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Net carbon dioxide assimilation rates (PN) were measured for individuals from two Texas honey mesquite populations grown under controlled water stress, temperature, and light treatments. Maximum PN observed for the various tests ranged from 0.69 to 0.82 mg l rne2se1. Net assimilation rates were significantly lower in the west Texas population than in the east Texas population under high and low water stress. Maximum PN at 1.35 mmol l rnm2se1 photon flux density were reached under low water stress at 20” C and 30 O C for east and west Texas populations, respectively. The response of PN to light was similar to responses reported for other CB species; and 81 to 99% of the variability in the data was accounted for by using a hyperbolic light response model. Light use efficiency was lower for the west Texas population, and for high temperature and water stress. High temperature and water stress treatments also caused substantial decreases in the estimated theoretical maximum net assimilation rates. Finally, the light saturation point of mesquite varied depending on population and water stress. Honey mesquite (Prosopis glandulosa Torr. var. glandulosa), a woody perennial of the Leguminosae, is adapted to a broad range of environmental conditions. It is distributed throughout much of the Southwestern United States and occurs on more than 50% of Texas rangelands (McCully 1973). Dye and Hanson (1978), Hanson et al. (1978), and Hanson and Dye (1980) have reported on photosynthetic rates and xylem pressure potentials of in situ populations of honey mesquite in west Texas. Net COZ assimilation rates (PN) of east and west Texas honey mesquite populations also have been compared (Hanson 1979). These studies of in situ mesquite, however, did not test between individual plants or plant populations because of the need to monitor complete diurnal cycles of I$. This study was conducted to investigate the response of PN to changes in water stress, temperature and light for individuals from two populations of honey mesquite grown under controlled environmental conditions. Concomitantly, effects of these variables on light response curves and light saturation points were investigated. Methods and Materials Seed Sources average yearly precipitation of about 100 cm. Plant Material Preparation Seeds, collected before abcission from a range of parental plants at both field sites, were scarified with sandpaper, planted in 4-liter plastic pots at a depth of 1.25 cm and grown in a controlled environment chamber. Fritted clay was used instead of soil because of its high water-retaining capacity and adequate aeration (van Bayel et al. 1978). The controlled environment chamber was set on a 16hour photoperiod to produce 0.50 mmolo. m-‘s-’ photon flux density (Ip, cumulative light in the 0.4-0.7 I_L m band) supplied by a combination of fluorescent and incandescent lamps. Two auxiliary lOOO-Watt multivapor lamps were added to the chamber to increase the chamber Ip to 0.60 mmol l rne2se1 for 10 hours each day. Plants were rotated daily to eliminate variance caused by proximity to the auxiliary lights. Temperature was 30” C during the day and 25°C during the night; relative humidity was held constant at 50%. Plants were watered every other day, alternating between 50% Hoagland’s solution and distilled water. The mesquite seedlings grew very well and were ready for experimentation within 6-8 weeks. From the 45 plants grown of each population, 24 plants of each population were randomly selected for PN determinations. Statistical Design and Analysis A 2 X 2 X 4X 5 factorial experiment arranged in a split-split plot design with three replications was used to determine environmental tolerances of PN from two populations of mesquite. The first classification factor represented the population effect (east and west Texas). The second factor was leaf water stress: low stress [about -0.5 macroPascals (MPa)] and high stress (about -3.0 MPa). The third and fourth factors were chamber temperature (20, 25,30, and 35” C)andincident light(0.10,0.25,0.35,0.60, and 1.35 mmol l m-‘s-l). Temperature was controlled within the assimilation chamber. Light was controlled by using different layers of cheesecloth filters over a IOOO-Watt multivapor lamp (Sestak et al. 1971). Water was withheld for 2 weeks from one group of plants of each population to produce high water stress (Mooney et al. 1977). Analysis was performed using standard analysis of variance techniques. Duncan’s multiple range test was subsequently used to test between treatment means. Seeds were collected from a site in west Texas and a site in east Texas to study PN in different populations of mesquite (Hanson Determining Assimilation Rates 1979). Crane County, representing the west Texas population, is A portable, open Co2 exchange system for measuring PN on located in the semiarid Trans-Pecos Region of west Texas and branches of woody plants was developed by Dye and Hanson receives an average yearly precipitation of about 32.2 