RELATIONSHIP BETWEEN YOUNG PANICLE LENGTH AT THE BEGINNING OF STEM ELONGATION STAGE AND HEADING TRAITS IN WHEAT (TRITICUM-AESTIVUM L) GROWNIN THE SOUTHWESTERN PART OF JAPAN

Early maturity has been one of the major breeding objectives in wheat (Triticum aestivum L.) in the southwestern part of Japan, because this trait is desirable for double cropping with summer crops such as rice , and for avoiding pre-harvest sprouting which often occurs in the rai ny season. However, early maturing cultivars usually belong to the spr ing type which is prone to frost damage in early spring because of the earliness in ear primordia initiation and stem elongation. The relati onship between ear primordia initiation, young panicle length, stem el ongation and heading traits, i.e. vernalization requirement, photoperi odic response and narrow-sense earliness, was assessed for 32 cultivar s and lines. Date of the beginning of stem elongation, which was defin ed as the time when the main stem length reached 20 mm, and young pani cle length at that time were estimated using an exponential curve base d on the records made at an interval of two weeks. There were large va rietal differences in the young panicle length at the beginning of ste m elongation. Cultivars and lines with a longer young panicle at the b eginning of stem elongation, headed earlier than those with a shorter one in the same stem elongation class (Fig. 1). In the cultivars and l ines with an early onset of stem elongation, the vernalization requrem ent was minimal (Fig. 2). Winter type cultivars and lines exhibiting a late onset of stem elongation were considered to be able to avoid fro st damage. Cultivars and lines with a longer young panicle at the begi nning of stem elongation exhibited a neutral response to photoperiod ( Fig. 3, Table 1), and many of them were the relatives of Chugoku 114 w hich has been used as a cross parent for its early maturity and displa ys rapid growth under short photoperiod and low temperature conditions (Yoshida et al. 1985). These results indicated that early heading cul tivars that can avoid frost damage, like the experimental line Aki 40 (Table 2), could be bred by introducing the character, which induces a longer young panicle at the beginning of stem elongation, into winter type cultivars and lines.