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Published in Agron J 99:1111-1118 (2007)
DOI: 10.2134/agronj2006.0360
© 2007 American Society of Agronomy
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Ecophysiological Yield Components of Maize Hybrids with Contrasting Maturity

Pedro R. Capristo, Roberto H. Rizzalli and Fernando H. Andrade*

Unidad Integrada INTA Balcarce, Facultad de Ciencias Agrarias UNMP, CC 276, 7620 Balcarce, Provincia de Buenos Aires, Argentina

* Corresponding author (fandrade{at}balcarce.inta.gov.ar)

Received for publication December 19, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The length of the growing cycle is one of the most important traits determining hybrid adaptability to the environment. The objective of this work was to study the development, dry matter accumulation, grain yield, harvest index, and sink–source relationship of 11 maize (Zea mays L.) hybrids with contrasting maturity. The durations of the cycle from emergence to flowering varied from 537 to 781 growing degree days and from emergence to physiological maturity from 1221 to 1656°Cd. Cumulative biomass from emergence to flowering increased linearly with hybrid cycle length. Long-season hybrids showed the highest cumulative interception but the lowest radiation use efficiency (RUE) during reproductive growth. Total aboveground biomass increased from 1624 to 2422 g m–2 with hybrid maturity class, and grain yields were lowest for short-season hybrids (832 g m–2) and similar between mid and long-season hybrids (avg. = 1256 g m–2). Increases in maturity class were associated with increases in grain number (from 2432 to 5078 grains m–2) and reductions in individual grain growth rate (from 9.1 to 4.9 mg grain–1 d–1). The sink–source relationship and the apparent reserve remobilization increased with hybrid maturity class. These results indicate that grain yield of short-season hybrids would be more limited by the capacity of the reproductive sinks during grain filling than their long-season counterparts. Hybrids Ax 840 and Experimental have a short developmental time from emergence to flowering but a long developmental time from flowering to physiological maturity. This resulted in the largest values of radiation interception during reproductive growth and in the greatest grain yields and harvest indexes.

Abbreviations: DAF, days or growing degree days after flowering • EGFD, effective grain filling duration • GGR, grain growth rate • GW, grain weight • PAR, photosynthetically active radiation • RUE, radiation use efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
BEST maize hybrids for a specific location are those that fully explore the potential growing season fitting the constraints of the local environment. Therefore, the longer the growing season, the longer the maturity group of adapted hybrids.

At low latitudes, temperature and radiation do not vary much along the year and long-season hybrids are generally the most suitable because they use the available resources more efficiently than shorter hybrids (Lafitte and Edmeades, 1997; Bruns and Abbas, 2006). Contrarily, at high latitudes, radiation and temperature decrease markedly during grain filling and grain yield usually decreases as hybrid maturity class increases (Ruget, 1993). Similarly, in the U.S. corn belt, cycle length of adapted hybrids declines from south to north (Neild and Newman, 1985).

In Argentina, the traditional corn belt is located between 32° and 35° S and full-season hybrids generally produce maximum grain yield (Otegui et al., 1995). During the last decades, the area cultivated with maize expanded south of parallel 37° S (Calviño et al., 2003). Compared with the traditional maize growing region, the new production areas present lower spring temperatures that limit early growth and a more rapid decrease in radiation and temperature toward the end of the growing season. Then, the grain-filling period of long-season hybrids would be more affected by the unfavorable environmental conditions during reproductive growth than that of short or mid-season hybrids. Short-season hybrids would place reproductive growth under more favorable environmental conditions than their long-season counterparts (Lauer et al., 1999). However, the former hybrids would intercept less amount of radiation than full-season hybrids (Edwards et al., 2005) because of low leaf area index and short growth duration.

