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II, Department of Biology, St. Michael's College, Winooski Park, Colchester, VT 05439 USA
gbyrd{at}smcvt.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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. Correlation between TE and leaf ash was inconsistent, with a negative relationship in the water stress experiment and a positive relationship in the N experiment. The results show differences in TE among switchgrass cultivars and show that SLW is consistently predictive of TE.
Abbreviations: SLW, specific leaf weight • TE, transpiration efficiency • 
, 13C discrimination
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Because photosynthesis and ultimately plant growth are strongly linked to water availability (Boyer, 1981), grass species displaying the C4 photosynthetic pathway show greater potential for maximizing growth under limited water. They are generally more efficient in water use when compared with C3 plants because higher CO2 assimilation rates result from the CO2 concentrating mechanism, which leads to CO2 saturation of rubisco (Hatch, 1987). Although C4 plants operate at great efficiency, water use may vary genetically within an individual C4 species (Kiesselbach, 1926; Peng and Krieg, 1992; Brown and Byrd, 1997a; Ranjith and Meinzer, 1997), making their improvement a distinct possibility.
Intraspecific variation in TE, defined as the weight of dry matter per unit of water transpired, has been demonstrated for several crop species, and there is a basic understanding of how TE is controlled in C3 plants. The stomatal discrimination against 13C () is useful in identifying C3 genotypes with high TE (Hubick and Farquhar, 1989; Wright et al., 1994). Farquhar et al. (1982) predicted a relationship between and TE from the ratio of photosynthetic C gain over water loss, or intrinsic TE, and its partial dependence on the intercellular relative to ambient CO2 concentrations (Ci/Ca), with differences in reflecting integrated differences of Ci/Ca. Thus, measurements of offer an effective method for predicting TE differences among genetic lines of C3 plants. However, measurements of may not be useful in predicting TE in C4 plants, in which differences in are determined by CO2 leakage from leaf bundle sheaths (Farquhar, 1983) or by a greater negative environmental impact on the C3 relative to the C4 cycle (Bowman et al., 1989).
Although values of C4 leaves may be negatively correlated with superior performance (Meinzer et al., 1994) and potentially with TE, other simpler and less expensive measures may better serve as correlatives to TE. Wright (1993) and Wright et al. (1994) found a correlation between TE and specific leaf area in peanut (Arachis hypogaea L.) genotypes. Brown and Byrd (1997a) also found a positive correlation between SLW (the inverse of specific leaf area) and TE among genetic lines of peanut and pearl millet [Pennisetum glaucum (L.) R. Br.]. On a leaf area basis, genotypes with greater SLW often assimilate CO2 more rapidly (Nelson, 1988), which would be expected from greater photosynthetic capacity. If greater CO2 assimilation is not accompanied by greater water loss from any increases in stomatal conductance, increases in TE may be related to enhanced C gain as SLW increases. Gutschick (1991) found a negative correlation between Ci and SLW in two alfalfa (Medicago sativa L.) cultivars, which may link TE to SLW through greater leaf biochemical capacity (or possibly greater mesophyll conductance) at greater values of SLW if unaccompanied by increased values of Ci. Thus SLW may be useful as a predictor of TE.
Mineral concentration in leaves may be influenced by differences in transpiration among plants, if water loss is proportional to mineral uptake. Masle et al. (1992) found that the sum of mineral concentrations and ash were both significantly correlated with 1/TE and in a range of species including the C4 species, sorghum [Sorghum bicolor (L.) Moench] Mayland et al. (1993) also found a negative correlation between ash and TE in crested wheatgrass [Agropyron desertorum (Fischer ex Link) Shultes] from well-watered environments. Brown and Byrd (1997a) also found a negative relationship between ash and TE in peanut and pearl millet, but suggested that SLW and ash were tied more closely than ash and TE. Even if the relationship is an indirect one, higher concentrations of minerals in leaf dry matter were observed in the genotypes that produce less dry matter per unit of water transpired (Masle et al., 1992; Mian et al., 1996; Brown and Byrd, 1997a). Although the theoretical basis for this relationship is not clear, determination of ash and possibly individual minerals in leaves may be a practical approach for screening plants for increased TE.
