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Climate change is likely to significantly alter productivity in arable crops. Increasing temperatures and varying day lengths are some of the possible outcomes that might result from this phenomenon. Recent studies indicate that varying sowing dates in winter wheat affects the crop's canopy development and ability to capture and convert available natural resources into biomass. However despite the influence of light and temperature on plant growth and development respectively, little is known about the adverse effects of untimely planting of winter crops in relation to climate change. The present study seeks to determine the effect of autumn sowing date on spring time canopy establishment in winter wheat cultivar Zebedee. This will be achieved by comparing total sun light used for plant growth, the leaf area and chlorophyll content, plant height, leaf and tiller number and shoot biomass produced by winter wheat sown on different autumn dates.
2.1 Origin and importance of wheat
Wheat (Triticum spp.) is classified as an annual grass, of the genus Triticum, originating several thousands of years from the "fertile crescent" region an area found within the drainage basins of present day Iraq and Syria. Wheat is ranked third, in terms of the world's cereal produced crop, to maize after rice. The three principal types of wheat used in modern food production are Triticum aestivum, T. durum, and T. compactum. However, T. aestivum is widely used for flour production in bread making and confectionary (Martin et al., 2006). T. aestivum is among the 0.007% crops used to satisfy most human requirements for food and fiber. The crop is believed to be central to the beginning of agriculture (Slafer and Satorre, 1999). Wheat is produced under diverse climatic conditions and cropping systems as a winter or spring crop. It yields relatively higher in the benign climates of Europe compared to the predominantly water deficit wheat growing areas (Dennett, 1999; Olivier and Annandale, 1998). The total are under wheat production worldwide is 216 million ha producing 626 million tonnes annually with an average of 2.8 t ha-1 (FAO, 2005). Winter wheat is the most widely grown arable crop in the UK accounting for over 40% of the tilled land in the eastern counties of England (Foulkes et al., 2001).
2.2 Resource capture in wheat
The capture and utilisation of resources by plants is largely influenced by their morphological characteristics and genotype. Wheat is a C3 species whose first product of photosynthesis is a 3-carbon compound phosphoglycerate with a photosynthetic efficiency of 1.4g MJ-1 (Sparkes, 2003). The crop is a planophile characterised with long, narrow and flat leaves. A fully headed-out wheat plant grows to a height of 2 to 5 ft and possesses a seminal fibrous root system with fine branches that extend downwards from 1 to 7 ft into the soil although an average depth of 4 to 6 ft is common (Peterson, 1965).
Growth (quantitative and irreversible changes in length, area, or weight of individual organs) and development (progression through a series of discrete changes in structure or number of individual organs) involves the capture and utilization of important resources from the environment by plants. Wheat exhibits a determinate growth habit which includes the following stages: germination, seedling growth, tillering, stem elongation, booting, ear emergence, flowering, milk development, dough development and ripening (Squire, 1990; HGCA, 2008). These growth processes are influenced by photosynthetically active radiation (PAR) whilst development is dependent on temperature and photoperiod; however, there is an interaction between growth and development (Olivier and Annandale, 1998; Sparkes, 2003).
The four most important resources for plant growth and development include: PAR, water, carbon dioxide and nutrients. The capture of these resources and their conversion into biomass depends on the crop canopy size and duration, root system, temperature and soil water status. In the UK 75 percent of wheat is sown in autumn since soils are warm with sufficient soil moisture content. This early sowing enables the crop to establish extensively and form a larger leaf canopy and a more extensive root system. In addition, as temperatures increase in the spring season the larger canopy intercepts more solar radiation and the deeper root system gives access to more stored soil water during the season. This ensures efficient capture and utilization of resources in the season (Spillane, 1968; Dennett, 1999).
Problem statement and justification
The increasing temperatures in this 21th century have been documented at global; continental, regional and local scales. Globally, the average surface temperature has been increased by 0.6 0C, with the 1990s recorded as the warmest decade (Sandras and Monzon, 2006). In the UK it is speculated that the country might experience wetter winters in the north and west and drier summers in the south and east (UKCIP, 2002). This might result into seasonal fluctuations which can affect the phenology of winter wheat that is currently cultivated on drought-prone soils (Foulkes et al., 2001). Timely sowing could be an important agronomic decision which determines the rate of wheat growth and development since it influences canopy architecture through tillering; leaf expansion as well as root development.
