Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 2004
Publication Date: July 20, 2004
Citation: Makus, D.J., Lester, G.E. 2004. Preliminary observations on the effect of light intensity and time of day on mustard greens leaf ascorbic acid and greenness at harvest. Hortscience 39(4):771.
Field-grown mustard greens, Brassica juncea, were used to validate several observations of a greenhouse study which reported nutrient changes in mustard greens grown, in part, under ambient and reduced light. The cultivar Florida Broadleaf was transplanted into a Hildago sandy clay soil near Weslaco,TX (26' 08' Lat.) on 06 Nov 03. Greens were fertigated with 30 kg N/ha on 1 Dec. Plants were grown under the following four light regimes 14 days before harvest: (1) continuous ambient light; (2) 7days of 50% shade then 7 days of ambient light; (3) 7 days of ambient light then 7 days of 50% shade; and (4) 14 days of 50% shade. Cumulative solar light was 28.9 and 19.4 kW/m2 during the first and second 7 days, respectively. Measured cumulative light, as PPFD, for the four treatments were 108, 67, 78, and 44 mM/s/m2, respectively. Shade during the last 7 days generally evoked the greatest responses. Increased shade duration did not significantly effect the agronomic performance, but did increase leaf total carotenoids, chlorophylls, water content, and reduced total ascorbate levels. As day progressed, sample plant wt, and avg. leaf wt decreased in shaded plants only. Free ascorbic acid, chlorophyll a:b ratio, and the chlorophyll to carotenoid ratio decreased with time of day. Cumulative sunlight, as PPFD, was significantly correlated with total ascorbate (fr wt basis), chlorophyll a:b ratio, and plant wt. (P<0.06) and negatively correlated with chlorophylls and total carotenoids (dry wt basis). Thus, cloudy weather prior to harvest can have a negative effect on leaf Vitamin C and alter leaf greenness.