The Effect of an Incandescent Supplement on the Growth of Tomato Plants in Low Light

Young tomato plants were grown at low light flux densities (21W m^-2 for 8 h days) in growth cabinets under three types offluorescent lamps or under a fluorescent/incandescent mixedsource. Whilst net assimilation rates under the fluorescentlamps were in agreement with those calculated from the lampcharacteristics and the photosynthetic action spectrum, therate under the mixed source was about 20 per cent higher thanexpected. Relative growth rates and relative leaf area growthrates were also higher and leaf area ratios lower under thefluorescent/incandescent lamp combination than under the purefluorescent sources. Small differences in stem elongation, leaftemperature and dry weight distribution which were associatedwith the addition of incandescent radiation were not consideredto be responsible for these increases. When the light flux densityfrom the mixed source was reduced by 20 per cent, the plantgrowth parameters were then similar to those in fluorescentlight alone.