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a Dep. of Botany and Plant Sci. and Air Pollution Research Center, Univ. of California, Riverside, CA, and Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648
b Statewide Integrated Pest Management Program, Univ. of California, Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648
* Corresponding author (david{at}uckac.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HO3, high O3 concentration • LAR, leaf area ratio (total leaf area/total shoot dry weight) • LO3, low O3 concentration • LWR, leaf weight ratio (total leaf dry weight/total shoot dry weight) • MO3, medium O3 concentration • OTC, open-top chamber • Rr, fine root respiration • SJV, San Joaquin Valley • SLW, specific leaf weight (total leaf dry weight/total leaf area) • WUE, water use efficiency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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60% of the land area under cotton cultivation (California Department of Pesticide Regulation, 2002), and increases yields by an estimated 2.5-fold (National Center for Food and Agricultural Policy, 2002). In a recent survey, 188 Mg of glyphosate, alone, was applied to 167000 out of 279000 ha planted to cotton (California Department of Pesticide Regulation, 2002). This competition management is increasingly restricted by regulation (Szmedra, 1997) and appearance of herbicide-resistant weed biotypes (Heap, 1997). A less recognized threat to current vegetation management strategies are a range of poorly characterized direct and indirect effects of global change (Fuhrer, 2003; Patterson, 1995), including tropospheric ozone (O3). This phytotoxic oxidant is increasing regionally and globally even as it declines in certain badly contaminated urban areas (North American Research Strategy for Tropospheric O3, 2000). It appears to be an aspect of ongoing climate change that is likely to have substantial impact on native and cultivated vegetated systems (Davison and Barnes, 1998; Heck et al., 1988; Krupa et al., 2001; Lefohn, 1992).
The SJV is a highly productive agricultural region in which crops absorb considerable O3 from the atmosphere (Grantz et al., 1994). As a consequence, yields of cotton are reduced substantially (Brewer et al., 1986; Oshima et al., 1979; Temple et al., 1988), particularly cultivars of long-staple Pima cotton (G. barbadense L.) selected elsewhere (Grantz and McCool, 1992; Olszyk et al., 1993). The older Pima cultivar, S-6, has been demonstrated to be relatively sensitive to current ambient concentrations of O3 in this production area (Grantz, 2003; Grantz and Yang, 1996, 2000; Grantz et al., 2003), with yield losses of 20% (Olszyk et al., 1993), relative to nonpolluted air. More recent Pima cultivars have been selected within the SJV and are reported to exhibit increased tolerance to O3, though exposure-response data are lacking.
Yellow nutsedge is one of the world's most problematic weeds in field and row crops (Holm et al., 1991; Mulligan and Junkins, 1976). It is a major pest in cotton under irrigated SJV conditions. As a C4 species, it is well adapted to hot, dry climates. Reproduction is largely or entirely by below-ground production of vegetative tubers.
A previous study (Shrestha and Grantz, 2005) of the interaction of tropospheric O3 and nutsedge competition with tomato (Lycopersicon esculentum Mill.) in the SJV found that the competitive ability of yellow nutsedge with respect to tomato could be reduced by current ambient levels of O3 air pollution, relative to nonpolluted air, but that further increases in O3 exposure could shift the advantage to nutsedge. The extent to which these results may be generalized to other competitive systems, and the impact of the O3 sensitivity of the individual species on the outcome of their interactions, remain unclear. Here we extend the previous results to the interaction of O3, nutsedge, and a cultivar of cotton that is shown here to be more sensitive to O3 than the tomato cultivars described previously (Shrestha and Grantz, 2005).
While O3 impacts on competitive interactions are potentially quite significant, little is known of interspecific interactions under elevated O3 (Fuhrer and Booker, 2003; Ziska, 2002). Under OTC exposure conditions, for example, blackberry (Rubus cuneifolius Pursh.) came to dominate an early successional community previously dominated by sumac (Rhus copallina L.; Evans and Ashmore, 1992), despite the great sensitivity of blackberry to O3. Grass–legume pasture communities (e.g., Lolium perenne L.–Trifolium repens L., Festuca arundinacea Schreb.–T. repens L., and Phleum pratense L.–Medicago sativa L.) have tended to simplify toward pure grass during O3 fumigation in both open air and chamber facilities (Nussbaum et al., 1995; Rebbeck et al., 1988; Wilbourne et al., 1995; Johnson et al., 1996). The degraded performance of the legumes and increasing competitiveness of the grasses may be explained by O3 inhibition of biomass allocation to storage roots of the former.
