Preharvest manipulation of rind pigments of Citrus spp.
Date
2006-12
Authors
Le Roux, Smit
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
Rind colour is one of the main cosmetic preferences consumers use when purchasing citrus
(Citrus spp.) fruit. To enhance the cosmetic quality of citrus fruit, attempts were made to
stimulate preharvest chlorophyll degradation and carotenoid biosynthesis to obtain a deeper,
more uniform, orange rind colour in early-maturing citrus cultivars. As part of a larger study
to stimulate rind colour enhancement, an initial study was conducted on ‘Eureka’ lemon [C.
limon (L.) Burm. f.] nursery trees to determine the concentration of various gibberellin
biosynthesis inhibitors required to obtain a biological response in citrus trees, as measured by
vegetative growth. Thereafter, different concentrations of prohexadione-calcium (ProCa;
Regalis®) were applied at various stages of fruit development on early-maturing citrus
cultivars to establish the concentration and timing of ProCa required to improve rind colour
by enhancing chlorophyll degradation and carotenoid biosynthesis. In addition, a search to
enhance rind colour development of early-maturing citrus cultivars was conducted by
screening various nutritional, hormonal and possible physiological stress-inducer products
and some combination treatments thereof.
Multiple applications of gibberellin biosynthesis inhibitors on ‘Eureka’ lemon nursery trees
significantly reduced internode length and hence vegetative growth. Regalis® applied at 4 to
8 g·L-1 and Sunny® (uniconazole) applied at 10 to 20 mL·L-1 had the greatest effect in
reducing internode length, and were therefore identified as potential candidates for further
field studies to test their effect on rind colour enhancement of citrus fruit.
The late, double applications (6 plus 3 weeks before anticipated harvest) of ProCa applied at
400 mg·L-1 consistently improved rind colour of all Citrus spp. tested. However, these effects were more pronounced after harvest, as ethylene degreening and cold-storage stimulated
additional chlorophyll degradation, unmasking the carotenoids, resulting in overall better
coloured fruit. In most instances in this study, ProCa stimulated chlorophyll degradation
allowing the underlying carotenoids to be expressed. Therefore, the improvement of rind
colour of citrus fruit following the application of a gibberellin biosynthesis inhibitor (400
mg·L-1 ProCa applied 6 plus 3 weeks before harvest) supports the hypothesis that there may
be a relationship between vegetative vigour and rind colour development of citrus fruit.
Preharvest applications of boric acid, Thiovit® (elemental sulphur), ammonium thiosulphate
(ATS) and half the recommended rate of Ethrel® (48% ethephon) in combination with
Thiovit® and ATS stimulated chlorophyll degradation in both orange- and yellow-rinded
fruit, and ColourUp® (neutralised calcium carbonate) and Figaron® (ethyclozate) stimulated
chlorophyll degradation only in orange-rinded fruit. Boric acid and the Thiovit®-ATSEthrel
® combination treatment stimulated carotenoid biosynthesis in orange-rinded fruit,
thereby improving the carotenoid to chlorophyll ratio. The screening of chemical products
which stimulate chlorophyll degradation in combination with chemical products which
stimulate carotenoid biosynthesis warrants further evaluation.
Worldwide, research on rind colour improvement has received attention for several decades,
particularly during the 1980s. Yet, rind colour still remains a problem at the beginning of
certain seasons. In the present study, the approach to improving rind colour was to manipulate
rind pigments through the reduction of vegetative vigour, which was hypothesised to be an
antagonist of chloro-chromoplast transformation. To this end, the preharvest application of
prohexadione-calcium stimulated chlorophyll degradation and carotenoid biosynthesis in
citrus fruit rinds. Furthermore, preharvest applications of various chemical products provides a novel approach to stimulate chlorophyll degradation and carotenoid biosynthesis. Together,
the results of this study provide potential commercial treatments that will result in deeper,
more uniform orange rind colour, thereby meeting consumer needs.
Description
Thesis (MscAgric (Horticulture))--University of Stellenbosch, 2006.
Keywords
Dissertations -- Horticulture, Theses -- Horticulture, Dissertations -- Agriculture, Theses -- Agriculture, Citrus fruits -- Coloring