of the regression analysis the chlorophyll concentration (Chl) of a leaf had as large positive influence as PAR on Pn under water stress free conditions. The partial regression coefficients for Chl were around 0.7. On the other hand the effect of VPD on Pn increased negatively under water stress conditions. The partial regression coefficients were around -0.5 (Table 3).