Photosynthetic rates of two species of malvaceae, Malvaviscus arboreus var. drummondii (wax mallow) and Abutilon theophrasti (velvetleaf).
Abstract: We examined two co-occurring species of Malvaceae in the savanna of central Texas to determine their photosynthetic response to varying levels of light. Abutilon theophrasti had a mean ([+ or -] 1 SD) density of 4 [+ or -] 4 plants/[m.sup.2] in the open-grassland phase of the savanna, and a density of 1 [+ or -] 2 plants/[m.sup.2] under canopy of woody mottes. Malvaviscus arboreus var. drummondii was not in the open-grassland phase and had a density of 3 [+ or -] 6 plants/[m.sup.2] under canopy of woody mottes. Mean midday levels of light in the open and canopy were significantly different at 2,004 versus 192 [micro]mol/[m.sup.2]/s, respectively. Maximum photosynthetic rate of A. theophrasti (34.6 [+ or -] 3.6 [micro]M C[O.sub.2]/[m.sup.2]/s) occurred at a photosynthetic-flux density of 2,000 [+ or -] 0.0 [micro]M/[m.sup.2]/s and was significantly greater than the maximum photosynthetic rate of M. arboreus var. drummondii (14.8 [+ or -] 2.2 [micro]M C[O.sub.2]/[m.sup.2]/s), which occurred at a photosynthetic-flux density of 1,350 [+ or -] 173.0 [micro]M/[m.sup.2]/s. Light saturation, light-compensation point, dark respiration rates, stomatal conductance, and transpiration rates for A. theophrasti were higher than rates of M. arboreus var. drummondii. These species have significant differences in most gas-exchange measurements, reflecting differences in their habitats. Based on these differences, M. arboreus var. drummondii is a sun-shade intermediate and A. theophrasti is a sun plant. However, maximum-photosynthetic-rate values and levels of light at maximum photosynthetic rate suggest that M. arboreus var. drummondii would do well in edge or partially shaded habitats.

Examinamos dos especies de malvaceas que conviven en las sabanas del centro de Texas con el fin de determinar su respuesta fotosintetica a diferentes niveles de luz. Abutilon theophrasti tuvo una densidad ([+ or -] 1 DE)de 4 [+ or -] 4 plantas/[m.sup.2] en la fase de pastizal de las sabanas, y una densidad de 1 [+ or -] 2 plantas/[m.sup.2] bajo el dosel en los parches boscosos. Malvaviscus arboreus var. drummondii no fue encontrado en los pastizales, pero tuvo una densidad de 3 [+ or -] 6 plantas/[m.sup.2] bajo el dosel en los parches boscosos. Los niveles de luz al mediodia en campo abierto y bajo del dosel fueron significativamente diferentes (un promedio de 2,004 versus 192 [micro]mol/[m.sup.2]/s, respectivamente). La tasa fotosintetica maxima de A. theophrasti (34.6 [+ or -] 3.6 [micro]M C[O.sub.2]/[m.sup.2]/s) ocurrio a una densidad de flujo fotosintetico de 2,000 [+ or -] 0.0 [micro]M/[m.sup.2]/s y fue significativamente mayor que la tasa fotosintetica maxima de M. arboreus var. drummondii (14.8 [+ or -] 2.2 [micro]M C[O.sub.2]/[m.sup.2]/s), que ocurrio a una densidad de flujo fotosintetico de 1,350 [+ or -] 173.0 [micro]M/[m.sup.2]/s. La saturacion de luz, el punto de compensacion luminico, las tasas de respiracion en la oscuridad, la conductancia estomatica, y las tasas de transpiracion fueron mayores en A. theophrasti que en M. arboreus var. drummondii. Estas especies tienen una diferencia significativa en la mayoria de los parametros involucrados en el intercambio gaseoso, lo cual refleja las diferencias entre sus habitats. Basados en estas diferencias, M. arbores var. drummondii es una planta entre sol y sombra, y A. theophrasti es una planta de sol. Sin embargo, las tasas fotosinteticas maximas y los niveles de luz a los que se produce esa tasa fotosintetica maxima sugieren que M. arboreus var. drummondii puede tener un buen desempeno en habitats de borde o en habitats parcialmente sombreados.
Article Type: Report
Subject: Malva (Environmental aspects)
Malva (Research)
Authors: Van Auken, O.W.
Bush, J.K.
Pub Date: 09/01/2011
Publication: Name: Southwestern Naturalist Publisher: Southwestern Association of Naturalists Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2011 Southwestern Association of Naturalists ISSN: 0038-4909
Issue: Date: Sept, 2011 Source Volume: 56 Source Issue: 3
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 269339102
Full Text: Communities are composed of species that occur together spatially and temporally (Begon et al., 2006). Determining the niche of individual species can be challenging, particularly in communities that are biphasic, such as savannas (House et al., 2003). Some species are restricted to the grassland phase, some to the woodland phase, and others occur in between or at the edge. Factors that cause these limitations could be biotic or abiotic, but are not always easy to delineate. Levels of light, depth of soil, moisture in soil, levels of nutrients, competition between species, or combinations of these factors are all potential limiting factors.