The quantification of the ecophysiological factors that determine grain yield is necessary to guide the selection of the most appropriate management practices to optimize the use of environmental resources in a specific location (Andrade et al., 2005; Edwards et al., 2005). The objective of this work was to study the development, dry matter accumulation, grain yield, harvest index, and sink–source relationship of maize hybrids with contrasting maturity growing in an area characterized by low average temperatures during the growing season.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Description of the Experimental Site and Hybrids
The experiments were conducted at the Instituto Nacional de Tecnología Agropecuaria Balcarce Experimental Station (37°45' S, 58°18' W, 130 m alt), during the 2000–2001 and 2001–2002 growing seasons. The soil was a fine-loamy, mixed, thermic Typic Argiudoll with a minimum effective depth of 1.5 m, and with an organic matter content of approximately 56 g kg–1 in the top 25 cm of depth. The area is characterized by low average temperatures during the growing season (17.8°C) and a frost-free period of about 150 d. More details about the climatic regime of the Balcarce region were presented in Andrade (1995). The experimental field was prepared with conventional tillage, consisting of moldboard plow pass on June and disk and field cultivator passes on September.

Maize hybrids KWS Romario, KWS Domingo, KWS Impacto, Pioneer P37P73, Nidera Ax 599, Dekalb DK 615, KWS Tandem, Nidera Ax 840, Nidera Experimental XPA 73811 (Exp), Dekalb DK 688, and Dekalb DK 752 were planted on 15 Oct. 2001 and 18 Oct. 2002 with a final density of 8.5 plants m–2. These hybrids varied greatly in growing degree days to flowering and to physiological maturity (Fig. 1).


Figure 1
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  		Fig. 1. Duration of the emergence–flowering and flowering–physiological maturity periods for 11 maize hybrids and 2 yr of experimentation, expressed as growing degree days. SE were 4.4 and 3.1°Cd from emergence–flowering and 26 and 29°Cd from emergence–physiological maturity,for the first and second growing seasons, respectively. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 
Trials were conducted without appreciable water or nutrient limitations. Plots were fertilized with 30 kg P ha–1 before planting and with 160 kg N ha–1 at V6 stage (Ritchie and Hanway, 1982) to provide adequate mineral nutrition. Irrigation was applied to keep soil water over 50% of maximum soil available water in the first meter of depth during the entire growing season. Weeds were controlled with the pre-emergence herbicides acetochlor (2-cloro-N-etoximetil-6'-etilacet-O-toluidida) (2.25 kg ha–1) and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] (2 kg ha–1).

The experiment was a randomized complete-block design with three replications and 11 treatments (hybrids). The size of the experimental unit was four rows 0.70 m apart and 14 m long. Daily values of incoming solar radiation and air temperature were obtained from a meteorological station located 500 m away from the experimental site.

Phenological Determinations
Duration of the emergence-flowering and flowering-physiological maturity stages were expressed in days and in growing degree days. Growing degree days for a specific period were calculated as the sum of daily average temperatures above 8°C (Cirilo and Andrade, 1996). Emergence was taken as the day when 50% of the plants showed visible coleoptile; flowering (R1; Ritchie and Hanway, 1982) was recorded when 50% of the plants showed visible silks; and physiological maturity (R6; Ritchie and Hanway, 1982) was taken as the day when maximum individual grain weight was achieved. Number of expanded leaves was recorded weekly from emergence to flowering. Grain samples were taken every 7 to 10 d starting at flowering and ending at least 6 wk after physiological maturity. In each sampling and experimental unit, a total of 25 grains were taken from the central portion of the ear of five randomnly selected plants of the central rows. Grains were oven-dried to constant weight and weighed.

Grain growth rate (GGR) during the effective grain-filling period (linear phase) was estimated using regression analysis. A linear with plateau model (Jandel Scientific, 1992) was fitted to the relationship between individual grain weight (GW) and days or growing degree days after flowering (DAF) for each hybrid:

Formula 1[1a]

Formula 2[1b]
where b is the slope of the GW–DAF relationship, that is, the grain growth rate (mg grain–1 d–1 or mg grain–1 °Cd–1), and c is DAF (d or °Cd) to maximum GW. The duration of the lag phase was estimated as the quotient between parameters a and b (absolute terms). The effective grain filling duration (EGFD), specifically, the linear phase of the grain-filling period, was estimated as the difference between c and the lag phase duration (Johnson and Tanner, 1972). Maximum GW was estimated as the product between GGR and EGFD.

