Physiologia Plantarum 22(5): 899-907 (May 1969)
Diurnal Periodicity and Plant Growth
H. J. Ketellapper
Department of Botany, University of California Davis, California

Abstract
The experiments were carried out to investigate whether other plant species, in addition to tomato plants, show injury symptoms when grown for 2 to 4 weeks in an aperiodic environment and to obtain information about physiological mechanisms involved in the response to the absence of environmental periodicity. The growth of seedlings of pea, peanut, and soybean, exposed to different daylengths at constant temperature, increased with increasing length of the light period up to 16 to 20 hours, defending on the species. Further lengthening of the photoperiod did not result in significant increases in dry matter accumulated. The absence of environmental periodicity did not cause injury in these three species. Tomato plants responded in an entirety different manner. The optimal photoperiod for dry matter production by tomato plants was 18 hours and photoperiods longer than 20 hours caused interveinal chlorosis. Thus, tomato plants have an absolute requirement for a daily periodicity, white the other species do not in short-term experiments. Under conditions of constant temperature development of chlorosis by tomato plants may be prevented by a daily dark period of 4 hours or longer or by a daily period of drastically lowered light intensity. Complete darkness is not essential, however. This suggests that development of chlorosis is not mediated through a photoperiodic response system. Involvement of a circadian oscillation may also be excluded. Aperiodic environmental conditions appear to affect the physiology of the tomato plant in a direct manner, possibly by influencing chlorophyll synthesis or degradation.

Nat Commun 5:4549 (2014)
A single locus confers tolerance to continuous light and allows substantial yield increase in tomato
Aaron I. Velez-Ramirez, Wim van Ieperen, Dick Vreugdenhil, Pieter M. J. A. van Poppel, Ep Heuvelink & Frank F. Millenaar

Abstract
An important constraint for plant biomass production is the natural day length. Artificial light allows for longer photoperiods, but tomato plants develop a detrimental leaf injury when grown under continuous light—a still poorly understood phenomenon discovered in the 1920s. Here, we report a dominant locus on chromosome 7 of wild tomato species that confers continuous light tolerance. Genetic evidence, RNAseq data, silencing experiments and sequence analysis all point to the type III light harvesting chlorophyll a/b binding protein 13 (CAB-13) gene as a major factor responsible for the tolerance. In Arabidopsis thaliana, this protein is thought to have a regulatory role balancing light harvesting by photosystems I and II. Introgressing the tolerance into modern tomato hybrid lines, results in up to 20% yield increase, showing that limitations for crop productivity, caused by the adaptation of plants to the terrestrial 24-h day/night cycle, can be overcome.