In central Texas, there are numerous savannas of various types including some associated with Juniperus-Quercus woodlands of the Edwards Plateau (Van Auken and McKinley, 2008). At least two species of Malvaceae occur in some of these savanna communities, especially those that have been disturbed. One species, Abutilon theophrasti (velvetleaf), is an introduced annual species that primarily occurs in open disturbed grassland habitats or at the edge of the woodland phase; while the other, Malvaviscus arboreus var. drummondii (wax mallow or turk's cap), is a native perennial species that primarily occurs below canopy of woodland associated with Quercus virginiana var. fusiformis.

Physiological differences between plants that are native to shady habitats and those native to sunny habitats have been widely discussed (Salisbury and Ross, 1992; Begon et al., 2006). Shade plants typically have lower photosynthetic rates at higher levels of light, saturate at lower levels of light (light saturation), and have lower light-compensation points (photosynthetic rate equals respiration rate) than sun plants (Boardman, 1977; Larcher, 2003). sun plants typically have higher rates of transpiration and stomatal conductance than shade plants (Young and smith, 1980). A few species exhibit adaptive crossover and are capable of acclimating to highlight or low-light environments, which may allow them to have a broader ecological niche (Givnish, 1988; Givnish et al., 2004).

The family Malvaceae (mallow) contains ca. 1,000 herbaceous and shrubby species in ca. 75 genera in tropical and temperate regions throughout the world (Correll and Johnston, 1979). Members of the genus Malvaviscus occur throughout the tropical and subtropical world; however, it is native to the Western Hemisphere (Turner and Mendenhall, 1993). Few studies have investigated physiological characteristics of members of this genus. systematics of Malvaviscus has been enigmatic. A revision of Malvaviscus delineated two widespread species, M. arboreus of North America and M. concinnus of South America, two localized species, M. achanioides of Mexico and M. williamsii of Peru and Colombia, and a widespread cultivar, M. peduliflora, of unknown origin (Turner and Mendenhall, 1993). Malvaviscus arboreus includes two varieties, M. arboreus var. arboreus and M. arboreus var. drummondii. Malvaviscus arboreus var. arboreus is widespread in tropical and subtropical habitats of Mexico, Central America, and the West Indies (Turner and Mendenhall, 1993). The other variety, the one included in this study, M. arboreus var. drummondii, occurs from Florida westward into Texas, occurring through eastern, central, and southern Texas along the Gulf Coast; and sparsely in northeastern Mexico (Turner and Mendenhall, 1993). it usually grows below a canopy in shaded habitat (Hanson, 1921).

