However, it is now well established that plant development responds to average 24-hour temperature rather than to either day or night temperatures alone.
In the experiments reported here, four tomato cultivars - Bigdena (beefsteak), Quest (beefsteak), Clarance (truss) and Conchita (cherry) - were grown in growth rooms with high-intensity lighting from 0600 to 1800 hours each day. The temperature between 2100 and 0600 hours ("night") was 18 degsC but in the preceding three hours of darkness (1800 to 2100 hours) the temperature could be anywhere between 9 degsC and 15 degsC.
The 24-hour average temperature in each treatment was set at 19.4 degsC and achieved by adjusting the "day" temperature appropriately. Being grown at 9 degsC, even for just three hours, was undesirable for all cultivars except Conchita because yields were reduced and blossom-end-rot increased.
The optimum temperature in this "pre-night temperature" period was 14.9 degsC for Conchita. The optimum pre-night temperature for Bigdena was 13.8 degsC in this period and for Clarance it was 12 degsC.
Response of Greenhouse Tomato to Varied Low Pre-Night Temperatures at the Same Daily Integrated Temperature by Zhang, Hao, Li and Jiang (2010). HortScience 45(11): 1654-1661. ISHS members can view HortScience at www.ishs.org.