The relationship between water status and chlorophyll a fluorescence in grapes (Vitis spp.)

A new minimum fluorescence parameter, as generated using pulse frequency modulation, compared with pulse amplitude modulation: F α versus F o

“…6c). However, as the grapes dehydrate, the efficiency of their photosystems declines (Wright et al 2008), the sharpness of the incline in fluorescence at the low f values is reduced, and the overestimation of the y-intercept also decreases, which may be interpreted (in terms as F α values) as a factor influencing the minimum fluorescence parameter downwards relative to mass loss (Fig. 6c-f).…”

“…The greater propensity for the PFM minimum fluorescence value to decline relative to grape mass loss compared with the PAM value likely occurred due to: 1) sensitivity to a decreasing F m factor, and 2) an overestimation of F α during periods of high curvilinearity in the f versus F plots. Both F m and F v have been shown to significantly decrease in both grape cultivars over the course of dehydration (Wright et al 2008). If the F α values were influenced by this decreasing F m parameter [there is evidence for this based on the similarity of the mass loss versus F α and mass loss versus vertex height (i.e.…”

“…A third factor, chlorophyll degradation, will also affect fluorescence levels. TS grapes undergo significant chlorophyll degradation during dehydration, while LAc grapes do not (Wright et al 2008). Chlorophyll content is positively correlated with minimum fluorescence (Smillie et al 1987;Toivonen and DeEll 1998).…”

“…Three cluster/scale systems were placed inside three PAM fluorometer units (Fig. 1b) (the grapes remained on the scales for the duration of the experiment); a typical point-and-click PAM unit was not used in this study since it had been determined that this design is sensitive to chlorophyll concentration in grapes (Wright et al 2008). The remaining three clusters were monitored with PFM fluorometer sensors positioned to the left and right of each cluster for a total of six sensors (Fig.…”

“…The physiological changes that occur in grapes (Vitis spp.) as they dehydrate over a long period of time induce osmotic stress and irreversible damage to the grape with a reduction in quantum efficiency (Wright et al 2008).…”