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CROP PHYSIOLOGY & METABOLISM

Heterosis for Leaf CO₂ Exchange Rate during the Grain-Filling Period in Maize

Dep. of Plant Agriculture, Crop Science Building, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1

^* Corresponding author (mtollena{at}uoguelph.ca)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Heterosis for grain yield in maize (Zea mays L.) manifests itself through its effects on the components of grain yield, dry matter accumulation at maturity, harvest index, and its effects on physiological processes underlying these components, such as leaf CO₂ exchange rate (CER). The objectives of this study were (i) to quantify the pattern of leaf CER throughout the grain-filling period in maize hybrids and their parental inbred lines, and (ii) to determine the mode of inheritance of leaf CER during the grain-filling period. Studies were performed with 12 F₁ hybrids and their seven inbred parents grown hydroponically in the field at the Cambridge Research Station, ON, Canada, in 2002. Data were recorded on leaf CER from silking to maturity, and grain yield, aboveground dry matter, and root dry matter at maturity. Mean leaf CER of hybrids was not different from that of their parental inbred lines at silking. However, significant differences became apparent 2 wk after silking and became increasingly larger as plants advanced toward maturity. In general, leaf CER differed among inbred lines but did not differ among hybrids. Combining ability analysis showed that predominantly additive genetic effects influence the expression of leaf CER late in the season. Finally, the maintenance of leaf CER throughout a plant's life cycle, rather than potential leaf CER, is positively associated with dry matter accumulation during the grain-filling period and grain yield.

Abbreviations: CER, CO₂ exchange rate • GCA, general combining ability • LAI, leaf area index • PPFD, photosynthetic photon flux density • SCA, specific combining ability

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
HETEROSIS FOR GRAIN YIELD in maize is attributable to heterosis for the physiological processes underlying grain yield. Results of studies with a set of 12 maize hybrids and their seven parental inbred lines have shown that heterosis for grain yield could be attributed to heterosis for the two components of grain yield, total above-ground dry matter at maturity, and the proportion of dry matter that was allocated to the grain (Tollenaar et al., 2004). Heterosis for dry matter accumulation was attributable, in part, to heterosis for light interception, that is, greater maximum leaf area index (LAI) and increased stay green of hybrids (Tollenaar et al., 2004). These results raise the question whether differences in leaf photosynthesis also have contributed to the observed heterosis for dry matter accumulation.

Seasonal crop dry matter accumulation is the integration across the growing season of net leaf CER across the crop leaf canopy. Maize genotypes have been shown to differ for leaf CER (Fousova and Avratovuková, 1967; Duncan and Hesketh, 1968; Heichel and Musgrave, 1969; Crosbie et al., 1977). Attempts to relate grain yield with leaf CER, however, have been unsuccessful. The correlation between grain yield and leaf CER was not significant in 64 maize inbred lines randomly derived from Iowa Stiff Stalk Synthetic (Crosbie et al., 1978b). A significant 39% increase in grain yield after seven cycles of recurrent selection for grain yield was not associated with a change in leaf CER in the improved hybrid (Facorede and Mock, 1978). Also, an 8% increase in leaf CER before silking and an 6.5% increase in leaf CER during the grain filling following five cycles of recurrent selection did not affect grain yield (Crosbie et al., 1981; Crosbie and Pearce, 1982). Similar results have been reported for soybean [Glycine max (L.) Merr.] and pea (Pisum sativum L.) (cf. Nelson, 1988). The failure to find a strong relationship between leaf photosynthesis and grain yield has led to the conclusion that leaf CER is not an important factor limiting grain yield and dry matter accumulation (e.g., Crosbie et al., 1978b; Facorede and Mock, 1978; Nelson, 1988). Instantaneous photosynthesis measurement on a single leaf under optimal conditions may only indicate maximum or potential photosynthesis and may not be a reflection of net photosynthesis throughout an entire growing season (Zelitch, 1982). Indeed, maize leaf CER varies with leaf age and leaf position (Thiagarajah et al., 1981) and maize hybrids with a similar potential leaf CER have been shown to differ in their leaf-CER response to low night temperature (Tollenaar et al., 2000; Ying et al., 2000, 2002). Previous work on the inheritance of leaf CER in maize has shown (i) that leaf CER is influenced mainly by additive genetic effects and (ii) that there is evidence for significant heterosis for leaf CER (Crosbie et al., 1978a; Albergoni et al., 1983).

