Agriculture Reference
In-Depth Information
colour are dependent upon peel pigment composition. In fruits growing under con-
stant high temperatures, chlorophyll levels remain high for oranges and mandarins
and the fruit peel remains green. But when temperature is as low as 15 °C, chlo-
rophyll is degraded and carotenoids synthesized. Carotenoid synthesis is reduced
above 15 °C but still occurs at temperatures conducive to chlorophyll degradation.
The ratio of TSS:TA (total soluble solid:total acidity; Brix: g/100 cc) is the most
widely used criterion for internal fruit maturity. Fruit from warm regions usually
have higher percentages of TSS and TA, and a higher TSS:TA ratio than those from
cooler regions. In tropical climates, TSS and TA are reduced in mandarin, orange,
grapefruit, tangelo and tangor, but not in lemon and lime. Advanced maturity of the
navel orange has been related to high heat summation (over 13 °C) in the spring,
mainly affecting TA. Furthermore, the higher the day/night temperature, the lower
the percentage of TA in Satsuma mandarins. This inverse relationship between tem-
perature and acid content was found in oranges, grapefruit, mandarin and pumme-
los under orchard conditions. This decrease in TA, due to high temperatures, has
been attributed to rapid respiration of organic acids at these temperatures. Unlike
TA, there is no clear relationship between TSS and temperature. Grapefruit and
pummelo cultivars, unlike most orange and mandarin cultivars, develop as high a
TSS concentration in juice in tropical as in subtropical climates, and temperature
regimes have little influence on TSS concentration.
Freeze-damage greatly reduces citrus fruit quality. Severely damaged fruit is
useless for fresh consumption and slightly damaged fruit gradually becomes par-
tially dry. The extent of damage depends on the cultivar, temperature, duration
of low temperatures and the TSS concentration in juice. Mandarins, lemons and
limes are more susceptible than oranges. Grapefruits and pummelos are moderately
susceptible. Citrus fruits suffer irreversible damage at temperatures of − 2.5 °C or
lower. Recovery from freeze damage is cultivar dependent, but if fruits are damaged
they have lower concentrations of TSS and TA in the juice. The peel of externally
injured fruit contains parts of completely desiccated tissue, typically around the
calyx. Damage to leaves and branches can also affect fruit quality and yield of new
crops. Low temperatures are also involved in the development of some physiologi-
cal disorders, which will be discussed in a later section.
Relative humidity (RH), together with the prevailing temperature regime, deter-
mines fruit set. Moderate temperatures contribute to improved fruit set, and low or
higher temperatures followed by sharp changes, promote fruitlet abscission during
cell division at the fruit growth stage.
Relative humidity also affects fruit size. Consistent low values through the night
reduce the growth rate of 'Valencia' oranges, of which 37 % RH is considered the
critical level. Relative humidity can reduce fruit quality dramatically, especially that
of mandarins.
Fruit shape is also affected by RH with grapefruit grown in humid subtropical
or tropical regions developing an oblate shape, while those grown in arid regions
becoming spherical. Grapefruits and mandarins attain better colour under high at-
mospheric conditions of RH.