The second species examined in this study, the non-native A. theophrasti, has been studied extensively because it occurs in cultivated fields and is considered a weed. The genus Abutilon contains ca. 160 species worldwide (United states Forest service, http://www.invasive.org/ weeds/asian/). Abutilon theophrasti is a native of China and was introduced into Virginia or Pennsylvania in the early 18th century as a source of fiber (Spencer, 1984). it now occurs in all 48 of the contiguous united states, and all of southern Canada. studies indicate that A. theophrasti tolerates a wide range of conditions of nutrients and light (Parrish and Bazzaz, 1985; Garbutt and Bazzaz, 1987).

in this study, we describe gas-exchange characteristics of M. arboreus var. drummondii (= Malvaviscus hereafter) and A. theophrasti (= Abutilon hereafter), two species that can grow in proximity to one another in central Texas. While there have been many studies on photosynthetic rates of species of agricultural Malvaceae (e.g, Gossypium hirsutum, cotton), few studies have been conducted on non-agricultural Malvaceae, and there is no study of photosynthetic rates of Malvaviscus.

MATERIALS AND METHODS--The field site was on the southern edge of the Edwards Plateau just south of the Balcones Escarpment on the eastern edge of the campus of the University of Texas at San Antonio in northern Bexar County, Texas. The site was a Juniperus-Quercus savanna with mottes (or clumps) of Juniperus-Quercus and other woody species mixed with gaps or patches of herbaceous vegetation. soils in this area are Crawford and Bexar and are in the Crawford series. They are stony clay in texture and are shallow to moderately deep over hard limestone with 0-3% slope (National Resource Conservation service, http:// websoilsruvey.nrcs.usda.gov/app/). The Crawford soils have a non-calcareous clay surface layer that is 20-22 cm thick and a subsurface layer that contains limestone ca. 66 cm thick (Taylor et al., 1962). Bexar soils have a cherty clay loam to gravelly loam surface layer that is 36-56 cm thick and a cherty clay subsurface 15-36 cm thick (Taylor et al., 1962). Climate has a mean annual temperature of 20[degrees]C with monthly means ranging from 9.6[degrees]C in January to 29.4[degrees]C in July (National Oceanic and Atmospheric Administration, http://www.ncdc. noaa.gov/oa/ncdc.html). Mean annual precipitation is 78.7 cm, bimodal, with peaks in May and September (10.7 and 8.7 cm, respectively), and with little rainfall and high evaporation in summer.

Vegetation is Juniperus-Quercus savanna and is representative of savanna and woodlands throughout this region, but higher in density of woody plants than savanna communities farther to the west (Van Auken et al., 1979, 1980; Smeins and Merrill, 1988). Dominant woody species are J. ashei (Ashe juniper) and Q. virginiana var. fusiformis (plateau live oak) with subdominants including Diospyros texana (Texas persimmon) and Sophora secundiflora (mountain laurel). Associated with these woodlands are sparsely vegetated intercanopy patches or gaps (Van Auken, 2000). The common herbaceous species below the canopy was Carex planostachys (cedar sedge; Wayne and Van Auken, 2008), but Malvaviscus occasionally was associated with Q. v. var. fusiformis on deeper soils. in gaps, Aristida longiseta (red three-awn), Bouteloua curtipendula (side-oats grama), Bothriochloa laguroides torreyana (silver bluestem), B. ischaemum var. songarica (King Ranch bluestem), various other [C.sub.4] grasses, and a variety of herbaceous annuals were common. Abutilon occasionally was in gaps, usually in new disturbances, and on slightly deeper soils. While levels of light are higher in gaps compared to canopy edge, there is no significant difference in surface-soil temperature or moisture in soil between gaps and canopy edge (Wayne and Van Auken, 2004).

We measured photosynthetic-flux density (PFD) in the open grassland phase and the woodland phase. We delineated all quadrats outside the canopy dripline as grassland phase and all those quadrats inside the dripline as woodland phase. We measured PFD in the center of 1-[m.sup.2] quadrats within [+ or -] 2 h of solar noon on nine transects. Each transect consisted of contiguous 1-[m.sup.2] quadrats that began under canopy of the woodland phase of the savanna, and extended outward into the adjacent grassland phase. Number of transects, length of transects, and number of quadrats varied for each transect, and was dependent on size and shape of canopy vegetation and associated grassland. Within the grassland phase, 91 quadrats were measured and, within the woodland phase, 126 were measured. We also measured density of Abutilon or Malvaviscus within each quadrat. We calculated mean PFD and density of Abutilon and Malvaviscus for each phase (grassland or woodland). Due to unequal variances, Welch's ANOVAs were used to compare mean PFD and density of Abutilon or Malvaviscus between grassland and woodland habitats (Welch, 1951).

