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Studies to understand the impact of thermal changes on phenological changes in wheat genotypes will be conducted at Agronomic Research Area, University of Agriculture, Faisalabad. There will be two experiments in the present study; in the first one wheat genotypes (viz. Sehar 2006, Farid 2006, Lasani 2008, Miraj 2008, Faisalabad 2008 and Chakwal 50) will be sown on six dates with 15 days interval (viz. 10 Nov, 25 Nov, 10 Dec, 25 Dec, 10 Jan and 25 Jan). In the second experiment, seeds primed with hydropriming, CaCl2 (-1.25MPa), ascorbate (2 mM) and salicylicate (50 ppm) will be sown on six dates with 15 days interval (viz. 10 Nov, 25 Nov, 10 Dec, 25 Dec, 10 Jan and 25 Jan). Both the Experiments will be laid out in randomized complete block design in split plot arrangements with three replications, keeping sowing dates in main plot and wheat genotypes and priming treatments in subplot in first and second experiment, respectively. During the course of investigation, data on soil and air temperature stand establishment, crop phenology, allometry, agronomic and yield related traits will be recorded following the standard procedures. Economic and marginal analysis will be done to evaluate the economic feasibility of experimental treatments. Experimental data will be analyzed using appropriate statistical package following Fishers analysis of variance technique and the treatment means will be compared by least significant test at 0.05 probability levels.
Global warming is an emerging threat to agricultural productivity all over the world. A rising temperature trend has been observed and predicted at many locations around the world during the last several decades and this trend will be projected to accelerate in future (IOCI, 2005). Bates et al. (2008) predicted that increasing temperature of our globe by current pace will boost up 1.2oC over the next 30 years. On this globe both minimum and maximum temperatures are rising but minimum temperature is rising faster than maximum during the recent decades (Menzel et al., 2001). Increasing minimum temperatures during the vegetative period positively affected wheat growth but increases in maximum temperatures during the reproductive period negatively affected kernel weight and grain yield (Wang et al., 2008) because each crop has its specific temperature requirement for growth and development.
Wheat is generally considered to enjoy an optimum temperature range of 17-23°C over the course of an entire growing season with a minimum temperature of 0°C and a maximum of 37°C beyond which growth stop (Porter and Gawith, 1999). Any fluctuation in temperature may reduce harvestable crop genetic potential (Subedi, 1998; Pressey et al., 2007) by shortening the growth period resulting in lowered crop yield (Gao et al., 1995). Steady rise in temperature from 1982 to 1998 has caused a considerable yield thrust in cereals (Lobell and Asner, 2003). Results of experiments under controlled conditions revealed that yield of wheat may decline up to 10% per 1°C rise in temperature (Mitchell et al., 1993). Temperature more than optimum has great potential to accelerate developmental processes in wheat (Bindi et al., 1993) and further reduction in yield may occur due to increasing photorespiration in C3 species (Polley, 2002).
Phenology is the study of cyclic and seasonal natural phenomena especially in relation to plant life and climate. Phenological phases of crops may respond to the changes of both minimum and maximum temperatures (Alward et al., 1999). Change in temperature has direct impacts on crop phenological seasonality in many ecological regions of the world (Chmielewski and Rotzer, 2002) so therefore understanding the relationship between temperature and phenological developmental process of crop plant is critical (Ye et al., 2002) because determination of production areas for introducing new species, information about climate change on phenological development is required (Zhang et al., 1997). Parmesan and Yohe (2003) observed that changing environment expected to lead changes in life cycle events. Environmental factors signal phenological processes such as flowering time could affect not only seed production but also food composition (Springer and Ward, 2007). Ample environmental factor relates with the genetics to influence the phenological and phyllochron development but no single environmental factor completely predicts development nonetheless temperature and occasionally photoperiod are obviously the most critical factors (Masle et al., 1989). Final number of leaves on the main culm dependent on photoperiod and time to heading linearly related final numbers of leaves (Slafer and Rawson, 1995a).
