Leaf Pubescence Effects on the Mass and Energy Exchange Between Soybean Canopies and the Atmosphere

Mass and energy exchanges with the atmosphere were compared in two soybean (Glycine max L. Merr. cv. Harosoy) isolines differing in pubescence density. The study was conducted in a field with a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll) during the summer of 1980 at Mead, Nebr. Mass and energy exchanges were determined by means of micrometeorological techniques. Evapotranspiration (reported in terms of latent heat flux) was reduced in the densely pubescent isoline. Canopy CO2 exchange was unchanged on a per unit land area basis. Water use efficiency (reported in terms of the CO2-water flux ratio) was, accordingly, greater in the densely pubescent isoline. The increase in pubescence did not significantly alter the net radiation balance, turbulent mixing, canopy CO2 exchange, or plant water status. Observed differences in the partitioning of net radiation into latent and sensible heat can be explained by greater penetration of solar radiation into the densely pubescent canopy. Leaf pubescence appears to alter the spectral characteristics of the leaf and, thus, to facilitate the penetration of solar radiation into the canopy.________ Additional index words: Photosynthesis, Evapotranspiration, Water use efficiency, Canopy aerodynamics, Glycine max L. C productivity is a function of genetic potential interacting with environmental conditions. In recent years, plant breeders have developed near-isogenic lines of cultivars that differ in morphological characteristics (Bernard and Weiss, 1973). One intent has been to alter the leaf characteristics and, as a consequence, the microenvironment of the leaf. The micro-environment of a leaf can be altered by changing characteristics that affect the radiation balance or the boundary layer resistance to energy and mass exchange. A morphological change which affects the environment of the leaf and may benefit crop productivity is increased leaf pubescence. Gausman and Cardenas (1973) found that leaf pubescence on detached soybean leaves decreased the reflectance of near-infrared radiation but had no effect on the reflectance of photosynthetically active 1 Published as Paper No. 6836, Journal Series, Nebraska Agric. Exp. Stn. The work reported here was conducted under Regional Research Project 11-33 and Nebraska Agric. Exp. Stn. Project 11-49. Received 31 Mar. 1982. 2 Formerly research associate (now Postdoctoral Fellow at the Atmospheric Turbulence and Diffusion Lab., P.O. Box E, Oak Ridge, TN 37830), associate professor, professor, professor and visiting scientist (Conicet, Argentina), Center for Agricultural Meteorology and Climatology and associate professor, Dep. of Agronomy, Institute of Agriculture and Natural Resources, Univ. of Nebraska, Lincoln, NE 685830728. 538 AGRONOMY JOURNAL, VOL. 75, MAY-JUNE 1983 radiation. Ehleringer e t al. (1976), Ehleringer and Bjorkman (197:3a, 1978b) and Ehleringer and Mooney (1978), on t h e o ther hand, reported that leaf pubescence on Enceliu farirzosa caused an increase in t h e leaf reflectance of solar radiation. Furthermore, Ehleringer a n d Bjorkman (1 978a) :showed t h a t leaf pubescence preferentially reflected near infrared radiation more t h a n photosynthetically active radiation. T h e r e is also evidence in the l i terature suggesting t h a t leaf pubeijcence may influence t h e leaf boundary layer. Woolley ( 1964) reported t h a t leaf pubescence increased t h e thickness of the boundary layer over soybean leaves. Ehleringer and Mooney (1978), on t h e other hand, present evidence suggesting that leaf hairs have only a small impact on the leaf boundary layer thickness of E. furinosa. The resultant effect of additional leaf pubescence has been reported t o be a reduction in transpiration (Woolley, 1964; Ghorashy e t al., 1971; Ehleringer and Mooney, 1978). This effect has been attributed to leaf hairs reducing t h e radiation load on the leaf. It is not yet clear as t o whether photosynthesis is affected by additional leaf pubescence. Ehleringer and Mooney (1978) found tha t an increase in pubescence reduced photosynthesis of E. furinosa while Ghorashy e t al. (1971) found no effect on soybean photosynthesis. Soybean yield, however, has been shown to be greater in a pubescent isoline of t h e