The relative O3–tolerance of competing species may not predict competitive outcomes in the presence of O3 exposure. The mechanistic details of competitive interactions between species, and the responses of the interactions to O3, must also be considered. Ozone affects cotton through direct oxidant damage to physiological processes in leaves, through inhibited carbohydrate translocation to developing sinks such as roots (Grantz and Farrar, 1999, 2000), and ultimately through reduced economic yield (Grantz, 2003). Biomass production is reduced, particularly in roots (Grantz and Yang, 1996). Nutsedge responds similarly to O3 (Shrestha and Grantz, 2005), though allocation to below-ground reproductive structures appeared to increase with O3 stress, a potentially significant finding that warrants further study. The ability of yellow nutsedge to compete with cotton under elevated O3 has not previously been described, though preliminary reports of these studies have appeared (Grantz and Shrestha, 2004, 2005). Here, we explore the growth and gas exchange responses of shoots and roots of cotton and nutsedge grown alone, and in a partial additive competition series, under a range of O3 exposures.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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1-cm deep in 6-40 mesh sintered clay (Quicksorb, A & M Products, Taft, CA). Juvenile individuals of yellow nutsedge (single tuber; shoot 6 cm tall; 2–3 leaf blades) were collected in the SJV from agricultural fields near Parlier. Soil was washed from the plantlets and several individuals transplanted into 9-L pots, simultaneously with planting of the cotton seed. Cotton was thinned to one uniform plant pot–1 and nutsedge to 0, 1, 2, or 3 plants pot–1, at about 1 wk after emergence of the cotton. Additional pots were planted with single plants of nutsedge, alone. Four pots of each of the five nutsedge–cotton population ratios were assigned randomly to each OTC. After cotton emergence, all pots were uniformly irrigated to run-through, by automated drip emitters, 1 to 3 times daily as required by the weather. A complete fertilizer solution (Miracle Gro, Scotts Miracle-Gro Products Inc., Port Washington, NY, 1.3 g L–1) was applied to run-through weekly, using an automated fertigation system.
Ozone Exposure
For controlled exposure to O3, experiments were conducted in the OTC (3.1-m diam. x 2.4-m height; Heagle et al., 1973) exposure facility at the University of California, Kearney Agricultural Center, Parlier, CA (103 masl, 36°35'53'' N, 119°30'11'' W). The O3 was generated by corona discharge (Model G22, Pacific Ozone Technology, Brentwood, CA) from oxygen (Model AS-12, AirSep Corporation, Buffalo, NY). The daily timecourse of O3 concentration was regulated in a single OTC using a dedicated O3 monitor (Model 49C, Thermo Environmental Instruments, Franklin, MA) interfaced to a computer for feedback control, as described previously (Grantz et al., 2003). The O3 concentration in each OTC was manually fixed relative to the regulated OTC and monitored every 15 min. Samples were obtained continuously from the center of each OTC through a Teflon dust filter and Teflon tubing attached to a multiport solenoid valve. All O3 concentration data were archived electronically.
The same nominal O3 profiles were administered each day. Ozone exposure was initiated at the time of sowing of cotton and transplanting of nutsedge. The LO3 treatment was charcoal filtered. The MO3 regime was charcoal filtered with O3 added to approximate the diurnal profile and maximal concentration observed on exceptionally polluted days at this location (see Grantz et al., 2003, for the shape of the diurnal timecourse). Daily maxima in this treatment, occurring near 1500 h PDT, were nominally 140 nL L–1. The HO3 regime was charcoal filtered, with O3 added to maintain 1.5-fold greater concentration than the MO3 treatment at each time point. In the current study, the LO3, MO3, and HO3 treatments were exposed to 12-h mean O3 exposures (± S.E.) of 12.8 ± 0.6, 79.9 ± 6.3, and 122.7 ± 9.7 nL L–1, averaged across the three replicate periods of growth.
Experimental Design
The experiment was replicated across time (three times), during spring and summer to allow generalization across the growing season. Planting dates were 1 July 2003, 20 April 2004, and 7 May 2004 with harvest 8 wk later. A block of 3 OTCs was selected at each planting date. One chamber in the block was exposed to each of the 3 different O3 concentrations. A different block of 3 OTCs was used for each replication to avoid possible confounding of treatment by chamber position. The experiment was thus replicated across time, each time in different OTCs (n = 3, with subsamples).
