218 E.J. Waters and C.B. Colby
the berries. Similar although less dramatic trends of reductions in protein levels were
seen in laboratory experiments in which otherwise healthy berries were inoculated
withB. cinerea(Girbau et al. 2004). Protein levels were also reduced in juice when
B. cinereawas grown in this medium (Girbau et al. 2004).
The reduction in protein levels in the juice from Botrytis-infected grapes could,
as suggested by Marchal et al. (1998), be due to proteolytic degradation of grape PR
proteins by enzymes ofB. cinerea,although there are other plausible explanations
for these observations. If these effects are due to the activity of proteolytic enzymes
fromB. cinerea, these enzymes have the potential to replace bentonite fining for
protein stabilization in oenology, a goal of many research efforts worldwide.
6C.4.2 Water Stress
The effect of water stress on the expression of PR proteins in grapes has been exam-
ined by analysing the PR protein content ofV. viniferacv. Shiraz berries from a
replicated irrigation trial (Pocock et al. 2000). The lack of irrigation gave clear
physiological signs of vine water stress but did not lead to elevated levels of PR
proteins in the berries. At a fixed amount of protein per berry, however, the protein
concentration in the juice from water stressed berries was higher than that from
irrigated berries because berries from irrigated vines were larger and thus berry
solutes were less concentrated. This effectof water stress on berry size is a general
phenomenon (Smart and Coombe 1983) and it is likely that anecdotal reports that
haze problems are greater in drought years are due to changes in berry sizes in these
years rather than a direct physiological response of the berries to water stress in the
form of enhanced PR protein production. Studies of gene expression under carefully
controlled experimental conditions involving water stress would add further support
to this hypothesis.
6C.4.3 Mechanical Harvesting
Paetzold et al. (1990) observed that mechanically harvested grapes produced must
with increased protein content compared to that of hand harvested fruit pressed
as whole bunches. They suggested that the lack of stems during crushing led to
lower polyphenolic content in the must from mechanically harvested fruit com-
pared to that from hand harvested fruit and therefore more protein was lost in
complexes with phenolics from must from hand harvested fruit. Dubourdieu and
Canal-Llaub`eres (1989) showed that wine made after maceration of destalked
grapes for 18 h contained more protein than wine made after immediate pressing of
whole bunches. Pocock and colleagues (Pocock and Waters 1998; Pocock et al. 1998)
examined the impact of mechanical harvesting alone, not destemming, on the PR
proteins in grapes and wine. Mechanical harvesting, with its associated prolonged
transport of the fruit, resulted in higher PR protein levels in the juice and wine.
Indeed mechanical harvesting of white grapes and subsequent transport was found