Like with most other experimental procedures, reported results of leaf CER measurements may be, in part, an artifact of the employed methodologies. Leaf CER measurement technology has changed substantially during the past three decades and caution should be exercised in comparing results of studies that have used widely different CER measurement methodologies. For instance, in the studies reported by Crosbie (e.g., Crosbie et al., 1981), leaves of field or indoor grown plants were excised, placed in moist paper towels, and transported to the laboratory. In the laboratory, leaves were cut into sections, preconditioned under 700 µmol m^–2 s^–1 photosynthetic photon flux density (PPFD) for 25 to 30 min, and a leaf measurement was made for about 3 min at 2000 µmol m^–2 s^–1 PPFD. In another study, leaf CER was estimated by measuring the uptake of ¹⁴CO₂ by leaf disks exposed to about 450 µmol m^–2 s^–1 PPFD and 350 to 700 µmol mol^–1 CO₂ in the laboratory (Albergoni et al., 1983). In more recent studies (e.g., Ying et al., 2002), leaf CER has been measured in planta in the field, on leaves exposed to and adapted to 2000 µmol m^–2 s^–1 PPFD. Leaf CER is influenced by most biotic and abiotic factors and, consequently, results of studies using more recent technology may differ from those using older technologies.

A study was performed with plants grown hydroponically in the field to examine the pattern of leaf CER in hybrids and their parental inbred lines during the grain-filling period. It is difficult to obtain reliable leaf-CER estimates of maize plants grown in the field under conventional production practices. To be able to make CER measurements on leaves that have been sunlit throughout most of the day and on plants that have not been exposed to abiotic stress during any part of the day, we grew the 12 F₁ hybrids and their seven inbred lines in a hydroponic system in the field (Tollenaar and Migus, 1984; Ying et al., 2000). The objectives of the study were (i) to quantify the magnitude of heterotic effects for leaf CER during the grain-filling period, and (ii) to determine the mode of inheritance of leaf CER during this phase of development.

	MATERIALS AND METHODS

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Materials, Experimental Design, and Measurements
Four inbred lines (CG57, CG58, CG59, and CG69) and three inbred testers (CG33, LH61, and LH145) were used to make 12 hybrid combinations in a factorial mating design. The CG lines were a sample from elite germplasm available for hybrid development at University of Guelph, and the LH lines were inbred lines developed by Holden's Foundation Seeds, Inc., Williamsburg, IA. The resulting hybrids along with their parental inbred lines were grown hydroponically in 2002 at the Cambridge Research Station, ON, Canada (43°39'N, 80°25'W, and 376 masl). An outdoor hydroponic system as described by Tollenaar and Migus (1984) and Ying et al. (2000) was used to grow the plants. In short, this system consisted of a water pump, a fertilizer injector, and plastic pipes that delivered the nutrient solution to 22.5-L plastic pails filled with "turface," a baked montmorillonite clay (International Minerals and Chemical, Blue Mountain, MS). Sufficient nutrient solution was supplied two or three times a day through the automatic irrigation system. Pails were spaced 0.35 m between pail centers within a row and 1.42 m between rows. The experiment was arranged in 25-pail rows, one row per hybrid or inbred line, and hybrids and inbred lines were grown in separate blocks with a border row on each side and one border pail containing two plants on either side of each row. Four seeds were planted per pail on 18 May 2002, and seedlings were thinned to two plants per pail at the three-leaf stage, resulting in a plant population density of 40000 plants ha^–1. This plant density is lower than that used in commercial maize production in Ontario (i.e., about 65000 plants ha^–1) to assure that leaf CER could be measured on sunlit leaves.

Leaf CER was measured with a portable, open-flow gas exchange system LI-6400 (LI-COR, Lincoln, NE) at 2000 µmol m^–2 s^–1 PPFD at the leaf surface using the 6400-02 LED light source (LI-COR). Measurements were taken on the first leaf above the topmost ear of six plants per entry (two plants per replication of three replications) at silking and 2, 4, 5, and 6 wk postsilking. Mean silking date for the hybrids was 24 July and mean silking date for the parental inbred lines was 27 July. The silking date for the latest maturing inbred line (i.e., 1 August) was taken as the silking date for leaf-CER measurements. Measurement of leaf CER of inbred lines at 6 wk postsilking was not possible because complete leaf senescence had occurred at that time. Observations were made at approximately 1100 to 1400 h Eastern Daylight-saving Time on cloudless days. The CO₂ concentration of the intake air was maintained at 350 µmol mol^–1 using the 6400-01 CO₂ injector (LI-COR). The CER was measured on a 6-cm² area of leaf that did not include the midrib, about 30 cm from the leaf tip. Leaf temperature was maintained at 28 ± 1°C by the Li-6400's Peltier thermoelectric coolers. Leaf CER was calculated by the LI-6400's operating software, which follows the method of Von Caemmerer and Farquhar (1981).

