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a USDA-ARS, Box 646402, Washington State Univ., Pullman, WA 99164 USA
b Institute of Soil and Water Conservation, Academia Sinica, Yangling, Shaanxi 712100, China
rcjohnson{at}wsu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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) has been correlated with the ratio of dry matter production to transpiration (water-use efficiency, WUE) in C3 plants and is potentially useful for breeding crops with improved WUE. Therefore, an assessment of the selection response of and its relationship to plant water status and forage production is needed. Divergent selection for high (low WUE expected) and low (high WUE expected) was completed for two cycles from a `Kentucky 31' tall fescue base (Festuca arundinacea Schreb.) population (Co ). Water relations, forage production, and were evaluated in Co and the selected populations in irrigated and dryland field environments in 1995 and 1996. Average realized heritability for was 0.49, suggesting that could be successfully manipulated in a breeding program. In 1995, leaf pressure potential (turgor) was higher in the populations selected for low , but in 1996, no differences in water relations measurements were observed. High- populations always had lower forage production than observed in Co, but the low- populations never produced more than the Co population. In greenhouse-grown plants, high- populations had higher internal substomatal [CO2] than Co, linking with mechanisms that cause lower WUE. However, the internal [CO2] of the low- populations and Co did not differ, suggesting that selection for low may not have increased WUE as expected. The results show that is a heritable trait in tall fescue, but an absence of increased production in populations selected for low may limit its utility in tall fescue breeding programs.
Abbreviations: Aa, photosynthetic CO2 exchange rate per leaf area • Ad, photosynthetic CO2 exchange rate per g leaf • gs, stomatal conductance to CO2 • PEG, polyethylene glycol • SLA, specific leaf area • WUE, water-use efficiency • , carbon isotope discrimination
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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may be increased by improving WUE, especially under drought (Briggs and Shantz, 1913; Ehleringer et al., 1993). Numerous studies of diverse C3 species have shown a negative correlation between WUE and , including Farquhar and Richards (1984), Hubick et al. (1986), Read et al. (1991), Johnson and Bassett (1991) and Johnson and Tieszen (1994). Thus, carbon isotope discrimination () appears to be a way to measure indirectly integrated WUE in plants with C3 photosynthetic metabolism.
The correlation between and WUE results from the relationship between and the ratio of internal substomatal [CO2] to ambient [CO2] integrated over time (Farquhar et al., 1989). A lower ratio of internal [CO2] to ambient [CO2] is associated with higher WUE. This link between , WUE, and the ratio of internal to ambient [CO2] has also been established in a number of species including tall fescue (Johnson and Bassett, 1991; Johnson, 1993).
Heritability estimates for vary considerably. Broad-sense heritability was estimated as 0.53 for peanut (Arachis hypogaea L.) by Hubick et al. (1988) and 0.61 for wheat (Triticum aestivum L.) by Ehdaie and Waines (1994). Menéndez and Hall (1996) reported intermediate values for broad-sense heritability (0.33–0.47) in cowpea [Vigna unguiculata (L.) Walp.], but realized heritability was low, ranging from 0.06 to 0.19. Realized heritability for bean (Phaseolus vulgaris L.) was also low, ranging from 0 to 0.12 under irrigated and rainfed environments (White, 1993). Read et al. (1993) reported narrow-sense heritability for that was greater than 0.75 for crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes], and narrow-sense heritability of in three wheatgrass species ranged from 0.47 to 0.63 (Frank et al., 1997).
A clearer understanding of the selection response to and its relation to agronomic and physiological factors is needed to assess the potential for using in breeding programs. Apparently, selection response has not been reported for in tall fescue. Our objectives were to (i) determine if could be altered through selection, and (ii) relate changes in to water relations, forage production, and gas exchange factors in tall fescue.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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Selection and Crossing
Seeds of PI 561430 were germinated in saturated vermiculite in a growth chamber at 25°C. Seedlings were transplanted into cardboard bands (50- by 50- by 80-mm height) containing a steam sterilized soil mix (2/3 Spofford soil, a fine-silty, mixed, mesic Natrixeroll, and 1/3 peat moss). The plants, contained within the cardboard plant bands and metal flats, were initially established and maintained in a greenhouse for about one month and then placed in a lath house for about 10 d prior to field transplanting.
