The aim of the study was to determine the most effective vernalization temperature for wheat, and establish whether this changes with cultivars and duration. Four cultivars of wheat were used, ranging in response 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 degrees 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 treatments. The most effective vernalization temperature differed depending on how the response to vernalization was assessed. Where maximum response 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 degrees 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 weeks. Using minimum leaf number to assess effects of temperatures on vernalization 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 Dollarbird. Consequently, even 12 degrees C can be rated as a vernalizing temperature. Final leaf number declined linearly with reduction in pre-treatment temperature towards 3 degrees C. In general, the very lowest temperatures took longer to effect vernalization than higher temperatures, but the eventual effects could be greater. Rate of emergence of leaves day(-1) measured under a 16-h photoperiod at 19 degrees C (22-16 degrees C) after the temperature pre-treatments were complete, was Linear, though the slopes differed depending on the pre-treatment temperature. Rate increased progressively in general accordance with the degree to which the pretreatment temperature effected full vernalization (assessed 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 that in any discussion of vernalization, the mode of assessment should first be defined. Additionally, predictions of final leaf number and date of heading may have to allow for the historical effects of temperature on rate of leaf appearance. (C) 1998 Elsevier Science B.V. All rights reserved.
