Agriculture Reference
In-Depth Information
increase in fruit size occurs only if fruit thinning is higher than 50-60 % of total fruit
number, and if it is performed early in fruit development.
Hormonal Control
The application of synthetic auxins at the beginning of the cell enlargement stage
increases final fruit size without fruit thinning. The 2-ethylexyl ester of 2,4-dichlo-
rophenoxipropionic acid (2,4-DP) or 3,5,6-trichloro −2-pyridyloxiacetic acid (3,5,6-
TPA), as free acid, at a concentration of 25 mg l −1 or 15 mg l −1 , respectively, applied
at the onset of cell enlargement stage, is recommended for Clementine and Satsuma
mandarins and for hybrid-like mandarins. It has been suggested that their effect is re-
lated to an increase of fruit sink strength since fruit dry weight is generally increased.
Accordingly, auxin treatment significantly increases carbohydrate contents in the
fruit; in fact, the direct increase in fruit size due to auxin treatment is associated with
an enhancement of cell enlargement, rather than cell division, which, in turn, produces
an increase in absolute juice content and also in pulp and rind content, i.e. dry matter.
Cell enlargement consists of two interrelated processes: the osmotic uptake of water,
driven by a water potential gradient across the plasma membrane, and extension of
the existing cell wall, driven by the turgor generated stress within the wall. Auxins
are related to both processes by: 1) increasing carbohydrate content that may reduce
osmotic potential, and 2) the use of the acid growth hypothesis that proposes protons
(H + ) as a mediator between auxin and expansins, the latter of which are induced plant
cell wall loosening proteins. But auxin treatment also increases fruit peduncle cross-
sectional area; this effect is partially due to the increase of fruit size promoted by the
auxin, as well as the direct effect of the auxin promoting the development of peduncle
vascular tissues, thus allowing for a larger water uptake into the fruit.
Agustí and Almela ( 1991 ), Agustí et al. ( 2002 ), El-Otmani ( 2006 ), and El-
Otmani et al. ( 2000 ) are recommended reviews for further knowledge.
Fruit ripening
Citrus fruit colour development is under the regulation of several factors, including
environment, nutritional factors and plant hormones.
Rind colour-break and colour intensity are markedly affected by both air and soil
temperature (see Section “Edaphoclimatic requirements for production and improved
fruit quality”). Based on these observations and his experiments conducted in vitro ,
Huff ( 1983 ; 1984 ) hypothesised that Citrus may degreen in response to a reduced
nitrogen flow into the fruit accompanied by an increased concentration of photo-
synthate to the epicarp. Under field conditions, soil temperatures below 20-22°C
reduce citrus root activity and nitrogen uptake and translocation to the fruit, which,
nevertheless, remains as a strong sink for photosynthate that both reduces nitrogen
uptake and increases sugar uptake in fruit colour development. Under Mediterranean
climates, fruit colour-break does not take place at a certain soil temperature, but after
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