Plant selection was conducted at the Central Ferry Research Farm located approximately 50 km southwest of Pullman, WA, in the Snake River Canyon. The site was cultivated prior to transplanting, and 50 kg N ha-1 and 15 kg S ha-1 were applied. The plot area was cultivated again to incorporate the fertilizer and rolled to smooth the soil surface prior to transplanting. Field transplanting was completed on 26 April 1991 in a selection block consisting of six rows spaced 1.5 m apart with each row consisting of 16 plants spaced 0.6 m apart. Plants were irrigated weekly with sprinklers for one month after transplanting, but not thereafter.
For determinations in the selection block, 56 individual plants were sampled from the interior four rows and the interior 14 plants of each row. Tissue from three or four recently fully emerged leaf blades were obtained on 26 June and 30 July 1991. Samples for each plant were combined, dried at 70°C to constant weight, and ground. Analysis for isotopic carbon composition (
13C)—the ratio 13C/12C relative to the PeeDee belemnite standard—was completed on each sample with an isotope ratioing mass spectrometer as described by Read et al. (1991). Values of 13C were converted to on the basis of a 13C value for air of -8 per mil (
) (Farquhar et al., 1989). Average for the two sampling dates and the 56 plants ranged from 19.2 to 16.9, with a mean of 18.0 and a variance of 0.35. Plants with the eight highest and eight lowest average values were selected as parents for the high- and low- populations.
On 13 Dec. 1991, plant material from the 16 selections was removed from the field. Six ramets from each selected plant were established in pots. The resulting 48 high- and 48 low- plants were placed in separate greenhouses, randomized, and allowed to intercross. Greenhouse fans provided air movement to facilitate crossing. Seed from each plant was harvested separately, and equal numbers of seeds from each plant were combined to provide seed of the first selection cycle (C1).
In 1993, selection blocks consisting of both the C1 high- and low- populations were established at Central Ferry as described above. Samples for leaf were collected on 10 July and 25 August. As before, the eight plants with the highest and lowest mean values were selected from their respective populations of 56. For the high- selection block, values ranged from 15.9 to 19.1 with a mean of 17.7 and a variance of 0.48. Values of in the low- selection block ranged from 15.6 to 18.1, with a mean of 16.7 and a variance of 0.37. Plant materials of high- and low- selections were removed from the field and crossed in greenhouses as described above to provide seed of the second selection cycle (C2).
Field Evaluation
In April 1994, plants from the C0 population, the C1 and C2 high- and low- populations, along with the check cultivars Alta = (PI 578712) and Fawn = (PI 578715), were transplanted to the field using the methods described above. The experiment was established at the Central Ferry Research Station in dryland and irrigated environments that were separated by about 50 m. For each environment, plots were randomized in complete blocks with six replications. Each plot consisted of a row of 10 plants spaced 0.6 m apart. Rows were spaced 1.5 m apart. For the irrigated plots, water was applied with sprinklers each week starting in May. The average irrigation was 27 mm applied over a 4-h period.
Leaf samples for water, solute, and pressure potentials were taken biweekly starting from 30 May 1995 until 20 Sep. 1995. In 1996, sampling started on 28 May and ended on 18 September. Each year and at each of the six sampling times, individually calibrated thermocouple psychrometers with an attached leaf punch (J.R.D. Merrill Specialty Equipment, Logan, UT1) were used to determine water, solute, and pressure potential of leaves. One 0.24-cm2 sample of leaf tissue was removed from each plant and six plants were sampled from each plot. Three psychrometers were used per plot with two plants sampled with each psychrometer. Samples were taken from recently fully expanded leaf blades between 1030 and 1130 h solar time, immediately sealed inside the psychrometer chamber, and transported to the laboratory. Wet bulb depression was measured to determine water potential (Johnson et al., 1986). Solute potential was determined by wet bulb depression after freezing and thawing the tissue in the psychrometers. Pressure potential (turgor) was determined as the difference between water and solute potential. Data from the two psychrometers, representing six plants from each plot, were combined to obtain average plot values.
