Publisher Summary The study of leaf anatomy and of the mechanisms of the opening and closing of stomatal guard cells leads one to suppose that the stomata constitute the main or even the sole regulating system in leaf transpiration. Meteorologists have developed a wide variety of formulae for estimating evaporation from vegetation that are based entirely on weather variables and take no account at all of the species composition or stomatal properties of the transpiring vegetation. These “potential evaporation” formulae are widely and, to a large degree, successfully used for estimating evaporation from vegetation that is not water-stressed. Transpiration depends on stomatal conductance, net radiation receipt and upon air saturation deficit, temperature, and wind speed. Saturation deficit and wind speed vary through leaf boundary layers, through canopies, and through the atmosphere above the canopies. The sensitivity of saturation deficit to changes in stomatal conductance depends on where the saturation deficit is measured. If all of the stomata on a single leaf change aperture in unison, there may be a substantial change in saturation deficit measured at the leaf surface but a negligible change in saturation deficit measured a centimetre or two away, outside the leaf boundary layer.

Stomatal Control of Transpiration: Scaling Up from Leaf to Region

Soybean [Glycine max (L.) Merr.] yield, when regressed on water needed to replenish 0 to 100% seasonal evapotranspiration (ET), generates an estimate of season-specific water-use efficiency (WUE). The impact of unpredictable water deficits might be lessened if high-yielding genotypes had a smaller beta. Our objective was to determine the genetic basis of beta and carbon isotope discrimination (CID), a theorized indicator of transpiration efficiency (TE). A Minsoy' Noir I' population of 236 recombinant inbred lines (RILs), genotyped at 665 loci, was evaluated in six water treatements (100, 80, 60, 40, 20, and 0% ET) for 2 yr. Water stress was mild in 1994, but high temperatures and no rainfall in 1995 led to a drought so severe that the 100% ET treatment required 41 cm of irrigation. The 1995 yield-to-water regression was highly linear (28 kg ha -1 cm -1 ), Genotype × water (G × W) interaction was due to genotypic heterogeneity in beta. The CH) vs. beta correlation was low (r = 0.26), so selection for better leaf TE may not improve crop WUE, Selection of low beta (less sensitivity to drought) will be difficult, given the yield beta vs. yield correlation (r = 0.71). The major quantitative trait loci (QTL) for yield beta, yield, and CID were coincident with maturity and/or determinancy QTLs. except for a CID QTL in linkage group U09-C2. but it had no effect on beta. Genetic improvement of soybean yield performance under drought would be better achieved by coupling a high-yield grand mean with a high- (not low-) yield beta.

Soybean response to water : A QTL analysis of drought tolerance

Publisher Summary This chapter reviews the wheat physiology/breeding work relative to the constraints of dryland cropping in Mediterranean environments and explores opportunities for additional yield improvement associated with morphological and physiological traits. The chapter discusses the environmental constraints to crop growth in the parts of the world that experience Mediterranean-type climates. Water stress is a major limitation to wheat growth and yield in Mediterranean and other environments. Grain yield (GY) is defined as the product of the biomass produced and the harvest index. Biomass can be increased by agronomic manipulation or by genetic means. Germination, ear initiation, terminal spikelet, and anthesis act as physiological switch for the allocation of assimilates to different organs of the plant, hence phenology and assimilate partitioning are closely related. Breeders have changed the structure of cereals considerably, both indirectly through changes in phenology and directly through the introduction of dwarfing genes. The chapter explains the effect of plant morphology and physiology on water usage. In water-limited environments, biomass production is a function of the water used by the crop (WU) and the efficiency with which it is converted into biomass (WUE). Biomass production can be defined by the amount of radiation intercepted (RI) and the radiation-use efficiency (RUE)—that is, the efficiency of the conversion of this radiation to dry matter.

Morphological and physiological traits associated with wheat yield increases in Mediterranean environments

Studies of heat and mass exchange between leaves and their local environment are central to our understanding of plant-atmosphere interactions. The transfer across aerodynamic leaf boundary layers is generally described by non-dimensional expressions which reflect largely empirical adaptations of engineering models derived for flat plates. This paper reviews studies on leaves, and leaf models with varying degrees of abstraction, in free and forced convection. It discusses implecations of finding for leaf morphology as it affects – and is affected by – the local microclimate. Predictions of transfer from many leaves in plant communities are complicated by physical and physiological feedback mechanisms between leaves and their environment. Some common approaches, and the current challenge of integrating leaf-atmosphere interactions into models of global relevance, are also briefly addressed.

