Bunchstem Development in Vitis Vinifera: Links to Berry Ripening and Composition

In grapevines, the bunchstem functions as a critical anchor point
for developing flowers and berries. It also provides the only transport
pathway between the vine and the berries for carbon, nutrient, water,
signal molecules and other metabolites. The size and architecture of
the bunchstem varies considerably between varieties and is
influenced by abiotic factors such as temperature and light, as well
as endogenous factors such as vine carbohydrate status.
Inflorescence flower number and fruit set also respond to these
factors, but it is uncertain how ovule and bunchstem development
are linked. Bunchstem morphology, anatomy as well as xylem and
phloem composition were investigated in Merlot and Cabernet
Sauvignon grapevines to gain better understanding of the role of
flower and berry number on its development. The link between
bunchstem development with berry number, size and composition
was also investigated.

In both Merlot and Cabernet Sauvignon, inflorescences that were
naturally small at flowering remained relatively smaller through until
harvest. Large inflorescences typically bore more berries than small
inflorescences, and at harvest these berries were higher in sugar
content, anthocyanins and phenolics. In bunchstems of both
varieties, phloem and xylem strands narrowed from the proximal to
the distal end of the bunchstem, and were most narrow in the laterals
and pedicels. The internal structures were linearly organised all
through the bunchstem, and additional secondary tissues, including
secondary phloem fibres and periderm were present in the hypoclade.

In order to gain better understanding of the link between flower
number and bunchstem development, 0 to 100% of flowers were
removed manually from an inflorescence one week prior to capfall.
Flower removal resulted in shorter bunchstems, with partial curving
of the bunchstem when more than half of the flowers were removed.
Complete flower removal resulted in the desiccation and dehiscence
of the bunchstem. Internally, flower removal resulted in the
development of disorganised vasculature with a decline in both xylem
and phloem area as the number of remaining berries decreased.
Flower removal was also accompanied by an inhibition and delay of
secondary structure formation. From these results it was
hypothesised that signals released from the flowers promoted normal
vascular development of the bunchstem. Flower removal increased
percentage fruit set of the remaining ovules and resulted in larger
berries, likely due to the greater amount of photoassimilate
availability per berry. Berry composition was, however, unchanged
with flower removal. In a separate experiment, complete leaf removal
one week prior to capfall did not alter the development of the
bunchstem despite a decrease in fruit set. However, the removal of
these carbon sources resulted in smaller berries, with significant
changes in their final composition including phenolics and potassium
concentrations as well as the amino acids profile.

Cytokinins and gibberellins, hormones critical for normal plant
development, were evaluated to determine their impact on
inflorescence and berry development. Application of the synthetic
cytokinin, BAP, one week before cap-fall, resulted in longer and
slightly thicker bunchstems, without a significant effect on fruit set.
The development of the vascular system was restricted by BAP in
Cabernet Sauvignon, but not in Merlot. Conversely, GA3 application
reduced fruit set, but increased the proportion of abnormal berries
retained on the bunch. The development of the vascular system was
not altered even though bunchstems were longer and thicker. Both
hormonal treatments increased berry size, but basic berry
composition parameters remained unchanged.

Finally, diurnal and seasonal trends in bunchstem xylem and
phloem composition were examined in Merlot and Cabernet
Sauvignon. Bunchstem phloem amino acid concentration was
greater than that of the xylem, and this was greater than that of
petioles. The predominant amino acids of the xylem and phloem sap
included glutamic acid, glutamine, aspartic acid, asparagine, proline
and arginine. In the phloem, these amino acids were higher in the
pre-veraison period than the post-veraison period; however,
seasonal trends in the xylem were dependent on the individual amino
acid. The predominant sugar in the phloem was sucrose, but glucose
and fructose were also present. There was little change diurnally in
the phloem sugar concentrations. Bunchstem phloem sugar
concentration was also greater than that of petioles.

In summary, bunchstem development is highly dependent on
flower number and is responsive to the application of gibberellins and
cytokinins. BAP limited vascular development in Cabernet
Sauvignon, but GA3 application did not have an effect on the
vasculature despite changes in overall morphology. Carbohydrate
deprivation by leaf removal did not impinge on bunchstem
development, but berry growth was hindered. Both the phloem and
the xylem transport amino acids through the bunchstem and these
change seasonally with berry development.