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CROP ECOLOGY, MANAGEMENT & QUALITY

Ear Position, Leaf Area, and Contribution of Individual Leaves to Grain Yield in Conventional and Leafy Maize Hybrids

Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre (ECORC), Central Experimental Farm, 960 Carling Avenue, Ottawa, ON, Canada K1A 0C6

^* Corresponding author (subedik{at}agr.gc.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The contribution of individual leaves and the extra leaves above the ear to dry matter (DM) accumulation and grain yield in Leafy maize (Zea mays L.) is not well documented. A field experiment was conducted for two growing seasons (2003 and 2004) at Ottawa, Canada, to determine whether additional leaves above the ear in a Leafy hybrid contribute more to grain yield than in a conventional hybrid and to assess the importance of individual leaves above and below the ear. At silking, 10 defoliation treatments were imposed in a conventional (Pioneer 3893) and a Leafy (Maizex LF850-RR) hybrid. Total number of leaves per plant, position of the primary ear height, and the area and DM of each removed leaf were measured at silk stage. At physiological maturity, number of kernels per plant, kernel DM, and whole plant DM were determined. Despite the Leafy hybrid having a 25% greater number of leaves, 26 to 40% more green leaf area, and 16 to 41% more total plant DM at silking than the conventional hybrid, there was no difference in total DM and grain yield at physiological maturity, indicating a situation of sink limitation in the Leafy hybrid. Removal of all leaves below the earleaf and earleaf alone in the conventional hybrid caused 19 to 26% and 17 to 25% reduction in grain yield, respectively, while there was no any notable effect of these treatments in the Leafy hybrid. When all leaves above the earleaf were removed, kernel number and kernel DM were reduced by 84 to 94% in the Leafy hybrid compared with a 40 to 50% reduction in the conventional hybrid. We conclude that the large number of leaves in the Leafy maize gave no additional advantage in terms of grain yield and total DM production and the earleaf and leaves below the ear-node were of less importance in the Leafy hybrids than in the conventional hybrid.

Abbreviations: DM, dry matter • LAI, leaf area index • PM, physiological maturity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
GENETIC MANIPULATION of leaf number and the proportion of leaves above and below the ear in maize has implications for yield improvement (Shaver, 1983). Maize with the Leafy trait has an increased leaf number per plant and a greater number of leaves above the ear (Begna et al., 2000) and consequently higher leaf area indices (Stewart and Dwyer, 1999). The Leafy trait is also associated with shorter internodes, highly lignified stalks, and leaves and low ear placement (Shaver, 1983; Dijak et al., 1999). Since the source of photosynthate for grain filling is largely from leaves around and above the ear-node, the new architecture of extra leaf canopy should have important implications for yield improvement (Shaver, 1983). The Leafy genotypes of maize have been shown to have increased yield in different areas (Andrews et al., 2000). Recently, the Leafy hybrids have been gaining popularity for silage production (Roth, 2003).

In maize, grain yield is closely associated with kernel number at harvest (Andrade et al., 1999). The physiological basis for higher yield potential in the Leafy genotypes was thought to be related to the photosynthate from additional leaves moving downward to the ear and higher average metabolic rate of younger leaves above the ear (Shaver, 1983). Dijak et al. (1999) found that there was approximately twice as much carbohydrate in the canopy at and above the ear in the Leafy genotypes than in the conventional check. Subedi and Ma (2005) observed that although the Leafy hybrid had overall greater DM, grain yield was not different from conventional hybrids. Andrews et al. (2000) reported that despite the greater amount of carbohydrate in the Leafy hybrids, grain yields were not greatly increased, indicating that the kernel component provided a weak sink. Both sink and source limitations occur in maize, and the particular combination of genotype and environment determines which limitation predetermines grain yield (Tollenaar, 1977).

Kernel set in cereals such as maize and wheat (Triticum aestivum L.) has been associated with intercepted radiation around anthesis (Lizaso et al., 2003). Position of the ear on the plant relative to the site of assimilate production also affects ear growth (Pendleton and Hammond, 1969). Similarly, the position of leaf relative to the ear influences the rate and direction of translocation (Palmer et al., 1973). Upper leaves export principally to the ear during the post-silking period, while lower leaves export relatively less to the ear and more to the lower internodes and roots (Tollenaar, 1977).