cm. Trees of (1978). The system consisted of an assimilation chamber, a ventilathe east Texas population were located in Brazos County, which is tion and temperature control unit, a measurement and recording in the temperate Blackland Prairie vegetation zone and receives an unit, and a power supply unit. The components allowed continuous measurement of chamber temperature, photon flux density, air Author was research assistant, Department of Range Science, Texas A&M Univerflow rates into the chamber and the difference between ambient sity, College Station; currently range scientist, USDA, Agr. Res. Serv., 8408 Hildreth Road, Cheyenne, Wyo. 82009. and chamber carbon dioxide concentrations. The system was modResearch reported in this paper was partially supported by an interagency contract ified for this study by using a 220 V AC motor, instead of a IO-hp with the University of Texas-Surface Leasing Department, University of Texas. Approved for publication by the Director, Texas Agricultural Experiment Station. gasoline engine, to drive the air conditioner compressor. JOURNAL OF RANGE MANAGEMENT 3!5(4), July 1982 455 Each potted mesquite seedling was sealed in the assimilation chamber, and the system was allowed to reach equilibrium to a predetermined temperature level. Subsequently, the five levels of Ip were randomly administered to the test plant. About 10 min additional equilibration time was given for the plant to respond to the new light level once a filter was placed over the light source. Net assimilation rates, in units of mg l me2sm1, were calculated from the relation PN = kF AC@? A-’ (1) where AC& was the C@ deficit within the chamber in ppm, F was the flow rate of air over the branch measured as liters per hour, and A was the leaf area (one side) in m2. The constant k was equal to 1.79 X 10e3 mg ppm-’ I-’ at 25’ C and 0.1 MPa. A photoelectric planimeter was used to estimate areas (Kemph 1976). Light Response Model Data were fit to a hyperbolic light response curve of the form PN = ~3 IP Pmax/(~~P + hmx) (2) where the parameters Pmax (mg l m-2s-‘) and a(mg mmol-‘) were the theoretical maximum PN and the light use efficiency coefficient, respectively (Thornley 1976). Light saturation points for solutions of the model using means grouped 7 ways were derived using the following procedure after a! and PN were estimated for Equation (2). Rewriting Equation (2) as IP = PInax PN/ [~(h~ax-PN)] (3) and setting PN = p P,,,, the light saturation point was defined as the incident light at which IP = pEn*x/[4l p)] (4) where p was arbitrarily set equal to 85%. Thus, the light saturation point was defined as the light intensity at which PN was 85% P max. Data were fit to Equation (2) to evaluate differences in Q, P,,, and light saturation point caused by temperature, site, and water stress conditions. Results and Discussion Response of Assimilation Rates to Water Stress Mean PN for the east and west Texas mesquite seedling populations across all levels of water stress, temperature, and light, were 0.39 and 0.27 mg COZ me2sA1, respectively (Table 1-A). The difference between mean PN for the two populations was significant (Prob > F = 0.024). Mean PN was significantly higher for the east Texas population than for the west Texas population at low and high water stress levels. Initially, it was hypothesized that PN ofthe west Texas population would exceed PN of the east Texas population when the populations were subjected to high water stress [Ho: PN(west) > PN(east) at high water stress] because the west Texas population originated from adrier region. Data lead to rejection of this hypothesis for the seedlings investigated, therefore it seems plants of the west Texas population may havedeveloped resistance to harsh environmental conditions at the expense of high PN. Pooling data from both populations, assimilation rates of mesquite were reduced from 0.42 to 0.24 mg rnm2se1 when the plants were subjected to high water stress conditions (Table 1-A). This 42% reduction was highly significant (Prob > F = 0.005). A plausible cause of the lower PN for highly stressed plants was reduced photochemical activity (Nir and Poljakoff-Mayber 1967, Hanson and Dye 1980). The reduced activity could result from a lack of COZ at the binding sites of the carboxylating enzyme, thereby implicating stomata1 reaction to water stress as a factor limiting carbon assimilation. Response of Assimilation Rates to Temperature The 35O treatment significantly reduced net assimilation rates as compared with the mean PN of 0.38 mg l me2s1 for the other three temperatures (Table 1-A). The lack of significant differences in PN 20,25 or 300 C averaged across all other treatments implies a broad range of optimal temperatures and roughly coincides with an optimal soil temperature for growth of 29°C (Meyer et al. 1973) and with the 30°C optimal air temperature for PN of in situ mesquite (Hanson and Dye 1979). PN for the east Tex

Effect of Light, Temperature and Water Stress on Net Photosynthesis in Two Populations of Honey Mesquite