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a Inst. Nacional de Tecnología Agropecuaria, EEA Paraná Ruta 11 km 12.5 (3101), Oro Verde, and Univ. Nacional de Entre Ríos, Facultad de Ciencias Agropecuarias, CC 24 (3100) Paraná, Entre Ríos, Argentina
b Univ. of Guelph, Dep. of Plant Agriculture, Guelph, ON, Canada N1G 2W1
* Corresponding author (mtollena{at}uoguelph.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: LAI, leaf area index • RLAI, percentage of green leaf area index during the grain-filling period relative to leaf area index at silking
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The progress of leaf senescence during the grain-filling period may vary as a result of water and nitrogen stress (Wolfe et al., 1988; Uhart and Andrade, 1995) and/or changes in the source-to-sink ratio, that is, the ratio between assimilate supply and the potential of the grain to accommodate assimilates (Tollenaar, 1977). The source-to-sink ratio during the grain-filling period has been indicated as an important factor in the regulation of leaf senescence at the whole-plant level (Tollenaar and Daynard, 1982). Thus, maize hybrids with an improved source capacity may stay green until late in the season (Rajcan and Tollenaar, 1999), which constitutes an advantage with respect to those hybrids that show early symptoms of leaf senescence (Hageman and Lambert, 1988). Sadras et al. (2000) reported that accelerated leaf senescence was associated with assimilate accumulation as a consequence of reduced grain set. Tollenaar and Daynard (1982) and Thomas and Smart (1993) have shown that leaf senescence may be accelerated as a consequence of both starvation or excess accumulation of assimilates. Thomas (1992) defined a window between an upper and lower threshold of accumulated assimilates and suggested that as long as a leaf is within this window, senescence is not accelerated. The concept of a window of accumulated assimilates for which leaf senescence is minimal is supported by a report by Rajcan and Tollenaar (1999), who showed that leaf longevity in maize hybrids during the grain-filling period was greatest when supply and demand of assimilates during the grain-filling period was approximately equal. The latter authors used the change in weight of the nongrain parts of the aboveground dry matter from silking to maturity as a measure to quantify the ratio of supply and demand of assimilates, or the source-to-sink ratio. However, exceptions to this concept have also been reported. For instance, Crafts-Brandner et al. (1984) found that one of the three hybrids they studied did not show accelerated leaf senescence even though assimilate accumulation was evident as a consequence of ear removal.
A profile of maize leaf senescence progressing from the bottom leaves up, as well as from the top leaves down, resulting in leaves centered around the ear remaining green longest has been observed under both controlled-environment and field conditions (Tollenaar and Daynard, 1978; Wolfe et al., 1988) and is exacerbated under water and nitrogen stress, or ear removal (Wolfe et al., 1988). In temperate maize, where yield is generally limited by assimilate supply, the presence of the top–bottom leaf senescence profile might be thought of as a significant limitation to increases in crop productivity. Indeed, although the top–bottom leaf senescence profile has been mainly identified from visual observations, studies where photosynthesis rates at different leaf positions were measured also suggest the existence of a functional top–bottom leaf senescence profile. Thiagarajah et al. (1981) reported that center leaves maintained a high photosynthesis rate for a longer period than leaves positioned above and below the center leaves. A similar result was reported in a study that included six maize hybrids (Dwyer et al., 1989). To the best of our knowledge, the top–bottom leaf senescence profile has not been examined in terms of genetic improvement (i.e., older vs. more recent maize hybrids).