Global warming is an inevitable phenomenon so adaptation is essential for risk management strategy (IPCC, 2007). Among different management practices seed priming has been found to modulate the phenological development in plant and help to plant withstand under various stresses (Farooq et al., 2009a). Khan et al. (2008) study the effect of seed priming (with polyethylene glycol) on phenology of two mungbean genotypes in semiarid climate. They observed that phenological events are delayed in NM-98 compared to NM-92 genotype and primed seed produced 12 % more grain yield as compared with control. Farooq et al. (2009b) also reported similar results in rice that primed crops grew more vigorously, flowered earlier and yielded higher. Among various priming techniques on-farm priming is a simple, easy and effective way to improve crop performance that resulted early flowering and higher yield (Harris et al., 1999). Seed priming changes growth attributes and help in timely accomplishment of phenological events in rice (Farooq et al., 2007) .It has also been reported that seed priming improves emergence, stand establishment, tillering, allometry, grain and straw yields, harvest index and decreases mean emergence time (Farooq et al., 2008a). The role of various seed priming agent on crop performance is of prime importance. Seed priming in salicylic acid solution enhanced the leaf area and dry matter in maize and soybean (Khan et al., 2003). Seed priming in lower concentration of salicylic acid has significantly increased number of leaves, fresh and dry weight per plant of wheat seedlings (Hayat et al., 2005). Further investigation revealed that salicylic acid when applied exogenously to wheat seedlings increased the size and mass of plantlets significantly, compared without treated seeds (Shakirova, 2007). Many studies revealed that salicylic acid plays a key role in providing tolerance against temperature stress and prevent plant from oxidative damage (Dat et al., 1998). In maize and rice, salicylic acid application was shown to increase accumulation of proline which trigger antioxidant system and prevent plant from oxidative damage, antioxidant enzymes being the most efficient mechanisms against oxidative stress (Farooq et al., 2008b, 2009c).
The present study will be therefore conducted to find the relationship between climate disparity, on the bases by enumerating temperature data (both soil and air temperature), and crop phenological events (leaf emergence rate, leaf elongation rate, tillering, plant length, time to flowering, anthesis and time to crop maturity) and yield of various wheat genotypes sown on different dates. We will also investigate the role of seed priming in improving phenological development and yield attributes of wheat at various sowing dates.
REVIEW OF LITERATURE
Impact of Temperature on Wheat Growth and Phenology
Phenological development is the most important trait involved in adaptation of crops to their growing environments. Wheat is adopted in a wide range of environmental conditions but it grew profusely and achieved genetic potential under optimum environmental conditions (Sadras and Trapani, 1999). Crop development has two unique features: phasic and morphological development, phasic development or phenology is an ordered sequence of processes punctuated by discrete events, such as sowing, emergence, floral initiation, anthesis, and maturity. It's implicitly assumed that the plant, or part of the plant, possesses developmental clock that proceed at given rate for each of the above phases dependant on temperature (Thornley and Johnson, 1990). Porter and Gawith (1999) observed that wheat require 17-23°C temperature over the course of entire growing season with a minimum temperature of 0°C and a maximum of 37°C beyond which growth stop however, wheat is less sensitive to temperature during its vegetative phase than the reproductive phase (Entz and Fowler, 1988). Temperatures more than optimum reduces the length of crop growing season, so less radiations are intercepted that results less photo-assimilation and ultimately lowered grain yield (Lawlor and Mitchell, 2000). The growth period of wheat from seedling emergence to stem elongation has shortened by 4.3 day while the growth period from stem elongation to booting was prolonged by 3.3 day for every 1.8°C of increase in minimum temperature during the growth period (Wang et al., 2008). In cereal duration of stem elongation phase control the number of fertile florets and it contributes in yield (Miralles et al., 2001). Photoperiod also determine the solar radiation interception period and it's direct relation with temperature play a key role in driving developmental processes, general responses of leaf initiation and appearance rates and leaf number are well documented (Slafer et al., 1994; Brooking et al., 1995) and are critical determinants of grain yield (Kantolic and Slafer, 2005).
Instead of air temperatures, literature revealed that soil temperature has to be involved in phenological development of plant because when the apical meristem are lying underground in the early crop's life cycle, it is possibly more acceptable to assume that development rate responds to soil temperatures at meristem depth. Seed germination and early growth are complex processes dependent on the interaction on soil temperature (Montieth, 1981). Hayhoe et al. (1996) observed that soil temperature determine the time and rate of seedling emergence in many crops. Temperatures sensitivity varies not only between plant components but also changes during the course of development (Musich et al., 1981). Thus base and optimum temperature thresholds increase with development (Slafer and Rawson, 1995b). The impact of increased specific leaf area on potential yield depended on temperature (Ludwig and Asseng, 2010). So it is more accurate to say that crop phenology is strongly associated to rhizosphere and crop canopy temperature (Jamieson et al., 1995). The proportion of both the plastochron and phyllochron are quite predictable based on temperature differences (Porter et al., 1987).