Environmental conditions were similar during the three growth periods (Table 1), with the major difference being 5°C greater day and night temperatures in the first (midsummer) growth period, relative to the others. Daylength and solar radiation were very similar, and skies were nearly always cloudless. Plant growth, competition, and O3 sensitivity were not significantly affected by these modest environmental differences (P 
0.1). In contrast, leaf and root gas exchange were significantly greater in midsummer.
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Analyses were conducted using PROC GLM (SAS Institute, 1990). Data were transformed to attain homogeneity of variance (Shapiro-Wilk; P > 0.05), and means back-transformed for presentation. Variances were not back-transformed and no measure of dispersion is presented.
A coefficient of competition (c) was obtained by linear regression analysis (Passini et al., 2003), as
 | [1] |
Use of OTCs as replicates reduced the statistical power of tests of the main treatment effect (Hunt et al., 2005), so we explicitly identify statistical significance at P 
0.01, 0.05, and 0.10 in the tables and figures. Nevertheless, some nonsignificant trends may reflect Type II errors due to the inadequate power of the design to reject (appropriately) the null hypothesis. These suggest potentially fruitful areas for further research.
For clarity in the main panels of the figures, the effect of competition was generally demonstrated by contrasting cotton alone (nutsedge–cotton ratio of 0:1) with the mean across all treatments containing both nutsedge at any density and cotton (All:1). To evaluate these contrasts, the effect of competition (across all population ratios) was assessed using a separate, single degree of freedom contrast. The responses to nutsedge density are shown explicitly in Fig. 2 and in the inset of Fig. 3.
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Growth Measurements
Growth of cotton and nutsedge were measured as biomass and plant height (cotton only), obtained by destructive harvest of the strictly vegetative cotton plants at 7–9 wk after planting. Occasional flowers were observed in nutsedge, but this was avoided by choice of harvest date as much as possible. Reproductive effort of nutsedge was restricted to production of vegetative tubers. Height was determined as mainstem length using a meter stick. Above-ground biomass was sealed into plastic bags and stored at 4°C for up to 2 d before separation into leaves (leaf blades plus petioles) and stems. Leaves of cotton were counted and their total area determined with a leaf area meter (LI-3000, LI-COR, Inc., Lincoln NE). Specific leaf weight (SLW, g m–2), leaf area ratio (LAR, m2 kg–1), and leaf weight ratio (LWR, kg kg–1) were calculated as (total leaf dry weight/total leaf area), (total leaf area/total above-ground dry weight), and (total leaf dry weight/total above dry weight). Leaves and stems of cotton, and the entire above-ground portions of nutsedge, were placed in separate paper bags and dried to constant mass at 70°C in a forced-air oven. All biomass is on a g species–1 pot–1 basis.
The root mass of each species grown alone was washed to remove as much of the sintered clay as possible. Tubers were separated from the roots and counted. Roots (including rhizomes of nutsedge) and tubers were placed in separate paper bags.
In pots containing both species, the root systems were intertwined, requiring that the total weight of the combined root system be determined. Tubers were separated and roots and tubers were dried, as above. Some sintered clay growth media remained attached to the root mass. Therefore, the dry mass of the root and of the tubers was recorded and combined and combusted at 800°C (Thermolyne Corp., Dubuque, IA). Total root plus tuber mass was then determined as the difference between the original dry weight of the root and the weight of the remaining ash (assumed to be only sintered clay).
The below-ground to above-ground biomass ratios (R:S) for cotton grown alone (0:1) and for nutsedge alone (1:0) were determined. For cotton the above-ground biomass corresponded to the shoot, and below-ground biomass to the root. In nutsedge this was more complex, as tuber and rhizome biomass (shoot tissues) were pooled with roots as a total below-ground biomass.
Physiological Measurements
These indicators of plant performance were assayed only on cotton or nutsedge plants grown alone to further characterize the O3–sensitivity of each species under these growth conditions.
A relative measure of chlorophyll content was determined on individual leaf laminae using a portable chlorophyll meter (Minolta SPAD-502, Spectrum Technologies, Plainfield, IL) on 29 Aug. 2003, 18 June 2004, and 28 June 2004 in the three replicates. In cotton, measurements were made on the young leaves in the upper canopy, fully expanded leaves in the midcanopy, and older leaves with visible symptoms of incipient senescence in the lower canopy (1 leaf pot–1 at each insertion level). In nutsedge, insertion level and vertical canopy position were more difficult to determine and measurements were made on representative exposed leaves from various parts of the plant canopy.
