Agriculture Reference
In-Depth Information
abscission process is, and the number of fruitlets remaining correlates negatively
with fruit growth rate. This is attributed to the high demand for carbohydrate by the
developing organs.
Several sources scoring demonstrate the positive relationship between carbo-
hydrates and fruit set: (1) leafy shoots set fruits in a higher proportion than leaf-
less shoots; (2) full or partial tree defoliation reduces carbohydrate concentration
in fruitlets and increases fruitlet abscission; (3) experiments involving translocation
of
14
C-metabolites and a CO
2
-enriched atmosphere show positive effects of carbon
availability on fruit development; and (4) girdling decreases fruitlet abscission by
increasing carbohydrate availability for the growing fruitlets. However the latter ef-
fect is mostly observed in leafy shoots through an increase in leaf ΦPSII. Girdling
also increases GA content in the ovary.
Girdling or scoring, which are the complete removal of a strip of bark from the
secondary branches of the tree or the performance of a cut around the complete
circumference of these branches, respectively, improves yield irrespective of the
parthenocarpic ability of a given cultivar. Its effectiveness increasing fruit set de-
pends on the date of treatment (35 days after anthesis being the optimum) and is
negatively related to flowering intensity.
Hormonal Control
Auxin concentration in the ovary is not a limiting factor controlling early fruit de-
velopment in citrus whereas there is strong evidence supporting the role of GA in-
ducing fruit set. Thus, in seeded sweet orange cultivars, pollination increases ovary
GA
1
concentration compared with un-pollination, which parallels a higher fruit set
of seeded fruits. When comparing a number of parthenocarpic mandarin cultivars
differing in their ability to set, the higher GA content at anthesis correlates with
the higher parthenocarpic ability. On the other hand abscisic acid (ABA) content is
higher in the ovaries of weaker parthenocarpic genotypes.
Citrus
seedless cultivars of weak parthenocarpic ability (mostly Clementine
mandarin cultivars and some hybrid-like mandarins) need GA
3
sprays to obtain a
commercial yield. The effectiveness depends on the date of treatment (petals fall)
(Fig.
6.2
) and concentration applied (5-10 mg l
−1
), and is negatively related to flow-
ering intensity.
Fruit Growth
Citrus fruit development follows a characteristic sigmoid growth curve and is di-
vided into three major stages: cell division, cell enlargement, and fruit maturation
(Bain
1958
; Fig.
6.2
). Stage I comprises the period of fruit set and extends from
anthesis to the end of the physiological fruit drop, and the increase in fruit size is
mainly due to peel growth. Juice vesicles are formed during this stage, and seed