The amount and distribution of leaf area and leaf angles in the canopy are major factors determining light interception by the canopy (Elings, 2000; Boedhram et al., 2001; Stewart et al., 2003). Because crop growth rate depends on the amount of intercepted photosynthetically active radiation (PAR), the leaf area index (LAI; i.e., leaf area per unit ground area) has an important role. The canopy structure of maize genotypes especially the Leafy types affects LAI (Stewart et al., 2003). It is interesting to know whether additional leaves above the ear in the Leafy types contribute more to grain yield or if the earleaf still maintains its dominant role. It is important to understand if the lower positioned earleaf in the Leafy types functions similarly to that of conventional types. A defoliation study with removal of leaves from different positions in conventional and Leafy maize hybrids was conducted with the objectives to (i) evaluate and compare the DM production and partitioning patterns in conventional and Leafy maize, (ii) assess the role of individual leaves in Leafy and conventional maize hybrids, and (iii) determine the effect of defoliation of leaves from different position of maize plant on grain yield and total DM production.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Experiment Management
One conventional (Pioneer 3893) and one Leafy maize (Maizex LF850 RR) hybrids were planted in a randomized complete block design with four replications at the Central Experimental Farm, Ottawa, Canada (45°22' N, 75°43'' W) during the 2003 and 2004 growing seasons. These hybrids were chosen on the basis of the fact that Pioneer 3893 is the most popular hybrid in the region, while Maizex LF 850 RR with a similar phenology is a newly registered Leafy hybrid. The experiment was planted within the same area but on different sections of the land in two years. The soil was a brown sandy loam (Typic Haplorthod). Before maize planting, soil samples from 0- to 30-cm depth were analyzed, which contained 6.6 µg g^–1 NO₃–N, 133 µg g^–1 phosphorus (P, Bray), and 128 µg g^–1 soil test potassium (K) with a pH value of 6.9 in water in 2003 and 5.3 µg g^–1 NO₃–N, 42.8 µg g^–1 P, and 101 µg g^–1 K with a pH value of 6.8 in 2004. Each hybrid was planted at 75000 plants ha^–1 in 12-row plot with 9 m in length and 76 cm between rows on 3 June 2003 and 13 May 2004. Before maize planting, P and K fertilizers were applied according to the soil test recommendation (0 kg P and 50 kg K ha^–1). At the V6 growth stage (Ritchie et al., 1993), N fertilizer as NH₄NO₃ was applied at 150 kg N ha^–1 in double bands in furrows. Herbicides Dual II Magnum (S-metolachlor/benoxacor) at 1.75 L ha^–1 + Field Star (flumetsulam) were applied pre-planting to control weeds.

Leaf Removal Treatments
At tasselling (VT), 20 uniform plants were selected with the same date of silking and marked from the middle portion of the four central rows of each plot. While selecting, plants without proper borders or with uneven spaces and plants with more than one ear per plant were avoided. The following 10 leaf removal treatments were imposed randomly on two plants within each plot at silking (R1): (i) control-no leaf removal, (ii) removal of all leaves below the earleaf, (iii) removal of all leaves above the earleaf, (iv) removal of earleaf, (v) removal of first leaf above the earleaf, (vi) removal of second leaf above the earleaf, (vii) removal of third leaf above the earleaf, (viii) removal of fourth leaf above the earleaf, (ix) removal of fifth leaf above the earleaf, and (x) removal of sixth and rest of the leaves above the earleaf. Altogether there were eight plants for each defoliation treatment (two plants x four blocks), and 160 plants treated in each year (two hybrids x 10 treatments x two plants per block x four blocks). For each defoliation treatment, only leaf blades were removed leaving the leaf sheath intact on the stalk. From each leaf removed treatment, the total number of leaves, and leaves below and above the ear-node were recorded. Areas of the removed leaves were immediately measured with a LI-COR Leaf Area Meter (Model LI-3000, LICOR, Lincoln, NE). The leaf sample was then oven dried at 80°C for >72 h, and DM was recorded. For the leaf area and DM of the whole plant (i.e., control plants) at silking, eight plants (two plants from each block) for each hybrid were cut at ground level. Total number of leaves, area and DM of leaves, and stalks were recorded. Leaf area index (LAI) was calculated as a ratio of total leaf area per plant (cm²) to ground area occupied by an individual plant (i.e., 0.133 m²). For the net LAI of each treatment, the area of the removed leaf in each treatment was subtracted from the total leaf area of the control plants. To assess whether the leaf removal treatments affected leaf senescence and leaf chlorophyll content of earleaf, leaf greenness was measured on the earleaf of each plant at silking (R1), R2, and R4 stages with a chlorophyll meter (SPAD–502; Minolta Camera Co. Ltd., Japan) in 2004 season. The measurements were made at three points (basal, middle, and top third) of each earleaf except in the Treatment 4, where the earleaf was removed. The average value of three readings was used at each time of measurement.