The objectives of this research were to examine intraspecific differences in TE in switchgrass and determine the relationship between TE and SLW and leaf ash in this C4 forage. Nine cultivars of switchgrass were screened for differences in TE, SLW, and leaf ash. Based on differences in TE and SLW, two switchgrass cultivars, Alamo and Greenville, were grown under conditions of limiting water or N to determine the stability of the relationship between TE and SLW and leaf ash as water and N varied.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Randomly selected seeds of each cultivar were established directly into 5-L closed containers and thinned to one plant per container after germination. To each container, 7.55 kg of 1:1 mix by volume of air-dried sand and soil [soil type classified as a fine montmorillonitic, thermic Vertic Ochraqualf (Mollisol), designated Bernard-Morey (Crout et al., 1965)] were added. To determine field capacity, three pots containing the mixture of soil and sand were overwatered and allowed to drain overnight. To reach field capacity, 1.56 L of water was needed. Each container was fitted with a lid and each seedling was allowed to grow through a small hole (2.5-cm diam.) in the lid. A second hole of the same diameter used for watering and fertilizing was plugged with a rubber stopper and removed when watering was needed.
To initiate the experiment, plants of the nine cultivars were watered with one-half strength Hoagland's solution (Epstein, 1972) to field capacity. Two cultivars, Cave-in-Rock and Pathfinder, were randomly selected for well-watered controls and were maintained near field capacity throughout the experiment. Three containers with no plants (blanks) were used to estimate water loss from the container. Plastic foam was inserted around the plants and in the holes of containers with no plants to reduce evaporation loss. All containers were weighed frequently and the amount of water used corrected by subtracting the small amount lost from these blanks. Stressed plants were allowed to undergo several cycles of drying and most reached approximately one-half field capacity before returning soil conditions to field capacity by watering with one-half strength Hoagland's solution.
The experiment was terminated after 11 wk of plant growth as plants became reproductive. Shoots were severed at the soil surface and roots were removed from the soil–sand mix by gently washing under tap water. For specific leaf weight, 10 leaves of similar age were selected from each plant and leaf area determined by multiplying lamina length by its average width. All plant material (shoots and roots) was dried at 70°C for 48 h. Transpiration efficiency was calculated by dividing total plant dry weight by water transpired. Ash concentration of leaf laminae was determined by combusting weighed samples at 500°C for 6 h in a muffle furnace.
Pots were arranged in a randomized complete-block design with three replications. Every 2 wk pots were rotated within blocks.
Correlative Traits Between Two Switchgrass Cultivars
Two outdoor pot studies were conducted during the summers of 1997 and 1998 in a mowed field on the campus of St. Michael's College in Colchester, VT (44°30'N, 72°51'W; elevation 144 m) using two switchgrass cultivars, Alamo and Greenville, selected because of their significant differences in TE and SLW (Table 1)
. In the summer of 1997, randomly selected seeds from the two cultivars were grown in 5-L containers in equal mixtures of soil, peat, and sand. Soil type was classified as a sandy-skeletal, isotic, frigid Typic Haplorthod (Spodosols) designated as Colton (USDA-NRCS Soil Survey Division, 1998). Containers were closed except for two 2.5-cm2 holes in the lid, one plugged except when adding nutrients and water and one for the shoot fitted with plastic foam to minimize water loss. A permanent rainout shelter was constructed consisting of transparent polyethylene attached to wooden stakes 
1 m above the soil surface. In the 1997 experiment, pots were arranged in a randomized complete block design with eight replications, and each pot contained one plant. A split-plot arrangement was used with two treatments (well-watered and water-stressed) assigned to whole plots and the two cultivars assigned to subplots. Four containers with no plants were designated as control pots. All containers were watered to near field capacity with a complete Hoagland's solution (Epstein, 1972). Plants were grown for 8 wk from early July to late August, allowing for three cycles of water-stress in the water-stressed treatment. Container weights were checked frequently (daily near the end of the experiment), and nonstressed plants were watered to field capacity each time. The amount of water transpired was determined by weighing each container frequently and correcting for the small amount of water lost from control pots.
