Characterization of Citrus Peel Maturation and the Effect of Water Stress, Growth Regulators and Date of Harvest

This research was carried out on ‘Marsh’ grapefruit and ‘Valencia’ orange, aims to understand the role of peel maturation in citrus as it relates to postharvest handling and storability.  Currently, citrus is harvested based on edibility. Peel changes have not been related to best harvest time, but most handling problems are peel related. Changes in juice TSS/acid ratio, peel color, peel turgidity, percent fruit drop, and fruit characteristics during storage, were measured over different at-harvest maturity and combined with changes in peel sugars and volatile components to determine if peel maturation and senescence can be monitored by some combination of physical and chemical characteristics in order to minimize fruit disorders resulting from immature or senescent peel or unusual stress levels that the peel may have been subjected to. Water stress and growth regulators were field applied to advance or delay maturity to see if any physical or chemical measurements induced corresponding changes in amount or rate. This would indicate the utility of these measurements for determining stage of peel development (immature, mature, or senescent). Results indicating changes in levels of peel sugars and volatile components during maturation coincided with changes in peel color and turgidity, percent fruit drop, and fruit characteristics during storage were promising. Peel sugar level increased as fruit maturation progressed, reaching a peak in mid-harvest season, and then decreasing toward the end of the season. Fruit can be harvested within this mid-season period with good juice quality and less senescent peel. Volatile components increased in senescent peel accompanied by a decrease in fruit firmness and increase in juice quality. These measurements may be useful for establishing harvest date guidelines to avoid harvesting too early (immature fruit) or too late (senescent fruit) so that less postharvest injuries occur while internal quality remains acceptable.  The suggested harvest period for ‘Marsh’ grapefruit was from January to March, and for ‘Valencia’ orange was from March to May. Growth regulators were more effective than water stress for changing peel characteristics, based on their effect on physiological processes related to peel maturation rate. Sugars and volatile components may be good indicators of peel senescence along with peel firmness and puncture force.