Relationship between Water Resistance of Crop and Photosynthesis by Scaling up from Leaf to Canopy

Q. Yu * X. M. Sun Y. Luo Z. Ouyang G. L. Zhang J. Li

Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101

Abstract

Crop transpiration is determined by the interactions between environmental and physiological elements. The relationship between canopy resistance, canopy photosynthesis and environmental elements was presented by scaling up the relation between stomatal resistance and photosynthesis from leaf to canopy. Measurements of CO₂ flux over wheat field were conducted by eddy correlation method at Yucheng, 36°57¢N, 116°36¢E, 28 m a.s.l., in the North China Plain in 1997. An experiment was designed to verify this relation, and the model is verified by using precise experimental data. The results demonstrate that there is a better relation between water vapor resistance of crop canopy (r_c) and water vapor pressure deficit from canopy to ambient air than that between r_c and air relative humidity.

Key words: stomatal conductance, canopy resistance, photosynthesis, transpiration, model

Plant transpiration is the physical process in which net radiation is converted into latent under physiological control by changes in stomatal aperture (Jarvis and McNaughton, 1986). In the Penman-Monteith evapotranspiration model based on energy balance, canopy resistance to water vapor diffusion is the sole factor to physiological actions (Thom, 1975). Therefore, the determination of resistance to water diffusion is a key point in the simulation of field evapotranspiration. As stomata are the main channel in CO₂ uptake and water loss through transpiration, there is a close relation between stomatal resistance influencing gas diffusion and photosynthesis and transpiration (Collatz et al, 1991; Leuning, 1995; Yu and Wang, 1998). Wong found a linear relation between stomatal conductance (reciprocal of stomatal resistance) and photosynthetic rate under changes in some environmental factors, such as solar radiation (Wong, 1979). On this basis, Ball et al. (1987) proposed a semi-empirical model after referred to as BWB model to describe the interactions between stomatal resistance, photosynthesis and ambient CO₂ concentration and humidity, which is widely accepted and used to evaluate plant productivity, biogeochemical cycling, and parameterization of land surface processes (Leuning, 1998; McMurtrie et al., 1992; Sellers, 1996; Hatton et al., 1992; Yu et al., 1998), in agriculture, ecology, geography and meteorology. The model is presented as:

(1)

in which, r_s, A_n are leaf stomatal conductance and photosynthetic rate respectively; a₁ and g₀ are parameter, h_s and C_s are relative humidity and CO₂ concentration over leaf surface.

As the driving force of transpiration is the vapor pressure deficit between stomatal pore to ambient air (VPD_s), rather than air relative humidity (h_s), Leuning (1995) applied VPD_s in stead of h_s to revise the BWB mode. Yu et al. (1998a, 1998b) obtained more realistic results by using the model revised by Leuning (1995), which was verified by observed data of wheat leaf photosynthesis under natural conditions (Yu, 2000). This revised form of BWB model is

(2)

in which, Γ is the CO₂ compensation point, and VPD₀ is a referenced VPD value.

Now many functions are being used to describe the relation between canopy resistance and environmental elements, such as the ‘Jarvis-type’ model (Jarvis, 1976). As leaf scale is the basic scale of models in physiological ecology and vegetated land surface processes, models with physiological meaning should be based on concepts and methods at leaf scale and by a scaling up method (Jarvis, McNaughton, 186). The objective of this study is to scale up the relation between stomatal conductance and photosynthesis from leaf to canopy, and to give a parameterization of canopy resistance which has physiological basis.

1. Experiment

Experiments were conducted at Yucheng Comprehensive Experiment Station (36°57¢N, 116°36¢E, 28 m a.s.l.), Chinese Academy of Sciences, from March to May, 1997. The items of the observation are the following: (1) Microclimate in field: solar radiation, air temperature, air humidity and wind speed over canopy at a reference height of about 1 m. Observations were made before and after 5 minutes intervals at recording time, data are collected over each 15 seconds, and then the average values were recorded. (2) Mass fluxes of CO₂ and water vapor over winter wheat field were measured by using eddy correlation method. Recording was made by eddy correlation system at 0.25 Hz, and recordings of each 30 min were averaged and stored in a data logger for later processing. Raw data of eddy correlation system was stored on a PC. As the noise of data was obvious, the value of the flux will be replaced by average of 4 nearby samples when deviation was over 20% of the average values.

For all wind directions the fetch was more than 500 meter. In the North China Plain, the terrain is flat over large areas. During the growing season, the leaf area index (LAI) was measured every 5 days.

2. Result

Based on leaf model, to integrate canopy resistance from leaf to canopy and obtain canopy resistance and photosynthesis. Crop canopy is supposed to be composed of a random array of leaves in the big-leaf model, canopy resistance consists of stomatal resistance of each leaf in the canopy (i=1, L,m)(r_c) (Shuttleworth, 1976):

(3)

canopy photosynthetic rate (P_cn) is the sum of photosynthesis of all leavels (i=1, L,m) (A_ni):

(4)

In the big leaf model, canopy is taken to be at the same temperature. To scale up the leaf model, Eq. 2, to the canopy level, and by comparing Eq. 3 and Eq. 4, we can obtain relationship between canopy resistance, canopy photosynthesis, ambient CO₂ and vapor pressure deficit in the reference height (VPD_c):

(5) in which, VPD₀ is a parameter, C_a the CO₂ concentration at the reference height, VPD_c the vapor pressure deficit at Hr. To define P_cn/[C_a(1+VPD_c/VPD₀)] the canopy resistance index.