Climate Change and Wheat Yield
Rising temperatures can potentially both increase or decrease crop yields (Peng et al., 2004). Analysis of climate risks for crops in 12 food-insecure regions based on weather projections through to 2030 indicate that South Asia and Southern Africa would be mainly susceptible to negative effects on wheat productivity if measures are not taken to improve crop adaptation (Lobell et al., 2008). Climate change has variable impact on crop production (Rubas et al., 2006) in addition to its impact on resource utilization (Parry et al., 2004). It is predicted that atmospheric temperature will increase from 0.5 to 2.0°C by 2030 with continued increases later in the century (IOCI, 2005).
Hay and Kemp (1990) observed that there are several active shoot apices within a plant, each apex as independently forming, growing and senescing leaves controlled by the environmental factors particularly temperature. Delay sowing of wheat reduces the grain yield up to 0.7% per day (Ortizmonasterio et al., 1994) because it causes delayed emergence, poor crop stand, less tillering and shorten growth, and less grain development period. Under controlled conditions it is estimated that wheat yield decreases by 4% for every 1°C rise in temperature above the optimum temperature (Wardlaw and Wrigley, 1994). High temperature stress during lateral stages of development i.e. anthesis and grain filling stages due to enzyme inactivity such as soluble starch synthase in wheat appears to be rate limiting at temperatures in excess of 20°C (Keeling et al., 1994). Furthermore, grain filling in wheat is seriously impaired by heat stress due to reductions in current leaf and ear photosynthesis at higher than optimum temperatures (Blum et al., 1994). Even so, as shown by Blum et al. (1994), in some wheat lines grain filling from mobilized stem reserves is a constitutive trait, which supports grain filling in heat stressed plant. High temperatures during grain filling (10 days after anthesis until ripeness) decreased wheat yield by reducing kernel weight (Stone and Nicolas, 1994). The largest reduction in kernel weight, 23%, occurred when temperature was raised from 20/15 to 40/15.8°C for 3 days beginning 30 days after anthesis (Stone and Nicolas, 1994).
Crop Management in Changing Climate
Recently, most research related to climate change has focused on impacts and mitigation (Salinger et al., 2005). Even though agricultural responses to climate change tend to be crop and location specific, but there are ample evidences that most agricultural systems has been reshaped. In some cases, expected changes in productivity will compel the farmers to implement different management practices, while in others the impacts of climate change will entail that current varieties no longer be a feasible economic alternative.
Role of seed priming in mitigating climate change
Drought and temperature extremes are perhaps the most evident impacts of change climate expected to effect the crop production. Several studies have indicated that seed priming techniques may be successfully employed to cope with the challenges of drought and extreme temperatures (Farooq et al., 2010). For example in a series of studies on rice, it was found that seed priming with salicylic acid, glycinebetain, nitric oxide and polyamines can substantially improve the drought resistance potential of crop (Farooq et al., 2009d, 2010).
Likewise in maize seed priming with KCl, CaCl2, salicylic acid and glycinebetain improved the crop resistance against temperature extremes (Farooq et al., 2009e). Ludwig and Asseng (2010) reported that plant growth depends upon early vigor and can potentially assist the crop to reaching high potential growth rate and also reduced its yield variability. Seed priming with various salts of calcium, potassium and/or priming with growth regulators or hydropriming proved best to enhance vigor of seed and also improve stand establishment (Basra et al., 2003). Priming with various osmoticum triggers such mechanisms that synthesized organic compounds (polyamines, glycinebetaine and prolene) in plant which are supportive to mitigate plant with abiotic stresses (Monika et al., 2001). Syntheses of polyamines are also triggered by exogenous application of salicylic acid in hydroponics under temperature stresses. The level of various polyamines (putrescine and spermidine) contents inside the plant tend to increased while spermine contents were decreased which increased tolerance in plant under temperature stresses (Monika et al., 2001).