To examine changes in PFD over a 24-h period, we used Spectrum Watchdogs (Spectrum Technologies, Plainfield, Illinois) to measure PFD every 10 min during 15-17 November 2008. We placed two sensors in an open grassland phase and six under canopy. More sensors were used under canopy due to greater variation from sunflecks. We calculated mean PFD at each 10-min time interval using each replication (2 or 6) over the 3 days.

We determined and plotted uptake of C[O.sub.2] as a function of PFD for Malvaviscus and Abutilon. We measured gas-exchange parameters from fully expanded, non-terminal leaves from flowering plants. There were four replications (individuals) for each species. Individuals were selected randomly. Average photosynthetic-flux density outside of the chamber during gas-exchange measurements for Malvaviscus was 1,086 [+ or -] 632 [micro]mol/[m.sup.2]/s and average photosynthetic-flux density during measurements of Abutilon was 1,475 [+ or -] 468 [micro]mol/[m.sup.2]/s. We made measurements during 29 April-20 May 2006 within [+ or -] 2 h of solar noon with a LI-COR 6400 infrared gas analyzer (LI-COR Environmental, Lincoln, Nebraska). Irradiances were generated by the LI-COR LED red-blue light source using the autolight-curve program of the LI-COR with a flow rate of 400 [micro]mol/s and a concentration of C[O.sub.2] of 400 [micro]mol/mol. We operated the LI-COR 6400 at ambient temperature (25[degrees]C), relative humidity (35-57%), and it was calibrated daily. Response data were recorded after [greater than or equal to] 2 min when a stable total coefficient of variation was reached (1%), usually in <3 min. We started light-response curves at a PFD of 2,000 [micro]mol/[m.sup.2]/s and then decreased levels of light to 1,800, 1,500, 1,200, 900, 600, 300, 150, 100, 50, 25, 10, 5, and 0 [micro]mol/[m.sup.2]/s. Net photosynthesis, stomatal conductance, and transpiration were measured over the 14 levels of light. We used a repeated-measures ANOVA to determine if net photosynthesis, stomatal conductance, and transpiration were significantly different between species when measured over the PFDs tested, with PFD as the repeat variable (Sall et al., 2001). We used Shapiro-Wilks tests to test for normal distributions and Bartlett's test to assess homogeneity of variances. Data were log transformed for analyses due to unequal variances.

We calculated [A.sub.max] (maximum rate of photosynthesis; [micro]mol C[O.sub.2]/[m.sup.2]/s), PFD at [A.sub.max] ([micro]mol/[m.sup.2]/s), transpiration at [A.sub.max] ([micro]mol [H.sub.2]O /[m.sup.2]/s), conductance at [A.sub.max] (mmol [H.sub.2]O/[m.sup.2]/s), light-saturation point ([micro]mol/[m.sup.2]/s), dark respiration ([micro]mol C[O.sub.2]/[m.sup.2]/s), light-compensation point ([micro]mol/[m.sup.2]/s), and quantum-yield efficiency ([micro]mol C[O.sub.2]/[micro]mol quanta) for each replicate, and we calculated means for each species. We fitted data for each replication (plant) to the model of Prioul and Chartier (1977) using the PC software package Photosyn Assistant (Dundee scientific, Dundee, Scotland). [A.sub.max] was the highest net rate of photosynthesis. Light-saturating photosynthesis was the PFD when slope of the initial rate line reached [A.sub.max]. Dark respiration was the gas-exchange rate at a PFD of 0 [micro]mol/[m.sup.2]/s (y-intercept of the line for the initial rate). Light-compensation point was calculated as the PFD when photosynthetic rate was 0 [micro]mol C[O.sub.2]/ [m.sup.2]/s (x-intercept of the line for the initial rate). Quantum-yield efficiency was calculated using the dark value and increasing PFDs until the regression coefficient of the slope decreased.

