PHENOLOGICAL DEVELOPMENT OF BRASSICA-CAMPESTRIS, BRASSICA-JUNCEA, BRASSICA-NAPUS AND B-CARINATA GROWN IN CONTROLLED ENVIRONMENTS AND FROM 14 SOWING DATES IN THE FIELD

This paper describes two studies of phenological development in plants of four Brassica species, The controlled environment study at 27/17 d egrees C and under low light showed that all species flowered earlier with increasing daylength and had greatest sensitivity between 12 and 14 h photoperiod though with no change in response at photoperiods sho rter than 12 h. Virtually all the response to photoperiod occurred bet ween plant emergence and visible flower buds. The field study, conduct ed at latitude 28.7 degrees N, considered the same lines of the same s pecies over two years with seven planting dates in each year. All spec ies completed their life cycles in less calendar time as planting date was delayed after October 13 with the time to maturity reducing on av erage by 0.62 day for every day's delay, There was no apparent respons e to photoperiod, but because photoperiod in the field during the peri od to the bud visible stage was always between 10.2 h and 11.3 h, this lack of response was in accord with the first study. The duration of the phase from plant emergence to the bud visible stage measured in th ermal time was, however, progressively reduced as mean temperature dec lined from 24 to 12 degrees C. Depending on species, these reductions in thermal time ranged from 22 to 41 degrees Cd > 0 degrees C for ever y 1 degrees C reduction in mean temperature. It is shown that the resp onse was akin to a vernalisation response being also correlated with s ummations of the reciprocal of daily minimum temperature. At equivalen t mean temperature, thermal time accumulation to budding was B. campes tris < = B. juncea < B, napus < B carinata; minimum observed accumulat ions between plant emergence and budding were approximately 200, 200, 350 and 600 degrees Cd > 0 degrees C respectively. Durations of the de velopmental phases after budding were apparently determined solely by temperature, and so, within a species, required the same thermal time for completion regardless of planting date. The results are discussed in relation to the determination of phenological development by photop eriod, temperature, vernalisation, radiation and the impact of develop ment on growth.