At physiological maturity (PM), control plants and treated plants were harvested at ground level, leaf blades and ears were separated from the stalk, and husks were added to the leaves. Length of ear (after husk removal), and ear diameter were measured in the fresh ear. All plant parts were oven dried at 80°C to constant weight. The number of kernels per ear was counted, and the DM of kernel, leaves, cobs, and stalk were recorded.

Data Analysis
The experimental data were subjected to analysis of variance using the general linear model (SAS Inst., 1996). The parameters of the control plants of two hybrids were compared by a t test, while the complete experimental treatments (i.e., two hybrids x 10 treatments) were analyzed as a split-plot design; hybrid as the main plot and defoliation treatments as subplots with four replications (Table 1). Within each treatment, average value for each parameter was obtained from the two plants per plot. Treatment mean differences within each hybrid were separated by the least significant test (LSD_0.05) when the F tests were significant (P 0.05).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

View larger version (30K): [in this window] [in a new window]   Fig. 1. Total precipitation (mm) and mean air temperatures (°C) in 10-d intervals during 2003 and 2004 growing seasons at the experimental site at Ottawa.

View larger version (30K): [in this window] [in a new window]   Fig. 2. Green leaf area (cm2 plant–1) of the removed leaf or leaves in different leaf removal treatments as compared with the control treatment (T1) at silking stage of a conventional (Pioneer 3893) and Leafy (Maizex LF850 RR) maize hybrid in 2003 and 2004. The sum of leaf areas below the earleaf, above the earleaf, and earleaf makes total leaf area of a plant. The ± error bars are the standard deviations of eight plants. The leaf removal treatments are: T1, no leaf removal (control); T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed.

View larger version (40K): [in this window] [in a new window]   Fig. 3. Final leaf area index (LAI) after different leaf removal treatments at silking stage of a conventional (Pioneer 3893) and Leafy (Maizex LF850 RR) maize hybrid in 2003 and 2004. The ± error bars are the standard deviations. The leaf removal treatments are: T1, no leaf removal (control); T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed.

View larger version (30K): [in this window] [in a new window]   Fig. 4. Leaf dry matter (g plant–1) of the removed leaf or leaves in different leaf removal treatments as compared with the control treatment (T1) at silking stage of a conventional (Pioneer 3893) and Leafy (Maizex LF850 RR) maize hybrid in 2003 and 2004. The ± error bars are the standard deviations. The leaf removal treatments are: T1, no leaf removal (control); T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed. Leaf dry matters per plant of the control plants were 28.2 g and 27.7 g for Pioneer 3893 and 39.3 g and 38.1 g for Maizex LF850 RR, respectively in 2003 and 2004.

Dry Matter Production and Partitioning
Generally, plants of both hybrids were bigger in size with greater total DM at PM in 2004 than in 2003 (Table 3), possibly because of longer growing season. However, the pattern of DM partitioning was consistent in both years. At the R1 growth stage, the Leafy hybrid had significantly greater leaf, stalk, and total plant DM than the conventional hybrid, but there was no difference in total DM between the two hybrids at PM (Table 2). Similarly, at PM, both leaf and stalk DM were significantly greater in the Leafy hybrid, but kernel DM was not different. Therefore, the total DM per plant was not different between the two hybrids. Although, the Leafy hybrid set significantly greater number of kernels per ear than the conventional hybrid in both years, this was offset by the smaller mass of individual kernels (Table 2). From R1 growth stage to PM, total plant DM in the conventional hybrid was changed by 156% in 2003 and 197% in 2004 as compared with only 86% in 2003 and 161% in 2004 in the Leafy hybrid (Table 2). Therefore, the final DM per plant was not different between the two contrasting hybrids. The smaller rate of change in DM production from R1 to PM in the Leafy hybrid was presumably associated with greater mutual shading.

