Growing greenhouse tomato and sweet pepper under supplemental lighting: Optimal photoperiod, negative effects of long photoperiod and their causes

This paper reviews the available information on the photoperiod aspects of the use of supplemental lighting for greenhouse tomato and sweet pepper production. Optimal growth and yields of tomato and sweet pepper were obtained under photoperiods of 14 and 20 hours, respectively. Longer photoperiods did not further improve growth and yields and even decreased growth and yields in some cases. Although long term use of continuous light is detrimental to tomato and pepper plants, vegetative growth and fruit production of both species can be improved by short term use (5 to 7 weeks) of continuous lighting. Compared to shorter photoperiods, continuous light (24-h photoperiod) increased the leaf levels of hexoses in tomato, of sucrose in pepper and of starch in both species. The accumulation of starch and sugar in leaves under continuous light indicate a limitation of tomato and pepper plants to export the photosynthate out of their leaves. Such a limitation would explain the fact that extra light energy provided by continuous lighting did not result into growth and yield gains. The increased leaf hexose levels in tomato and increased leaf sucrose in pepper suggest that the limiting steps of the export of photosynthate are respectively the synthesis of sucrose and the loading of sucrose in the phloem. Under greenhouse conditions, continuous light caused leaf chlorosis in tomato but not in sweet pepper. Development of leaf chlorosis in tomato under continuous light was related to a decrease of the chlorophyll concentration in the leaves. Compared to tomato, higher levels of carotene and xanthophylls (photoprotective pigments) in pepper leaves probably provided a better protection of the photosynthetic apparatus against excessive light, thus preventing the destruction of chlorophylls and the development of leaf chlorosis in pepper. The severity of leaf chlorosis varied with the type of lamps (high pressure sodium, HPS versus metal halide, MH) used to provide the supplemental light, indicating that the spectral composition of the light received by plants may also play a role in the development of leaf chlorosis. Under continuous light, the response of tomato and pepper plants to HPS lamps versus MH in the greenhouse differed from the response in growth chambers. These differences between greenhouse and growth chamber could be related to the light spectral quality (presence or absence of natural light) and/or the daily variation in the climatic conditions (larger day/night differential in greenhouse).