Growth and Yield Determinations
Aboveground dry matter was measured at flowering, 15 d after flowering and physiological maturity, by taking samples of 8 to 10 plants from the central rows and leaving borders between adjacent harvests. The samples were separated in stems + sheaths + tassels, leaves and ears (when visible), oven-dried (with air circulating at 60–70°C) to constant weight, and weighed. These data were used to calculate average crop and plant growth rates during different periods.

Grain yield was determined at physiological maturity by hand harvesting all the ears from 7.15 m of the two center rows of the plot. Grain moisture percentage was recorded and grain yield was expressed on a dry-weight basis. Weight per grain was determined for each experimental unit by weighing two samples of 500 grains each. The number of grains per unit surface area and per plant were calculated as the quotient between grain yield and individual grain weight, both on a dry-weight basis. Harvest index was calculated as the ratio between grain yield and total aboveground biomass at physiological maturity.

Photosynthetically active radiation (PAR) interception, expressed as percentage of the incident radiation, was estimated according to Gallo and Daughtry (1986) using a lineal sensor (LI-COR, Lincoln, NE). Percentage radiation interception was calculated as

Formula 3[3]
where It is the incident radiation just below the lowest green layer of leaves and I0 is the incident radiation at the top of the canopy. The measurements were confined to the midday period (1100–1300 h). At least five determinations per plot were made at noon (±1 h) placing the sensor perpendicular to the rows. Corrections by solar angle were small and neglected. Measurements were taken every 15 to 20 d during the entire growing cycle.

Daily total incident PAR obtained from a nearby experimental station was multiplied by the corresponding fraction of PAR interception and accumulated to obtain total PAR intercepted by the crop during a period (i.e., emergence to flowering, flowering to physiological maturity). Daily values of fraction PAR interception were obtained by linear interpolation between measurements. The RUE was calculated as dry matter accumulated in a period divided by the total amount of PAR intercepted during the same period. Dry matter accumulation, intercepted radiation, and RUE were calculated for the emergence–flowering and flowering–physiological maturity periods.

Apparent reserve remobilization during grain filling (AR) (defined as the relative decrease in weight of nongrain organs during grain filling) was calculated as

Formula 4[4]
were DMF is the aboveground dry matter at 15 d after flowering and RDMPM is the nongrain biomass at physiological maturity.

Sink activity during grain filling was estimated as the product between the number of grains m–2 and grain growth rate during the linear phase of the effective grain-filling period. The ratio between sink activity and crop growth rate from 15 d after flowering to physiological maturity (both expressed as g m–2 d–1 or g m–2 °Cd–1) was taken as an indicator of sink–source relationship during grain filling. The product of grain number by grain growth rate is a better indicator of daily sink activity than grain number alone because it includes the concept of individual grain activity. In maize, grain growth rate is little affected by source capacity (Echarte et al., 2006) and is closely associated with maximal grain water content, which is a good indicator of potential individual grain weight (Borrás and Westgate, 2006; Sala et al., 2007).