Recently fully emerged leaf blades were sampled from each plant within a plot, including those taken for water relations data, and combined to determine on a per-plot basis. Samples for were taken on three dates in 1995 (30 May, 9 August, and 18 September) and in 1996 (11 June, 29 July, and 16 September).
Forage production was determined twice in 1995 (9 August and 21 September) and three times in 1996 (24 June, 5 August, and 23 September). For 1995 the first cutting was after anthesis and in 1996 the first cutting was during anthesis. Plants were gathered at the base and cut about 6 cm above the ground as described by Johnson and Bassett (1991). Plant material was placed in large paper bags and dried to constant weight in a greenhouse. Yields from each harvest were summed to obtain total forage production per plot each year.
Forage production data from the irrigated and dryland environments were analyzed together each year as described by Snedecor and Cochran (1967). Water relations and data were also analyzed for irrigated and dryland environments together, with sampling time as a repeated measure. The analyses were completed with general linear models (SAS Institute, 1985). To ensure that pooling data from the separate irrigated and dryland environments was appropriate, F-tests for homogeneity of error at P < 0.05 were conducted (Snedecor and Cochran, 1967). The only factor that showed a significant difference in residual error variance between environments was solute potential in 1995, but this had no important effect on the analysis. The error term for comparisons involving populations was the entry x block variance pooled across environments. The residual error was used as the denominator in the F-test for comparisons involving sampling date. When the appropriate F-value was significant at P < 0.05, mean differences were declared significant at P < 0.05 by the LSD.
Realized heritabilities (Falconer, 1960) and associated standard errors for high- and low- populations, were calculated as outlined by McLean and Watson (1992). For each selection cycle, selection response was obtained from 1995 and 1996 field data and selection differential from the 1991 and 1993 field selection blocks. Selection response was the difference between the mean of the polycross progeny and the mean of the parent population; selection differential was the difference between the mean of the individuals selected from the parent population and the mean of the parent population. The selection response was divided by the selection differential to obtain realized heritabilities.
Gas Exchange
On 1 March 1996, seeds from C0 and the C1 and C2 high- and low- populations were germinated in saturated vermiculite and transplanted to plastic cones (33-mm diam at the top tapering to 25 mm at the bottom) containing Grower's Mix A soil mix (Black Gold Inc., Hubbard, OR). Plants were maintained in the greenhouse and periodically watered and fertilized with 0.25-strength Hoagland's solution. The plants, one per plastic cone, were randomized in 10 complete blocks with selection populations and polyethylene glycol (PEG, 8000 MW) stress as experimental factors. Solutions of 10% (w/v) PEG were prepared in 0.25-strength Hoagland's solution and applied to designated pots daily until solution drained from the base of the cones. For well-watered treatments the same procedure was followed using 0.25-strength Hoagland's solution without PEG. Water potential of the 10% PEG solution was -0.38 MPa. This was determined by dipping filter paper discs in the PEG solution and measuring wet-bulb depression on six individually calibrated thermocouple psychrometers.
Leaf photosynthetic characteristics were measured starting on 24 June 1996 by means of a steady-state gas-exchange system described by Johnson et al. (1987). For those plants receiving the PEG treatment, stress was initiated on a different randomized block of plants each day and continued for 14 d before gas exchange measurements were made. Each day, 10 plants representing a single block were moved from the greenhouse to the laboratory, and measurements were made on a single, recently fully emerged leaf from each plant. Standard measurement conditions were 1800 µmol m-2 s-1 PPFD, 210 mL O2 L-1, 25°C, and an ambient [CO2] of 350 µL L-1. After steady-state conditions were achieved on a given leaf blade, which usually took about 15 min, the blade area inside the reaction chamber was removed, two 0.24-cm2 samples were taken with two psychrometers, and total blade area determined. Water relations were determined as described above. Three additional leaves at the same physiological stage were also removed from the plant, leaf area determined, and combined in an envelope with the leaf blade on which gas exchange was measured. The leaf material was dried to constant weight at 70°C, weighed, and SLA (specific leaf area, cm2 g-1 leaf) determined. The material was then ground for analysis as described above.