Harosoy cultivar ( H a r t u n g et al., 1980). The effects of pubescence on mass and energy exchange are understood on the scale of t h e leaf. It is uncertain, however, whether these effects are extendable to a full plant because of the complex geometry of the canopy. Here we report measurements of mass and energy exchange between the atmosphere and t h e canopies of two near-isogenic lines of a soybean cultivar differing only in pubescence density. Mechanis t ic explanations for t h e differences found in mass a n d energy exchange are proposed. MATERIALS AND METHODS Experimental Details This study was conducted between 18 July and 7 Aug. 1980 at the Univ. of Nebraska Agricultural Meteorology Laboratory at Mead, Nebr. (41 e 09’ N ; 96’ 30’ W; alt. 354 m above msl). Soybeans (Glycine max L. Merr. cv. Harosoy) were planted in adjoining portions of an experimental field of Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll). The east side of the field (65 m E-W by 210 m N-S) was planted with an isoline of Harosoy cv. with normal pubescence (HN). The west lield (85 m E-W by 210 m N-S) was planted with an isoline possessing as single dominant gene which increases the density of the pubescence on the leaves, stems and pods (HPD). Border fields to the east, south, and west of the main experimental fields were planted with the H N isoline. The soybeans were planted in 0.75 m wide rows oriented north-south. The soybean isolines were developed by R. L. Bernard (USDA and University of Illinois) and agronomic differences are discussed by Hartung et al. (1980). Bernard and Weiss (1973) state that leaf hairs on the HPD isoline are about four times more numerous per unit leaf area than on the H N isoline. Gausman and Cardenas (1973) reported that pubescent density on soybean leaves is greater on the abaxial side of the leaf. Air temperature and vapor pressure were measured over both plotsat 1.25, 1.50, 1.75,2.25,2.75,and 3.25 m above theground with an automatic, self-checking, multilevel psychrometer (Rosenberg and Brown, 1974). Once each hour the psychronieter assembly rotated automatically into a horizontal position for calibration. Air was sampled to determine C02 concentration in both fields with multilevel manifolds a t 0.30, 0.50, 0.70, 0.90, 1.25, 1.50, and 1.75 m above the surface. The manifolds had six air intakes a t each level. These intakes were spaced equi-distantly over a horizontal distance of 2.5 m. The C02 concentrations were measured with a system which employed an absolute and a differential infrared gas analyzer (see Rosenberg and Verma, 1976, for details). Once each hour both analyzers were Calibrated automatically with standard gases of known concentration. Wind speed was measured over each canopy a t 0.25 In intervals between 1.25 and 2.50 m with Cayuga three-cup anem o m e t e r ~ . ~ The anemometers were calibrated in a wind tunnel before and after the growing season. Net radiation (Rn) was measured a t 1.85 m above the ground with a Swissteco net r a d i ~ m e t e r . ~ Net radiation was measured within the canopy at 0.20, 0.40, 0.60, and 0.80 m using strip net radiometers (SNR). Each SNR was 0.35 m long and 45 mm wide. To account for spatial variability within the canopy, Rn was measured at six locations a t elevations of 0.60 and 0.80 m, a t four locations a t 0.40 m and a t two locations a t O.;!O m. Soil heat flux was measured with three soil heat flux plates5 placed at a depth of 10 mm in the soil. The output of all micrometeorological sensors was measured with a computer-controlled data acquisition system and data were recorded on magnetic tape. Counts from cup anemometers were integrated over 5 min periods. The C02 concentrations were measured once every 7% min. All other sensor-produced voltages were measured a t the rate of about two times per minute. All data were later averaged over the first 45 min of each hour. The remaining 15 min of each hour were reserved for automatic calibrations of the psychrometers and infrared gas analyzers. Plant water potential was measured with a pressure chamber6 on an hourly basis. Four to six sunlit leaves from the upper canopy were selected for this purpose. Each leaf, after excision, was placed in a plastic bag full of moist air. The bag was immediately inserted into the pressure chamber. Stomatal resistance was measured on the top and bottom sides of six sunlit leaves with a steady state porometer.’ The leaves were randomly selected from the upper portion of the canopy. The mean stomatal resistance (r,) was computed on the assumption that resistances of the tops and bottoms of the leaves act in parallel. Both soybean isolines had reached full development by the time measurements reported here were started. The H N and HPD isolines were, respectively, about 1 .OO and 1.07 m tall and had leaf area indices of about 3.8 and 4.5. Analytical Considerations Fluxes of COz, latent heat, and sensible heat were computed using flux-gradient theory: fluxes were computed as the products of the appropriate vertical gradients and exchange coefficients. Calculations of both C02 flux (F,) and latent heat flux (LE) 3 Cayuga Development, Ithaca, New York, Model WP-I. Swissteco Pty. Ltd., Melbourne, Australia, Type S-I. Concept Engineering, Old Saybrook, CN, Model F-080-4. Soil Moisture Equipment Corp., Santa Barbara, CA, Model 3005. ’ Lambda Instrument Co., Lincoln, NE, Model LI-1600. * Baldocchi, D.D. 1982. Mass and energy exchanges of soybeans: Microclimate-canopy architecture interactions. Ph.D. Diss. Available from: Ann Arbor, Mich. Univ. of Nebraska-Lincoln. (Diss. Abstr. DA8228144) 206 p. Nielsen, D. C., 9. L. Blad, and S. B. Verma. 1981. Influence of soybean pubescence on radiation balance. 15th Conf. on Agri,culture and Forest Meteorology, Anaheim, Calif. 46 p. Negative values indicate that the flux is directed away from the surface. All fluxes were computed on a ground area basis. BALDWCHI ET AL.: LEAF PUBESCENCE EFFECTS ON SOYBEAN 539 Table 1. Statistics from the paired t-tests comparing the HN and HPD soybean isolines. t is the computed sample t-statistic, is the value for one-tailed test at 5% level of significance and n is the sample size. Sample means and standard deviations (SD) for the r e spective isolines are given. Means of the paired differences and their respective 90% confidence intervals (C.1.) are also included. HN HPD units of Mean of the Variable variable t t0.06 n Mean SD Mean SD differences C.I. m s-’ 0.68 1.70 28 0.51 0.13 0.51 0.14 W 5.73 1.70 28 106 62 132 65 26 1.7 W rn-l -4.98 -1.10 28 328 103 306 98 22 7.4 -1.23 -1.72 21 0.39 0.35 0.44 0.38 P MPa -0.99 -1.73 19 1.50 0.31 1.48 0.33 r.9 8 m-’ 0.22 1.66 85 1.89 142 193 151 “I mgrn-’s-l Fc CWFR mg g-’ 2.40 1.72 21 2.93 2.58 3.82 2.94 -0.89 0.64 were corrected for the effect of water vapor exchange on the density of dry air (Webb et al., 1980). The exchange coefficients for COz (KJ, water vapor (K,), and sensible heat (KH) were assumed identical (K, = K, = KH) and were computed by means of the Bowen-ratio energy balance (BREB) method. Water use efficiency was expressed in terms of an index-the CO2-water flux ratio (CWFR)-which is the ratio of the mass flux of COz to that of water vapor. Details of the computational procedures are given in Baldocchi (1982).8 A discussion of the errors associated with the BREB method for computing the fluxes of water vapor and C 0 2 are given in Blad and Rosenberg (1974) and Verma and Rosenberg (1975). Under non-advective conditions BREB estimates of COz and latent heat flux have accuracies on the order of 10 to 20%. The manner in which a crop extracts momentum from the air flowing over it can be characterized by means of the friction velocity (u.). The magnitude of U. is indicative of the effectiveness of turbulent exchange between the crop and the atmosphere. Friction velocities were computed for both canopies using drag coefficients measured close to the canopy top (see Deacon and Swinbank, 1958; Bradley, 1972; Verma et al., 1976, for further details). The drag coefficients were computed from wind profile measurements. A common data set was constructed to determine whether leaf pubescence influenced mass and energy exchange. Paired observations (each representing 45 min averages) were selected from periods during which the fetch to height ratio exceeded 70 to 1 and no instrument malfunctions occurred. This data set contains observations made between 18 July and 7 Aug. 1980. The analysis did not include any data collected after 7 August since a storm lodged the crop on 10 August. Variables were compared by means of a paired t-test with significance determined at the 5% level of probability. The comparisons are shown in Table 1.