The objective of this study was to quantify the visual symptoms of leaf senescence across the vertical profile of leaf area during the grain-filling period in an older and two more recent maize hybrids. In addition, effects of source-to-sink ratio on leaf senescence were examined by growing the hybrids across a wide range of plant population densities.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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An older (Pride 5) and two more recent maize hybrids (Pioneer 3902 and Pioneer 3893) were grown at 1, 3.5, and 12 plants m–2. The plant population densities in this study were selected based on results reported by Tollenaar (1992) showing that the relative difference in grain yield and dry matter accumulation between older and newer hybrids was smallest at 3.5 plants m–2 and that the relative difference between older and newer hybrids increased both when plants were grown at very low plant population densities (i.e., spaced plants) and when plants were grown at supraoptimal plant population densities. It was assumed that the three hybrids used are representative of breeding efforts in different eras, that is, Pride 5 in the 1950s, Pioneer 3902 in the 1980s, and Pioneer 3893 in the 1990s. The experimental layout was a split-plot design with the main plots (plant population density) arranged in a randomized complete block and replicated four times. Hybrids were assigned to subplots. Each subplot was 54.7 m2 (six 0.76-m wide rows, 12 m long) and two additional rows were used to separate plots of low and supraoptimal plant population density treatments. Two sample areas of 4.56 m2 were marked in each subplot for future harvests. The sample area consisted of two 3-m-long center rows, separated on each side by a 2-m border.
Data Collection and Measurements
Crop development was monitored weekly on 10 tagged plants per subplot. Silking was recorded when the first silks emerged from the husks in 50% of the tagged plants. At silking, plants from a 4.56-m2 sample area were cut at ground level and divided into sample and subsample portions. The subsample consisted of either five plants (1- and 3.5-plants-m–2 treatments) or 10 plants (12-plants-m–2 treatment) selected randomly from the plants in the sample. The subsample was separated into leaves, stem plus leaf sheaths, tassels, and ears (ears and shanks). Area of all green leaves on five plants were measured separately for each leaf with a LI-3000 leaf area meter (LI-COR, Lincoln, NE) and the mean area for each leaf position was computed from the topmost leaf to lowest leaf that was still green. The LAI was calculated by summing the area of all leaves in the subsample and dividing by the ground area of the sampled portion. After the fresh weights of the sample and subsample were recorded, moisture content of the subsample was determined by drying the subsample at 80°C until weight did not change for two consecutive weighing dates. Total weight of the sample area was estimated by multiplying total sample fresh weight and dry matter percentage of subsample. After physiological maturity, plants from a 4.56-m2 sample were separated into stover and ear portions, dried at 80°C, and weighed. Ears were counted and threshed to determine grain yield (0% grain moisture). Dry matter mobilization during the grain-filling period was calculated as change in stover weight between silking and maturity, where change in stover weight is the difference of nongrain aboveground dry weight between final harvest at maturity and the harvest at silking. Change in stover weight is an indication of the source-to-sink ratio (Tollenaar and Daynard, 1982; Rajcan and Tollenaar, 1999).
Leaf senescence was estimated during the grain-filling period for each individual leaf on the 10 tagged plants per plot with a methodology described by Tollenaar and Daynard (1978). Starting at approximately 1 wk after silking, each leaf was scored for the fraction that remained green at 10-d intervals. For each leaf position, mean green leaf area was estimated by multiplying the mean fraction remaining green by the leaf area at silking. Green LAI at each measuring date was estimated by summing mean green leaf area at all leaf positions and multiplying green leaf area per plant and plant population density. During the grain-filling period, the proportion of the leaf area that remained green relative to LAI at silking was termed relative leaf area index (RLAI). Inspection of the data revealed that the decline in RLAI displayed a biphasic pattern consisting of two linear curves, similar to that reported by others (e.g., Borras et al., 2003). Linear regressions were fitted (i) between RLAI and number of days from silking, starting at silking, and (ii) between RLAI and number of days from the last date of measurement of RLAI during the grain-filling period, starting at the last date of measurement of RLAI and working backward. The breakpoint between the two linear curves was the date at which both curves intersected.
