Studies of Energy and Gas Exchange within Crop Canopies (9)

Abstract

The canopy photosynthesis and the CO2 environment within a crop canopy are studied numerically by employing the model based on transfer equation of CO2 within the canopy. The transfer of CO2 can be expressed by-d/dz(KdC/dz)=-fL(z)p(z)+fL(z)rp, where p(z) is CO2 uptake intensity of leaves, rp respiration intensity of leaves, fL(z) leaf area density function, K transfer coefficient and C concentration of CO2 in the air. Numerical computations were made on an electric computer (Tosbac 3400) to give profiles of CO2 concentration and cumulative CO2 flux profiles. Iterative formulae (5) and (6) were used for numerical integration of the transfer equation of CO2. The following relations are assumedp=Dc, max⋅C(Q/a+Q), rp=const=αp∞, K=KH-δ(H-z), Q=QHk/1-τexp{-k∫HzfL(z′)dz′}, where Dc, max is the effective exchange velocity for CO2 exchange between ambient air and photosynthetic action center in leaves under sufficiently high light intensity, Q intensity of short-wave radiation on leaves, KH transfer coefficient at the canopy top (z=H), QH intensity of short-wave radiation at the canopy top, k extinction coefficient, τ, transmissibility of leaves for short-wave radiation and a, α and δ proportionality constants. The boundary conditions used in the computations are expressed by Eq. (4).1. CO2 profiles within a model canopy are presented in Fig. 1. The top half of Fig. 1 shows CO2 profiles within the canopy (LAI=Ft=4.0) as a function of radiation intensity and transfer coefficient. The CO2 profiles as influenced by the soil CO2 flux are shown in the bottom half of Fig. 1. In the case of low transfer coefficient and high intensity of short-wave radiation, CO2 concentration profiles are characterized by a minimum in the middle layer of the canopy. The minimum value droppes to 41⋅10-8g CO2/cm3 (i.e. down to 74 per cent of the normal concentration). A similar drop of CO2 concentration was also observed in a corn canopy on calm and sunny days. The height (zm) of the minimum concentration moves towards the canopy top with decreasing radiation intensity (see Fig. 2). The dependence of the height zm on radiation intensity was well approximated by an exponential formula.2. Fig. 3 shows that the daily range of CO2 concentration decreases rapidly with increasing wind velocity and approaches about 10ppm with wind velocity of about 6m/sec. A similar conclusion was reached in a slightly different approach to the problems (UCHIJIMA et al. 1967). The relation between the daily range of CO2 concentration at the canopy top and the leaf area index was well fitted to a rectangular hyperbolic equation (Eq. 4). Increase of the extinction coefficient and the transfer coefficient leads to decrease in the maximum daily range of CO2 concentration at the canopy top. The daily range of CO2 concentration within the canopy increased gradually with the canopy depth.3. Fig. 8 gives profiles of cumulative flux of CO2 in the canopy in relation to the canopy denseness, radiation intensity and extinction coefficient. The profiles presented hear are of the same general patterns as those determined experimentally in a corn canopy (INOUE et al. 1968).