Net carbon assimilation (An) and stomatal conductance (gs) were determined on recently fully expanded leaves of cotton (1 leaf pot–1), located in the mid- to upper canopy, and on young, fully exposed leaves of nutsedge from the center of the plant canopy. Measurements were obtained using a steady state gas exchange system (LI-6400, LI-COR Inc., Lincoln, NE), in situ in the OTC. Illumination was controlled at 1000 µmol photons m–2 leaf s–1, provided by 80% red and 20% blue light emitting diodes. All measurements were obtained at steady state, with ambient CO2 concentration in the cuvette controlled at 400 µL L–1. Net carbon assimilation and gs were expressed relative to projected leaf area. The measurements were taken on 25 Aug. 2003, 16 June 2004, and 24 June 2004 in the three replicates.
For determination of fine root respiration (Rr), fine root samples were obtained from distal portions of the intact root system of cotton and nutsedge plants growing alone (0:1 and 1:0), immediately following destructive harvest. The Rr was determined in liquid phase at 25°C using a Clark-type oxygen electrode (Delieu and Walker, 1972), according to the procedure previously described (Grantz et al., 2003). Four respirometer chambers (Oxygraph Oxygen Electrode System; PP Systems, Haverhill, MA) were run in parallel, interfaced with a computer for data acquisition and analysis. The Rr was expressed relative to the blotted wet weight (g fresh wt) of root in each chamber, obtained following the measurement.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In the absence and presence of nutsedge (Fig. 1), HO3 reduced above-ground biomass of cotton, by a similar 76 and 75% relative to LO3, respectively. At LO3 and HO3, nutsedge competition caused a 48 and 46% decline, respectively. The combined effects were additive, with a reduction at HO3 and All:1 (Fig. 1) of 87%. The interaction was not significant (Table 2).
An alternative method of evaluating this interaction was derived from the nonlinear responses of cotton biomass to competition observed at all levels of O3 exposure (Fig. 2A). The inversion of a simple hyperbolic model (Eq. [1]) of this response of biomass to competition resulted in significant linear relationships at all levels of O3 (Fig. 2B), and in a clear separation of the slope at HO3 from those at LO3 and MO3. The slopes at the two lower concentrations of O3 did not differ from each other. The separation was even greater when the calculation was restricted to the three lowest nutsedge densities, increasing the slope at HO3 and reducing the slope at LO3 to make it even more similar to the slope at MO3 (Fig. 2B; excluding 3:1). The significant difference between these coefficients of competition (Passini et al., 2003) indicates that exposure to the highest O3 concentration altered the competitive interaction between cotton and nutsedge.
In nutsedge grown alone (1:0), above-ground biomass (shoot minus rhizomes and tubers) was not significantly impacted by O3 (Table 2 and Fig. 1, open squares). In this and the previous study (Shrestha and Grantz, 2005), nutsedge exhibited the same response to O3, with biomass maximal at MO3 and trending downward at HO3 by 24% in the present study and declining significantly by 52% in the previous study (at similar O3 exposures of 122.7 nL L–1 and 142.3 nL L–1, respectively).
Nutsedge biomass (g pot–1) was significantly affected by the density of nutsedge plants per pot (Table 2). Nutsedge biomass was lowest at the 1:1 population ratio at all levels of O3, increasing at 1:0, with competition from the single cotton plant removed, and also increasing with nutsedge density (data not shown).
Cotton had no effect on above-ground biomass of nutsedge (Table 2) at any level of O3. Although the present experimental design was not optimized to detect impacts of cotton on nutsedge, the results contrast with the competitive interaction previously observed between yellow nutsedge and tomato (Shrestha and Grantz, 2005). In that case the presence of tomato significantly decreased the above-ground biomass of nutsedge at all levels of O3, and even a 3:1 population advantage did not increase shoot biomass of nutsedge to the level observed in the absence of tomato competition.
Canopy Properties
Other measures of shoot development in cotton were similarly affected more strongly by nutsedge competition than by O3 exposure at 7 to 9 wk after planting, despite the relatively great sensitivity to O3 of this cultivar. Mainstem length of cotton was not significantly affected by O3 across all competition treatments (Table 2), but when grown without competition plant height was reduced by about 25% at HO3 (Fig. 3A
, open circles). In contrast, both the presence of nutsedge (Table 2 and Fig. 3A, cf. open, closed circles) and increasing nutsedge density (Fig. 3A, inset) reduced mainstem length of cotton at all levels of O3. At LO3 and MO3, increasing nutsedge density reduced plant height in cotton, but this was less apparent at HO3 (Fig. 3A, inset).