A pooled t-test was used to detect significant difference in [A.sub.max], light saturation, respiration, transpiration at [A.sub.max], conductance at [A.sub.max], and quantum-yield efficiency between species (Sall et al., 2001). Due to unequal variances, we used Welch's ANOVAs (Welch, 1951) to determine differences in light compensation and PFD at [A.sub.max]. We used an alpha value of 0.05 for all tests. In addition, we used a t-test to detect significant differences between photosynthetic rate, stomatal conductance, or transpiration rate between species at each PFD.

[FIGURE 1 OMITTED]

Results--Diurnal measurements of PFD in the open grassland phase ranged from 0 [micro]mol/[m.sup.2]/s between 1740-0700 h to 1,436 ([micro]mol/[m.sup.2]/s at 1150 h during 15-17 November 2008 (Fig. 1). In the woodland phase, mean PFD ranged from 0 [micro]mol/[m.sup.2]/s between 1650-0740 h to 380 [micro]mol/ [m.sup.2]/s at 1150 h during the 3 days (Fig. 1). PFD within quadrats (within [+ or -] 2 h of solar noon) in the grassland phase was significantly greater and less variable (2,004 [+ or -] 77 [micro]mol/[m.sup.2]/s) than in the woodland phase (192 [+ or -] 200 [micro]mol/[m.sup.2]/s; Table 1). Density of Abutilon was significantly greater in the grassland phase than the woodland phase, and conversely, density of Malvaviscus was significantly greater in the woodland phase when compared to the grassland phase (Table 1).

Photosynthetic response was significantly different between species over light-levels measured (repeated-measures ANOVA; Fig. 2a). At PFDs >300 [micro]mol/[m.sup.2]/s, Abutilon had significantly higher photosynthetic rates than Malvaviscus (t-test; P < 0.05), while at PFDs <300 [micro]mol/ [m.sup.2]/s, Malvaviscus had significantly higher rates than Abutilon (t-test, P < 0.05). Stomatal conductance and transpiration indicated that species responded differently over light-levels measured (repeated-measures ANOVA; Figs. 2b and 2c). At all levels of light tested, Abutilon had a significantly higher stomatal conductance and transpiration rate than Malvaviscus (t-test; P < 0.05).

The [A.sub.max] of Abutilon (34.6 [micro]mol C[O.sub.2]/[m.sup.2]/s) occurred at the maximum PFD measured (2,000 [micro]mol/[m.sup.2]/s; Table 2). The rate was about twice that of Malvaviscus (14.8 [micro]mol C[O.sub.2]/[m.sup.2]/s), which occurred at a PFD of 1,350 [micro]mol/[m.sup.2]/s. Light saturation for Abutilon was 633 [micro]mol/[m.sup.2]/s, which was significantly higher than light saturation of Malvaviscus (279 ([micro]mol/[m.sup.2]/s). The light-compensation point of Abutilon (41 [micro]mol/[m.sup.2]/s) was four times greater than the light-compensation point of Malvaviscus (12 [micro]mol/[m.sup.2]/s; Table 2). Likewise, the dark respiration of Abutilon (2.4 [micro]mol C[O.sub.2]/[m.sup.2]/s) was four times greater than the dark respiration of Malvaviscus (0.6 [micro]mol C[O.sub.2]/[m.sup.2]/s). Quantum-yield efficiency was not significantly different between species (Table 2). Finally, both conductance and transpiration at [A.sub.max] was significantly higher for Abutilon compared to Malvaviscus (Table 2).

