EFFECTS OF HEADING DATE ON AGRONOMIC PERFORMANCE OF WINTER-WHEAT ISOLINES

Heading and photoperiod response are key processes affecting adaptiven ess of wheat (Triticum aestivum L.). Few soft white winter (SWW) wheat cultivars of the U.S. Pacific Northwest (PNW) express early heading. Earliness offers several advantages including early harvest, reduced p re-harvest sprouting loss, and increased operational efficiency. The o bjective of this study was to determine if early heading genotypes war rant greater breeding emphasis. Near-isogenic lines (NILs) were develo ped by transferring early heading character from Early Blackhull and a n Early Blackhull derivative to 'Nugaines' (NGN), an adapted PNW SWW c ultivar. The NILs comprise a near-continous range of phenotypes that h ead 0 to 10 d earlier than NGN. The parents and NILs were tested durin g two to four yr in three diverse field environments. Results showed t hat genotypes 3 to 7 d earlier than NGN warrant increased breeding emp hasis because they had high yield potential across all environments an d often had enhanced test weight, kernel weight and harvest index. The earliest NILs had limited yield potential in the cool-moist medium gr owing season environment, but they had high yield potential in the tem perate long-growing season environment. Grain-fill duration of NILs wa s strongly influenced by environment and only partially associated wit h heading date. Two early-heading NILs were less sensitive to photoper iod than NGN, but less sensitivity was not a requirement for early hea ding. Rate of apical meristem development was partially associated wit h early heading date at the distinct blister-like spikelet stage. Bree ding programs centered in cool-moist environments of the PNW should co nsider increasing emphasis on early heading SWW germplasm.