Plant leaf area and the number of leaves per plant were significantly affected by O3 (Table 2). These parameters were reduced at HO3 compared with LO3 and MO3 (Fig. 3B, 3C, open circles), consistent with previous observations with this cultivar (Grantz and Yang, 1996) as well as with upland cotton (G. hirsutum L.; Oshima et al., 1979; Miller, 1988; Olszyk et al., 1993; Temple et al., 1988; Temple, 1990a, 1990b).
Nutsedge density also affected these parameters (Table 2; Fig. 3B and 3C, insets), with responses similar to those observed with shoot biomass (cf. Fig. 3, Fig. 1, open, closed circles). The magnitude of reduction by nutsedge competition was largest in LO3 (Fig. 3A, 3B, 3C; cf. 0:1, All:1), but relative impacts were similar at all O3 concentrations. Cotton was sufficiently sensitive to O3 that competition only slightly further reduced productivity at HO3 in absolute terms. This was also apparent from the effect of increasing nutsedge density on these growth parameters at each of the O3 exposures (Fig. 3A, 3B, 3C, insets).
In contrast to shoot biomass, the interaction between O3 and nutsedge density for these parameters was significant (Table 2). Leaf area and leaf number in cotton were reduced by the presence of nutsedge (Fig. 3B and 3C, cf. open, closed circles). As in mainstem length, the reduction in cotton leaf area and leaf number by nutsedge density was generally more evident at LO3 and MO3 than at HO3 (not shown). Canopy development of cotton was progressively reduced by increasing nutsedge competition (Table 2).
Both O3 and competition impacted these parameters (Fig. 3), in contrast to tomato, in which plant height was reduced while leaf area was unaffected. This reflected a substantial O3–induced compensatory leaf initiation in tomato (Shrestha and Grantz, 2005). In Pima cotton, a slight acceleration of leaf emergence was apparent (not shown), but this was insufficient to compensate for the O3–induced reduction in leaf expansion (final leaf size; calculated from Fig. 3B, 3C) and acceleration of leaf abscission (Grantz and Yang, 2000). In part because of these canopy scale responses in tomato, nutsedge was found to be a relatively weak interspecific competitor with that species, but a strong intraspecific competitor (Santos et al., 1997). Tomato was not competitive with nutsedge at an intermediate O3 exposure. In contrast, in cotton the interspecific competitiveness of nutsedge increased with increasing exposure to O3, as well as with increasing nutsedge density.
Cotton was relatively less affected by nutsedge at LO3 and MO3 than at HO3, despite the observation that nutsedge shoots exhibited a visibly more erect growth habit in the LO3 and MO3 treatments than in the HO3 treatment (not shown). This erect habit would improve nutsedge competitiveness under many field conditions, in which yellow nutsedge is a weak competitor for light (Santos et al., 1997), but was inadequate in the present case to overcome the increased plant stature in cotton at low [O3]. The competitive outcome between these the two species was adequately predicted by the biomass ratios of the two species grown alone at various O3 concentrations (cf. Figs. 1A, 1B, open symbols). The outcome was visually apparent as overtopping and consequent shading of cotton by nutsedge at higher O3 exposures.In contrast, tomato exhibited a competitive advantage over nutsedge in light interception at all O3 concentrations (Shrestha and Grantz, 2005). The nutsedge plantlets had a slight developmental advantage over the direct-seeded cotton, but an early disadvantage with respect to the transplanted tomato. Outcomes under field conditions may thus depend on the complex interplay of O3 concentration, weed propagule density, and the environmental and cultural factors that determine the relative phenologies of crop and weed species.
Leaf Properties
Leaf morphology of cotton was altered by exposure to O3 (Table 3). The SLW decreased with increasing O3. This reduced leaf density on an area basis may indicate thinner leaves, and is consistent with the observed O3–induced reductions in photosynthetic pigments (Fig. 4A
), the commonly reported reductions in photosynthetic enzymes per unit leaf area (e.g., Calatayud and Barreno, 2001), and the decline in leaf starch content previously observed with this cultivar (Grantz and Farrar, 1999). Similar reductions in SLW were observed in tomato leaves exposed to O3 in this system (Shrestha and Grantz, 2005).
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Nutsedge competition had little effect on LAR of cotton (Table 3). In contrast, SLW and LWR of cotton declined significantly with competition (Table 3), and with increasing nutsedge density (not shown).