DISCUSSION--As hypothesized, Malvaviscus arboreus var. drummondii, which commonly grows in shaded woodlands, had an [A.sub.max] lower than that of Abutilon theophrasti, which typically grows in open disturbed areas. In addition, the [A.sub.max] of Malvaviscus occurred at a lower irradiance than Abutilon. Other photosynthetic parameters, including light saturation, light compensation, respiration, stomatal conductance, and transpiration rate, were significantly lower for Malvaviscus than for Abutilon. These responses are consistent with other sun and shade plants (Boardman, 1977; Hull, 2002; Larcher, 2003; Givnish et al., 2004; Begon et al., 2006).

While Malvaviscus is a widespread native species in the Gulf Coast region of the united states and is used as an ornamental over much of its range, little is known about its photosynthetic capabilities. No study has examined physiological responses of this genus, including the two varieties of M. arboreus. Maximum photosynthetic rate ([A.sub.max] = 14.8 [micro]mol C[O.sub.2]/[m.sup.2]/s) for Malvaviscus is a rate that would indicate that this species is an intermediate between sun and shade species, and can grow in intermediate-light habitats.

[FIGURE 2 OMITTED]

Some understory herbaceous species have much lower gas-exchange rates than those in the current study for Malvaviscus. Photosynthetic rates of three herbaceous species that grow in understory of montane spruce (Picea) forests in central Europe all had lower rates (3.4-5.5 [micro]mol C[O.sub.2]/[m.sup.2]/s) than Malvaviscus (Hattenschwiler and Korner, 1996). In addition to these lower [A.sub.max] rates, species reached light saturation at a lower irradiance (ca. 200 [micro]mol/[m.sup.2]/s) than Malvaviscus (279 [micro]mol/[m.sup.2]/s). Arnica cordifolia, an herbaceous perennial of the family Compositae, which grows in understory of lodgepole pine (Pinus contorta) forests in southeastern Wyoming, also had much lower photosynthetic rates (3.5-4.2 [micro]mol C[O.sub.2]/[m.sup.2]/s) than Malvaviscus, but light saturated at 350 [micro]mol/[m.sup.2]/s (Young and smith, 1980). Polygonum virginianum, an herbaceous perennial that occurs in understory and at the edge of forests in the eastern United states, had an [A.sub.max] of ca. 3 [micro]mol C[O.sub.2]/[m.sup.2]/s at a light saturation of ca. 500 [micro]mol/[m.sup.2]/s (Zangerl and Bazzaz, 1983).

While Malvaviscus typically grows in understory or edge of woodlands, its high [A.sub.max] compared to other herbaceous shade plants would suggest it is an intermediate-light species, and perhaps, could grow in a variety of light environments including edge habitats. A recent study of a biennial herbaceous species that grows in edge and understory habitat of some of these central-Texas savannas (but was in our study site) indicated that Verbesina virginica (frost weed) had an [A.sub.max] of ca. 12 [micro]mol C[O.sub.2]/[m.sup.2]/s (Gagliardi, 2008).

Plants can acclimate to variability of the light environment in which they live, particularly early successional species or plants from disturbed (open) communities (Bazzaz and Carlson, 1982). Polygonum pensylvanicum, a colonizing annual of open fields, had an [A.sub.max] of ca. 12 [micro]mol C[O.sub.2]/ [m.sup.2]/s at ca. 1,500 [micro]mol/[m.sup.2]/s when plants from a shaded habitat (200 [micro]mol/[m.sup.2]/s) were measured (Bazzaz and Carlson, 1982; Zangerl and Bazzaz, 1983). However, the rate was ca. 24 mmol/[m.sup.2]/s at ca. 1,800 [micro]mol/[m.sup.2]/s when plants from a full-sun habitat were measured (Bazzaz and Carlson, 1982). Individuals we sampled were growing in an area that received 1,087 [micro]mol/[m.sup.2]/s (ca. 50% full sunlight). We might expect that individuals from higher light environments would have a higher maximum rate, while those from lower light environments would be lower. Further studies would be needed to determine if Malvaviscus does acclimate to variability in light environment as reported for other species (Givnish, 1988; Givnish et al., 2004).

