Temperature and photoperiod response of vernalization in near-isogenic lines of wheat

Abstract

Vernalization, a requirement for plants to experience a period of cool conditions to accelerate flowering, is an important determinant of flowering date in winter wheat. Although recent work has provided much better quantitative descriptions of the process, questions remain about the temperature response of vernalization, and how vernalization and daylength responses interact. Hence, controlled environment studies were carried out to quantify the response of vernalization rate to temperature for two near-isogenic lines of the wheat cultivar Batten, and to determine the effects of photoperiod during vernalization. The lines differed in vernalization-sensitivity, with Spring Batten (SB) being essentially vernalization-insensitive and Winter Batten (WB) being strongly vernalization responsive. Vernalization treatments were 1 °C (in darkness), and 5, 8, and 11 °C under both 16 and 8 h photoperiods for treatment durations of up to 12 weeks. A 25/20 °C, 8 h photoperiod treatment was included to test for short day vernalization. Post-treatment conditions for all treatments were 25/20 °C with 16 h photoperiod. Plants were sampled for dissection at intervals during the treatment and post-treatment period, recording leaf stage and cumulative primordium number until the flag leaf could be distinguished. Unvernalized WB flowered with 19 leaves, reducing to a minimum of 8.1 leaves when fully vernalized by the 5 °C/16 h treatment. The progress of vernalization was monitored through both changes in the pattern of primordium initiation at the shoot apex and changes in the final number of leaves initiated. Based on the duration to saturation of the vernalization response, the rate of vernalization under 16 h photoperiods increased linearly from 1 to 8 °C then declined again to a projected zero rate at 16 °C. The vernalization rate under 8 h photoperiods was greater than the rate at 16 h photoperiods for all temperatures, with a maximum rate at 8 °C. The existence of a short day vernalization response at 25/20 °C meant that the vernalization rate under 8 h photoperiods declined only slightly between 8 and 22 °C. In all cases, a significant lag period preceded any response to vernalization, ranging from 6 days in the 25/20 °C, 8 h treatment to 45 days in the 1 °C treatment. The work reported here suggests that the recently developed framework for describing vernalization is sound, but the details may need modification.