Phenological development of Brassica campestris, B. juncea, B. napus and B. carinata grown in controlled environments and from 14 sowing dates in the field

Abstract

This paper describes two studies of phenological development in plants of four Brassica species. The controlled environment study at 27/17°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 shorter than 12 h. Virtually all the response to photoperiod occurred between plant emergence and visible flower buds. The field study, conducted at latitude 28.7°N, considered the same lines of the same species over two years with seven planting dates in each year. All species completed their life cycles in less calendar time as planting date was delayed after October 13 with the time to maturity reducing on average by 0.62 day for every day's delay. There was no apparent response to photoperiod, but because photoperiod in the field during the period 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 thermal time was, however, progressively reduced as mean temperature declined from 24 to 12°C. Depending on species, these reductions in thermal time ranged from 22 to 41°Cd > 0°C for every 1°C reduction in mean temperature. It is shown that the response was akin to a vernalisation response being also correlated with summations of the reciprocal of daily minimum temperature. At equivalent mean temperature, thermal time accumulation to budding was B. campestris < = B. juncea < B. napus < B carinata; minimum observed accumulations between plant emergence and budding were approximately 200, 200, 350 and 600°Cd > 0°C respectively. Durations of the developmental 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 photoperiod, temperature, vernalisation, radiation and the impact of development on growth.