TEMPERATURE The optimal temperature for net assimilation rates in tomatoes is between 25 and 30 celsius (Khavari-Nejad, 1980) It does not seem to matter greatly in young plants if day and night temperatures differ. Relative growth rates of young plants responded relatively independently to the day and night tem peratures with an optimum for both at around 25 celsius (Picken et al., 1986). The effects of changing night temperature on the relative growth rate are about half those of changing day temperature (Picken et al., 1986).