Carbon Assimilation
Characterization of the mechanistic bases of O3 impacts on interspecific competition will allow prediction of such effects in future environments and across a broader range of competitive systems than may be investigated directly. Competition above-ground is likely to be dependent on foliar dominance of the light environment and on the capacity of the exposed leaves for net carbon fixation. At the lowest O3 exposure, these young cotton plants contained greater concentrations of photosynthetic pigments in the older, lower-canopy leaves than in the upper- or midcanopy leaves (Fig. 4A). However, chlorophyll content was much more sensitive to increasing O3 exposure in these lower leaves than in the mid- or upper canopy. This reflects the effect of leaf age and reduced light interception on leaf senescence in the lower canopy, and the effect of O3 in inducing accelerated senescence (Glater et al., 1962; Ting and Dugger, 1971; Grantz, 2003). Visible chlorosis and reddish pigmentation also increased with depth in the canopy (not shown), while leaf chlorophyll concentrations declined with depth at MO3 and HO3 (Fig. 4A), but not at LO3.
Nutsedge leaves, sampled from throughout the exposed canopy, exhibited reduced chlorophyll content with increasing O3 exposure that was comparable with losses observed in midcanopy leaves of cotton (Fig. 4A, squares), though chlorophyll concentrations were higher than in cotton. These results are very similar to the responses of nutsedge to O3 observed previously (Shrestha and Grantz, 2005). There was considerably less O3–enhanced senescence and abscission of older leaves in nutsedge than in cotton.
Chlorophyll content was a useful predictor of leaf gas exchange performance in cotton. In the midcanopy leaves, for example, chlorophyll content (Fig. 4A, triangles) declined by approximately one-third across this range of O3 exposure, matched by a similar decline in net assimilation (An; Fig. 4B, circles). In tomato, a similar decline in leaf chlorophyll was not associated with a significant decline in An or in gs. This was suggested to reflect the more rapid rate of leaf renewal in tomato than in cotton in response to high levels of O3 exposure (Shrestha and Grantz, 2005).
The response of gs in cotton to O3 was similar to that of An (Fig. 4C, circles). This is consistent with substantial reductions in gs caused by O3 exposure in general (Mansfield, 1973) and in Pima (Grantz and McCool, 1992) and upland cottons (Temple, 1986), specifically. In the study of Grantz and Yang (1996), stomatal conductance of Pima cotton was reduced by 41%, while hydraulic conductance (a root property) was reduced by only about 35% across a similar range of O3 concentrations. These responses were found to enhance shoot water status.
Net assimilation generally declines with increasing exposure to O3, along with reductions in gs, activities of Calvin-Benson Cycle enzymes, and eventually electron transport (Calatayud and Barreno, 2001; Dann and Pell, 1989; Farage and Long, 1995; Hill and Littlefield, 1969). These generalizations have been developed largely for C3 species such as cotton.
Gas exchange of nutsedge reflected its C4 photosynthetic pathway. Rates of An were similar to those observed in cotton, a C3 species (Fig. 4B, squares), but gs was 75% lower than in cotton (Fig. 4C, squares). This reduced calculated intercellular CO2 concentration (Ci) as well as stomatal conductance to O3. Intrinsic WUE of nutsedge (An/gs) was about 2.8-fold greater than in cotton at LO3 but only about 1.8- to 1.9-fold greater at MO3 and HO3 (calculated from Fig. 4B, 4C). Stomatal conductance of nutsedge was relatively insensitive to O3, declining by about 15% at HO3 compared with >50% in cotton at HO3. Sensitivity of An in nutsedge was similar to that of cotton at MO3, but little further inhibition was observed at HO3 (Fig. 4B).
The contrasting magnitudes of gs in these competing species suggest that the actual O3 doses absorbed by photosynthetic tissues may differ under equivalent environmental conditions, with the lower gs in nutsedge than in cotton excluding more O3 at moderate O3. The photosynthetic sensitivities of nutsedge and cotton to O3 must be compared in equivalent toxicological units of absorbed O3 dose. To facilitate this comparison, an apparent O3 dose (OD) was calculated as
 | [2] |
The absolute sensitivities of An to OD were very similar in cotton and nutsedge. The response in cotton was about 6% greater than in nutsedge. However, nutsedge operated across a lower and narrower range of OD than did cotton. Across the range of O3 exposures in the present study, An declined by 20% in nutsedge and by 33% in cotton. At LO3, OD was 3.0-fold larger in cotton than in nutsedge, due to higher gs (Fig. 4C). This declined to about 1.6-fold larger in cotton at HO3 due to more vigorous stomatal closure in response to O3 in this species.
