Abstract
Supplemental lighting with high-pressure sodium (HPS) lamps has been commercially used for greenhouse tomato production in Quebec, Canada since more than a decade. This very particular situation results from low rates of off-peak electricity combined to cold climate allowing an efficient recuperation of the heat released by the lamps. Early research conducted at Laval University showed that 100 umol m-2 s-1 would be an optimal economic photosynthetic photon flux (PPF) under our climatic conditions. We also showed that a progressive photoperiod from 14hOO in September to 16h00 in December and January and then to 14h00 till April lead to the highest yields, without causing physiological disorders such as leaf chlorosis. Starch accumulation in tomato leaves submitted to long photoperiods was partly attributed to a reduced sucrose phosphate synthase (SPS) activity. More recently, we showed that genetically-modified tomato lines for higher SPS activity produced higher yields. We also found that overwintering tomato plants under HPS supplemental lighting were not able to fully take advantages of higher PPF at Spring as a result of important limitations of the leaf water status.