Citrus Production - Horticulture: Plants for People and Places - page 176

However, in Citrus there are a lot of seedless cultivars; therefore, other major
regulating factors, aside from seed derived stimuli, must also be implicated. In seed-
less cultivars, fruit set appears to be developmentally regulated, probably operating
also through the synthesis and action of gibberellins in ovary walls.
In Citrus , gametic sterility and homogenetic sterility are the main genetic mecha-
nisms that produce parthenocarpy, or the natural or artificially induced production
of fruit without fertilization of ovules. Navel orange and Satsuma mandarin are
species of gametic sterility, Clementine mandarin, hybrids-like mandarins (i.e. 'tan-
gors', 'tangelos', etc.) and pummelo are species of homogenetic sterility.
Natural parthenocarpy can be obligate or facultative , depending on the fertil-
ity of the flower. When the flower is sterile, obligate parthenocarpy takes place
without any external stimulation. Navel orange and Satsuma mandarin are species
of obligate parthenocarpy. When the flower is fertile, facultative seedless fruit is
produced if fertilization does not occur. Clementine mandarin presents facultative
parthenocarpy.
Factors Determining Fruit Set
Environmental Control
As mentioned above, temperature and relative humidity determine Citrus pollina-
tion. Their effect may be either indirect, through modifying bees ( Apis mellifera )
activity, or direct, inducing pollen sterility or modifying pollen tube growth.
Under Mediterranean conditions, the effective pollination period of a flower var-
ies between 8-9 days for sweet oranges and Clementine mandarin and 2-3 days
for Satsuma mandarin. High temperatures during flowering accelerate pollen tube
growth, and stigma and ovule maturation, and low temperatures slow pollen tube
growth and extend ovule viability.
Fruit set also shows a significant and negative correlation with daily mean leaf-
temperature during the physiological fruit drop.
Relative humidity may influence pollination in Citrus through its effect on stig-
ma longevity. The Citrus flower has a wet-type stigma with unicellular and pluri-
cellular papillae covered with a conspicuous secretion that plays a part in stigma
receptivity to pollination. Low relative humidity, together with high temperatures,
increase tree transpiration, which may increase the physiological fruitlet drop, espe-
cially when soil moisture and tree water status are low.
Nutritional Control
In Citrus trees, many flowers and fruits abscise during flowering or during the
physiological fruitlet drop period. It is of great importance in some sweet orange
cultivars in which the higher the flowering intensity the earlier and heavier the