Foliar 2,4-D Application after Physiological Fruit Drop Reduces Fruit Splitting of Mandarin

Abstract

<jats:p>Various mandarin (<jats:italic>Citrus reticulata</jats:italic>) cultivars are prone to fruit splitting, a physiological disorder that entails cracking of the rind, starting from the stylar end of the fruit, with eventual splitting of the endocarp and abscission of the fruit. On two mandarin cultivars, Marisol and Mor, foliar applications of 2,4-dichlorophenoxyacetic acid (2,4-D), calcium (Ca), and potassium (K) were evaluated over two growing seasons for efficacy to reduce fruit splitting in Paarl, South Africa (lat. 33°69′S, long. 18°95′E). Foliar treatment of 10 mg·L<jats:sup>−1</jats:sup> 2,4-D directly after physiological fruit drop (APFD) compared with later dates in January and February, either alone or in combination with K, increased rind thickness and reduced fruit splitting of ‘Marisol’ and ‘Mor’ by up to 50%, without negatively affecting internal fruit quality. Treatments increased rind thickness and rind strength throughout fruit development in addition to fruit diameter, length, and growth rate, with no significant effect on rind coarseness. There was a slight reduction in juice content and titratable acidity (TA), but no effect on the total soluble solids (TSS). Application of 2,4-D APFD is thought to increase rind integrity due to a direct strengthening effect on the rind during early stages of fruit development.</jats:p>

Pre-Harvest Fruit Splitting of Citrus

“…The occurrence of any stress to trees and/or to fruitlets during the cell division stage was key in determining susceptibility to PFS because it was at this time that the fruitlets' flavedo formed and structural integrity was largely determined [13,43,47]. Any small lesion at the stylar end of the rind was thought to be a starting point for fruit splitting [49]. Observations of the process of stylar abscission in relation to environmental or other conditions may prove worthwhile as an "early warning" for PFS.…”

“…In genetically predisposed cultivars where fruits typically possess relatively large or protruding navels and/or "open" stylar regions, the first signs of small, radiating cracks may sometimes be seen during flowering, petal fall, and at stylar abscission [48]. Although different patterns for the development of PFS were described, it usually begins as small cracks (or "microcracks" [3,49]) at the fruit's stylar end [13]. However, in a few cases, it started at the fruit's equator [8,29].…”

“…Combinations of foliar GA 3 and/or 2,4-D (two foliar applications of each) reduced PFD [3,5], an effect attributed to increased peel resistance to puncture rather than to increased skin thickness [7,43]. However, two foliar applications of 20 ppm 2,4-D (at full bloom and at petal fall) altered fruit shape (increased fruit length) and increased rind thickness especially at the stylar end [13,49]. The resulting thicker rind showed development of solid, compact tissue under the stylar-end flavedo tissue and reduced PFS.…”

“…While nutrition affects rind strength, it is less clear how best to measure that strength as it relates to PFS. Attempts have been made to quantify strength by puncturing [30,49], and resistance to cutting [44]. Other mechanical properties may also be important, such as the ability to stretch [5].…”

Postharvest Application of Oligochitosan and Chitosan Reduces Calyx Alterations of Citrus Fruit Induced by Ethephon Degreening Treatment

“…2,4-Dichlorophenoxyacetic acid (2,4-D), a synthetic auxin (plant growth regulator), has been used as a preharvest treatment to reduce fruit splitting as well as the premature drop of mandarin fruit and also has been used as a postharvest packhouse treatment to retard calyx abscission (to repress postharvest decay) or calyx senescence, occurring as a consequence of ethylene-degreening treatment, thereby maintaining the quality of citrus fruits. − 2,4-D reduces PG and Cx activities and increases the lignin and water contents of fruit peels. , Currently, European Union and U.S. legislations have restricted the use of 2,4-D in the citrus postharvest industry because of concerns over its effects on human health and environmental safety. Therefore, determining an alternative to this auxin in controlling calyx senescence is necessary.…”