MATERIALS AND METHODS

The experiment was conducted under field conditions at the Agronomic Research Area, University of Agriculture, Faisalabad during Autumn 1996. The treatments comprised four nitrogen levels (0, 100, 200 and 300 kg N ha^–1) and three plant densities (11.11, 8.33 and 6.67 plants m^–2). The experiment was laid out in randomized complete block design with split plot arrangement and three replications. Nitrogen levels and seeding densities were placed in main and sub plots, respectively. Net plot size was 2.4×7 m.

The crop was sown on August 10, 1996. A basal dose of phosphorus was applied at 90 kg ha^–1. All the phosphorus and one-third of nitrogen was incorporated in the soil at sowing, while remaining nitrogen was applied in two equal splits at first irrigation (10 days after sowing) and at tassel initiation. The crop was sown with the help of a dibble at plant spacings of 60×15, 60×20 and 6×25 cm with 2 seeds per hill and subsequently thinned out at 3-4 leaf stage to obtain one plant per hill in order to attain the plant densities as per treatments. All other agronomic practices were kept normal and uniform for all the treatments.

Number of plants m^–2 at harvest, number of cobs per plant, number of grains per cob, 1000-grain weight, grain weight per cob, leaf area index, harvest index, plant mortality and sterility were recorded by using the procedures described by Saeed et al. (1997), while protein content was determined by the method of sulphuric acid digestion and distillation by Micro-Kjeldahl method (Jackson, 1960). Data collected were analysed statistically using the Fisher’s analysis of variance technique and treatment means were compared by using the least significant difference (LSD) test at 5% level of probability (Steel and Torrie, 1984). Correlation and regression analysis were carried out using the Haward Graphic (HG3) computer programme.

RESULTS AND DISCUSSION

Since both the individual and interactive effects of treatments on all the parameters under study were significant (Table 1 and 2), only the interactive effects are discussed.

Number of plants m^–2: The maximum number of plants m^–2 (11.4) at harvest was recorded in the crop raised at a density of 11.11 plants m^–2 and fertilized at 300 kg N ha^–1 (N₃D₁) that was, however, statistically on a par with the cob raised at 11.11 plants m^–2 but fertilized at 200 kg N ha^–1 (Table 1). On the contrary, the lowest number of plants m^–2 was in the control crop raised at 6.67 plants m^–2 (N₀D₃). This variation in plants m^–2 was due to the variable plant density and probably because of different nitrogen supply as nitrogen has been reported to affect plant survival in maize.

Table 1:	Effect of nitrogen level and plant density on number of total and cob bearing plants m–2 at harvest, leaf area index, plant mortality and sterility
Any two means not sharing the same latter differ significantly from each other at p = 0.0.5

Table 2:	Yield, yield components and grain protein content of maize cv. "Golden" as affected by nitrogen level and plant density
Any two means not sharing the same latter differ significantly from each other at p = 0.0.5

Fig. 1:	Relationship between a) number of cobs per plant, b) No. Of grains per cob c) 1000-grain weight and grain yield of maize respectively at different nitrogen levels and planting densities.

A similar decrease in plant stand in response to increased plant density and at lower nutrient availability has also been reported by Kang and Park (1986).

Number of cob-bearing plants m^–2: The maximum number of cob-bearing plants (10.2) was produced by the crop fertilized at 300 kg N ha^–1 with 11.11 plants m^–2 (N₃D₁) (Table 1), while the minimum number of cob-bearing plants (5.8) was recorded in the control crop raised at 6.67 plants m^–2 (N₀D₃) (Table 1). The results suggested that elevated nitrogen levels along with higher plant density increased the number of cob-bearing plants m^–2. Greater number of cob-bearing plants m^–2 at higher nitrogen level and higher plant density was ascribed to greater number of total plants m^–2 at harvest. Significant interactive effects of nitrogen and plants density have also been reported by Ali et al. (1986).

Plant mortality: The maximum plant mortality (11.79%) was observed in the control crop grown without N fertilization and with 11.11 plants m^–2 that was however, statistically on a par with the control crop with 8.33 plants m^–2 (Table 1). On the contrary, the lowest mortality was recorded in the crop with 6.67 plants m^–2 and fertilized at 200 and 300 kg N ha^–1, respectively. The results suggest that decreased nitrogen level along with higher plant density increase the mortality rate.

Plant sterility: The maximum sterility (9.1%) was observed in the control crop with 11.11 plants m^–2 (Table 1). On the contrary, no sterility was observed in the crop fertilized at 300 kg N ha^–1 along with 6.67 plants m^–2. The results suggest that higher nitrogen and lower plant density cause a significant reduction in plant sterility. Kang and Park (1986), also reported reduced sterility with increased nitrogen supply and at lower plant density.

Leaf area index: The maximum leaf area index (LAI) (5.34) was recorded in the crop fertilized at 300 kg N ha^–1 with 11.11 plants m^–2 (N₃D₁), followed by N₂D₁ and N₃D₂ (Table 1). On the contrary, the minimum LAI (1.62) was observed in the control crop with 6.67 plants m^–2 (N₀D₃). It is obvious that both reduced supply of nitrogen and lower plant density caused a substantial reduction in LAI of maize. These results are in line with that of Kang and Park (1986), who reported a substantial increase LAI by increasing nitrogen and plant density. The variable LAI in treatment combinations was attributed to different leaf area and ground area per plant. These results suggest that adequate nitrogen supply and proper plant density are essential for attaining the optimum LAI, the most important indicator of size of the assimilatory system in maize to maximize harvest of the incident solar radiation.