Data were analyzed by ANOVA procedures and regression analysis SAS/STAT (SAS Institute, 1996). Appropriate standard errors of the means were calculated and the parameters of the lineal with plateau models were compared using t test (p ≤ 0.05; Steel and Torrie, 1960).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Weather Conditions and Phenological Characterization of the Hybrids
Hybrids differed for duration of emergence to flowering, flowering to maturity, and emergency to maturity (expressed in days or in thermal time) (p ≤ 0.0001; Fig. 1). The durations of the cycle from emergence to physiological maturity varied between 115 and 157 d or 1221 and 1656°Cd depending on the hybrid (2-yr avg.). Increments in total cycle length were related to increases in the duration of the emergence to flowering and flowering to physiological maturity periods. Exceptions to this rule were hybrids Ax 840 and Exp, which had a long total cycle but a short vegetative period (Fig. 1). Pooling the data from the 2 yr, the emergence–flowering period was associated with the maximum number of leaves (R2 = 0.77, n = 22, p ≤ 0.001), which ranged from 15.6 for Romario to 21.0 leaves for DK 752.

Since all hybrids were sown at the same date (mid-October), physiological maturity dates were reached by the end of February, mid-March, and end of March for the short, intermediate, and long-season hybrids, respectively. Increases in cycle length resulted in a higher average temperature during the emergence–flowering period and lower average temperature from flowering to maturity (Table 1). Average temperature during vegetative growth varied from 16.3 to 17.6°C according to the hybrid. Contrarily, average temperature during reproductive growth decreased from 21.0°C for short-season hybrids to 19.5°C for the long-season counterparts. Increases in hybrid maturity did not affect average daily incident radiation from emergence to flowering (23.7 MJ m–2 d–1), but reduced it from flowering to maturity (23.5–19.5 MJ m–2 d–1).


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  		Table 1. Mean temperature and cumulative incident and intercepted PAR during the emergence–flowering and flowering–maturity periods for 11 maize hybrids cultivated in 2000–2001 and 2001–2002 growing seasons. SE, standard error.

 
Intercepted Radiation and Cumulative Biomass
Biomass at flowering increased linearly with the hybrid cycle length expressed as growing degree days from emergence to flowering (p ≤ 0.01; Fig. 2A). Cumulative intercepted radiation during vegetative growth also increased with hybrid maturity class (Table 1). Cumulative intercepted PAR around the critical flowering period was also positively associated with growing degree days from emergence to flowering (Fig. 3; p ≤ 0.001). A significant and positive association between cumulative biomass and cumulative intercepted radiation occurred for the emergence–flowering period (R2 = 0.85, p ≤ 0.001). The slope of this regression, forced through the origin (2.52 g MJ–1), represents the average RUE, which was not affected by the maize cycle length. Differences among hybrids in cumulative intercepted radiation during the vegetative period were mainly explained by differences in the duration of this period (Fig. 1) and in the fraction of incoming radiation intercepted by the plants. The fraction of incoming radiation intercepted ranged from 0.32 for Romario to 0.46 for DK 752 (SE = 0.036).


Figure 2
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  		Fig. 2. Cumulative dry matter (A) from emergence to flowering and (B) from flowering to maturity as a function of the duration of their respective growing periods in degree days. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Lines show the linear regression of pooled data in A, and the regression for each season in B. SE were 38.5 and 32.9 g m–2 for cumulative dry matter from emergence to flowering, and 92 and 105 g m–2 for cumulative dry matter from flowering to physiological maturity, for the first and second growing seasons, respectively. Data from DK 615 second season (y = 1804, x = 706) not included in B. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 

Figure 3
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  		Fig. 3. Cumulative intercepted PAR from 15 d before to 15 d after flowering as a function of hybrid maturity expressed as growing degree days from emergence to flowering. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Line shows the linear regression for the pooled data. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 
Cumulative biomass from flowering to maturity increased in response to total cycle length in 2000–2001 and up to intermediate season hybrids in 2001–2002 (p ≤ 0.001; Fig. 2B). Hybrids that flowered late received lower daily incident radiation, but intercepted a greater fraction of the incident radiation than earlier hybrids (0.95 vs. 0.77 for DK752 and Romario, respectively; SE = 0.04). The greatest cumulative intercepted radiation during the flowering–physiological maturity period was observed in Exp (2000–2001) and Ax 840 (2001–2002) the hybrids with the longest reproductive periods; contrarily, the lowest values corresponded to the short-season hybrids (Table 1). The RUE for the reproductive period decreased in 2001–2002 in response to increments in hybrid maturity class expressed as growing degree days from emergence to physiological maturity (R2 = 0.51; p ≤ 0.05), but not in 2000–2001 (Fig. 4).


