Agriculture Reference
In-Depth Information
of the secondary buds can develop further and replace it in the following spring. In
susceptible cultivars such as Shiraz, a physiological disorder called primary bud
necrosis (PBN) can lead to the death of up to 60 % of primary buds, and a signifi-
cant loss of yield potential in the following season. Other causes of bud loss include
mechanical damage, mites, fungal pathogens and severe winter freeze events.
Formation of Inflorescence Primordia
Inflorescence development in grapevines commences with the separation of an an-
lage or uncommitted primordium from bud apex when three to seven leaf primordia
have been produced (Srinivasan and Mullins
1981
). The exact timing varies ac-
cording to cultivar and climate, but may range between 2 and 6 weeks after bud-
break (Vasconcelos et al.
2009
). Anlagen at this point can develop into a tendril,
inflorescence or sometimes an intermediate form, with commitment of the anlage
to a pathway of inflorescence development promoted by cytokinins and inhibited
by gibberellins (Boss and Thomas
2002
; Srinivasan and Mullins
1981
). In hot cli-
mates, inflorescence primordia may be initiated up to 3 weeks before flowering
at the basal node positions, and this process continues in an acropetal direction
along the shoot through spring and early summer (Swanepoel and Archer
1988
).
The development of the inflorescence primordia continues through the season, and
at the onset of dormancy several orders of branch and subtending bract primordia
can be observed. Individual flowers are not formed until bud-break in the follow-
ing spring, but variation observed between inflorescence differentiation in cool and
warm climates suggests that yield may be influenced by the extent of inflorescence
development in the season of initiation (Watt et al.
2008
).
The number of inflorescences initiated per bud can range between one and three,
although for most cultivars two is more common. Environmental factors that fa-
vour the formation of inflorescence primordia over tendrils include temperatures
above 20 °C and less than 35 °C in the weeks leading up to flowering, and well
exposed shoots that ensure a good supply of assimilates for the developing buds
(Buttrose
1969
; Sanchez and Dokoozlian
2005
). Carbohydrate reserves, or at least
carbohydrate reserve mediated effects on canopy development may also influence
bud-fruitfulness (Smith and Holzapfel
2009
), and the perennial nature of stored car-
bohydrates potentially extends the influence of management practice and climatic
conditions on yield to a 3 year cycle or beyond. Varying the number of buds retained
after pruning offers one of the main avenues for yield regulation in grapevines, and
where bud dissections are undertaken to assess bud fruitfulness during winter, prun-
ing may be adjusted to target a specific number of bunches in the following season.
