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Ezzedine Derbali, Louis-Philippe Vezina, and Joseph Makhlouf

Objectionable off-odors are produced by broccoli (Brassica oleracea, L.) when it is held under anaerobic conditions. These off-odors were attributed to sulfur volatile compounds mainly methanethiol (MT) and hydrogen sulfide. The present study was undertaken to investigate the effects of anaerobic conditions on the metabolism and emission of sulfur volatiles by broccoli. Inhibition assays using aminooxyacetic acid (AOA)—a potential inhibitor of pyridoxal-phosphate-dependent enzymes-confirmed the enzymatic origin of these volatiles. However, anaerobic atmosphere had no inducible effect on the enzymes cystine lyase, cysteine desulfhydrases and S-alkylcysteine lyase. These pyridoxal-phosphate-dependent enzymes thought to catalyze the respective degradation of cystine, cysteine and S-methyl-L-cysteine to sulfur volatiles showed no significant activity increase. Storage of sterile broccoli seedlings under anaerobic atmosphere resulted in an important increase of the content of sulfur amino acids that corresponded to an increased emission of sulfur volatiles. Cysteine and methionine content increased particularly at 24 hours and decreased later. Whereas, S-methyl-L-cysteine content increase was more obvious after 48 hours. The results suggest a possible involvement of the pathways for synthesis and breakdown of sulfur amino acids via methionine.

Free access

Linda Gaudreau, Josée Charbonneau, Louis-Philippe Vézina, and André Gosselin

`Karlo' and `Rosana', two Boston-type lettuce (Lactuca sativa L.) cultivars, were subjected to various light treatments in greenhouses equipped with one of two propane heating systems. Photoperiods of 16, 20, 24, or 24 hours for 2 weeks after transplanting and then 16 hours (24–16) and photosynthetic photon flux of 50 or 100 μmol·m–2·s–1 provided by supplementary lighting (high-pressure sodium vapor lamps) were compared to natural light during four experiments performed in greenhouses between Sept. 1989 and May 1990. Using supplementary lighting resulted in significant increases in biomass (≤270%), head firmness, and tipburn incidence and decreases in production cycle length (≈30%). Treatment effects were most pronounced during the months when natural-light levels were low. Fresh weights were higher for `Karlo' than `Rosana'; however, `Rosana' was less susceptible to tipburn than `Karlo'. In general, the radiant heating system resulted in earlier crop maturity and a higher incidence of tipburn than the hot-air system.

Open access

Régis Larouche, Louis-Philippe Vézina, and André Gosselin

Abstract

The activity of nitrate reductase (NR) and glutamine synthetase (GS) was followed during 4 weeks in various tissues of tomato plants (Lycopersicon esculentum Mill. cv. Vedettos) grown in growth chambers under two photosynthetic photon flux (PPF; 125 and 250 μmol·s−1·m−2, high-pressure sodium lamps) conditions for 18 hr and with four N fertilization regimes (5, 10, 15, and 20 meq·liter−1). In roots, NR increased with increased PPF but not with increasing N. Leaf NR activity was stimulated by increased PPF. Leaf NR increased over time in all treatments, but the highest values were obtained at lower N concentrations. Glutamine synthetase was stimulated by both light and N increases; its activity also increased throughout the 4 weeks of treatment. Nitrate reductase activity was highly correlated with the fruit fresh weight : leaf fresh weight ratio. On a mature tomato plant, NR activity was found mainly in leaves opposite developing fruits (sixth and seventh leaves), while GS activity was concentrated in the upper portion of the plant (second and third leaves).

Open access

Régis Larouche, André Gosselin, and Louis-Philippe Vézina

Abstract

Tomato plants (Lycopersicon esculentum Mill. cv. Vedettos) were grown in growth chambers under two photosynthetic photon flux (PPF; 125 and 250 μmol·s−1·m−2, high-pressure sodium lamps) conditions for 18 hr and four N fertilization regimes (5, 10, 15, and 20 meq·liter−1) to characterize the ratio of vegetative to reproductive growth. At the lower PPF, vegetative growth was limited and did not respond to N increments in the nutrient solution. At 125 μmol·s−1·m−2, maximum yields were obtained at the lowest N fertilization level. At 250 μmol·s−1·m−2, maximum leaf dry weight and yields were obtained at intermediate N fertilization levels (10 and 15 meq·liter−1). Light levels did not alter the nitrate content of stem sap (but high PPF resulted in nitrate accumulation in leaves) but, at low PPF, nitrate accumulated in leaves at 20 meq N/liter. Nitrate in stem sap varied only slightly with N increments in the nutrient solution. The ratio of fruit fresh weight to leaf fresh weight decreased as N increased in the nutrient solution.