Dark respiration of Malvaviscus (0.5 [micro]mol C[O.sub.2]/[m.sup.2]/s) is similar to other shade-adapted plants. Dark respiration for shade-adapted species typically is lower than sun-adapted species, due to lower metabolism of shade-adapted species (Bjorkman, 1968; Bazzaz and Carlson, 1982). Polygonum pensylvanicum grown at 200 [micro]mol/[m.sup.2]/s had a respiration rate of ca. 0.5 [micro]mol C[O.sub.2]/[m.sup.2]/s, although the rate was twice as high when plants from full sun were measured (Bazzaz and Carlson, 1982).

Although little is known about physiology of Malvaviscus, many studies have examined photosynthetic parameters of Abutilon. Early studies indicated that optimum temperature for growth of Abutilon was 30[degrees]C and that the maximum rate of ca. 15 [micro]mol C[O.sub.2]/[m.sup.2]/s was reached at a PFD of 1,600 [micro]mol/[m.sup.2]/s (Bazzaz, 1979). However, subsequent studies have shown that both higher and lower rates occur under varying conditions. Rates of ca. 20 [micro]mol C[O.sub.2]/[m.sup.2]/s were recorded at a fertilizer concentration of 30 kg N/ha and this rate dropped to <5 [micro]mol C[O.sub.2]/[m.sup.2]/s at a fertilizer concentration of 5 kg N/ha (Lindquist and Mortensen, 1999). A photosynthetic rate of ca. 10 [micro]mol C[O.sub.2]/[m.sup.2]/s was detected when simulated herbivory (clipping) was compared to plants that were unclipped; these measurements were taken at a PFD of 250 [micro]mol/[m.sup.2]/s and a temperature of 24-30[degrees]C (Stafford, 1989). Photosynthetic rates of Abutilon grown in mixture with Glycine max (soybeans) was 22.73 mmol C[O.sub.2]/[m.sup.2]/s, while rate was 27.27 mmol C[O.sub.2]/[m.sup.2]/s when grown in monoculture. However, rates in monoculture and mixture were dependent on time of day when measurements were taken; perhaps, due to changes in water stress (Munger et al., 1987b). Decreased photosynthetic rates of Abutilon occur with increased water stress (Wieland and Bazzaz, 1975; Munger et al., 1987a). Increased photosynthetic rates of Abutilon were detected with increased levels of C[O.sub.2] (Hirose et al., 1997). We recorded a maximum rate of photosynthesis at 34.6 [+ or -] 3.6 [micro]mol C[O.sub.2]/[m.sup.2]/s at a PFD of 2,000 [micro]mol/[m.sup.2]/s.

Other photosynthetic parameters reported in this study for Abutilon are similar to other studies. Respiration (2.4 [micro] 0.7 [micro]mol C[O.sub.2]/[m.sup.2]/s) was similar to the range reported (0.4-2.3 mmol C[O.sub.2]/[m.sup.2]/s) for a study with varying treatments of fertilizer and C[O.sub.2] (Hirose et al., 1997). In addition, quantum-yield efficiency (0.059 [+ or -] 0.003 [micro]mol C[O.sub.2]/ [micro]mol quanta) reported here for Abutilon is similar to previous reports (0.035-.052 [micro]mol C[O.sub.2]/ mmol quanta; Hirose et al., 1997). stomatal conductance and transpiration reported in the current study were similar to other studies; however, many factors affect these parameters (Wieland and Bazzaz, 1975; Zangerl and Bazzaz, 1984; Yun and Taylor, 1986; Munger et al., 1987a, 1987b; Stafford, 1989). Stomatal conductance and transpiration of Abutilon increased with simulated herbivory (Stafford, 1989) and decreased with increasing water stress (Munger et al., 1987a).

