September 2020 | Volume 33 | Issue 3 | Page 550 Pakistan Journal of Agricultural Research Research Article Introduction H igh temperature is becoming an important limiting factor for crop productivity especially under the changing climate scenario and is projected to be increased up to 1.5-4.5 ˚C by 2050 (Houghton et al., 2001). Wheat (Triticum aestivum L.) productivity is greatly infuenced by temporal variation in sowing in rice-wheat cropping system in South Asian countries owing to variability in temperature regimes with respect to its phenophases (Fujisaka et al., 1994; Hussain et al., 2012a). Critical dates of wheat sowing than optimum exposes the crop to temperature extremes and consequences in altered heat unit accumulation and photoperiod which becomes limiting factor for yield (Arduini et al., 2009). Tis necessitates that management practices viz. time of sowing and selection of appropriate variety which could impose negative or positive efects on yield and yield formation must be studied for making better Abstract | Climate change has afected optimum sowing time of wheat and yield formation by fuctuation in temperature regimes during sowing season of crop. However, diferent varieties could difer in their potential for yield formation in response to sowing time. Tis 3-years study was conducted to ascertain impact of sowing dates on grain yield and critical yield component of diferent wheat varieties. Wheat varieties (Lasani-08, Faislabad-08, Millat-11, AARI-11 and Punjab-11) were sown on diferent sowing dates (November 01, November 15, November 30 and December 15). Yield and related traits of all varieties were decreased by delayed sowing. Early sown crop (November 01) had maximum emergence which decreased gradually with delayed sowing. Tillers m -2 , grain weight, grains per spike and yield of all varieties was greater on November 15 sowing and decreased by sowing afterwards (November 30 - December 15). Although Lasani-08 and Punjab-11 both were high yielders but grain yield of AARI-11 was least afected by sowing time. Linear regression revealed that sowing time manifested negative relations with emergence and grain weight of wheat varieties indicating them the most afected traits in response to temporal variation in sowing. Sowing time had negative relation with number of tillers for only Lasani-08 and Punjab-11, while non-signifcant relation with grains per spike. Nonetheless, tillers m -2 and grains per spike were correlated positively with wheat yield indicating them most crucial yield imparting traits. In conclusion, wheat sowing on November 15 produced maximum yield; and number of tillers was critical yield determining component followed by grains number per spike in response to sowing dates. Javed Iqbal 1 , Ali Zohaib 1 *, Muzzammil Hussain 1 , Iftikhar Ahmad 1 , Adnan Bashir 1 , Wardah Muzafer 2 , Naeem Faisal 1 , Muhammad Tahir Latif 1 and Sami Ullah 1 1 Adaptive Research Farm, Gujranwala 52200, Pakistan; 2 Sugarcane Research Institute, Ayub Agricultural Research Institute, Faisalabad 38000, Pakistan. Received | April 17, 2020; Accepted | July 03, 2020; Published | August 01, 2020 *Correspondence | Ali Zohaib, Adaptive Research Farm, Gujranwala 52200, Pakistan; Email: alizohaib208@gmail.com Citation | Iqbal, J., A. Zohaib, M. Hussain, I. Ahmad, A. Bashir, W. Muzafer, N. Faisal, M.T. Latif and S. Ullah. 2020. Grain yield and critical yield determining component of bread wheat varieties in response to sowing dates. Pakistan Journal of Agricultural Research, 33(3): 550-560. DOI | http://dx.doi.org/10.17582/journal.pjar/2020/33.3.550.560 Keywords | Grain yield, Sowing time, Varieties, Wheat, Yield formation Grain Yield and Critical Yield Determining Component of Bread Wheat Varieties in Response to Sowing Dates