Number of cob per plant: The maximum number of cobs per plant (1.33) was produced by the crop with 6.67 plants m^–2 and fertilized at 300 kg N ha^–1 (N₃D₃) that was, however, statistically equal to N₂D₃ (1.27) (Table 2), while the minimum number of cob per plant (0.90) was recorded in the control crop at a density of 11.11 plants m^–2 (N₀D₁). Greater number of cobs per plant at lower plant density could be due to less plant competition for different production factors such was water, nutrients, etc. The suppressive effect of higher density on number of cobs per plant has also been recorded by Putnam at al. (1986) and Ramison and Lucas (1982). Similarly nitrogen deficiency results in less number of cobs per plant of maize (Short at al., 1982).

A positive but weak correlation was observed between grain yield and number of cobs per plant (Fig. 1a). Regression analysis also indicated dependence of grain yield on number of cobs per plant.

Number of grains per cob: The maximum number of grain per cob (544) was found in the crop with 6.67 plants m^–2 and fertilized at 300 kg N ha^–1 (N₃D₃) that was, however, statistically on a par with N₂D₃ (536 and N₃D₂ (530)) (Table 2). On the contrary, the minimum number of grains per cob (220) was produced by the control crop with 11.11 plants m^–2 (N₀D₁). The increased nitrogen supply and reduced plant density enhanced the number of grains per cob. These results are in line with findings of Liang at al. (1992) who reported that high plant density and lower nitrogen level suppressive effect on number of grains per cob as nitrogen plays an important and essential role in grain formation (Arnon, 1972).

A strong positive correlation was observed between grain yield and number of grains per cob (Fig. 1b). Regression analysis indicated a strong dependence of grain yield on number of grains per cob.

1000-grain weight: The crop raised at 6.67 plants m^–2 and fertilized at 300 kg N ha^–1 (N₃D₃) had the maximum 1000-grain weight (128.6 g) that was, however, statistically on a par with N₃D₂ (127.2 g) and N₂D₃ (126.9 g) (Table 2). On the contrary, the minimum 1000-grain weight (95.3 g) was recorded in the control crop with 11.11 plants m^–2 (N₀D₁). The results indicated that grain weight increased gradually by increasing nitrogen level but decreasing plant density. More 1000-grain weight at elevated nitrogen levels and reduced plant density was probably attributed to the better growth facilities especially more nitrogen availability at lower plant density then that at elevated plant density and inadequate supply of nitrogen. Similar results have been reported by Amano and Salazar (1989) who stated that more 1000-grain weight was associated with reduced plant density and higher nitrogen rate.

A strong positive correlation was observed between grain yield and 1000-grain weight (Fig. 1c). Regression analysis showed a strong dependence of grain yield on 1000-grain weight.

Grain yield: The maximum grain yield (7.11 t ha^–1) was produced by the crop with 11.11 plants m^–2 fertilized at 300 kg N ha^–1 (N₃D₁) that was, however, statistically on a par with that of N₂D₁ (6.97 t ha^–1) (Table 2). On the contrary, the minimum grain yield (2.03 t ha^–1) was recorded in the control crop with 6.67 plants m^–2 (N₀D₃). Thus an increase in the nitrogen supply and plant density enhanced maize grain yield. These results concur with the findings of Nedic at al. (1991). The maximum grain yield ha^–1 at the highest nitrogen level and plant density was primarily attributed to the most dense plant stand and not to the other individual yield components which were suppressed at higher plant densities due to competition among plants for various development determinants such as light, water nutrients (other then nitrogen) etc.

Harvest index: The maximum harvest index (HI) of 41.59% was recorded for the crop raised at 11.11 plants m^–2 and fertilized at 300 kg N ha^–1 (N₃D₁) that was, however, on a par with N₂D₁ (41.25%) and N₃D₂ (40.77%) (Table 2) the contrary, the lowest HI (18.88%) was observed for the crop with 11.11 plant m^–2 and no nitrogen fertilization (N₀D₁). The suppressive effects of lower nitrogen level and elevated plant density on HI may be ascribed to the immense interplant competition which exhibited more adverse effect on dry matter accumulation by against than other plant parts. These results are in line with Asare et al. (1989) who observed the maximum HI at higher plant density and nitrogen level.

Grain protein content: The maximum grain protein content (10.53%) was produced by the crop fertilized at 300 kg N ha^–1 and with the lowest plant density (N₃D₃) that was however, statistically equal to N₃D₂ (Table 2). On the contrary, the minimum grain protein content (6.15%) was recorded in N₀D₁. These results are in line with those of Arnon (1972) who also reported high grain protein content at elevated nitrogen levels and lower plant density. Reduced protein content at lower nitrogen level and dense plant stand might be due to the limited nitrogen availability. However, higher nitrogen level minimize the interplant competition for nitrogen and thus resulted in high grain protein content.

CONCLUSION

Application of 200-300 kg N ha^–1 along with 11.11 plants m^–2 should be used to harvest the maximum grain yield of cv. "Golden" under the edaphic and climatic conditions matching to those of the experiment.