Removal of leaves above the earleaf had much greater effects in the Leafy hybrid, especially for cob size and kernel set (Fig. 5 and 6) , kernel DM (Fig. 7) , and total DM production (Table 3). The percentage reduction in kernel number, kernel DM, and total DM per plant as compared with the control plant are shown in Table 3. The removal of all leaves above the earleaf virtually impaired kernel set in the Leafy hybrid: there were some ears without any grains (Fig. 5), while some plants set a few grains but the overall size of the ear, cob, and kernels were much reduced. Compared with the non-leaf removal control plants, the number of kernels for the leaf removal treatment (i.e., removal of all leaves above the earleaf) was reduced by 84% in 2003 and 94% in 2004 in the Leafy hybrid, while the corresponding reduction in the conventional hybrid was only 33 to 37% (Table 3). It was interesting to note that removal of any individual leaves above the earleaf had no significant effect on kernel number in the conventional hybrid in both years. However, removal of the third leaf above the earleaf had significantly reduced the number of kernels in the Leafy hybrid in both years. Similarly, the removal of the sixth leaf and all other leaves above it had substantial effect on kernel set (Fig. 5 and 6). Similar to kernel number, kernel DM was not reduced by the removal of any individual leaf above the ear in 2003, but in 2004, removal of earleaf + 2 had significantly fewer kernels in the conventional hybrid. However, in the Leafy hybrid, removal of the third leaf above the earleaf in 2003, and removal of the second, third, and fourth leaves above the earleaf in 2004 caused significant reduction in kernel DM (Fig. 7; Table 3). Removal of the sixth leaf and rest of the above leaves (i.e., additional 4–5 leaves than the non-Leafy maize) also reduced the kernel DM by 31% in 2003 and 39% in 2004 (Table 3). There was also a significant effect of different leaf removal treatments on total DM (Table 3). Removal of the earleaf, all leaves below the earleaf, all leaves above the earleaf, earleaf + 1 and earleaf + 4 had significant effects on total DM of the conventional hybrid in 2003, primarily as an effect of reduced cob and kernel weights. In 2004, there was also a significant effect on the DM because of removal of the first, third, fourth, and fifth leaf above the ear. In the Leafy hybrid, removal of all leaves above the ear resulted in the greatest reduction in total DM, by 49% in 2003, and 59% in 2004. The other treatments that caused significant reduction in total DM of the Leafy hybrid were removal of the third leaf above the earleaf in 2003 and second, third, and fourth leaves above the earleaf in 2004. The removal of the sixth leaf and rest other leaves above it reduced total DM by 18% in 2003 and 34% in 2004 as compared with the control treatment in the Leafy hybrid (Table 3). This indicated that the topmost quarter of leaves also played an important role for yield and total DM production in the Leafy hybrid.

View larger version (119K): [in this window] [in a new window]   Fig. 5. Ears of maize from one of the blocks at harvest: Pioneer 3893 (upper row) and Maizex LF 850 (lower row) as affected by different leaf removal treatments (T1, no leaf removal; T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed). The Pioneer 3893, was limited up to T9.

View larger version (41K): [in this window] [in a new window]   Fig. 6. Effect of different leaf removal treatments on the number of kernels per ear in Pioneer 3893 and Maizex LF850 RR maize hybrids in 2003 and 2004. The leaf removal treatments are: T1, no leaf removal; T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed. The Pioneer 3893 was limited up to T9. The bars with the same letter in each hybrid are not significantly different by LSD0.05 test.

View larger version (44K): [in this window] [in a new window]   Fig. 7. Effect of different leaf removal treatments on the kernel dry matter in Pioneer 3893 and Maizex LF850 RR maize hybrids in 2003 and 2004. The leaf removal treatments are: T1, no leaf removal; T2, all leaves below the earleaf removed; T3, all leaves above the earleaf removed; T4, earleaf removed; T5, earleaf + 1 removed; T6, earleaf + 2 removed; T7, earleaf + 3 removed; T8, earleaf + 4 removed; T9, earleaf + 5 removed, and T10, earleaf + 6 + all other leaves removed. The Pioneer 3893 was limited up to T9. The bars with the same letter in each hybrid are not significantly different by LSD0.05 test.

View larger version (26K): [in this window] [in a new window]   Fig. 8. Relationships between percentage of leaf area removed and percentage reduction in grain yield (kernel DM) in the two hybrids of maize in two years. The bold line is the linear fit for the Maizex LF 850 RR while the thin line is for the Pioneer 3893.