Tansley Review No. 59 Leaf boundary layers

Plant stress is a major limitation to crop yield. The amelioration of crop environments is either impossible or costly. The breeding of crop varieties resistant to environmental stress is the most effective economical means to improve and stabilize yield under conditions of stress. Crop resistance to various environmental stresses is being improved by traditional and costly breeding methods that involve the stability of yield performance over different environments as a major criterion. Recent advances in stress physiology allow embarkation upon breeding programs that employ distinct physiological selection indices for stress tolerance. Using physiological selection indices in breeding work requires the definition of the importance and effectiveness of given physiological attributes under stress conditions, the design of a proper selection scheme within the logistic framework of the breeding program and the development of rapid and effective selection techniques. These requirements, the progress made and ...

Breeding crop varieties for stress environments

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. 

Leaf Pubescence Effects on the Mass and Energy Exchange Between Soybean Canopies and the Atmosphere1

A micrometeorological-physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well-watered and water-stressed soybean (Glycine max (L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully-developed. Measurements of CO2, latent heat and sensible heat flux were made using the Bowen-ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well-watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two-thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water-stressed, stomatal closure appears to have been the primary cause of the reduction in LE. Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO2 exchange appears to have been caused by a combination of high stomatal resistance limiting CO2 diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO2-water flux ratio) was greater when the crop was well-watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO2-water flux ratio. Additional index words: Glycine max (L.) Merr., Micrometeorology, Water use efficiency, Photosynthesis, Evapotranspiration, Canopy CO2 exchange, Environmental physiology. E and physiological variables affect mass and energy exchanges between a crop canopy and the atmosphere in a mutualistic, synergistic, or antagonistic manner. Micrometeorological measurements, supported with physiological data, provide one of the best means of examining these complex processes. The ad1 Published as Paper No. 6779, 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 28 Jan. 1982. 2 Formerly research associate (now postdoctoral fellow at Atmospheric Turbulence and Diffusion Laboratory, P. O. Box E, Oak Ridge, TN 37830), associate professor, professor, professor and visiting scientist, Center for Agricultural Meteorology and Climatology, and associate professor, Dep. of Agronomy, Institute of Agriculture and Natural Resources, Univ. of Nebraska, Lincoln, NE 68583-0728. vantage of using micrometeorological techniques is the ability they afford to measure spatially integrated exchange rates on a continuous basis, without influencing the local microclimate. Over the past 20 years, most comprehensive micrometeorological-physiological studies have been performed over crops in a well-watered condition—corn, wheat, alfalfa, and sugar beets (e.g., Lemon, 1960; Monteith and Szeicz, 1960; Denmead, 1969; Saugier, 1970; Brown and Rosenberg, 1971; Baldocchi et al., 1981a, 1981b). Only a few such studies have been attempted with water-stressed crops (Begg et al., 1964; Biscoe et al., 1975). There have also been very few detailed studies of the mass and energy exchanges between the atmosphere and soybeans, an economically important crop. Bailey and Davies (1981) is an exception to this statement. The objective of the study reported here is to examine the diurnal exchanges of mass and energy of well-watered and water-stressed soybean canopies. The influence of environmental and physiological variables on these exchanges are discussed in detail. Results are compared with findings derived from chamber studies on soybeans. MATERIALS AND METHODS Experimental Details The study was conducted during the summer of 1980 at the Univ. of Nebraska Agricultural Meteorology Laboratory at Mead, Nebr. (41° 09' N; 96° 30' W; alt. 354 m above msl). Soybeans (Glycine max L. Merr. cv. Harosoy) were planted on 22 May 1980, in an experimental field (65 m E-W by 210 m N-S) in 0.75 m wide north-south rows. Border fields to the east, south, and west were also planted in Harosoy cv. soybeans. Data used in this study were selected from periods when the fetch to height ratio exceeded 75 to 1. Between 20 July and 10 August the canopy was erect but was water-stressed. A storm on 10 August relieved the water stress but caused lodging in the crop. Air temperature and vapor pressure were measured over the field at 1.25, 1.50, 1.75, 2.25, 2.75, and 3.25 m with an automatic, self-checking, multilevel psychrometer (Rosenberg and Brown, 1974). Once each hour the psychrometer assembly rotated automatically into a horizontal position for calibration. Air was sampled to determine CO2 concentration with a multilevel manifold at 0.30, 0.50, 0.70, and 0.90 m within the crop 544 AGRONOMY JOURNAL, VOL. 75, MAY-JUNE 1983

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A. Garay

Papers

Leaf Pubescence Effects on the Mass and Energy Exchange Between Soybean Canopies and the Atmosphere1

Influence of Water Stress on the Diurnal Exchange of Mass and Energy between the Atmosphere and a Soybean Canopy1

Water Use Efficiency in Soybean Pubescence Density Isolines— A Calculation Procedure for Estimating Daily Values'1