Data Analysis
All data were analyzed by procedures included in the SAS package (SAS Institute, 1997). Data from 3 yr were combined and analyzed by the PROC MIXED procedure. Both density and hybrid were assumed as fixed effects and year as random effect. Analysis of variance for each individual year was executed on transformed data (Fernandez, 1992) by PROC GLM procedure. When the ANOVA indicated the presence of significant differences, simple mean comparisons were made with the LSD test. Regression coefficients were obtained by the PROC REG procedure. The ANOVAs for the rates of leaf senescence during the first half of the grain-filling period were performed on root-square transformed data.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Rate of Leaf Senescence
The decline in green leaf area during the grain-filling period showed a biphasic pattern, consisting of a period of a relatively low linear decline during the first half of the grain-filling period and a period of a much more rapid linear decline during the second half of the grain-filling period. Rate of leaf senescence was defined in this study as the change per day in green leaf area relative to maximum LAI at silking (RLAI). Mean rate of leaf senescence across years, hybrids, and plant population densities was 0.44% d–1 during the first half of the grain-filling period and 1.87% d–1 during the second half of the grain-filling period (Tables 4 and 5). Analyses of variance for rate of leaf senescence (data not shown) showed highly significant effects for the main treatments and their two-way interactions (P < 0.01), except for the hybrid x plant-population-density interaction during the first half of the grain-filling period (P > 0.16). The three-way interactions were not significant (P > 0.36).
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A moderate inverse relationship between rate of leaf senescence during the second half of the grain-filling period and change in stover weight from silking to maturity was apparent for two out of the three growing seasons (r = –0.53 in 1999; r = –0.33 in 2001; P < 0.05, n = 36), indicating that remobilization of dry matter from the stover to the grain occurred concomitantly with accelerated leaf senescence (Table 6). Rate of leaf senescence during the second half of the grain-filling period was also negatively associated with grain yield in the 2000 (r = –0.58; P < 0.05, n = 36) and 2001 (r = –0.56; P < 0.05, n = 36) growing seasons, accounting for approximately one third of grain yield variation in each year. The slope of this relationship was greater in 2000 than 2001, that is, during the second half of the grain-filling period grain yield declined 139 kg ha–1 in 2000 and 79 kg ha–1 in 2001 for each percentage of increase in the rate of leaf senescence (data not shown). In contrast, the rate of leaf senescence during second half of the grain-filling period was not significantly associated with grain yield (P > 0.05) in 1999, the growing season with the highest yield. Changes in stover weight from silking to maturity were not associated with grain yield in any of three growing seasons (P > 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Leaf senescence progressed at a low rate between silking and approximately the midpoint of the grain-filling period and rate of senescence during the second half of the grain-filling period were approximately four times greater than those during the first half. Borras et al. (2003) also reported a bilinear pattern of senescence for maize hybrids grown in Argentina. Although they used Celsius degree-days rather than calendar days as the denominator in their calculation of rate of leaf senescence, their results were similar to our results as their rates of leaf senescence during the second phase were 3.5 times greater than those during the first phase.
The response of rate of leaf senescence to maize grown at plant population densities including spaced plants (i.e., 1 plant m–2), a low plant population density (i.e., 3.5 plants m–2) and a supraoptimal plant population density for grain yield (i.e., 12 plants m–2) was surprising. Rates of leaf senescence during the first half of the grain-filling period were about two times greater for plants grown at 3.5 vs. 12 plants m–2 (Table 4), which is consistent with the contention that leaf senescence increases when plants are exposed to stress. Borras et al. (2003) reported that rates of leaf senescence during the first phase of the bilinear decline in LAI were almost two times greater when maize was grown at 12 compared with 3 plants m–2 and increased rates of leaf senescence have also been reported when maize has been exposed to N stress (Pearson and Jacobs, 1987) or water deficiency (Muchow and Carberry, 1989). During the second half of the grain-filling period, however, rates decreased from the highest to the lowest plant population density for Pride 5, and rates at the highest and lowest plant population density were similar and rates at the medium plant population density were lowest for the newer hybrids (Table 5). The 80% increase in rate of leaf senescence from 3.5 to 12 plants m–2 for the newer hybrids, however, is similar to results reported by Borras et al. (2003) for Argentinean maize hybrids. The grain-yield ratio between the newer hybrids and the older hybrid was smallest at the 3.5-plants m–2 plant population density (Table 2), which is consistent with results of an earlier report (Tollenaar, 1992), and which may be attributable, in part, to the different response of rate of leaf senescence in the two groups of hybrids to plant population density.
