Michael Treeby

Calcium-accumulating cells in the meristematic region of grapevine root apices

Apical roots of grapevines were examined by cryo-SEM (scanning electron microscopy) and the intracellular distribution of Ca was demonstrated by X-ray microanalysis in different regions of the primary root. We show that large amounts of Ca are accumulated as raphide crystals in the vacuoles of specialised cortical cells (idioblast cells) of the root apex. These crystal idioblast cells appeared to form a discontinuous cone of cells in the outer region of the root meristem. The raphide crystals within these cells were less apparent in older regions of the root, 10-12 mm basipetal to the root tip. We suggest that the raphide crystals could initially act as another Ca sink involved in the regulation of Ca levels in root apices. In older regions of the root these cells are spaced at intervals around the periphery of the cortex and the subsequent disappearance of the raphides may be indicative of remobilisation, perhaps in the zone of elongation where cell wall synthesis occurs and Ca demand is high. Calcium-accumulating cells were also observed in the older regions of the root, forming endodermal protrusions extending into the cortex. These cells may play a part in regulating Ca delivery to the xylem stream by sequestration of Ca from the radial flow of water at the endodermis. The observed distribution of Ca in root apices was different from the other major cations (e.g. K) and anions (e.g. Cl) because high concentrations were localised to specific cells. We interpret the results in the context of a model of the dynamics of grapevine root growth and cell differentiation, and the temporal balance of solute supply from the protophloem and the external medium.

Influence of Pre- and Postharvest Factors on β-Carotene Content, Its in Vitro Bioaccessibility, and Antioxidant Capacity in Melons

Fresh and stored melons were analyzed for total and bioaccessible beta-carotene content, as well as total antioxidant potentials, to investigate the effects of genotype, season, sowing time, and storage on nutritional quality. There were significant cultivar effects on beta-carotene content (P < 0.001), ranging from 1 to 23 mg kg(-1) of fresh weight (FW), its bioaccessibility (P < 0.005), ranging from 79 to 94.5%, and for antioxidant potential (P < 0.001), ranging from 1569 to 2267 micromol of Fe(2+) kg(-1) of FW of melons. Sowing time significantly affected beta-carotene contents (P < 0.017) and antioxidant potentials (P < 0.001), but these were highly cultivar dependent, with season having no effect. Postharvest storage over 4 weeks at 7 degrees C resulted in losses of antioxidant potential and beta-carotene content independent of cultivar. Cultivar and postharvest management were major determinants of nutritional quality of the melons evaluated.