A typical chlorosis in the leaves of tomato appears at photoperiods longer than 18 hrs. when incandescent lamps are used as the sole source of radiant energy. This chlorosis occurs at 20[degree]C when a 15-hr. photoperiod is lengthened to 24 hrs. with relatively low irradiances of 4 and 20 fc. When the plants are grown at 12[degree]C, the length of the photoperiod has relatively little effect on the chlorophyll concn. of the leaves and the typical severe chlo-rotic pattern developed at higher temps. under 24-hr. photoperiods is absent.