Cob DM was also highly affected by the leaf removal treatments. When all leaves above the ear were removed, cob DM was reduced by as much as 90% in the Leafy hybrid as compared with 54 to 58% in the conventional hybrid. In 2004, cob of the Leafy hybrid was not developed at all when all the leaves above the ear were removed (Fig. 5). This indicates that even if the earleaf and all leaves below the earleaf were present, they did not contribute to cob development.

Effect on Leaf Greenness
The measurement of leaf chlorophyll content (i.e., SPAD) in the earleaf at R1, R2, and R4 in 2004 showed no effect of leaf removal treatments (data not shown). However, the two hybrids differed significantly and the stage of measurement had a significant effect on SPAD readings (Fig. 9) . The conventional hybrid had significantly higher SPAD values than the Leafy hybrid at all stages of measurements, and the SPAD readings were smaller as the crop advanced from R1 to R4 in both hybrids.

View larger version (17K): [in this window] [in a new window]   Fig. 9. Differences in SPAD readings in the earleaf of two maize hybrids (Pioneer 3893 and Maizex LF850 RR) as affected by the stage of crop development (R1, R2, and R4 growth stages) in 2004. The vertical bars are the LSD0.05 values to separate the two hybrids at each growth stage.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Why the large leaf area and LAI in the Leafy hybrid was not more productive than the conventional hybrid could be explained by the fact that in the Leafy hybrid, 69% of stalk length and 60 to 67% of leaf area were located above the ear-node. The larger leaf area in the upper part of the canopy probably caused more mutual shading as the earleaf was positioned only at 31% of the total plant height compared with at 47% in the non-Leafy hybrid. Thus, the lower contribution of the earleaf in the Leafy hybrid could be explained with three possible reasons individually or in combination: (i) the loss of the earleaf was compensated by the large number of other leaves above the ear, (ii) the earleaf was far below in the canopy, which was heavily shaded by the other leaves, and (iii) probably the photosynthetic efficiency of the earleaf was low during the grain filling since it emerged and senesced earlier than the earleaf of the conventional hybrid.

As evidenced by the lower SPAD readings in the Leafy hybrid at all stages of measurement, it can be argued that either the photosynthetic potential of earleaf in the Leafy hybrid was low or assimilate in the earleaf was not translocated to the growing cob. However, the lower SPAD value in the Leafy maize was an inherent phenotypic character even when that genotype was grown under adequate nitrogen supply (Subedi and Ma, 2005). Therefore, it can be argued that the lower leaves in the canopy were still green, but their contribution to kernel fill was minimal. The nonsignificant treatment difference in SPAD readings due to leaf removal treatments indicates that removal of any single leaf or leaves above or below the earleaf did not alter the greenness of the earleaf.

The different defoliation treatments created substantial effects on kernel set and total DM production. However, the interaction between hybrid and treatment was significant in kernel set and total DM production. The importance of the earleaf differed between the two types of architecture: in the Leafy hybrid, removal of the earleaf had no significant effect on kernel yield and yield components, while there was a substantial yield reduction in the conventional hybrid. When the rate of increase in total DM from R1 to PM was calculated, the conventional hybrid had a rate of 3.1 g d^–1 in 2003 and 3.5 g d^–1 in 2004, but only 2.5 g d^–1 in 2003 and 3.3 g d^–1 in 2004 in the Leafy hybrid (Table 2). The lower rate of change in DM from R1 to PM in the Leafy hybrid, especially in 2003 was possibly because the leaf removal treatments in 2003 were initiated after tassels were exerted. We noticed that in the Leafy hybrid, silks were emerged at least 3 to 4 d earlier than the tassel. Therefore, the ears of the Leafy hybrids were slightly advanced at the time of leaf removal. This could be the reason why the DM of the Leafy hybrid at R1 in 2003 was greater than in 2004, although the total DM at PM was much greater in 2004. Another explanation for the lower rate of DM production after R1 in the Leafy hybrid could be because of the over-crowding and mutual shading of the lower leaves in the canopy, which resulted in an overall lower efficiency of the leafy canopy and a counterproductive leaf area.

