Abstract:
Several effects of xylem discontinuity in Pinot noir
and Merlot grape berries were studied. There was a
reduction in the amount of apoplastic dye (Eosin Y or
azosulfamide) uptake through cut pedicels into soft
versus firm berries, suggesting a reduction in maximal
xylem flow at that time. Both greenhouse and field grown
Pinot noir berries took up less dye after softening.
Merlot berries, collected on one date from the field and
separated by hand into four developmental categories,
took up different amounts of azosulfamide dye according
to category. Soft green and just colored berries took up
30%, and fully colored berries 70%, less dye than firm
green berries. The reduction in xylem conductivity was
related to the developmental stage of each individual
berry and not the cluster as a whole.
The accumulation of K⁺ (primarily a phloem
transported element) and Ca²⁺ (primarily a xylem
transported element) differed in field grown Pinot noir
berries during maturation. On a per berry basis K⁺
increased after veraison, suggesting greater phloem
activity, but Ca content remained stable after
veraison, suggesting little xylem activity.
Berry diameters on pre-veraison clusters on well
watered vines increased slightly; those on unwatered
vines decreased, losing 0.87 mm in diameter during day 3
of the experiment. Pre-veraison berry deformabilities
were 380% higher in unwatered versus watered vines on day
4. Bagging pre-veraison clusters to slow transpiration
reduced the rate of berry diameter loss and softness only
slightly. Pre-veraison berry shrivelling occurred before
vine wilting. Post-veraison Pinot noir berry diameters
and deformabilities in the greenhouse were not
significantly affected by short term vine water stress.
Heat girdling cluster peduncles to block phloem flow
reduced pre-veraison berry growth rates to near zero and
increased the rate of diameter loss significantly in
post-veraison berries. Girdling had little effect on
pre-veraison berry deformabilities, but a large effect on
those of post-veraison berries.
The different berry responses to vine water stress
and peduncle girdling before and after veraison suggested
a change in xylem activity. Prior to veraison there was
rapid water loss from the berry to a stressed vine, but
after veraison berries were more isolated, showing little
response to vine water stress. Blocking phloem transport
in the cluster peduncle prior to veraison reduced berry
growth to almost zero but affected deformability little,
which suggested that xylem was maintaining berry size and
firmness. Under the same conditions post-veraison
berries lost both size and firmness rapidly, which
suggested little xylem activity.