Transpiration, photosynthetic CO2 exchange rate per leaf area (Aa), stomatal conductance to CO2 (gs), and internal substomatal [CO2] were calculated on a per-unit leaf area basis according to Von Caemmerer and Farquhar (1981). The CO2 exchange rate per gram leaf was also determined. The ratio Aa/gs, directly related to Ci/Ca (Meinzer et al., 1990), was calculated as a measurement of "inherent" gas exchange WUE. Data were analyzed as a randomized complete block design by SAS general linear models (SAS Institute, 1985). Differences between water treatments and populations were declared significant at P < 0.05. When the F-value among populations was significant, mean differences were declared significant at P < 0.05 by the LSD.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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, which was related to a change in the ranking of the cultivar Fawn from the first to later sampling dates. This did not affect sampling date differences and the rankings of other germplasm entries were consistent among dates.
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declined with sampling date in both years (Table 1). In 1995, there were differences among sampling dates for water relations (Table 2), but no clear seasonal trend toward decreased or increased water, solute, or pressure potential with sampling dates. Each sample was collected near midday and under sunny to mostly sunny conditions, but differences in temperature, wind, humidity, and proximity to rainfall events may explain some of the variation associated with sampling dates. The precipitation total for 1995 was 326 mm. Rainfall amounts during the sampling period were 0 mm in May, 58 mm in June, 14 mm in July, 7 mm in August, and 14 mm in September. Water and pressure potential tended to decrease through 18 July, even in the irrigated plots (Table 2). The 7 mm of rainfall on 7 August, which accounted for all the rainfall that month, provided some drought relief and led to an increase in dryland water and pressure potential on 8 August. In 1995, total forage production in dryland plots (130 g plant-1) was similar to that in irrigated plots (160 g plant-1). But symptoms of drought, including leaf rolling and wilting, were observed in the dryland environment, especially from 10 July to 6 August 1995, when there was no rainfall.
Similar to 1995, the water relations data for 1996 showed sampling date variation without clear seasonal trends (Table 3). Total precipitation for 1996 was 438 mm, including 32 mm in May, but only 4 mm in June, 3 mm in July, 0 mm in August, and 18 mm in September. Water potentials averaged over the season were 31% lower in the dryland than in the irrigated environment, and drought stress symptoms (leaf rolling and wilting) were apparent in the dryland environment. Total forage production in the dryland environment was 77% (820 g plant-1) less then that of the irrigated environment.
Field Evaluation
For both 1995 and 1996, significant differences (P < 0.05) in were observed among entries. For both and water relations measurements, the entry x environment interaction and the entry x environment x sampling date interaction were not significant either year. Therefore, comparisons among entries were based on average values across sampling dates and environments. In both 1995 and 1996, leaf of the selected C1 and C2 populations differed from Co in both directions (Fig. 1)
. Realized heritabilities after Cycle 1 were
in the high direction and
in the low direction. From Cycle 1 to Cycle 2, realized heritabilities were
and
in the high and low directions, respectively. Except for the second cycle, high- value, realized heritabilities were sufficiently high to suggest that was a heritable trait that could be manipulated in a breeding program.
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population differed from C0. Solute potential of the C1 high- population also differed from Co. There were no differences, however, among high- and low- populations for solute potential. For pressure potential, the difference between C0 and populations at C1 and C2 did not differ, but low- populations had consistently higher pressure potentials than high- populations (Fig. 2). As determined by mean values of cultivars and selection populations, significant correlations were observed between pressure potential and dry matter production
and between pressure potential and
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, were not consistent across years.
Compared to C0, high- populations (low WUE expected) had reduced forage production in both 1995 and 1996 (Fig. 3)
. For low- (high WUE expected) populations, however, forage production did not differ from C0 in 1995, but in 1996, forage production was lower for the C2 low- population than for C0 (Fig. 3). Thus, selection for high always decreased forage production but selection for low had either no effect, or led to decreased forage production. On the basis of mean values of cultivars and selection populations in 1995, the correlation between and forage production was negative
. Because forage production was low for the low- population at C2 in 1996, dry matter production and were not correlated. The relatively low dry matter production of Fawn in 1995 was associated with its susceptibility to stem rust (Puccinia graminis Pers.: Pers subsp. graminicola Z. Urban), which was apparent in dryland and irrigated environments (Fig. 3). Only light to moderate stem rust infection was observed on the other germplasms and in 1996.
