ISHS Acta Horticulturae 481: International Symposium on Growing Media and Hydroponics
SUPPLEMENTAL LIGHTING OF GREENHOUSE VEGETABLES: LIMITATIONS AND PROBLEMS RELATED TO LONG PHOTOPERIODS |
| Authors: | D. Demers, A. Gosselin |
| Keywords: | Tomato, Lycopersicon esculentum, Pepper, Capsicum annuum, chlorosis, growth, yields, starch, sugars |
| Abstract:
In northern regions, the major factor limiting winter greenhouse production is low natural light. However, it has been demonstrated that using supplemental light from high-pressure sodium vapor (HPS) lamps from November to March achieves increased growth, productivity and gaseous exchanges of vegetable crops such as tomato (Dorais, 1992; Dorais et al., 1991; Vézina, 1989; Vézina et al., 1991), cucumber (Turcotte and Gosselin, 1987, 1989), pepper (Demers, 1990; Demers et al., 1991) and lettuce (Gaudreau, 1991; Gaudreau et al., 1994). Most research on supplemental lighting has concentrated on the effects of the amount of light on crops; little work has been done on the influence of the photoperiod. In practice, greenhouse vegetable growers generally use photoperiods of 14 to 17 hours. Greenhouse tomato plants (Lycopersicon esculentum Mill.) exposed to photoperiods of 20 and 24 h developed chlorosis on their young leaves (4th to 7th leaf from apex of plant) (Bradley and Janes, 1985; Vézina et al., 1991). Moreover, the productivity of tomato plants subjected to a 17-h period did not increase compared with a 14-h photoperiod (Vézina et al., 1991). Extending the photoperiod to 20 to 24 hours even reduced plant productivity (Vézina et al., 1991). Foliar chlorosis and reduced productivity also occurred among tomato plants exposed to a photoperiod of 17 h where the 7-h nocturnal period had been separated by a 3.5-h light period into two short nights of 3.5 h. (Vézina et al., 1991). Extending the photoperiod to 20 h resulted in increased growth and productivity of greenhouse sweet pepper (Capsicum annuum L.) plants (Demers et al., 1991). Compared with a photoperiod of 15 h, continuous lighting (24 h) resulted in earlier flowering and fruiting of pepper plants, giving better early yields, while final productivity was identical for the two photoperiods (Costes and Milhet, 1970). In addition, continuous lighting resulted in smaller average volume of the fruit harvested (Costes and Milhet, 1970). Blistering of the leaves was observed in pepper plants exposed to a photoperiod of 24 h (Costes and Milhet, 1970) and even 20 h (Demers et al., 1991). Slight internerval discoloration of the leaves was noted among pepper plants receiving 24-h light (Costes and Milhet, 1970). Also, foliar chlorosis and loss of leaves was observed in pepper plants grown under uninterrupted light (Nilwik, 1981). We found that exposing tomato and pepper plants to long photoperiods (tomato: over 17 h, pepper: over 20 h) resulted in negative effects such as chlorosis or blistering of leaves. In addition, long photoperiods also reduced growth and productivity. Where long photoperiods do not have negative effects on plant growth and productivity, the plants do not appear to benefit or utilize the additional light. Many questions arose from these early measurements: Why leaf chlorosis does appear? Why tomato plants cannot be cultivated using photoperiods exceeding 17 h, while pepper plants tolerate a photoperiod of 24? Which physiological processes are affected by long photoperiods? | |
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