Effects of heat stress on the biological Maillard reaction, oxidative stress, and occurrence of internal browning in Japanese radish (Raphanus sativus L.)

Impact of changes in root biomass on the occurrence of internal browning in radish root

The purpose of this study was to investigate the effects of root biomass during the later stage of growth on fatty acid composition and lipid peroxidation, and to clarify the physiological mechanisms by which these differences affect internal browning (IB) development in radish roots. Therefore, we controlled the enlargement of roots by changing the thinning period and generated plots composed of roots with different biomass in the latter half of growth. The earlier the radish seedlings were thinned, the more vigorous the root growth from an earlier stage was achieved. Earlier thinning caused IB from the early stage of root maturation, and IB severity progressed with subsequent age progression; however, IB damage did not occur when root size during the later growth stage was kept small by later thinning. Higher levels of hydrogen peroxide, peroxidase activity, NADPH-dependent reactive oxygen species (ROS) burst-related genes, and carbonyl compounds were detected in earlier-thinned large-sized roots compared to later-thinned small-sized ones. Compared with the latter small-sized roots, the former large-sized roots had a lower ratio of linoleic acid (18:2) and a higher ratio of α-linolenic acid (α-18:3). Furthermore, in earlier-thinned large-sized roots, higher levels of phospholipase- and/or lipoxygenase-related genes were detected compared to later-thinned small-sized ones. These facts suggest the possibility that root biomass in the later stage of growth affects the desaturation of membrane fatty acids, ROS concentration, and activity of fatty acid degrading enzymes, and controls the occurrence of IB injury through membrane oxidative degradation.

Role of Different Abiotic Factors in Inducing Pre-Harvest Physiological Disorders in Radish (Raphanus sativus)

Radish, one of the important root vegetables, is widely grown in the world due to its easy cultivation, short duration, growing habit, and adaptability to various growing conditions. However, it is still extremely difficult to produce good quality radish roots due to its vulnerability to different preharvest physiological disorders. Important physiological disorders that significantly reduce the yield and quality of radish are forking, pithiness/sponginess, cracking/splitting, hollowness, and internal browning. Different abiotic factors like moisture stress, temperature fluctuation, growing medium, nutrient imbalance, plant density and harvesting time cause a disturbance in the metabolic activities of root tissues that produce non-marketable roots. Therefore, this review provides a detail insight on the causes, physiology of these disorders, and the management practices to prevent them to produce commercial quality roots. This comprehensive knowledge will not only help the growers, but it will provide relative information for researchers as well to control these disorders through breeding innovations and biotechnological tools.

Inhibitory mechanism of low-oxygen-storage treatment in postharvest internal bluing of radish (Raphanus sativus) roots

Oxidative stress in radish roots causes internal blue discoloration and decreases vegetable quality. Accordingly, the effects of different oxygen concentration treatment on this coloration during storage was investigated; 4-hydroxyglucobrassicin content (a precursor of the blue component); the reactive oxygen species (ROS) superoxide (O₂^-) and hydrogen peroxide (H₂O₂); the antioxidants ascorbic acid (AsA) and glutathione (GSH); and the activities and gene expression levels of the enzymes catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione peroxidase (GPX), were monitored under normal and low-oxygen conditions. The results indicated that packaging radish roots under 10% O₂ prevents blue discoloration by decreasing the activity and expression of the oxidant enzyme POD, increasing the levels of antioxidant and reducing substances, and upregulating antioxidant enzymes, all of which act to decrease the generation of ROS (O^2- and H₂O₂).