Figure 4
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  		Fig. 4. Radiation use efficiency from flowering to maturity as a function of hybrid cycle length expressed as growing degree days from emergence to physiological maturity. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Line shows the linear regression for the 2001–2002 growing season. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 
Grain Yield and Harvest Index
There were significant differences among hybrids for total biomass at maturity and grain yield in both years of the study (p ≤ 0.001) (Fig. 5A and 5B). Total biomass increased with cycle length expressed as thermal time from emergence to physiological maturity (Fig. 5A, p ≤ 0.001). Short-season hybrids yielded less than the rest of the hybrids. Increments in cycle length beyond that corresponding to intermediate hybrids, however, did not consistently increase grain yield. Grain yield was highest for Ax 840 (2001–2002) and Exp (2000–2001), the hybrids with the longest EGFD (Table 2) and flowering–maturity period (Fig. 1).


Figure 5
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  		Fig. 5. (A) Cumulative dry matter from emergence to physiological maturity, (B) yield, and (C) harvest index as a function of the hybrid cycle length expressed as growing degree days from emergence to physiological maturity. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Lines show the quadratic regressions for each growing season in A and B and for the pooled data in C. SE were 80 and 81 g m–2 for cumulative dry matter, 33.8 and 20.3 g m–2 for grain yield, 0.008 and 0.006 for harvest index, for the first and second growing seasons, respectively. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 

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  		Table 2. Grain number (GN), individual grain weight (GW), grain growth rate (GGR), and effective grain filling duration (EGFD) for 11 maize hybrids cultivated in 2000–2001 and 2001–2002 growing seasons. SE, standard error. GW was determined by weighing 500 grains at harvest. GGR and EGFD were determined by Eq. [1].

 
Among all hybrids, grain yield was positively associated with final grain number per unit area (R2 = 0.76, n = 22, p ≤ 0.001). When only intermediate and long maturity hybrids were included in the regression, there was no relationship between yield and grain number. Weight per grain differed among hybrids (p ≤ 0.001; Table 2) and showed a slight negative correlation with number of grains per m2 (R = 0.55, n = 22, p ≤ 0.01). There were significant differences among hybrids in harvest index in both years (Fig. 5C; p ≤ 0.001). Harvest index was also maximum for hybrids Ax 840 and Exp.

Activity of the Reproductive Sinks, Sink–Source Relationship, and Apparent Remobilization of Reserves
Increases in hybrid maturity class resulted in a greater number of grains and in a lower grain growth rate expressed on a daily or thermal time basis (Table 2). Daily sink activity, expressed as the product of number of grains per m–2 and grain filling rate were lowest for the short-season hybrids (22.5 g m–2 d–1 or 1.80 g m–2 °Cd–1) and highest for mid-season hybrids (27 g m–2 d–1 or 2.24 g m–2 °Cd–1) (2-yr avg., SE = 0.85 g m–2 d–1, 0.07 g m–2 °Cd–1). The effective grain filling duration expressed in d or in °Cd was shortest for Romario and Domingo and longest for Exp and Ax 840 (Table 2).

The sink–source relationship expressed as the ratio between reproductive sink activity and crop growth rate (both expressed on a daily or growing degree basis) during the period from 15 d after flowering to maturity increased with hybrid maturity class (p ≤ 0.01 for both years; Fig. 6). Apparent remobilization also increased with hybrid maturity class (p ≤ 0.01; Fig. 7). Finally, the apparent reserve remobilization was positively associated with the sink/source ratio during the period from beginning of the linear phase of grain filling to maturity (R2 = 0.59, n = 22, p ≤ 0.001).