At maturity, five randomly selected pails per entry were used to determine aboveground dry matter accumulation, grain yield, and dry root weight. Each pail, containing two plants, was treated as a replication. Plants were cut off at the base of the stem, ears were removed, and roots were carefully washed from the turface medium. Roots, ears, and leaves plus stems were dried separately to constant weight at 80°C. Ears were mechanically shelled and dry weights of grain were measured and recorded at 0% moisture. Harvest index was computed as the proportion of grain dry weight to total aboveground dry weight (i.e., leaves, stems, and ears) and the root-to-shoot ratio was computed as the dry weight of the root over the dry weight of total aboveground dry weight.

Statistical Analysis
Data for all traits were analyzed using a completely randomized design with three replications for leaf CER and five replications for other traits. Analysis of variance was performed for each trait using PROC GLM of SAS v. 8.2 (SAS Institute, Cary, NC).

Design II analyses were performed for F₁ hybrids according to Comstock and Robinson (1948)(1952) to determine the mode of inheritance for leaf CER. The following linear model was assumed:

where y_ijk is the kth observation on i x jth progeny, µ is the general mean, g_i is the effect of the ith male, g_j is the effect of jth female, s_ij is the interaction effect, and e_ijk is the error associated with each observation. On the basis of this model, the hybrids sums of squares were partitioned into sources of variation due to females, males, and the female x male interaction to estimate general combining ability (GCA) and specific combining ability (SCA) effects. The main effects of females and males are equivalent to GCA_females and GCA_males, whereas the female x male interaction is equivalent to SCA effect (Hallauer and Miranda, Fo., 1988). For each character, GCA effect for each inbred line and SCA effect for each F₁ hybrid was calculated according to Beil and Atkins (1967). Two-tailed t tests were used to test the significance of the GCA and SCA effects, where t = GCA/SE_GCA and t = SCA/SE_SCA, respectively (Singh and Chaudhary, 1977). Midparent heterosis was calculated for each trait as [(F₁ – MP)/MP] x 100, where F₁ is the mean of the F₁ hybrid performance and MP = (P₁ + P₂)/2, in which P₁ and P₂ are the means of the inbred parents, respectively. Statistical significance of heterosis value for each trait was determined by appropriate t tests.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dry Matter Accumulation
Dry matter accumulation at maturity and grain yield of the 12 hybrids and their seven parental lines for plants grown in the field hydroponic system was similar to that for plants grown under conventional field conditions (Tables 1 and 2). Mean aboveground dry matter accumulation of hybrids at maturity was 360 g plant^–1 (14.4 Mg ha^–1) and mean grain yield was 178 g plant^–1 (7.12 Mg ha^–1), resulting in a harvest index of 0.493. These values are similar to values obtained in the 3-yr field study using the same genetic material [12.1 Mg ha^–1 and 6.1 Mg ha^–1, respectively (Tollenaar et al., 2004)]. For inbred lines, mean aboveground dry matter accumulation at maturity was 195 g plant^–1 (7.8 Mg ha^–1) and mean grain yield was 71 g plant^–1 (2.84 Mg ha^–1), resulting in a harvest index of 0.366. Again, data for dry matter accumulation and grain yield at maturity for inbred lines were similar to those in the 3-yr field study [6.5 Mg ha^–1 and 2.36 Mg ha^–1, respectively (Tollenaar et al., 2004)]. Although total dry matter accumulation and grain yield of plants grown in the field hydroponic system was similar to that of field-grown plants, it should be noted that plant population density in the 3-yr field study was 7 plants m^–2, compared with 4 plants m^–2 in the current study.

View larger version (9K): [in this window] [in a new window]   Fig. 1. Heterosis based on midparent values for leaf CO2 exchange rate (CER) at silking (CER0), 2 wk postsilking (CER2), 4 wk postsilking (CER4), and 5 wk postsilking (CER5), and grain yield, total dry matter (TDM), aboveground dry matter (ADM), root dry matter (RDM), root-to-shoot ratio (R/S), and harvest index (HI). * Significantly different from zero at P < 0.05; ** Significantly different from zero at P < 0.01; and NS = not significantly different at P < 0.05.