The greater OD at LO3 in cotton resulted in a much smaller relative increase in OD (3.4-fold) than in nutsedge (7.1-fold) over the range of O3 exposures. In these relative terms, An was >3.0-fold more sensitive to O3 in cotton than in nutsedge.
The C4 pattern of gas exchange (large An–gs ratio) was protective of An in nutsedge, despite similar intrinsic sensitivities to O3 in the two species. This provides one potential mechanism for the observed interaction of O3 and competition in productivity of cotton (Table 2). This interplay between O3 impacts on physiological processes and biotic stressors such as weed competition may be an important driver of plant community dynamics (Fuhrer and Booker, 2003; Ziska, 2002).
During daylight hours, canopy conductance, dominated by gs, is usually smaller than the atmospheric conductance, and effectively controls the rates of both water loss and O3 deposition to plants (Grantz et al., 1994; Massman and Grantz, 1995; Massman et al., 1993, 1994; Wesely et al., 1978). The contrasting stomatal sensitivities to O3 observed in these studies suggest that in future environments the C4 advantage in WUE may be reduced by increasing ambient O3 concentrations.
Below-Ground Biomass Productivity
Reduced allocation of photosynthate to sink tissues and the associated inhibition of root development (Andersen, 2003; Andersen et al., 1991; Coleman et al., 1995; Oshima et al., 1979; Barnes et al., 1998; Miller, 1988; Kostka-Rick et al., 1993; Taylor and Ferris, 1996), are common but not universal (Cooley and Manning, 1987; Reiling and Davison, 1992; Miller, 1988; Barnes et al., 1998) impacts of O3 exposure. This response has been well documented in Pima (Grantz and Yang, 1996; Grantz, 2003) and upland (Olszyk et al., 1993; Oshima et al., 1979; Temple, 1990b) cottons. In the current study, root production in cotton grown alone (0:1; Fig. 5A
, circles) declined with increasing exposure of the shoot to O3 (Table 2). At HO3 root biomass was reduced by 85%. This is very similar to the reduction in root biomass observed previously with this cultivar (Grantz and Yang, 1996, 2000).
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Reduced allocation to shoot tissues such as stolons (Wilbourn et al., 1995; Barnes et al., 1998) has also been observed. Below-ground biomass of nutsedge contains both shoot and root vegetative tissues as well as shoot-borne reproductive tubers. Nutsedge grown alone produced considerably greater below-ground biomass than did cotton (Fig. 5A). Allocation to these diverse below-ground organs was not significantly affected across all [O3] in nutsedge (Table 2; Fig. 5A, squares), but exhibited a clear minimum at MO3. A similar pattern of minimal biomass at MO3 with recovery at HO3 was observed previously with this species (Shrestha and Grantz, 2005). The nearly opposing responses of above- and below-ground biomass (cf. Fig. 1B, 5A, open squares) resulted in a substantial reduction in the below-ground to above-ground biomass ratio (R:S) at MO3 (Table 3), with a complete recovery at HO3 (Table 3). This also reproduces the response observed previously (Shrestha and Grantz, 2005).
Total tuber production, as both biomass and number of tubers per plant, increased with increasing nutsedge population (Table 2) after an initial decline from the 1:0 to 1:1 population ratio treatment (not shown). The presence of a single cotton plant reduced the number of tubers relative to nutsedge grown alone at all O3 concentrations. In the study of Morales-Payan et al. (2003), the number of tubers was reduced by 50% when yellow nutsedge plants were grown in competition with tomato, and the size of individual nutsedge tubers was reduced by 40%. Tuber production increased at MO3 but declined from MO3 to HO3, and the present experiment did not resolve a statistically significant relationship between tuber production and O3 exposure (Fig. 5B). This may be a Type II error, as our initial experiments with nutsedge and cotton (Grantz and Shrestha, 2004, 2005) strongly suggested a stimulation of tuber production at all [O3]. As enhanced tuber production would lead to greater distribution and persistence of this weedy species in cultivated and other systems, this possibility remains a serious concern and an important subject of future investigation.