Ranges of sowing dates are applied within the winter wheat production system depending on the climate and cropping system used. It has been reported that wheat germination, emergence, leaf initiation, appearance and expansion are significantly influenced by temperature and day length (Gallagher 1979). The temperature for optimum emergence has been related to thermal time with 1500Cd being quoted for normal seeding depths of winter wheat in England (Dennett, 1999).
The growth and development of young wheat seedlings is influenced by temperature after sowing. For autumn sown wheat it's necessary to ensure that the plants grow sufficiently and not excessively in order to avoid winter damage. In addition, a more vigorous wheat canopy established prior to snow fall is susceptible to lodging, winter injury and desiccation leading to plant death (Peterson, 1965). The time of sowing has been reported to largely influence phyllochoron development in winter wheat. This affects canopy development, resource capture and conversion in winter wheat (Sparkes, 2003).
In spite of its influence on winter wheat phenology, little is known about the adverse effects of untimely planting of winter crops in relation to climate change and its influence on canopy development. This study therefore seeks to determine the impact of sowing date on the crop's spring time canopy establishment. The knowledge of sowing date for winter crops is important in minimising the deleterious effects associated with the winter season culminating into low crop yields. Thus sowing time must be optimised so that growth and development are matched to favourable conditions in order to maximise light interception by the canopy during the spring season.
2.4. Objectives and hypotheses
The aim of the study is to determine the effect of autumn sowing date on spring time canopy establishment in winter wheat cultivar Zebedee.
The hypotheses of the study include:
The total PAR intercepted by the canopy, leaf chlorophyll content and shoot dry matter are higher in the early than the late drilled winter wheat.
The leaf area index (LAI) and fractional interception of light (f) are higher in the early than the late drilled winter wheat.
The plant height, number of tillers and leaves are higher in the early than the late drilled winter wheat.
Soil moisture content significantly affects plant growth and development
3.0 MATERIALS AND METHODS
3.1 Experimental set up
The study will be conducted at the University of Nottingham, School of Biosciences, Sutton Bonington campus farm field number S02 (pH 6.8) with two treatments replicated three times. They include; early and late drilled winter wheat (Triticum aestivum var. zebedee) which will be sown on the 14/09/2009 and 19/10/2009 respectively at a seed rate of 111.3 kg/ha. A randomised complete block design (RCBD) will be used in the study. Eight, 24x1.625 m plots will be established with two discard plots to avoid edge effects. Prior to drilling the field will be cultivated by ploughing on the 07/09/2009 followed by harrowing on the 09/09/2009 using a power harrow. Rolling will be done following drilling to ensure proper seed germination and establishment. Agronomic practices like weeding, herbicide and fertilizer application will be done appropriately.
3.2 Meteorological data
Data of the daily incident solar radiation, day length, rainfall and temperature will be obtained from the Sutton Bonington weather station from September 2009 to May 2010 (http://ssbbap3.nottingham.ac.uk/metdata/) and used to determine thermal time. The accumulated thermal time for each day will be calculated using the following formula;
(Tmax + Tmin)/2 - Tbase
Where Tmax = daily maximum air temperature
Tmin = daily minimum air temperature
Tbase= base temperature
Destructive and non-destructive measurements will be taken. The non destructive include; PAR, fractional interception (f), plant height, leaf and tiller number, leaf chlorophyll content and soil moisture content. The destructive measurements include; LAI and shoot dry matter. All measurements will be done once a week for every five randomly selected plants within a 0.5m2 squared quadrat per plot for four consecutive weeks.
(a) Photosynthetically active radiation (PAR)
The radiation intercepted and used by the canopy for photosynthesis (PAR) is a shortwave electromagnetic radiation with wavelengths between 400 to 700 nm waveband (Pearcy et al., 1989). PAR will be measured as photosynthetic photon flux density (PPFD) (Î¼molm-2s-1) at different canopy depths using a PAR Quantum Sensor (model LI-190SA) (LI-COR Instruments) (Pearcy et al., 1989). The actual incident and transmitted PAR values will be obtained by averaging the readings taken at many horizontal points above, within and below the crop canopy.
