INTERNATIONAL JOURNAL OF AGRICULTURE & BIOLOGY 1560–8530/2004/06–6–1119–1123 http://www.ijab.org Effect of Sowing Date and Plant Density on Growth, Light Interception and Yield of Wheat under Semi Arid Conditions AFTAB WAJID, ABID HUSSAIN, ASHFAQ AHMAD, MUHAMMAD RAFIQ, A.R. GOHEER AND MUHAMMAD IBRAHIM Department of Agronomy, University of Agriculture, Faisalabad–38040, Pakistan ABSTRACT Growth and yield of wheat at different sowing dates (10 Nov., 25 Nov., 10 Dec.) with different plant densities (200, 300 and 400 plants m -2 ) were analyzed in terms of solar radiation intercepted by the leaves during 1998-99 and 1999-2000. Leaf area index, radiation interception and biomass accumulation were measured throughout the growing seasons. The relationship between dry matter production and intercepted photosynthetically active radiation (PAR) was highly significant (r = 0.93**) and linear throughout the growing season for all treatments, and a common regression line (slope) represented a constant mean seasonal efficiency in the conversion of radiation into biomass. The radiation use efficiency (RUE) was 3.65 g MJ -1 for the pooled data. Results showed that the highest yields were obtained from early (November) sowings and a plant density of 300 plants m -2 ; yield variations among treatments were caused by affecting both the amount of intercepted PAR and RUE. Key Words: Sowing date; Plant density; Radiation use efficiency; Yield; Wheat; Pakistan INTRODUCTION Key to increasing the productivity of field crops is to maximize the amount of radiation they intercept (Monteith, 1977). Interception of radiations on leaf surface can not be controlled but can be manipulated for their maximum use by crop husbandry means. The fraction of radiation intercepted by crops increases hyperbolically with LAI; in many crops 80-85% is intercepted when LAI is between 3.0 and 4.0, and 95% when LAI reaches 5.0 (Scott & Jaggard, 1978; Milford et al., 1980). Crop growth can be analyzed in terms of its efficiency to use intercepte radiatins. This approach has been applied to many field crops (Scott et al., 1973; Gallagher & Biscoe, 1978; Muchow, 1989; Hussain & Field, 1993; Hussain et al., 1998; Hussain et al., 1998, 1999, 2002). The relationship has been used as a basis for theoretical investigations into tropical crop productivity, modeling climate effects and the importance of light as a limiting factor in crop performance (Monteith, 1972, 1973, 1981). Under adequate supply of water and nutrients, wheat yield has been shown to be closely related to the amount of radiation intercepted during the growing season. Gallagher and Biscoe (1978) showed that 3 g dry matter (DM) of wheat was produced by each mega joule (MJ) of photosynthetically active radiations (PAR) absorbed until ear emergence. For the whole season about 2.2 g DM was produced per MJ absorbed. Such measurements provide a useful index of the production efficiencies of crops in different regions. Scanty information is available on these aspects of productivity of wheat or other crops in Pakistan. This paper presents the effects of sowing date and planting density on growth, light interception and yield of wheat under semi arid conditions. MATERIALS AND METHODS The study was conducted at the Agronomic Research Area, University of Agriculture, Faisalabad, (31º.76 ,  75º.06 , 104.4 m) during 1998-1999 and 1999-2000. The experiment was laid out in a randomized complete block design with split plot arrangement; sowing dates in main plots and seeding rates in sub-plots. There were three replications having a plot size of 1.6m x 10m. The treatments were three sowing dates (10 and 25 Nov., and 10 Dec.) and three planting densities (200, 300 and 400 plants m -2 ). The seed was sown with the help of a single row hand drill. Nitrogen and phosphorus @ 120 and 100 kg ha -1 , respectively, were applied in both the years. Half of the nitrogen and full phosphorus was applied at sowing and remaining half of nitrogen was top dressed with first irrigation. All other practices such as hoeing, irrigation, weeding, etc. were kept uniform in both seasons. Estimates of dry weight (oven dry) and leaf area were made weekly from harvesting 20cm long row from each plot at ground level. Leaf area was measured from a sub sample of 10 g of green leaf lamina by an area meter (Li- cor, Model 3100). Tube solarimeters were used to measure the radiation interception by the crop canopy. Accumulated intercepted radiation was calculated by summing the products of proportional interception values, and half of the total daily incoming solar radiation was considered as PAR (Szeicz, 1974).