The species we investigated clearly had distinct and varied photosynthetic responses. Use of resources is partitioned spatially among species along complex environmental gradients, such as changes in levels of light, surface temperatures, moisture in soil, and nutrients in soil from open areas to woodland or forest edges. Differential physiological response of these species probably is related to the continuum of life-history traits of r-strategist to k-strategists (MacArthur and Wilson, 1967). Ability of the perennial Malvaviscus to reach a maximum photosynthetic rate at lower levels of light, and its lower light-saturation and light-compensation points indicate that it is more of a k-strategist than the annual Abutilon. These characteristics may allow it to exist in these edge and understory communities. At levels of light <300 [micro]mol/[m.sup.2]/s, data suggest that Malvaviscus would be able to out compete Abutilon, as indicated by its significantly higher rate of photosynthesis at lower levels of light. At levels of light >300 [micro]mol/[m.sup.2]/s, Abutilon would be better able to compete and dominate; in part, because it has significantly higher photosynthetic rates than Malvaviscus.

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Submitted 1 January 2009. Accepted 12 June 2011.

Associate Editor was Michael L. Kennedy.

O. W. Van Auken and J. K. Bush *

Department of Biology, University of Texas at San Antonio, San Antonio, TX 78249

* Correspondent: janis.bush@utsa.edu
TABLE 1--Photosynthetic-flux density ([micro]mmol/[m.sup.2]/s
within [+ or -] 2 h of solar noon) and density (plants/[m.sup.2])
for Abutilon theophrasti and Malvaviscus arboreus var.
drummondii in grassland and woodland habitat of a
central-Texas savanna. Means with the same letter
indicate no significant differences between habitats
(Welch's ANOVA, P < 0.05). Errors are [+ or -] 1 SD.

                                              Habitat

Parameter                      Grassland               Woodland

Photosynthetic-flux
  density                2,004 [+ or -] 77 (a)   192 [+ or -] 200 (b)
Density of Abutilon
  theophrasti              4 [+ or -] 4 (a)        1 [+ or -] 2 (b)
Density of Malvaviscus
  arboreus var.
  drummondii               0 [+ or -] 0 (a)        3 [+ or -] 6 (b)
n                                 91                     126

TABLE 2--Mean [+ or -] 1 SD for photosynthetic variables of
Abutilon theophrasti and Malvaviscus arboreus var. drummondii.
Means with the same letter are not significantly different. All
means were compared using a pooled t-test except photosynthetic-
flux density at maximum photosynthetic rate and light
compensation, which were analyzed using Welch's ANOVA due to
unequal variances. Data were generated from the model of Prioul
and Chartier (1977).

                                                     Malvaviscus
                             Abutilon               arboreus var.
Parameter                  theophrasti                drummondii

Maximum
  photosynthetic
  rate ([micro]mol
  C[O.sub.2]/
  [m.sup.2]/s)        34.6 (a) [+ or -] 3.6     14.8 (b) [+ or -] 2.2
Photosynthetic-flux
  density at
  maximum
  photosynthetic
  rate ([micro]
  mol/[m.sup.2]/s)   2,000 (a) [+ or -] 0      1,350 (b) [+ or -] 173
Light saturation
  ([micro]mol/
  [m.sup.2]/s)         633 (a) [+ or -] 77       279 (b) [+ or -] 34
Light compensation
  ([micro]mol/
  [m.sup.2]/s)          41 (a) [+ or -] 12        12 (b) [+ or -] 2
Respiration
  ([micro]mol
  C[O.sub.2]/
  [m.sup.2]/s)         2.4 (a) [+ or -] 0.7      0.6 (b) [+ or -] 0.1
Quantum-yield
  efficiency
  ([micro]mol
  C[O.sub.2]/
  [micro]mol
  quanta)            0.059 (a) [+ or -] 0.003  0.055 (a) [+ or -] 0.003
Conductance at
  maximum
  photosynthetic
  rate (mol
  [H.sub.2]O/
  [m.sup.2]/s)         0.6 (a) [+ or -] 0.1      0.2 (b) [+ or -] 0.1
Transpiration at
  maximum
  photosynthetic
  rate (mmol
  [H.sub.2]O/
  [m.sup.2]/s)         9.6 (a) [+ or -] 2.5      2.4 (b) [+ or -] 1.5
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