Figure 6
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  		Fig. 6. Sink–source relationship as a function of hybrid maturity class expressed as growing degree days from emergence to physiological maturity. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Lines show the linear regressions. SE for the sink/source ratio were 0.14 and 0.12 for the first and second growing seasons, respectively. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 

Figure 7
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  		Fig. 7. Apparent remobilization as a function of hybrid maturity class expressed as growing degree days from emergence to physiological maturity. Data from 11 maize hybrids cultivated in 2000–2001 (closed symbols) and 2001–2002 (open symbols) growing seasons. Line shows the linear regression for the pooled data. SE for apparent remobilization was 0.06 for both growing seasons. Growing degree days were calculated as the sum of daily average temperatures above 8°C.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The evaluated hybrids showed different duration of the emergence–flowering and flowering–physiological maturity stages. These results differ from those observed by Derieux and Bonhomme (1982a, 1982b) for the North region of Europe and by Major et al. (1983) in Canada, who found that the differences in the hybrid cycle were mainly explained by changes in the duration of the period from emergence to flowering. Differences in durations of the grain-filling period among hybrids were also found by Maddonni et al. (1998).

Radiation interception at flowering increased with hybrid maturity mainly because long-season hybrids had higher number of leaves than their short-season counterparts as observed by Chase and Nanda (1967) and Lafitte and Edmeades (1997). These results are similar to those of Giauffret et al. (1997) and Lafitte et al. (1997).

The greatest biomass production at flowering observed for the long-season hybrids resulted from a greater radiation interception efficiency and a more extensive duration of the emergence–flowering period compared with other hybrids. Similar results were observed by other authors (Ruget, 1993; Bolaños, 1995; Lafitte et al., 1997). In contrast, cumulative dry matter from flowering to maturity was lowest for short-season hybrids, increased with hybrid cycle length in 2000–2001 and was quite similar among mid and full-season hybrids in 2001–2002 (Fig. 2). Cumulative intercepted radiation during reproductive growth was mainly modified by changes in the duration of the period (R2 = 0.92, p ≤ 0.001) since daily incident radiation and interception efficiency varied inversely resulting in a similar daily average radiation interception for the different hybrids. Long-season hybrids had the highest cumulative interception (Table 1) but the lowest RUE (Fig. 4) during the flowering–maturity period. Daily average temperatures decreased far below 20°C during the last part of the grain-filling period in the full-season hybrids. Decreases in temperature below this value reduce RUE in maize (Andrade et al., 1992; Wilson et al., 1995). Thus, the small response of cumulative biomass during reproductive growth to total cycle length beyond intermediate maturity hybrids would be explained by low daily incident radiation and low RUE during the last part of the growing cycle.

Grain yields were lowest for short-season hybrids and did not vary much between mid and long-season hybrids. Thus, the response of grain yield to maturity class was curvilinear in both years (Fig. 5B). These results do not agree with data obtained at other sites. At Pergamino, a location with an average temperature during the growing season 3°C higher than in Balcarce, grain yield increased linearly and continuously along a similar range of hybrid maturity classes (Cirilo Alfredo, personal communication, 2006). Accordingly, Otegui et al. (1995) reported that full-season hybrids generally produced maximum grain yield at that location. Contrarily, at the high latitudes of northern Europe, grain yield decreased with hybrid cycle length (Ruget, 1993). These contrasting results would be explained by the deterioration of the environmental conditions during the last part of the growing season, which is more pronounced as latitude increases. Based on the results of this work, it can be concluded that mid-season hybrids are appropriate for the Balcarce region because they have similar grain yields but allow earlier harvesting with lower grain moisture compared with their long-season counterparts.

