Pak. J. Bot., 43(3): 1621-1627, 2011. IS PHOTOTHERMAL QUOTIENT DETERMINANT FACTOR FOR SPRING WHEAT YIELD? M. AHMED *1 , FAYYAZ-UL-HASSAN 1 , ABDUL RAZZAQ 1 , M.N. AKRAM 1 , M. ASLAM 2 , S. AHMAD 3 AND M. ZIA-UL-HAQ 4 1 Department of Agronomy, PMAS Arid Agriculture University Rawalpindi-46300, Pakistan; 2 Ministry of Food and Agriculture Islamabad, Pakistan; 3 Department of Agronomy, Bahauddin Zakariya University, Multan-60800, Pakistan 4 Department of Pharmacognosy, University of Karachi, Karachi-75270, Pakistan, Abstract Photothermal Quotient as combined effect of temperature and solar radiation were studied as determinant factor for spring wheat grain yield. The data obtained at anthesis and maturity for grain m -2 , grain weight and grain yield were examined in experiments involving three wheat genotypes under five different environmental conditions (E) provided in the form of planting windows (PW’s) during 2008-09 & 2009-10 at National Agricultural Research Center (NARC), Islamabad. The mean temperature at anthesis (T1) and maturity (T2) was calculated by averaging all temperature from germination till anthesis and maturity, respectively. Similarly, solar radiation at anthesis (SR1) and maturity (SR2) was calculated- with the Angstrom formula while Photothermal quotient (PTQ) was calculated at anthesis (PTQ1) and maturity (PTQ2). The data obtained was subjected to STATISTICA 8 software and scatter plot regression model was developed- at 95% confidence interval with crop data and climate variables (T1, T2, SR1, SR2, PTQ1 and PTQ2). The b (regression coefficient) recorded were 623.73 (GM vs T1), 0.0037 (GW vs T1), -5.97 (Y vs T1), -0.028 (GW vs T2), -1356.28 (GM vs T2), -57.86 (Y vs T2), 29.53 (GM vs SR1), 0.0005 (GW vs SR1), 3.95 (Y vs SR1), 19.90 (GM vs SR2), 0.0003 (GW vs SR2), 2.57 (Y vs SR2) 314.20 (GM vs PTQ1), 0.0049 (GW vs PTQ1), 49.88 (Y vs PTQ1), 0.0037 (GW vs PTQ2), 31.87 ( Y vs PTQ2), 237.40 (GM vs PTQ2) while, b for E and PW with yield was -872.51 and -309.75, respectively. Direct relationship between PTQ and yield parameters confirmed that it determined crop yield and its management for variable environmental conditions need to be opted by adopting suitable sowing time. Introduction Climatic factors like temperature, solar radiation and rainfall impact on crop yield all over the world. Changes in climatic factors like rise in temperature and decline in rainfall were reported in the report of the Intergovernmental Panel on Climate Change (IPCC, 2007a, b). Low temperature in the growing season may reduce germination, retard vegetative growth by inducing metabolic unbalances and can delay or prevent reproductive devolvement (Mohsen & Yamada, 1991). You et al., (2009) observed significant reduction in yield due to rise in temperature and it was concluded that with 1.8 o C rise in temperature caused 3-10 % reduction in wheat yields. The impact of climatic variables can be assessed by studying climatic parameters like temperature, solar ration and Photothermal Quotient (PTQ) during crop life-cycle and by developing a quantitative relationship among climatic variables and crop yield. Li et al., (2010) observed significant change in yield of wheat due to variation in temperature and solar radiation. Similarly, 0.6 to 8.9% reduction in wheat yield per 1 o C rise in temperature has been recorded by Lobell & Field, (2007). Ormerod et al., (2003) reported that agricultural production is more complex because of the need to balance global food security, optimum production, technological innovation, preservation of environmental functions and protection of biodiversity. The ecological knowledge achieved in the present study enables considerable compliance with most of those objectives. Development of genotypes resistance to extreme climatic events needs to be considered as potential option for wide range of conditions with agronomic managements like sowing dates (Bedo et al., 2005). Limited availability of climatic resources has generated marginal environments where wheat production drops to 70% of optimum yield (Rajaram, 2005). Photothermal quotient (PTQ) can be defined as the ratio of total solar radiation in MJ m -2 day -1 to the mean daily temperature minus a base temperature (4.5 o C for Spring Wheat). Nalley et al., (2009) identified, PTQ to improve the explanatory power of statistical regression models on grains per meter square (GM), grain weight (GW) and yield under climate change scenario. Similarly, Khichar & Niwas (2007) concluded direct relationship with PTQ and growth of wheat under different planting systems. However, Loomis & Amthor (1996) documented that crop growth and yield were derived from photosynthesis, therefore dependent on receipt and capture of solar radiation. Hence studies are needed to understand and quantify the crop response and its relationship with varied combination of temperature and solar radiation to workout suitable planting windows for wheat. This is only possible if the canopies are tested under varying circumstances of temperature and radiation. The practical way to record such reading is to alter the sowing time of the crop within recommended growing season. The objectives of this study were to evaluate the effect of mean temperature, total solar radiation and PTQ at anthesis and maturity stage of wheat crop during different environments (2008-09 & 2009-10) and thereafter to develop a regression model for climatic variables with grain weight, grains per meter square and yield of wheat crop for wide range of conditions. Materials and Methods Experimental site and field conditions: Five different planting windows (PW’s) were used to provide variable climatic conditions at anthesis and maturity of wheat for two environments i.e. 2008-09 & 2009-10. The planting windows were; PW1 (Sowing on 20-10-2008 & 23-10-2009), PW2 (Sowing on 28-10-2008 & 05-11-2009), PW3 (Sowing on 05- 11-2008 & 19-11-2009), PW4 (Sowing on 19-11-2008 & 27- 11-2009) and PW5 (Sowing on 05-12-2008 & 10-12-2009). The study was conducted at National Agriculture Research Center (NARC), Islamabad, Pakistan, located at latitude of 33 o 42 / N and longitude of 73 o 10 / E. The soil series of study site was Rajar, with great groups Ustorthents and soil order Entisol. The experiments were laid out using randomized complete block design (RCBD), replicated four times in 4.5 x 10 m plot with row spacing of 25 cm. Wheat genotypes Chakwal-50, Wafaq-2001 and GA-2002 were used in the study. Sowing was done by hand drill using seed rate of 50 kg acre -1 . Prior to sowing particular field remained fallow during summer which was ploughed once with soil inverting implement and thereafter thrice with tractor mounted cultivator. Recommended doses of fertilizer Nitrogen and Phosphorus at the rate of 100 kg ha -1 was added with last cultivation in the form of Urea and DAP. * Corresponding author’s email: shakeel.agronomy@gmail.com