To validate the model, observation of all variables, i.e canopy photosynthetic rate, transpiration rate and canopy microclimate are needed. Substituting r_c calculated by Eq. 5 to the Penman-Monteith equation:

(6)

in which s is the slope of saturate water vapor pressure with temperature, r air density, g pychrom constant, D_a vapor pressure deficit at reference height, r_a aerodynamic resistace. Under near neutral conditions, r_a can be calculated by

(7)

in which z_r is the reference height, u the wind speed, d the zero plane displacement, k Karman constant. The big leaf model is applicable for closed canopy, in which soil evaporation is small and can be neglected. In the meantime, CO₂ released from soil is neglected, therefore, CO₂ flux over canopy is assumed to be equal to canopy photosynthetic rate.

Data observed on some typical clear days are chosen in the verification, to fit the relation between canopy resistance and canopy resistance index in Eq. 5. The days are April 26, 27 and May 17, 18, 19, 1997 at each o’clock from 8:00 to 18:00. To fit Eqs 1, 2 and 3 with experimental data, CO₂ compensation point is 50mmol mol^-1，to adjust the parameter VPD₀, so that the relation between stomatal conductance and stomatal conductance index (the algebraic formula on the right of equations including environmental and physiological variables) achieves its best. Similar method was used to fit the relation between canopy resistance and canopy resistance index based on BWB model to judge the performance of the improved model. Figure 1 is a typical example of diurnal changes in CO₂ and water fluxes in relation to the changes in solar radiation. Solar radiation shows regular change, similar to global radiation, with its maximum at noon. Water vapor flux is basically synchronous to solar radiation. Maximum photosynthesis occurs at 10-12 h, due to relative high solar radiation and suitable temperature.

Figure 1. Typical diurnal changes in CO₂ and water fluxes in synchronous with solar radiation (Yucheng, from 5:00-19:00, April 26, 1997)

Figure 2. Relation between canopy resistance and canopy resistance index P_cnh_s/C_a of wheat at Yucheng, 1997.

Figure 3. Relation between canopy resistance of wheat and canopy resistance index P_cn/[C_a(1+VPD_c/VPD₀)] at Yucheng, 1997.

In the verifying canopy resistance index scale up from BWB stomatal model (Eq. 1), with observational data, shown in Fig. 2, parameters of the model set at: a₁=0.369, VPD₀=2.1 (kPa), 1/r_c0=0.0039. That 1/r_c0 is near 0, indicates canopy resistance goes to infinity when net photosynthetic rate is near 0. The correlation coefficient between canopy resistance and canopy resistance index is 0.84 (n=45), with considerable scattering of points. The index coming from Eq 2 gives a better simulation, with a correlation coefficient is 0.91 (n=45), there is an obvious improvement of simulation precision in Fig 3. It is because stomata constraints respond to water loss, the relation between rate of water loss and vapor pressure deficit is closer than that between water loss and leaf surface humidity (Jarvis, 1976; Sheriff, 1984). In gaseous diffusion equation proposed by Aphalo and Jarvis (1993), stomata respond to vapor pressure deficit from stomatal pore to ambient air more than that in air. Under natural conditions, when there is temperature difference between leaf and air, the vapor pressure deficit from stoma to air and that in the air are different, consideration of influence of driving force of VPD_s to the transpiration on stomatal conductance accords with natural conditions. Mott and Parkhurst (1991) found that there is a better relation between stomatal opening (also the stomatal conductance) and rate of water losing, i.e., transpiration rate, than that between stomatal conductance and water vapor pressure in air.

3. Discussion

In the Penman-Monteith model, as the definition of canopy resistance is made from the model itself, which induce criticism for repetition of definition (Lhomme, 1991). Empirical models describe the effect of influencing factors by multiplication. As this relation lacks of physiological regulation and feedback, that means the model has less physiological meaning than some relation based on experimental observation. In this study, we proposed a relation between canopy resistance and some physiological and environmental factors, therefore, it is a semi-empirical expression of the resistance, which combines canopy resistance for water vapor with photosynthesis. The convenience of using Penman-Monteith equation lies in that use of leaf temperature need not be known. After the development of remote sensing, however, canopy temperature can be measured over large areas, and leaf temperature data can be used to estimate the role of evapotranspiration, to monitor crop water consuming, photosynthesis and water use efficiency. The result of this study may induce application of remote sensing to evaluate regional evapotranspiration and photosynthesis of vegetation. As the model is limited in suitable water supply, it is necessary to include influence of water stress on stomatal resistance.

Equation 5 is a canopy resistance model scaling up from leaf to canopy, which contains double relation with photosynthesis and transpiration. This relation is based on a great volume of experimental result. Wong (1979) found a linear relation between photosynthesis and stomatal conductance. Monteith (1995) concluded 271 published experimental results, 234 of which demonstrated negative linear relation between transpiration and stomatal conductance. At leaf scale, in a leaf chamber of photosynthesis system, g_s comes from E, both are not independent, therefore the application of Eq. 5 is more realistic and reasonable (Yu et al, 2000). In this study, the suitability of this relation at canopy scale is tested. The validation of models differs in many ways on leaf scale with a leaf chamber and on canopy scale, where CO₂ flux is measured in open field. In the relation between stomatal conductance, photosynthetic rate and transpiration, Eq. 5 is comprehensively considered positive relation of photosynthesis and negative relation of transpiration with stomatal conductance.

To construct the relationship between canopy resistance and canopy photosynthesis and water vapor deficit from canopy to ambient air coupled canopy photosynthesis and transpiration. To combine canopy photosynthesis model made, to construction combined photosynthesis and transpiration model to serve analysis for water use efficiency.

Acknowledgement

This work is supported by Natural Science Foundation of China with project number 49890330 and the Chinese Academy of Sciences (KZ95T0401 and CXIOG-C00-03).

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