Results of this study do not elucidate physiological mechanisms underlying the senescence pattern in older and newer hybrids. We postulated that differences in rates of senescence would be negatively associated with the source-to-sink ratio during the grain-filling period in newer vs. older hybrids, as a low source-to-sink ratio could result in self destruction of leaf area to satisfy the demand for assimilates by the grain (Tollenaar and Daynard, 1982). The plant population density treatments resulted in different source-to-sink ratios, as was indicated by the relative change in stover weight (Table 6). These results are similar to those reported by Rajcan and Tollenaar (1999), who utilized defoliation treatments to change the source-to-sink ratio. Although there was a negative association between the relative change in stover weight and rate of leaf senescence during the second half of the grain-filling period, results were too variable to be conclusive.
A distinct vertical profile for maximum LAI and for progress of leaf senescence during the grain-filling period was apparent. Although LAI differed markedly among plant population densities at silking, the vertical distribution of leaf area across the canopy was rather stable for each group of hybrids. Approximately one half of total green leaf area was positioned in the central section of the canopy for all three hybrids, a result that agrees with results reported by Tollenaar and Daynard (1978). Differences between the older and the newer hybrids were found in both the top and bottom sections of the canopy: the older hybrid had a greater proportion of green leaf area in the top section and a lower proportion in the bottom section. These small but significant changes in the vertical leaf area distribution reflect a plant architecture closer to a Christmas-tree shape in more recent hybrids, a trait thought be more favorable for light interception (Troyer, 2000). A relatively smaller leaf area in the top section results in greater penetration of light to the ear leaf (Dwyer et al., 1992) in less mutual shading and, possibly, in a delay in senescence of bottom leaves as plant population density increases. Troyer and Rosenbrook (1983) speculated that the Christmas-tree shape vertical distribution of leaf area might be a consequence of selection under high plant population density, a key component in the apparent increase in stress tolerance in modern temperate maize germplasm (Tollenaar and Wu, 1999). A greater leaf area above the ear has been also associated with lower optimum plant population density for grain yield (Dwyer et al., 1992).
When the progress of leaf senescence across the canopy was examined, a top–bottom profile of leaf senescence during the second half of the grain-filling period was observed in two out of three growing seasons, and this profile was more distinct in the two newer hybrids than in the older hybrid. In this profile, senescence occurred earlier in leaves positioned in the top section than in leaves in the central section, despite the fact that the upper leaves were younger and were in a more advantageous position for photosynthesis (Tollenaar and Daynard, 1978). The top–bottom profile of leaf senescence is also in agreement with previous results demonstrating that leaves in the vicinity of ear have greater photosynthesis rate and senesce more slowly than the other leaves (Dwyer and Stewart, 1986). Although the top–bottom profile of leaf senescence was associated with higher-yielding growing seasons and higher-yielding hybrids, the reason for the apparent advantage of this senescence profile is not clear.
In conclusion, delaying leaf senescence has been a prime target for crop improvement (Thomas and Howarth, 2000) and it continues to be an avenue for increasing the total amount of carbon fixed by the crop. Results of this study show that differences in grain yield between older and more recent maize hybrids could be accounted for, in part, by a differential progress in visual symptoms of leaf senescence. Our study identified two distinctive periods delimited by a sudden increase in the rate of leaf senescence occurring around the middle of the grain-filling period. During the first period, visual senescence progressed in the older hybrid and was almost negligible in newer hybrids. During the second period, the rate of visual senescence accelerated in both the older hybrid and the more recent hybrids. A top–bottom profile of leaf senescence became apparent during the second period, with leaves in the central section of the canopy being the last leaves to senesce. This profile was evident in growing seasons characterized by more favorable conditions for grain yield and the profile was more distinct in newer hybrids and, consequently, this profile may represent an ideal type of plant for visual senescence and grain yield.
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ACKNOWLEDGMENTS |
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Received for publication June 25, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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during the first half of the grain-filling period of three maize hybrids grown at three plant population densities in 1999, 2000, and 2001.
