Plant and Soil 249: 401–416, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 401 A coupled model of photosynthesis-transpiration based on the stomatal behavior for maize (Zea mays L.) grown in the field Gui-Rui Yu 1,4 , Tatsuaki Kobayashi 2 , Jie Zhuang 1 , Qiu-Feng Wang 1 & Le-Qing Qu 3 1 Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 3 Datun Road, ChaoyangDistrict, P.O. Box 9717, Beijing 100101, P. R. China 2 Faculty of Horticulture, Chiba University, Mat- sudo 648, Matsudo-city, Chiba, 271-8510, Japan 3 Institute of Genetics, Chinese Academy of Sciences, 3 Datun Road, Chaoyang District, Beijing 100101, P. R. China. 4 Corresponding author ∗ Received 6 March 2002. Accepted in revised form 14 October 2002 Key words: maize (Zea mays L.), photosynthesis, stomatal conductance, stomatal behavior-based coupled model, transpiration Abstract The study presents a theoretical basis of a stomatal behavior-based coupled model for estimating photosynthesis, A, and transpiration, E. Outputs of the model were tested against data observed in a maize (Zea mays L.) field. The model was developed by introducing the internal conductance, g ic , to CO 2 assimilation, and the general equation of stomatal conductance, g sw , to H 2 O diffusion, into models of CO 2 and H 2 O diffusion through the stomata of plant leaves. The coupled model is easier for practical use since the model only includes environmental variables, such as ambient CO 2 concentration, leaf temperature, humidity and photosynthetic photon flux received at the leaves within the canopy. Moreover, concept of g ic , and factors controlling A and E were discussed, and applicability of the model was examined with the data collected in the maize field. Introduction In order to utilize water resources effectively, and to evaluate plant production and interaction between vegetation and environment quantitatively, material- and energy-exchange processes within soil–plant– atmosphere system through photosynthesis and tran- spiration have to be modelled. The processes of photo- synthesis and transpiration occur through an identical path in stomata of plants leaves, since the same open- ing and closing movement of the stomata controls both processes. Therefore, it is very important to under- stand responses of stomata to environmental factors and to simulate the stomatal conductance. In these aspects, numerous researchers have contributed to the study (e.g., Jarvis, 1976; Kelliher et al., 1995; Rochette et al., 1991). In a series of our studies, the response characteristics of stomatal conductance ∗ FAX No: +86-10-6488-9399. E-mail: yugr@igsnrr.ac.cn/yuguirui@cern.ac.cn. of maize to particular environmental factors were measured (Yu, 1999; Yu et al., 1996), and a com- bination model for estimating stomatal conductance over the long term was presented (Yu, 1999; Yu and Nakayama, 1997; Yu et al., 1998). However, photosynthesis activity is actually acted on stomatal behavior, simultaneously, the stomatal be- havior is subject to the feedback of photosynthesis activity (Ball et al., 1987; Jones, 1992; Leuning, 1990, 1995; Lloyd, 1991). Although transpiration and pho- tosynthesis both occur through stomata, they show different behaviors (Kosugi, 1997; Yu, 1999). For ex- ample, the leaves may adjust stomatal conductance to maximize carbon assimilation for a given daily loss of water vapor (Cowan and Farquhar, 1977; Leuning, 1990). It is, therefore, important to comprehensively understand both of these processes. Moreover, if tak- ing diffusion of H 2 O and CO 2 as a combined physical and physiological phenomenon through the stomata of plant leaves, then, it may be expected to develop a