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from 21.0 to 20.7. As expected, PEG stress caused a reduction in Aa, Ad, and in gs (data not shown).
Greenhouse-grown plants showed the same response to divergent selection for (Fig. 4)
as observed for field-grown plants (Fig. 1). Thus, the selection response for was consistent in all environments tested. Internal leaf [CO2] and Aa/gs in the low- populations did not differ from corresponding values in C0 (Fig. 4). At C2, however, internal leaf [CO2] was higher and Aa/gs was lower for the populations selected for high than for C0. The difference between high- and low- populations for internal leaf [CO2] was significant at C2 but not at C1. But there was a divergence between populations selected in the high- and low- directions. The difference between selection populations at C1 and C2 was significant for Aa/gs (Fig. 4). Overall, was positively correlated with internal leaf
and negatively correlated with
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and either Aa or gs (Fig. 4 and 5)
, and correlations between and Aa or gs were not significant. This contrasts with results reported by Johnson (1993) under similar conditions, when positive correlations were found between and Aa, and between and gs among tall fescue plants with high and low . Selecting for high did result in higher photosynthetic assimilation rate when expressed as Ad, and in higher SLA (Fig. 6)
. For low- populations, selection did not result in significant changes in Ad and SLA compared to values for C0, but high- and low- populations did differ for Ad (Fig. 6). There was also a difference between high- and low- populations for SLA at C2. Overall, values of were positively correlated with
and negatively correlated with SLA
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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appeared to be a heritable trait that could be manipulated in a traditional breeding program by mass selection. This was apparent in the selection response in the dryland and irrigated field environments and in the greenhouse environment (Fig. 1 and 4). Further cycles are needed to determine if continued selection will result in continued divergence in .
Work with tall fescue (Johnson and Bassett, 1991; Johnson, 1993) and other C3 crop species including wheat (Farquhar and Richards, 1984), peanut (Hubick et al., 1986), and crested wheatgrass (Read et al., 1991, 1993), has shown that is inversely related to WUE, defined as unit dry matter produced per unit transpiration. Johnson (1993) linked Aa/gs (gas exchange WUE) and internal [CO2] to measurements of in tall fescue accessions. Thus, the evidence indicates that selection for low should lead to higher production of dry matter per unit transpiration, higher Aa/gs, and lower internal [CO2]. And in the current study, Aa/gs was negatively correlated with as expected. Despite this, the expected increase in Aa/gs and reduction in internal [CO2] for low- selections was not observed in the greenhouse grown plants, even though was clearly reduced in low- populations (Fig. 4). The explanation for this could relate to at least two factors.
First, the gas exchange data were "instantaneous" measurements of Aa/gs and internal [CO2], but measurements are integrated estimates of Aa/gs, which in the long term are usually reflected in changes in dry matter per unit transpiration. Thus, instantaneous gas exchange measurements should not be expected to exactly reflect . However, gas exchange was optimized under carefully controlled light and temperature conditions to obviate the potential variability in instantaneous measurements. In a preliminary gas exchange experiment without PEG stress, overall results for Aa/gs and internal [CO2] in selection populations were the same as shown in Fig. 4, so results for internal [CO2] were consistent under the conditions of this experiment.