The combined root biomass decreased with increasing O3, averaged across all pots containing both cotton and nutsedge (not shown) and increased, though marginally, with increasing nutsedge density. A single cotton plant grown with a single nutsedge plant (1:1) exhibited combined root biomass (Fig. 5A, triangles) that was substantially greater than that of cotton alone (Fig. 5A, circles) but somewhat less than that of nutsedge alone (Fig. 5A, squares). The two species inhibited each other to a similar extent, since at a population ratio of 1:1, root mass fell midway between 0:1 and 1:0 (Fig. 5A, triangles), particularly at MO3. In tomato, the combined root biomass was similar to that of tomato alone (0:1), but substantially less than that of nutsedge alone (1:0), at all concentrations of O3 (Shrestha and Grantz, 2005). Thus, tomato inhibited nutsedge productivity substantially more than nutsedge inhibited tomato. Whereas cotton biomass (0:1; Fig. 5A, circles) declined monotonically with increasing [O3], the mixed treatment (1:1) exhibited a minimum at MO3. This reflected the influence of nutsedge biomass in the combined total, as nutsedge alone (1:0) exhibited the same pattern. It will be informative in future experiments to evaluate the distribution of below-ground biomass between the two species as a function of [O3], but this was not possible in the present study as the two root systems could not be separated quantitatively when grown together.
Root Respiration
The Rr averaged across all O3 treatments was substantially greater in nutsedge (85.9 nmol g fresh wt min–1) than in cotton (51.7 nmol O2 g fresh wt min–1). The respiratory rates tended to converge with increasing shoot exposure to O3. The Rr of cotton increased by almost 20% with O3 exposure (Fig. 6
, circles), approximately the same magnitude of increase previously observed with this cultivar (Grantz et al., 2003). In contrast, Rr of nutsedge declined by about 20% with increasing O3 (Fig. 6, squares). O3 exposure was found to slightly improve plant water status under transpiring conditions in this cv. of Pima cotton (Grantz and Yang, 1996). If this causes an increase in the relative water content of roots with increasing O3 concentration, then the upward slope in cotton is likely to be slightly underestimated and the decline in nutsedge slightly overestimated. The increase in Rr in cotton contrasts not only with nutsedge but also with tomato (Shrestha and Grantz, 2005), both of which exhibited declines in Rr with increasing O3, and both of which are considerably more tolerant of O3 than this cultivar of Pima cotton. This root respiratory response of Pima cotton is of potential interest as a screening tool for O3 resistance.
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20% increase in Rr (not significant here but significant in the study of Grantz et al., 2003) with declining root growth represents biomass that has been translocated below-ground but that is unavailable for root system development. Together, these responses provide at least a partial mechanism for the pronounced decline in root growth consistently observed in this cotton cultivar following exposure of the shoot to O3. The O3–induced inhibition of root system development indicates a disruption in the coordination between root and shoot functions, that generally serves to balance nutrient and water relations and phytohormone distribution. Reduced hydraulic conductance has been observed in several species exposed to O3 (Lee et al., 1990; Ogata and Maas, 1973; Grantz and Yang, 1996). This could represent a principal effect of O3 exposure, as observed previously (McLaughlin et al., 1982; Grantz et al., 1999). Differential sensitivity to O3 of the altered allocation that leads to these outcomes could be an important determinant of future competitive interactions among plant species. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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These exposure chamber experiments indicate that the productivity of cotton will be reduced in areas where ambient O3 concentrations are substantial and where nutsedge competition is intense. If tropospheric O3 levels increase further, yield of cotton will be further reduced and the competitiveness of cotton with species such as yellow nutsedge will decline. In contrast, Shrestha and Grantz (2005) found that nutsedge was most competitive with tomato at an intermediate elevated concentration of O3, and that nutsedge density did not affect tomato productivity. O3 tended to enhance tuber production in nutsedge in this and the previous studies with tomato. As tuber production is key to nutsedge proliferation and persistence in the environment, this merits further research. The greater sensitivity of cotton than nutsedge, and impacts of O3 on their competitive interactions, suggest that further increases in O3 concentration in the ambient atmosphere may increase requirements for nutsedge control to maintain yields of cotton and other sensitive crop species.
It remains to be seen whether the observed importance of the O3 sensitivity of individual species grown in isolation, as observed in the comparisons of cotton and of tomato with yellow nutsedge, are observed in sexually propagated weedy species. O3–tolerance might be expected to emerge more rapidly in such species than in a vegetatively propagated species such as nutsedge. These characteristics of individual communities and competitive interactions will likely determine the impact of future O3 environments on invasion biology and vegetation management. Mechanistic studies will be required to provide adequate predictive capacity to respond appropriately to these evolving challenges.
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ACKNOWLEDGMENTS |
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Received for publication May 12, 2006.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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  D.A. Grantz and H.-B. Vu O3 Sensitivity in a Potential C4 Bioenergy Crop: Sugarcane in California Crop Sci., March 17, 2009; 49(2): 643 - 650. [Abstract] [Full Text] [PDF]
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