(b) Fractional interception (f)
The proportion of incident radiation intercepted by the crop canopy (f) is affected by the leaf are index (LAI) and leaf angle. The factional interception of the whole canopy will be measured using a Ceptometer placed above the canopy to measure incident light (Io) and then at the bottom of the canopy to measure the radiation transmitted (I). The radiation reflected by the canopy (r) will be determined by inverting the probe above the canopy. The fractional interception (f) of the canopy will be calculated from the following equation.
f=1- (I + r)/I0
The radiation use efficiency (RUE) or light use efficiency (LUE) is the total amount of biomass produced per unit of radiation intercepted. RUE (gMJ-1) =Dry matter (gm-2)/PAR (MJ)
Scaling up to obtain values for the whole field.
(c) Number of leaves and tillers
The number of leaves and tillers for five randomly selected plants with in the 0.5m2quadrat will be counted for each plot and recorded.
(d) Plant height
The plant height for five randomly selected plants within the 0.5m2 quadrat for each plot will be determined using a meter rule placed to the nearest cm from the ground level.
(f) Leaf chlorophyll content
The leaf chlorophyll content indicates the greenness of the leaf and this determines the green area available for light interception. This will be determined using a SPAD meter for five randomly selected plants within the 0.5m2 quadrat per plot.
(g) Leaf Area Index (LAI)
Squire (1990) defined LAI as "leaf area per unit ground area". It is expressed as L= Npnsas where Np is the number of plants per unit ground area, ns the number of leaves per plant and as their mean area. The Np depends on the seed rate and percentage establishment influenced by environmental and management factors. N1 depends on the sowing date and leaf emergence which is heavily influenced by temperature and nitrogen availability (Sparkes, 2003).
The leaf area (LA) will be determined by using a leaf area meter and LAI will be calculated from the following equation. LAI= LA/GA where GA is the ground area (1m2).
The relationship between LAI and f described by the extinction coefficient of the canopy (k) as will be established by Monsi-Saeki equation: I=I0 e-k L
Where: I0 is the irradiance above the crop canopy; I is the irradiance at a point in the crop canopy and L is the leaf area index (Azam-Ali and Squire, 2002). Extinction coefficient (k) is analogous to the absorption coefficient in Beer's Law (Baret et al., 1993). It describes the plant canopy characteristics that affect light interception, mainly leaf angle, shape, thickness and vertical stratification of the leaf area. These morphological characteristics influence the transmission, reflection and absorption of light (Monteith, 1965).
(h) Shoot dry matter
Photosynthetic efficiency is defined as the amount of dry matter produced per unit of light intercepted by the canopy. The shoot dry matter tends to be proportional to the total dry matter produced by the crop in terms of yield (harvest index). Plants will be harvested by cutting stems at ground level using a pair of scissors and the material sorted into leaves and stems and packed into paper bags. Their fresh weight will be determined immediately using an analytical balance and transferred to the oven for drying at 70 0C for 48 hours. The total shoot dry matter will be calculated as the difference between the fresh and dry weights.
(i) Soil moisture content
The soil moisture content measures the amount of water available in the soil. However, there is an inverse relation between its availability and the tenacity at which it's held. Plants need water for their metabolic activities like photolysis during photosynthesis. Uptake of water by plants depends on the gradient in the plant water potential (Azam-Ali and Squire, 2002). The soil moisture content for each plot will be determined using a calibrated Theta Probe Soil Moisture Sensor-ML2x (Delta T Devices) inserted in the soil.
3.4 Data analysis
The data generated will be subjected to regression and analysis of variance (ANOVA) to compare treatment effects using statistical package GenStat Release 12.1 (VSN International, 2009). A probability value of 0.05 or less (Pâ‰¤0.05) will be considered significant.
EXPECTED STUDY OUTCOMES
The knowledge generated from this study will be useful to the farmers in optimising sowing date to achieve a larger green canopy size with a longer duration so as to maximise productivity. The data will be useful to physiologists and agronomists in predicting models that affect resource capture and conversion in this era of increasing climate change due to global warming. Further research needs to be conducted to investigate the effect of sowing date on the anthesis and dry matter portioning in winter wheat and models simulated to predict the likely impact of climate change on sustainable winter wheat productivity.