The cultivation of wheat (Triticum spp) reaches far back into history. Wheat was one of the first domesticated food crops and for 8000 years has been the basic staple food of the major civilizations of Europe, West Asia and North Africa. Wheat is grown across a wide range of environments and is considered to have the broadest adaptation of all cereal crop species. This is, to a large extent, due to its tolerance to cold. Winter and spring wheats are terms that have been adopted for different types of wheat depending on the sowing time. A ‘winter wheat' is sown before winter and‘spring wheat' is sown in spring, however, according to Crofts (1989), the term winter and spring wheat is used based on the presence or absence, respectively, of genes controlling the vernalization requirement (Vrn) of wheat. Unlike spring wheat, most winter wheat is grown under rainfed conditions and is grown on 75 million ha of the 220 million ha devoted to wheat worldwide. Areas requiring wheat cultivars with high levels of winter hardiness include eastern, central and northern Europe, eastern Turkey, northwest Iran and China (Curtis et al., 2002).
The agricultural and physiological specialities of the winter wheat cultivars have more important effect on grain yield. The profitability of winter wheat can be maximised with changes made in such factors as sowing date, coupled with the selection of breed suitable for the area and with the right farming practices (seeding rate). It has been observed by many researchers that wheat breeds give different crop yield by sowing different seed quantity, however the growing season and the sowing date causes significant changes in the yield. Environmental circumstances have been identified as determinative factors in the productivity and yield safety of winter wheat cultivars. For example, the role played by environmental factors in determining the phyllochron, or rate of leaf appearance in grass crops such as wheat has been the subject of extensive research (Mcmaster et al 2003; Bassu et al. 2009).). It has been observed that temperature primarily drives the phyllochron with light (photoperiod and to a lesser extent, quality and intensity) being a secondary factor. Effects of temperature on leaf appearance rates are usually quantified using some form of thermal time and according to Mcmaster et al. (2003), air temperature above the canopy has most frequently been used to calculate thermal time (in growing degree days). Temperature and light regime have a seasonal variation implying that the optimisation of temperature effects on plant growth and development require that plants are sown at well defined times. It is reported in the wheat growth guide (2008) that the rate at which wheat passes through its life cycle may only be managed through the choice of sowing date and of variety. In addition to determining the timing of tillering, sowing date influences the speed of development with later sowings said to develop quicker giving less time for tillering. According to the authors, establishment decreases from around 70% for sowings in September to less than 50% for sowings in November or later and the time between the emergence of two successive leaves measured in thermal time (phyllochron) varies with wheat variety and sowing date. Late sowing has been identified to decrease both the phyllochron and total leaves emerged. Hay and Porter (2006) have illustrated the effect of seasonality on wheat phenology highlighting the seasonal patterns of leaf area index (L) in annual temperate crops. Leaf expansion has been observed to be depressed by low temperature until April/May, when there is a rapid increase in L, associated with higher temperatures and larger individual leaf sizes up to a pronounced peak in June/July. In this report, the authors identified that wheat crop produced an earlier and larger peak L and thereby expressed an earlier advantage over other cereals mainly because the crop, established in the autumn, was able to respond more rapidly to favourable spring temperatures, irrespective of other soil conditions. Albeit the enormous amount of knowledge available regarding the effect of temperature on wheat growth and development, research on the effect of autumn sowing date on springtime canopy establishment is relatively still in the primary stages. It is important therefore to define the optimal sowing date of winter wheat, due to the climate-change of habitats, not only from agronomic, but also from economic point of view to assess the potential for further optimisation of growth and development of cereals following the spring-time temperature rise.
RADIATION CAPTURE OF WHEAT
The canopy architecture and radiation capture characteristics are vital in determining the rate of canopy establishment in any crop. The interception of incident radiation is determined by the spatial arrangement of the above-ground organs of the plants, the distribution of the plants in the community, by the position of the sun and by the proportion of diffuse radiation. An efficient description of the plant canopy structure is therefore required to predict the pattern of radiation interception and to understand the processes adopted by plants to regulate this interception. Hay and Porter (2006) have indicated that with some important exceptions, crop biomass production depends upon resource capture rather than resource utilisation. Muurinen and Peltonen-Sainio (2006) highlighted that biomass production in non-stress conditions is determined by the amount of photosynthetically active radiation (PAR) intercepted by the canopy and the efficiency with which it is used to produce dry matter, referred to as radiation-use efficiency, (RUE). Radiation interception by the crop can be further described in terms of the total amount of incident radiation and the fraction of it that is intercepted by the canopy (f). This is a function of the green leaf area index, (GLAI) and the efficiency with which the green leaf area intercepts solar radiation, described by the light extinction coefficient (k). Monteith (1969) has observed that crops grow and use water and other resources because they intercept radiation from the sun, the sky, and the atmosphere. Diurnal changes of solar radiation dictate the diurnal course of photosynthesis and transpiration, and the vertical gradient of radiant flux in a canopy is a measure of the absorption of energy by foliage at different heights. In addition to the primary function of radiation in providing energy for photosynthesis, other less familiar aspects of radiation distribution may influence the pattern of growth and development in a field crop. Consequently, it is not an overstatement to say that, the distribution of radiation within a plant community is the most important single element of microclimate that influences canopy establishment. Thus, the seasonality of the quality and intensity of radiation produces varying effects on crops sown at different times necessitating the need to investigate the effects of sowing date on canopy establishment.
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Curtis BC, Rajaram S, Macpheson HG (2002) Bread Wheat: Improvement and production. FAO Corporate Documentary Repository - Plant Production and Protection Series. Pp 567.
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