Leaf CO₂ Exchange Rate
Heterosis for leaf CER changed during the grain-filling period, increasing in magnitude from silking to physiological maturity (Fig. 1). Mean leaf CER declined during the grain-filling period for both hybrids and their parental lines, but the decline was more rapid for the inbred parents than for the F₁ hybrids (Tables 1 and 2, Fig. 2) . The difference in mean leaf CER between the hybrids and their parental inbred lines was not significant at silking (P > 0.05), but the difference was significant (P < 0.05) at all other measurement dates. The difference in mean leaf CER between hybrids and their parental inbred lines increased from 1.7 µmol CO₂ m^–2 s^–1 at silking to 8.9 µmol CO₂ m^–2 s^–1 at 5 wk postsilking. Leaves of the inbred lines had completely senesced at 6 wk postsilking, whereas mean leaf CER of hybrids was 18 µmol CO₂ m^–2 s^–1 at that date. Heterosis increased from 10.7% at 2 wk postsilking, to 23.5% at 4 wk postsilking, and to 51.2% at 5 wk postsilking. Values of heterosis for leaf CER during the grain-filling period for the data reported herein are underestimated as mean silking date of inbred lines was 3 d later than that of the hybrids; hence, at each measuring date, hybrids were in a slightly more advanced stage of development. Our results clearly show that differences in leaf CER between hybrids and their parental inbred lines increase from silking to maturity, and they confirm results reported by Albergoni et al. (1983) on studies with leaf photosynthesis of hybrids and inbred lines. The increasing difference in leaf CER during the grain-filling period between hybrids and their parental inbred lines is similar to the increasing difference in leaf CER during the grain-filling period between newer and older maize hybrids in Ontario (Ying et al., 2000, 2002) and the U.S. Corn Belt (Tollenaar et al., 2000).

View larger version (11K): [in this window] [in a new window]   Fig. 2. Leaf CO2 exchange rate ±SE of hybrids and their parental inbred lines at silking and at 2, 4, 5, and 6 wk postsilking.

Results of the genetic analyses showed that only the additive genetic effects associated with male parents were a significant source of variation for CER during later phases of the grain-filling period. A significant positive GCA effect was exhibited by LH61 at 6 wk postsilking (Table 3), indicating that LH61 transmitted high CER to its F₁ progeny late in development compared with other parents. The significant positive GCA effect of LH61 for CER at 6 wk postsilking, however, did not translate into a significant GCA effect of this parent for dry matter accumulation at maturity (Table 3). However, results of the 3-yr field study showed that this line conferred significant positive effects on dry matter accumulation during the grain-filling period and stay green to its F₁ progenies (Ahmadzadeh, 2003), but neither of these traits was measured in the present study. A significant negative GCA effect was exhibited by CG33 at 5 and 6 wk postsilking (Table 3). The negative GCA effect of CG33 for leaf CER was associated with negative GCA effect for dry matter accumulation and grain yield at maturity (Table 3). In addition, results of the 3-yr field study showed that this inbred line had negative GCA effects for dry matter accumulation during the grain-filling period and stay green (Ahmadzadeh, 2003). The SCA effects for CER were not significant at any stage of development (data not shown). This implies that the variation observed among the hybrids in this study in respect to dry matter production and grain yield could not be attributed to variation in leaf CER within the nonadditive portion of the genetic variance. Furthermore, by examining the proportion of the hybrid sums of squares attributed to male and female effects, leaf CER appears to be controlled predominantly by additive genetic effects: from 60% at silking to 78% at 6 wk postsilking, similar to what other studies have reported (Crosbie et al., 1978a; Albergoni et al., 1983).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Part of a dissertation submitted by A. Ahmadzadeh in partial fulfillment of the requirements for a Ph.D. Financial support, in part, from the Ontario Ministry of Agriculture and Food, Natural Sciences, and Engineering Research Council, and Ontario Corn Producers' Association. A. Ahmadzadeh was supported by a postgraduate scholarship from CIMMYT.

Received for publication October 15, 2003.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in Crop Science Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Ahmadzadeh, A. Articles by Tollenaar, M. Search for Related Content PubMed Articles by Ahmadzadeh, A. Articles by Tollenaar, M. Agricola Articles by Ahmadzadeh, A. Articles by Tollenaar, M. Related Collections Crop Physiology & Metabolism Maize