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For both 1997 water stress and 1998 N experiments, temperature of the air and within the container was logged every 6 h and ranged from 18 to 35°C during the day and from 8 to 22°C at night. Relative humidity during the day ranged from 46 to 100%; no relative humidity data were collected at night. For each replicate of the 1997 experiment, SLW (g m-2 leaf) was determined by measuring the lamina area and dry weight on two young, fully mature leaves each week during the last 5 wk of the experiment. For the 1998 experiment, SLW was determined on 10 randomly selected leaves for each replicate at the completion of the experiment. Plants were separated into leaves, leaf sheaths, stems, and roots and then oven-dried upon termination of each experiment. Transpiration efficiency was expressed as grams of dry plant weight produced per kilogram of water transpired by the plant. In both experiments, leaf ash concentration of leaf laminae used to determine SLW was determined after their combustion at 500°C for 6 h. Total leaf area was estimated by dividing leaf weight (leaf lamina plus leaf sheaths) by the average values of SLW.
Data Analysis
Data were subjected to analysis of variance and linear regression using the statistical package SYSTAT, and pairwise differences were compared with Fisher's protected LSD values. Correlation coefficients were calculated with transpiration efficiency as the dependent variable and leaf ash and specific leaf weight as the independent variables.
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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. Cultivars showed no significant variation in leaf ash concentration when expressed on a dry matter basis (69–84 g kg-1), but on a leaf area basis, leaf ash among cultivars was significantly different (Table 1). There was no correlation between TE and leaf ash on a dry weight or leaf area basis. There was also no significant correlation between SLW and leaf ash on a dry weight or leaf area basis (data not shown).
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For the 1997 water stress experiment, plant dry weights (shoots and roots) ranged from 4.5 to 20.5 g, and mean weights of water-stressed plants were 87 to 89% of the mean weight of well-watered plants. Water stress had no significant effect on any of the measured variables. Alamo transpired more water than Greenville and also produced more biomass and leaf area when grown under both well-watered and water-stressed conditions (Table 2) . Alamo exhibited a greater TE and SLW than Greenville; however, there was no treatment effect on TE or SLW for each cultivar. A greater range of values of TE and SLW was observed in this experiment than in the initial and N experiments. When compared with Greenville, Alamo displayed a lower leaf ash concentration (g kg-1) only when grown under water stress conditions. However, when leaf ash was based on area, no significant differences were found between cultivars or between treatment.
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With decreasing N in solution, TE also decreased in both cultivars. When compared with Greenville, Alamo exhibited higher values of TE when each was grown at 10 mM N and at 1.0 mM N, but not at 0.3 mM N. The two cultivars had similar SLW values across treatments, with the exception of the higher SLW for Alamo when grown under 10 mM N.
For the water stress experiment, TE was positively correlated with SLW within treatments and for the combined analysis of the two cultivars (Table 3) . A negative relationship was found between TE and leaf ash on a weight basis when switchgrass was subjected to water stress and for the combined data for water-stress and well-watered plants. The relationship between TE and leaf ash on a leaf area basis was significant only under water stress conditions.
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for the combined analysis (Table 3) and the relationship tended to be positively related even within each treatment for the two cultivars. When the relationship between TE and SLW was examined for each cultivar for the combined treatments, Greenville showed a close relationship between the two traits; however, the correlation between TE and SLW was not significant for Alamo. In contrast to the water-stress experiment, the relationship between TE and leaf ash tended to be positive rather than negative in the N experiment. Furthermore, a significant positive correlation was found between TE and leaf ash only when ash concentration was based on leaf area rather than leaf weight. This area-based relationship was significant for the combined analysis of each cultivar and when data from all treatments and cultivars were combined
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As shown in Fig. 1, TE was positively related with SLW in each of the three experiments and the regressions were similar. For each cultivar, there was more than a twofold range in TE, from 4 to 10 g kg-1 for Alamo and from 3.5 to 8.9 g kg-1 for Greenville. The range in SLW for each cultivar was also substantial, with a 1.8- and 1.7-fold change in SLW for Alamo and Greenville, respectively. When data were pooled across the experiments, the correlation between TE and SLW improved and formed the overall linear equation: TE = 0.20SLW - 2.366
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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The relationship between TE and SLW in switchgrass was consistent across all experiments, and the correlations are in general agreement with several studies in the literature for peanut (Wright, 1993; Wright et al., 1994; Brown and Byrd, 1997a) and for pearl millet (Brown and Byrd, 1997a). Virgona et al. (1990) also found that leaf C/leaf area was related to TE in sunflower (Helianthus annuus L.). In contrast, Ismail and Hall (1993) reported no relationship between SLW and TE in cowpea [Vigna unguiculata (L.) Walp.] even though TE was negatively correlated with , suggesting that for some species other factors are more closely related to TE than is SLW.
