Mechanism of Stimulating Inflorescence Development and Nitrogen Assimilation in Cool-Weather Broccoli in Relation to Temperature and Photoperiod.

High-value, cool-weather broccoli (Brassica oleracea var. italica), a hardy vegetable, can only thrive in low temperatures (10-20^oC). In temperate Nova Scotia, growing season is short but summer temperatures have exceeded 30ºC in recent years. The interactions of temperature and photoperiod duration on broccoli inflorescence development and plant nitrogen assimilation were examined in a greenhouse study. The treatments consisted of two temperatures (20 and 30^oC) and two photoperiods (8 and 16 hours per day). The broccoli cultivar was ‘Everest’, an early variety. A completely randomized design was used to arrange the treatments with 10 replicates in the greenhouse. Plants were seeded in peat moss in 2-L pots and maintained at 22^oC until shoot tips of 7 mm then the treatments were applied in the growth chambers for 30 days. For the higher temperature treatment (30^oC), plant were heat-treated for 4 hours per day and under 20^oC for the rest of time. All plants received the same quantity of water and liquid fertilizers throughout the experiment. Susceptibility of broccoli to heat was detected using shoot tip and crown status examined using scanning electron microscopy (SEM). Nitrogen assimilation of broccoli plants was determined at harvest using a LECO CNS Analyzer. The microscopic images showed that plants were heat damaged by exposure to the high temperature of 30^oC 4 hours per day for 30 days. The interaction of temperature and photoperiod were significant on broccoli leaf nitrogen and carbon accumulations (P 0.001). The combination of temperature at 20^oC and extended photoperiod (16 hours per day) resulted in highest plant dry matter content and N/C ratio. It is suggested that the relatively low temperature of 20^oC can stimulate broccoli plant nitrogen assimilation and carbon accumulation, which is the mechanism of promoting broccoli inflorescence development.