Submitted to: Subtropical Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2005
Publication Date: 1/17/2005
Citation: Makus, D.J., Lester, G.E. 2005. Light intensity and time of day at harvest effects ascorbic acid concentrations and mineral nutrient content and leaf greennessof field-grown mustard greens. Subtropical Plant Science. Interpretive Summary: Many pre-harvest environmental factors, such as soil type, soil fertility, and weather (temperature, light intensity, and duration) can effect vegetable nutritional quality. Mustard greens are an excellent source of dietary mineral nutrients and Vitamin C. We report here on the effects of simulated cloudy weather and harvest time (of day) on Vitamin C, leaf pigments, and mineral nutrient levels in mustard greens. Cloudy weather reduces Vitamin C levels, but increases leaf greenness and many mineral nutrients. Leaves contain more Vitamin C and less nitrate when plants are harvested later in the day. To optimize Vitamin C content in mustard greens, processors and fresh market producers should harvest greens during periods of high light intensity.
Technical Abstract: 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 degr 08' Lat.), on 06 Nov 03. Beginning 14 days prior to harvest, plants were grown under the following four light regimes: (1) continuous ambient light; (2) 7 days 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*m**-2 during the first and second 7 days, respectively. Measured cumulative light, as photosynthetic photon flux density (PPFD), was 101, 67, 78, and 44 mM*s-1*m**-2 for treatments 1-4, respectively. Plants were harvested at 0800, 1100, and 1400 hrs on 02 Jan 04. Shade during the last 7 days generally evoked the greatest responses. Increased shade duration did not significantly affect the agronomic performance, but did increase leaf total carotenoids, chlorophylls, water content, most mineral nutrients (dry wt. basis), and reduced total ascorbate (fresh wt. basis) levels. As daylight progressed, sample plant wt. and avg. leaf wt. decreased in shaded plants only. Free ascorbic acid, NO3, Mg, 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, chlorophyll a:b ratio, and plant wt. (P<0.06) and negatively correlated with NO3, Fe, chlorophylls and total carotenoids. Thus, cloudy weather prior to harvest and, to a lesser extent, the time of day, can reduce leaf Vitamin C, mineral nutrients, and alter leaf greenness.