Mostafa et al. (2010) studied the effect of foliar application of arginine and putrescine (1.25 and 2.5 mM each) on growth and yield of wheat crop sown on recommended and delay sowing. They observed that the normal and delayed sowing wheat exhibited significant increments in the growth and all yield parameters in comparison to the late sowing plants or the untreated control sown at normal date. Primed wheat crop complete their growth phases earlier took less time than the unprimed to reach tillering, jointing, heading and flowering (Kant et al., 2004. Primed plants also had significantly more tillers, panicles and grains per panicle than non-primed plants in rice (WARDA, 2002). Farooq et al. (2006) observed that seeds osmohardened with CaCl2 had the highest number of tillers, 1000 kernel weight, and kernel yield, which was also observed in seeds exposed to hydropriming, osmohardening with KCl, hardening, and ascorbate priming. Sharma et al. (1993) obtained higher yield from soybean, also observed early floral initiation, recorded more flowers and pods per plant in salicylic acid primed seed. Du and Tuong (2002) examined that seed priming, with 14 % KCl solution and saturated CaHPO4 solution, increased established plant density, number of final tiller, and grain yield compared with the unprimed treatment. Kaur et al. (2005) reported that priming resulted higher grain yield even under various stresses.
Performance of genotypes under varying climatic conditions
It is essential to explore the potential of any crop under stressful environment to adopt such management practices or strategies or develop such genotypes which retain optimum growth and development from germination till maturity under temperature instability. Various hypothetical climate change adaptation options for agriculture have been suggested, such as improved changes in cropping patterns, or use of traditional knowledge (Salinger et al., 2005). In addition, developing new crop cultivars is one of the options often used as a possible adaptation to changing environment (Humphreys et al., 2006). Crop biomass is important in contribution of total crop yield which is highly affected under various environmental conditions. Fodor and Palmai (2008) found that wheat produced less biomass when it was late sown. Shafi et al. (2006) observed that different varieties of cereals respond to environment differently and seed priming had a significant effect on seed germination. Mahfoozi and Aminzadeh (2006) conducted experiments using number of wheat genotypes which were planted on different dates and concluded that sowing date - cultivar interaction used as a tool to determine the optimal sowing date of crops of cold regions. More grain yield (optimum) was obtained from early planting wheat (optimum sowing date) generally compared with late planting (Donaldson et al., 2001). Tahir et al. (2009) compared the performance of various genotypes sown on different dates. They observed that there were a significantly difference in yield among varieties on different sowing dates. Inqlab-91 produced significantly maximum yield (3550.44 kg ha-1) while minimum yield (2932.59 kg ha-1) was obtained by AS-2002. In case of sowing dates maximum grain yield (4289.54 kg ha-1) was obtained when crop was sown on 1st December against the minimum grain yield (2109.50 kg ha-1) in case of late sowing i.e. 30th December. Wajid et al. (2004) also conducted experiment on wheat crop to see the effect of sowing date and plant population on biomass, grain yield and yield components. They scrutinize higher grain yield in early sowing and it was due to higher number of ears m-2 and mean grain. Anwar et al. (2007) reported that the reduction in grain yield after November 10 planting was 14.45, 24.26, 36.71 and 48.04% from crop planted on November 25, December 10, December 25 and January 10, respectively. The varieties Uqab-2000 and Iqbal-2000 for 1000 grain weight and Chenab-2000 and MH-97 for number of tillers showed stability.
In conclusion, it is said that due to global warming temperature is expected to increase rapidly in days to come. The previous studies revealed that ongoing warming trends in climate having some measurable impacts on the development and production of field crops. Increasing temperatures shortened the reproductive growth period, reduced the length of the growing season, and decreased the grain weight of wheat. Various priming techniques may be helpful in mitigating the adversities of climate change. In addition, genotype screening for future climates is also of vital importance and must be given due consideration. This study is aimed to achieve the aforementioned targets therefore.
VI) MATERIALS AND METHODS
The proposed studies pertaining to the impact of thermal changes on phenological development in wheat comprises of two experiments. Both experiments will be carried out at Agronomic Research Area, University of Agriculture, Faisalabad using randomized complete block design in split plot arrangements with three replications in 2009-10 and whole study will also be repeated next year. In both the experiments certified seed of different wheat genotypes will be used @ 125 kg ha-1. Sowing will be done in well pulverized soil with the help of hand drill keeping 22.5 cm spaced rows. The net plot size will be 2 m x 5 m. Fertilizers 100-90 NP kg ha-1 will be used, respectively. Irrigation will be applied according to the general recommendation of each cultivar. Appropriate plant protection will be adopted if any disease or weeds infestation or insect pests attack.