This study also supported the earlier findings (e.g., Tollenaar, 1977; Boyle et al., 1991; Lizaso et al., 2003) that post-silking assimilates are the most important source for kernel set. There was a severe shortage of assimilate following leaf removal, which resulted in a ceased cob growth and very high kernel abortion, especially in the Leafy hybrid (Fig. 5). Kernel abortion in maize has been linked to a shortage of current assimilate supply to developing kernels (Boyle et al., 1991; Lizaso et al., 2003). The stage bracketing silking is the most susceptible to stress for kernel number (Tollenaar and Daynard, 1978). The Leafy hybrid had higher stalk DM at silking (Table 2), but this reserved carbohydrate did not support kernel set or growth when all leaves above the earleaf were removed. The removal of leaves not only affected the kernel set, but there was also a significant reduction in cob size and individual kernel size, indicating an insufficient supply of assimilates for grain fill. Although we did not analyze the carbohydrate content in this study, the effect on grain-fill especially when all leaves above the earleaf were removed indicates that assimilate reserved in the stalk before silking was not remobilized to meet the requirement for grain filling.

In the Leafy hybrid, the lower leaves including the earleaf did not contribute much to grain filling. In contrast, in the conventional hybrid, the earleaf had an important role. When there were no top leaves above the earleaf, 50% of the kernels were reduced, and when there was no earleaf, 15 to 25% kernels were reduced in the conventional hybrids. This indicates that the photosynthetic efficiency and contribution to kernel development of the earleaf in the conventional hybrid was greater than in the Leafy counterpart. The contribution of individual leaves other than the earleaf to grain DM was also detected in this study. The leaves up to the third leaf above the ear were of primary importance (Fig. 6). Rajcan et al. (1999) also found that the effect of defoliation was greater for earleaf and earleaf + 3 than earleaf-3. Pendleton and Hammond (1969) demonstrated that the relative photosynthetic potential of maize leaves in the top one-third of the canopy was twice as high as the middle leaves and five times as high as the bottom third. Tollenaar (1977) also suggested that the lower leaves export relatively less to the ear and more to the lower internodes and roots. This also implies in our study that contribution of the additional leaves at the top (i.e., the topmost five leaves) to kernel yield was significant in the Leafy hybrid. This is most likely due to the fact that the upper leaves in the Leafy hybrid were younger, had higher photosynthetic and metabolic capacity, and less shading compared with lower leaves.

In summary, this study using a unique methodology, demonstrated the roles of leaves from different position in the Leafy and conventional maize hybrids to grain yield and total DM production. This study rejected the hypothesis that large number of leaves above the ear in the Leafy hybrid contribute to higher grain yield and total biomass production. It can be concluded that the Leafy maize maybe suitable for silage production because of its larger portions of leaf biomass relative to silage yield, but it may not be suitable for high grain yield because of physiological limitations as discussed above. The earleaf and lower leaves in the Leafy hybrid were probably photosynthetically less efficient than in the conventional hybrid. The individual leaves above the earleaf played significant roles in grain filling and final kernel DM but the overall contribution of the large canopy was not evident, which was most likely due to a higher level of mutual shading in the canopy. In both years and both types of hybrids, the DM of cob, kernel, stalk, and the entire plant were greatly influenced by the leaf removal treatments. However, there was a contrasting response of the two hybrids: earleaf and all leaves below the earleaf had significant contributions to ear development and final grain yield in the conventional hybrid, but these leaves played nonsignificant roles in the Leafy hybrid. On the other hand, for the Leafy hybrid, removal of individual leaves above the ear appeared to play a large role in grain filling, while removal of all the leaves above the ear jointly showed the greatest effect; there was virtually no grain set or grain filling when all leaves above the ear were removed, nor was there any compensation through increased individual kernel weight. In addition, earleaf and all other leaves below the ear were not able to support kernel development in the absence of all leaves above the earleaf in the Leafy hybrid. These findings have significant implications in physiological studies, such as crop modeling, development of more efficient leaf architecture in future breeding work, and best management practices for grain and silage production through modification in planting pattern and population density. Consequently, further studies with manipulation of plant population density, planting arrangements, and nitrogen amendments are needed to improve the light interception and DM partitioning in the Leafy maize.

	ACKNOWLEDGMENTS

 
The excellent technical assistance of D. Balchin and L. Evenson of Agriculture and Agri-Food Canada is gratefully acknowledged. ECORC Contribution No: 04-460.

Received for publication November 9, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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