The relationship between TE and SLW may be based on increased photosynthetic capacity, since SLW has been positively correlated with photosynthesis per unit of leaf area for genotypes of many species (Nelson, 1988). Photosynthesis has been positively related to N, protein, and photosynthetic enzyme levels when all parameters are leaf area based (Bowes et al., 1972; Hesketh et al., 1981). Most increases in SLW are accounted for by increases in cell wall components, nonstructural carbohydrates, and protein (data from van Arendonk and Poorter, 1994, in Brown and Byrd, 1997b). Any increases in leaf components that would lead to improved performance could also enhance TE. Significant positive correlations (
for the water stress experiment and
for the N experiment; data not shown) were found between leaf soluble protein and SLW, implying that increases in leaf protein contribute to increases in SLW and therefore potentially to TE. Bowman et al. (1989) suggested that changes in plant water status were linked to decreases in the activity of C3 photosynthetic enzymes relative to C4 enzyme activity. Whether the connection between leaf soluble protein and TE can be explained by altered coordination between the two photosynthetic cell types in switchgrass is not known since enzyme activities or levels were not determined.
The environment has a major impact on SLW and whether this trait can remain predictive of TE across a wide range of growth conditions is unclear. For example, nutrition, particularly N supply, can affect SLW (Dijkstra, 1989). When Greenville and Alamo were grown in different solutions of N, their differences in TE and SLW were amplified at higher N, but no differences were observed between the two cultivars at the lowest N (Table 2, N experiment). The similar values of TE and SLW between the two cultivars at the lowest N level suggest that as factors other than water limit growth SLW may no longer be predictive of TE. At the highest N (10 mM) Alamo showed a greater improvement in TE, which also coincided with increased SLW. If TE in switchgrass is improved by greater photosynthetic performance, it may not be expressed under very low N.
The most important criterion for selecting genotypes with greater TE would appear to be plant performance during water stress. However, no differences in TE were found between well-watered and water-stressed treatments for either Alamo or Greenville. Conditions for this experiment were cool and humid, and both cultivars showed a wide range in both TE and SLW. It is not clear whether the range was a result of greater intracultivar variation for this experiment or experimental conditions. Cooler conditions tend to increase SLW as nonstructural carbohydrates accumulate, and more humid conditions would lead to greater TE associated with lower water vapor pressure differences. Brown and Byrd (1997a) found lower correlations between TE and SLW for water-stressed peanut and pearl millet, which indicates that water stress may actually hinder the observed relationship. Our observation that water treatment had no effect on SLW or TE suggests that SLW may be predictive of TE under conditions where water is not limiting or during periods of mild water stress.
Plant improvement in water use may result from drought-induced changes in shoot and root biomass and leaf area. With greater genotypic reduction in water use compared with biomass, Ismail and Hall (1993) reported increases in TE for cowpea. In that experiment, leaf area in cowpea was also substantially reduced by drought, while transpiration efficiency and were only moderately linked with the root/shoot ratio even though genotypes differed in this ratio (Ismail and Hall, 1993). In the two switchgrass cultivars examined under different water treatments (Table 2), leaf area and water use were unaffected by water stress, although total biomass in Greenville was reduced compared with well-watered plants. Reduced leaf area relative to total dry matter could lead to greater TE only if increases in biomass can be maintained with lower water use.
Improving TE could show potential value not only in water-limited environments but also when water is optimal (Johnson, 1994), yet the difficulty has often been determining a useful and simple method of predicting TE in field environments. From the data reported herein and from the work of Wright (1993), Wright et al. (1994), and Brown and Byrd (1997a), it can be concluded that SLW shows promise as a useful and inexpensive predictor of TE. Although variability in the relationship is likely under different conditions of growth and for different species, selecting for greater SLW from a field population could lead to potential increases in TE. Since values of SLW in this report were determined on individual plants, scaling to field situations is still unclear and more research is needed. Gutschick (1988) maintained that any photosynthetic gain from increases in SLW would be smaller for an entire canopy because leaves at lower levels receive less light, and furthermore, that greater SLW would eventually reduce leaf area per unit of plant biomass. Whether there is an optimum SLW for plant canopies above which plant yield is reduced will need to be carefully considered in using SLW measurements to select for improvements in TE.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Received for publication September 2, 1999.
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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