Experiment No. 1. Studying the impact of thermal changes on wheat (Triticum aestivum L.) growth and phenology.
There will be two sets of experimental treatments, 1) six sowing dates (viz. 10 Nov, 25 Nov, 10 Dec, 25 Dec, 10 Jan and 25 Jan) placed in main plots; 2) wheat genotypes (Sehar-2006, Fareed 2006, Chakwal 50, Miraj 2008, Lasani 2008, Faisalabad 2008), which will be placed in sub-plots.
Experiment No. 2. Exploring the role of seed priming on wheat performance in changing climate
The experiment will comprised of six sowing dates (viz. 10 Nov, 25 Nov, 10 Dec, 25 Dec, 10 Jan and 25 Jan) placed in main plots and four seed priming treatments viz. hydropriming, seed priming with CaCl2 (1.2%), ascorbic acid (2mM) and salicylic acid (50 ppm), each for 12 h, placed in sub-plots. For seed priming, seeds will be soaked in respective aerated solution keeping seed: solution ratio 1:5 (w/v) at 25±2 °C. Thereafter seeds will be removed and will be given three surface washings and will be re-dried with forced air near to its original weight. Untreated seeds will be used as control treatment.
Following observations regarding crop stand establishment, allometry, agronomic traits and yield attributes will be recorded by following standard procedures in both the experiments.
Daily soil and air temperatures will be recorded 8:00 am by using digital thermometer.
(b) Stand establishment
Numbers of emerged seeds will be counted daily according to the seedling evaluation Handbook of Association of Official Seed Analysts (1990). Time taken to 50% emergence of seedlings (E50) will be calculated following the formulae of Coolbear et al. (1984) modified by Farooq et al. (2005). Mean emergence time (MET) will be calculated following Ellis and Robert (1981). Energy of emergence (EE) will be determined on fourth day of seed sowing (Farooq et al., 2006). Coefficient of uniformity of emergence (CUE) will be calculated using the formulae of Bewley and Black (1985).
Leaf emergence (Haun, 1973), leaf expansion, leaf area index (LAI), leaf area duration, crop growth rate (CGR), and net assimilation rate (NAR) will be calculated using the formulae of Hunt (1978). Leaf area will be measured by using leaf area meter.
(e) Plant biomass components:
Plant biomass components will be determined following the standard procedure.
Data regarding phonological traits like number of tillers, number of leaves, Leaf expansion rate, leaf emergence rate, tiller emergence rate, daily plant height, Time taken emergence to heading (days), time taken heading to maturity (days) will be recorded by following standard procedure.
(g) Agronomic traits and yield components
Agronomic parameters like, plant height at maturity (cm), no. of fertile tillers per plant, no. of grains per spike, spike length, 1000 grain weight (g), biological yield (t ha-1), grain yield (t ha-1), harvest index (HI %) will be done by using standard procedures.
(h) Plant water relations
Leaf water potential (Ψw) (-MPa) will be determine by using pressure chamber and leaf osmotic potential (Ψs) (-MPa) by osmometer while turgor potential (Ψp) (-MPa) will be calculated from Ψw and Ψs. Stomatal conductance will be determined by using infra red gas analyzer (IRGA).
(i) Reserve translocations
Reserve mobilization in stem and grain filling rate will be determine by using standard procedures.
(j) Biochemical analysis
Total proteins, lipid peroxidation, starch contents and antioxidants will be determined by following standards protocols.
(h) Statistical analysis
Standard procedures will be followed to collect the data. The collected data will be analyzed statistically by employing the Fisher analysis of variance technique (Steel et al., 1997) and treatment means will be compared by using Least Significance Difference (LSD) test at 5% probability level.
(j) Economic and marginal analysis
Net field benefits will be calculated by subtracting the total variable cost from the total benefits of each treatment combination. Input and output cost for each treatment combination will converted into Rs. ha-1. Similarly marginal rate of return (MRR) will be calculated according to CIMMYT (1988).
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