EFFECT OF SEEDLING TEMPERATURE AND ITS DURATION ON DEVELOPMENT OF WHEAT CULTIVARS DIFFERING IN VERNALIZATION RESPONSE

The aim of the study was to determine the most effective vernalization temperature for wheat, and establish whether this changes with cultiv ars and duration. Four cultivars of wheat were used, ranging in respon se to vernalization from zero (Dollarbird), through moderate response (Oxley, Osprey), to that typical of North American winter wheats (JF87 %014). They were exposed to temperature pre-treatments of 12, 10, 8, 6 degrees C in the light or 5, 3, 0 degrees C in the dark for 0, 2, 3, 6, 8, or 10 weeks before being transferred to 19 degrees C (22-16 degr ees C) to complete their growth. Control plants remained in the 22-16 degrees C regime throughout. A photoperiod of 16 h was used in all tre atments. The most effective vernalization temperature differed dependi ng on how the response to vernalization was assessed. Where maximum re sponse was equated with fewest days from sowing to heading, i.e., the sum included the time elapsed during the period of pre-treatment, the most effective temperature was 19, 8-10, 6-10 and 6 degrees C for the respective cultivars. Using summed thermal time above a base of 0 degr ees C to describe this same period, the most effective temperature was 6 degrees C for all cultivars. The generalised ranking for effect was 6 > 8 > 10 > 3 > 12 > 19 degrees C. The ranking for 0 degrees C rose with duration of treatment, with peak effects occurring after 8-10 wee ks. Using minimum leaf number to assess effects of temperatures on ver nalization showed 3 degrees C as generic optimum, but all temperatures below 19 degrees C (22-16 degrees C) reduced leaf number relative to that at 19 degrees C, even marginally in the spring cultivars Dollarbi rd. Consequently, even 12 degrees C can be rated as a vernalizing temp erature. Final leaf number declined linearly with reduction in pre-tre atment temperature towards 3 degrees C. In general, the very lowest te mperatures took longer to effect vernalization than higher temperature s, but the eventual effects could be greater. Rate of emergence of lea ves day(-1) measured under a 16-h photoperiod at 19 degrees C (22-16 d egrees C) after the temperature pre-treatments were complete, was Line ar, though the slopes differed depending on the pre-treatment temperat ure. Rate increased progressively in general accordance with the degre e to which the pretreatment temperature effected full vernalization (a ssessed for each treatment as nearness to minimum number of leaves for the cultivars). Rate of leaf appearance reached maximal values in the winter types as they approached full vernalization. It is proposed th at in any discussion of vernalization, the mode of assessment should f irst be defined. Additionally, predictions of final leaf number and da te of heading may have to allow for the historical effects of temperat ure on rate of leaf appearance. (C) 1998 Elsevier Science B.V. All rig hts reserved.