Agriculture Reference
In-Depth Information
primordial growth is characterized by a curvature in the micropylar end, and by
expansion in the chalazal end.
Stage II is a rapid growth period of fruit development due to cell enlargement and
water accumulation. During this period the increase in fruit size is mainly due to pulp
growth, juice vesicles reach their maximum size storing water, sugars and acids, and
zygotic embryo matures. This stage comprises from the end of the physiological fruit
drop until the onset of fruit colour change, its duration depending on the cultivar.
In stage III, growth is mostly arrested and fruits undergo a non-climacteric rip-
ening process. During this stage fruit metabolism changes occur that determine the
final external and internal fruit quality. The process of external fruit ripening mainly
involves the progressive loss of chlorophyll and the gain of carotenoids, thus chang-
ing peel colour from green to orange. The process of internal fruit ripening involves
a decline in acidity, mostly due to the catabolism of citric acid, and an increase in
sugars, both determining the maturity index. External and internal ripening not al-
ways coincides in time.
Factors Determining Fruit Growth
Environmental Control
Fruit growth depends on the tree water status and carbohydrate partitioning. In fact,
the fruit serves as a storage organ of water and its final size depends directly on the
availability of water. Drought provokes biochemical inhibition of citrus photosyn-
thesis that reduces fruit carbohydrate supply and can stop fruit growth. But fruit
continue to be a strong carbohydrate sink and apparently continues to accumulate
and is available for fruit growth after the drought is relieved. This decline in fruit
development is almost irreversible and fruit become smaller. Drought also reduces
peel turgidity and consistency and, consequently, there is resistance to handling and
consumer acceptance.
Nutritional Control
Final fruit size is closely linked to the number of developing fruits. Competition
for photosynthate among fruit is the hypothesis that has prevailed for many years
to explain the relationship between fruit size and fruit number. Accordingly, fruit
thinning is widely used to increase the final size of the remaining fruit on the tree.
This effect, based on a reduction of competition among developing fruit, has been
explained as being due to a modification of source-sink relationship. In
Citrus
,
synthetic auxins have been widely used as thinner agents. Auxins temporarily in-
duce photosynthetic disorder that leads to reduction in photosynthate production
and fruitlet uptake that temporarily slows its growth, triggering ethylene produc-
tion and fruitlet abscission. Afterward, the remaining treated fruit overcomes this
effect, increases growth rate, and reaches a larger size than untreated fruit. Accord-
ing to the non-linear relationship between fruit size and fruit number, a significant