Grain yield was more closely associated with grain number than with GW. The number of grains was positively related to the length of the vegetative period (R2 = 0.67, n = 22, p ≤ 0.001) and to the amount of radiation intercepted during 30 d around flowering (R2 = 0.62, n = 22, p ≤ 0.001). Thus, it appears that the high number of grains at harvest observed in some hybrids resulted from high potential number of grains and high crop growth rate during the critical flowering period (Andrade et al., 1999; Otegui and Andrade, 2000). On the other hand, high GW was observed either in a short (Romario) or a long (Exp) season hybrid, resulting from high GGR in the former or high EGFD in the latter (Table 2).

Increases in maturity class increased grain number but reduced grain growth rate (Table 2). Since grain number increases were relatively larger than grain growth rate reductions, daily sink activity during the effective grain-filling period increased with hybrid maturity class from short- to mid-season hybrids. The lowest sink/source ratio and dry matter remobilization from vegetative organs during grain filling were observed for the short-season hybrids (Fig. 6 and 7). These results indicate that grain yield of short-season hybrids would be more limited by the capacity of the reproductive sinks during grain filling compared with their long-season counterparts. A low potential grain number (Ritchie and Hanway, 1982) and adequate environmental conditions during grain filling in the short-season hybrids contribute to explain these results. Accordingly, Uhart and Andrade (1991), working in the same environment, observed dry matter accumulation in vegetative tissues during grain filling in a short-season hybrid and remobilization from those tissue during the same stage in a long-season hybrid. Moreover, Ruget (1993), working at higher latitudes (50° N), found increases in dry matter remobilization from stems as hybrid maturity class increased.

Based on the results of this work, it is possible to quantify the contribution of each ecophysiological yield component that causes lower yield in short-season cultivars. Percentage radiation interception from flowering to physiological maturity was 78 and 90% for short and mid-season hybrids, respectively (Table 1). Improving percentage radiation interception of the short-season hybrids up to that observed for the midseason ones would increase daily amount of solar radiation intercepted by 15% (90/78). In irrigated maize, biomass production during reproductive growth would respond proportionally to this improvement in resource capture provided sink limitation does not limit canopy photosynthesis. Since short-season hybrids showed a trend toward sink limitation during reproductive growth, grain sink activity of these hybrids must be increased proportionally to the increments in source capacity (15%) to maintain their original sink/source ratio and to obtain similar relative increases in grain yield. If sink/source ratio increases to values similar to those observed for mid-season hybrids (Fig. 6), grain yield would increase another 10 to 15%. Further increments in yield must be achieved through extension of the reproductive period. An increment in the length of this period would render an increase in incident radiation (611 vs. 687 MJ m–2 of incident PAR for short and intermediate hybrids; Table 1), that would be translated into a proportional increase in grain yield if RUE was not negatively affected by lower temperatures toward the end of this extended reproductive period and if the grains can accommodate the extra available assimilates.

The knowledge of the physiological components of growth and yield assist in the design of the most appropriate strategies to increase grain yield of a specific cultivar (Andrade et al., 2005). Plant density is a way to ameliorate the limited capacity of short-season hybrids to intercept radiation and to set reproductive sinks (Beech and Basinski, 1975; Tetio-Kagho and Gardner, 1988; Modarres et al., 1998; Edwards et al., 2005). Accordingly, Sarlangue et al. (2007) found that short-season hybrids showed the greatest response in total biomass and harvest index to increases in plant density. Row distance reductions would also improve radiation interception of those hybrids (Westgate et al., 1997; Andrade et al., 2002). Finally, yield potential can be further increased through the lengthening of the flowering–physiological maturity period keeping total cycle length constant, provided management practices assure full interception is achieved by the critical flowering stage. Hybrids Ax 840 and Exp are examples of this strategy. Both are short-season hybrids considering the emergence–flowering period, but are the ones that presented the longest flowering–physiological maturity period, which resulted in the largest values of radiation interception during reproductive growth, grain yields, and harvest indexes.


   ACKNOWLEDGMENTS
 
The technical assistance of Diego Gaitán is greatly appreciated. This work was supported by INTA, University of Mar del Plata, and CONICET.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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