Research Article WHEAT PHASIC DEVELOPMENT UNDER DIFFERENT SOWING WINDOWS UNDER RAINFED CLIMATE MUHAMMAD ASIM 1 *, MUHAMMAD ASLAM 2 , SYED HAIDER ABBAS 3 AND ABID MAJEED 4 1 Plant Sciences Division, Pakistan Agricultural Research Council, Islamabad, Pakistan. 2 Ministry of National Food Security and Research, Islamabad, Pakistan. 3 Wheat Programe, Crop Sciences Institute, National Agricultural Research Centre, Islamabad Pakistan. 4 Crop Sciences Institute, National Agricultural Research Centre, Islamabad Pakistan. Email: hakazmi79@gmail.com Received:17 July 2013, Revised and Accepted:18 August 2013 ABSTRACT In rainfed areas of Pakistan, erratic rainfall causes variation in sowing window of wheat starting from mid of October and extends until the end of December. The development of wheat plant is a continuity of vegetative phase, reproductive phase and the grain-filling phase. So, to determine the changes in timing of developmental events during the crop-growing season Wheat cultivar, Wafaq-2001, was planted in four sowing windows (22 October, 13 November, 4 December and 24 December) in the field area of National Agricultural Research Centre (NARC), Islamabad. Observations of crop phenology began at the emergence of first leaf and continued until the maturity when yield data was measured. The study revealed that environmental factors play an important role in determining the duration of each phase in the given agro-ecological conditions. The duration of these phases is strongly influenced by the changes in sowing dates. The major components of the environment that affect development are temperature and photoperiod. There are association between sowing window and the yield and also between the duration of phases and yield. So, it becomes important to be able to manipulate the duration of these phases to customize cultivars for specific environments and get the potential yield in a given environment on sustainable basis. Keywords: Wheat, phasic development,phenology, sowing windows Introduction Crop development is a sequence of phenological events controlled by external factors (Landsberg, 1977). These developmental phases controlled by genetics are also affected by environmental factors. The effects of certain environmental factors on crop growth differ depending upon the developmental stages when these factors act. The development of wheat plant is a continuity of vegetative phase, when the leaves are initiated: the reproductive phase, when floret development occurs until the number of fertile florets (the number of grains) is determined; and the grain-filling phase, when the grain first develops the endosperm cells and then grows to determine the final grain weight (Miralles and Slafer, 2000). The duration of each phase is generally determined by interactions between genetic and environmental factors. Changes in sowing dates can strongly modify the duration of different developmental phases (Hay, 1986). Wheat sown at different dates pass through each developmental stage at slightly different times and, therefore, under different environmental conditions. Thus any one of the developmental stages, which determine the components of yield, could occur under more (or less) favorable conditions. The major components of the environment that affect development are temperature and photoperiod. Temperature has the most significant influence on the rate of leaf appearance and thus the concept of thermal time is frequently used to analyze the dynamics of leaf appearance (Slafer and Rawson, 1997). Nix (1976) showed that although temperature and radiation influence plant processes differently, their combined effects can be usefully described by a photothermal quotient (PTQ) which is the ratio of total solar radiation to the mean daily temperature minus a base temperature. It assumes development rate linearly relates to mean temperature. Many investigators have reported a highly significant linear correlation of leaf number on the main stem of wheat and cumulated degree-days (CDD) (Baker et al., 1980; Klepper 1982; et al., Kirby et al., 1985). Objectives of this study were to determine the changes in timing of developmental events during the crop-growing season in field- grown plants and determine the effects of major environmental factors controlling the timing and duration of different developmental phases by comparing between crops that were sown in the field at different dates. Materials and Methods Wheat cultivar, Wafaq-2001, was planted in four sowing windows in the field at National Agricultural Research Centre, Islamabad (33o 43’N, 73o 06’E and 547 m above sea level). Three 4.5-by-10 m plots per sowing window were sown on 22 October, 13 November, 4 December and 24 December. Row spacing was 15 cm and plant population was 250 plants m-2. 100 kg/ha of N and P (as urea and DAP) was applied. Weed control was done manually. The fields were irrigated whenever the soil surface appeared dry to maintain an adequate water supply. Observations of crop phenology were made, using Zadok’s scale (Zadok, 1974), of ten plants per plot. These measurements began at the emergence of first leaf and continued until the maturity when yield data was measured from each plot and averaged to get the mean. Weather data (maximum and minimum daily air temperature, rainfall and sunshine hours) was recorded at weather station at National Agricultural Research Centre. Daily growing degree-days (DD) were calculated as: DD = [{(T max + T min) / 2} – T b Where T max and T min are maximum and minimum air temperatures and T b is a base temperature. We use 5°C as the base temperature. PTQ was calculated following the algorithm described by Monasterio et al., (1994): If T > 10 PTQ day-1 = solar radiation/ (T-4.5); If T ≤ 4.5 PTQ day-1 = 0; and If 4.5 < T ≤ 10. PTQ day-1 = solar radiation * [(T – 4.5 /5.5]/5.5 Where T is the daily mean temperature [(max + min)/2] and PTQ was expressed as MJm-2 day-1oC-1. Vol 1, Issue 2 , 2013 ISSN: 2321-6832