Long-day effects on growth and flower initiation of tomato plants in low light

Young tomato plants (cv. Minibelle) were grown in plant growth cabinets in 575 kJ m^-2 (400–700 nm) daily radiation. Plants grown in an 8 h day were then compared with those in which 10% of the radiation was taken from the main 8 h light period and supplied over the next 8 h period. After 41 days from sowing the 16 h day plants had almost twice the dry weight of those in short days and a 55 % greater leaf area. Net assimilation rate, relative growth rate and relative leaf area growth rates were all greater in long days, although the differences in growth diminished with time. The long-day treatment also increased the proportion of dry weight in the leaves, a function which is usually relatively stable in different environments. The beneficial effect of the long days may arise from a reduction in night respiration, or an increase in rate of photosynthesis through the observed increase in chlorophyll content. The 8 h light period resulted in flower initiation one or two nodes lower than the 16 h period so that the cultivar is a quantitative short-day plant.