Glucosinolates In Pakchoi (Brassica Campestris L. Ssp. Chinensis) As Affected By Nutrient Status And Postharvest

Glucosinolates (β-thioglucoside N-hydroxysulfates, GSs) are nitrogen- andsulfur-containing anionic natural products, one group of plant secondary metabolitesfound in Cruciferae. Glucosinolates were well known because of the strong flavorsand tastes. In the past decade, glucosinolates have been identified as the self-defencecompounds for plants and cancer-prevention agents for human beings with in-depthstudy. Therefore, how to get more beneficial glucosinolates from these vegetables hasbecome a hot research topic in recent years.The profiles of glucosinolates are usually determined by genotypes, howeverglucosinolate contents are also affected by plant growth conditions and environmentalfactors. In the process of harvest, storage and cooking etc, the glucosinolates areeasily hydrolyzed into a variety of degradation products due to its contact withthioglucosidases [myrosinases (EC 3.2.3.1)] upon the plant tissue disruption. Theunsuitable postharvest treatments induce unnecessary loss of glucosinolates.Pakchoi (Brassica campestris L. ssp. chinensis var. communis), a traditionalcruciferous vegetable originated from China, is widely grown in the Yangtze Riverregion. The effects of nitrogen (N) form, the supply levels of phosphorus (P),magnesium (Mg) and iron (Fe), storage temperature and cooking method on theprofiles and contents of bioactive compounds (such as GSs) are studied. So, it will beimportant to find the best nutrient composition, plant growth condition andprocessing method for the high content and good retention of glucosinolate inpakchoi. The main results as follows:(1) The effects of different N forms on GS contents were studied in pakchoi plantsgrown in nutrient solution. Five treatments with the same total N content (8 mM) but different ammonium/nitrate ratios were used (NH₄⁺-N: NO₃^?-N, 0:8, 2:6, 4:4, 6:2 and8:0). The highest plant biomass was achieved by mixed supply of both forms of N,especially in 25% ammonium-N supply. Nitrate accumulated at a lower level whennitrate-N was replaced with ammonium-N. GSs contents were higher whenammonium-N supplied than other treatments, however, the worse plants growth wasin an opposite way.(2) The effects of P supply levels (1.00 and 0.01 mM) and different lightintensities (normal and 60% shading) on plant growth and GSs contents wereinvestigated in pakchoi plants grown in nutrient solution. P deficiency decreasedshoot biomass and the pigments content. Plant could grow better whereas the pigmentcontent decreased under normal light intensity. That might because the values werebased on dry weight and/or a dilution effect in this case. P deficiency and high lightintensity increased the root/shoot ratio. P deficiency increased the contents ofindividual and total GSs content especially under normal light intensity. Higher lightintensity increased the content of aliphatic GS, however light supply has nosignificant effect on indolyl and aromatic GS, provided P supply was sufficient. Onthe other hand, the GS precursor - amino acid (AA) was also affected by differentlevels of P supply and light intensities. However, changes in the content of free AA donot result in altered GS contents.(3) The effects of Mg²⁺ levels (1.00 and 0.02 mM) and different light intensitieson plant growth and GSs content were studied in pakchoi plants grown in nutrientsolution. Mg²⁺ deficiency inhibited pakchoi plant growth and reduced pigmentsaccumulation. The influence of Mg supply was not uniform for different types of GSconsidered, as positive (indolyl GS), negative (aliphatic GS), and neutral (aromaticGS) effects under normal light intensity. However, the effect was not significantunder low light intensity. (4) Two treatments for Fe supply (50 and 5μM) were used. The results indicatethat Fe deficiency decreases the plant biomass and pigments content under both lightintensity conditions. However, GSs contents were increased. Light intensity affectedGS content obviously, and shown an interaction effect with Fe supply.(5) Influences of two temperatures (4 and 20℃) on weight loss, carotenoids andGS contents in pakchoi shoots were investigated during storage. The results suggestthat low temperature was beneficial to reduce the loss of carotenoids (β-carotene,lutein and chlorophylls) and maintained the visual quality as well as GS content oreven increased the content of total GS, specifically total aliphatic GS, and individualGS glucoalyssin, gluconapin, glucobrassicanapin and 4-methoxyglucobrassicin at theend of experiment. That could be due to the increased glucosinolate synthesis and/or arelease of bound compounds during storage.(6) The influences of common culinary proceeding (blanching, boiling, steaming,stir-frying and microwave) on GS profile and contents in pakchoi shoots weredetermined. Blanching retained the highest glucosinolate content. The data alsoshowed that the GS content was not reduced by steaming, stir-frying and microwavesignificantly. On the contrary, significant loss of GSs was resulted from boiling byleaching into cooking water. Aliphatic GS has a better thermal stability than indolyland aromatic GS. Therefore, the best cooking method for pakchoi to maintain higherGS content should be blanching, followed by steaming, stir-frying and microwave. Itis not good for health to throw away the cooking water when pakchoi was boiled inwater for a long time.