Agriculture Reference
In-Depth Information
and heat stress early during berry development may result in a similar decrease in
berry size. Berry volume is also highly dependent on cell expansion both early and
late in development (Ojeda et al.
1999
).
During the pre-veraison stage water flow into the berry is primarily through the
xylem but after veraison water inflow is chiefly through the phloem and this is
accompanied with a switch from symplastic to apoplastic unloading (Zhang et al.
2006
). The decline of water flow through the xylem may be the result of a reduction
in the hydrostatic gradient between the pedicel and the xylem (Tilbrook and Tyer-
man
2009
). Under certain conditions, the xylem, can however, continue to contrib-
ute up until late ripening (Rogiers et al.
2001
; Rogiers et al.
2006c
). The volume of
the berry is a function of water inflow through the xylem and phloem and the water
lost through transpiration or backflow. Diurnal cycling in berry volume is the result
of these opposing influences (Greenspan et al.
1994
; Rogiers et al.
2006a
). Berries
expand during the night when vine water status is high and when berry transpiration
is low. Conversely, berries will shrink during the day when high evaporative de-
mand drives transpirational water loss from the berry's surface. Vine water status is
therefore one prime determinant of berry volume and this is dependent on environ-
mental factors such as readily available water in the soil and evaporative demand.
Water loss through transpiration from the berry's surface is also dependent on the
position of the berry in the bunch and the degree of bunch compactness. Berry age
is another determinant since transpiration rate on a surface area basis decreases
rapidly after veraison (Poni et al.
2001
; Rogiers et al.
2004
), possibly as a result of
changes in the physical structure of the epicuticular wax layer.
Some varieties such as Shiraz are prone to shrinkage during late ripening, affect-
ing composition and overall yield per hectare. It is thought that exposed berries have
higher transpiration rates and this contributes to shrinkage since water flow into
the berry is often reduced at this later stage. There is also some evidence of water
movement out of the berry and back to the shoot through the xylem in the process
called backflow (Lang and Thorpe
1989
; Tyerman et al.
2004
; Keller et al.
2006
).
This can potentially occur when the water status of the vine is more negative than
that of the berry. On the opposite extreme, splitting of the berry skin can occur after
a rain event or on humid days. In Shiraz, susceptibility to splitting is highest shortly
after the onset of ripening and remains high for about 1 month. After this stage, the
susceptibility to splitting declines due to a loss in cell vitality and the turgor generat-
ing capacity of these cells (Clarke et al.
2010
). There are currently few management
options to control splitting but bunch zone aeration to increase transpiration and
cultivar choice upon vineyard establishment are important considerations.
Berry Composition
During grape berry ripening the flesh accumulates carbohydrates, organic acids and
amino acids while the skin and seeds are sites of colour and tannin accumulation.
Glucose and fructose are the predominant sugars accrued within the berry. Tartaric
and malic acid are the main organic acids while proline and arginine are the pre-