Second, selection for low may have changed fractionation in a way unrelated to internal substomatal [CO2]. The cause of isotope fractionation in plants involves differences in diffusion and carboxylation of 13CO2 compared to 12CO2. Most fractionation in plants with C3 metabolism is by ribulose1,5-bisphosphate carboxylase-oxygenase (Rubisco), but phosphoenol pyruvate (PEP) carboxylase, respiration (light and dark), translocation, and diffusion of CO2 to carboxylation sites are also involved (Farquhar et al., 1989). It is generally felt that fractionation constants for the numerous factors involved are relatively constant so that changes in are the result of changes in the ratio of internal to ambient [CO2], and therefore related to WUE (Farquhar and Lloyd, 1993). However, Vitousek et al. (1990) found 13C increased from -30 to -24 (i.e., decreasing ) in Metrosideros polymorpha Gaudich with increases in elevation and leaf thickness (deceased SLA). This variation in 13C was not related to changes in the internal substomatal [CO2] to ambient [CO2] ratio. They hypothesized that a decrease in internal conductance to CO2 diffusion to carboxylation sites caused 13C to become less negative. It is possible that selection pressure could lead to relatively small changes in one or more of the fractionation constants. If this occurred, significant changes in could be unrelated to changes in internal to ambient [CO2], and therefore, unrelated to WUE.
The correlation between SLA and Aa/gs in this study suggests that SLA may be a predictor of WUE in tall fescue as it was in peanut (Rao and Wright, 1994). However, a relationship between SLA and is not always observed and appears to depend on species and environment (Ismail and Hall, 1993; White et al., 1990; Brown and Byrd, 1996). This is certainly relevant with tall fescue in that SLA data was collected only on greenhouse grown plants. Nevertheless, the link between SLA and Aa/gs observed in this study does suggest that SLA should be examined further as a predictor of WUE in tall fescue and other grasses.
The relationship between and plant productivity is variable, with both positive and negative correlations observed (Condon and Richards, 1993; Johnson and Bassett, 1991). Inconsistencies in the association between production and in grain and oil seed crops were observed over different water regimes and years (Matus et al., 1996). The relationship between and production in grain crops is complicated by correlations of with harvest index (Johnson et al., 1995; Menéndez and Hall, 1996) and plant development (Menéndez and Hall, 1995). Since grain is not the product of interest in perennial forage and turf crops such as tall fescue, some of this complexity is circumvented. But difficulties still remain. Read et al. (1993) found that forage yield and were uncorrelated and suggested the two traits were under separate genetic control. In the current study, selection for high was detrimental to forage production. The lower Aa/gs observed in high- populations is strong evidence that WUE was also lower in high- populations. However, there was no corresponding increase in forage production or clearly higher Aa/gs in populations selected for low . The low resulting from selection was not linked to physiological factors that are known to increase WUE. Thus, when is used to select for improved WUE, whole plant or gas exchange WUE should be examined in advanced generations to ensure that changes in lead to the expected changes in WUE.
Although appeared heritable, selection for may have unforseen consequences associated with the complex interactions among photosynthetic factors, water relations, and dry matter productivity. A clearer understanding of the interaction of complex gene systems controlling these factors, along with environmental interactions, may be necessary before breeders can effectively manipulate to improve tall fescue productivity.
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Received for publication June 5, 1998.
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13C in Hawaiian Metrosideros polymorpha: a case of internal resistance?. Oecologia 1990;84:362-370.[ISI]This article has been cited by other articles:
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  R. C. Johnson, A. A. Hopkins, and M. A. Evans Carbon Isotope Discrimination, Selection Response, and Forage Production of Tall Fescue in Contrasting Environments Crop Sci., May 1, 2008; 48(3): 1048 - 1054. [Abstract] [Full Text] [PDF]
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J. S. Ebdon and K. L. Kopp Relationships between Water Use Efficiency, Carbon Isotope Discrimination, and Turf Performance in Genotypes of Kentucky Bluegrass during Drought Crop Sci., September 1, 2004; 44(5): 1754 - 1762. [Abstract] [Full Text] [PDF]
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R. C. Johnson, V. L. Bradley, and M. A. Evans Inflorescence Sampling Improves Effective Population Size of Grasses Crop Sci., July 1, 2004; 44(4): 1450 - 1455. [Abstract] [Full Text] [PDF]
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R. C. Johnson, V. L. Bradley, and M. A. Evans Effective Population Size during Grass Germplasm Seed Regeneration Crop Sci., January 1, 2002; 42(1): 286 - 290. [Abstract] [Full Text] [PDF]
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