Title

Enhancement of naturally occurring chemopreventive compounds in salad greens through environmental manipulation during crop growth

Date of Completion

January 1998

Keywords

Agriculture, Plant Culture

Degree

Ph.D.

Abstract

Studies were conducted to characterize the effects of key environmental factors on the concentration of omega-3 fatty acid $(\alpha$-linoleic acid, LNA) in purslane leaves and phenethyl isothiocyanate (PEITC) in watercress leaves. In one study, purslane was grown at day/night temperatures of $27/22\sp\circ$C, photoperiod of 20-h. d$\sp{-1}$, photosynthetic photon flux (PPF) of 299 or 455 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$ and PPF duration of 8, 12, 16 or 20-h. d$\sp{-1}$. Thus plants received a cumulative PPF of 8.6, 12.9, 17.2, and 21.5 mol.m$\sp{-2}$.d$\sp{-1}$ for low PPF (299 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$) and 13.1, 19.7, 26.2, and 32.8 mols.m$\sp{-1}$.d$\sp{-1}$ for high PPF (455 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$). At low PPF, purslane leaves grown under 16-h PPF duration (17.2 mol.m$\sp{-2}$.d$\sp{-1}$) produced the highest concentration of LNA, while at high PPF, purslane leaves grown under 8 and 12-h PPF durations (13.1 and 19.7 mol.m$\sp{-2}$.d$\sp{-1}$) produced similar LNA concentrations and decreased at longer(16 and 20-h) PPF durations (26.2 and 32.8 mol.m$\sp{-2}$.d$\sp{-1}$ respectively).^ In another study, two purslane cultivars (green-leafed and Goldberg) were grown in nutrient solutions containing 200 $\mu$g.mL$\sp{-1}$ N provided as NO$\sb3$-N:NH$\sb4\sp+$-N ratios of 1:0, 0.75:0.25, 0.5:0.5, and 0.25:0.75. Although both cultivars produced similar LNA concentrations per g leaf dry mass (DM), Goldberg had 24% higher LNA than green-leafed cultivar per unit leaf area. Results showed that a NO$\sb3\sp-$-N:NH$\sb4\sp+$-N ratio of 0.4:0.6 would maximize LNA concentration in both cultivars. The first study with watercress examined the effect of three N:S ratios (1:0.32, 1:0.64 and 1:0.96) and two days to harvest (18 and 28-d) on leaf and stem PEITC concentration. At 28-d watercress leaves grown with 1:0.64 and 1:0.96 N:S ratio contained 84 and 61% (DM basis) higher PEITC concentration respectively than those grown with 1:0.32 N:S ratio; leaf PEITC concentration was higher (157%) than stems. In another study, watercress was grown at 15 or $25\sp\circ$C, 8 or 12-h.d$\sp{-1}$ photoperiod and PPF of 265 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$. One week before harvest half of the plants were exposed to a higher PPF of 435 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$, while the remaining plants continued to receive 265 $\mu$mol.m$\sp{-2}$.s$\sp{-1}$ until harvest. Results showed that PEITC concentration is influenced by higher PPF before harvest, but only for plants grown under a short (8-h) photoperiod. ^