3.4. Enzyme activities
Cell wall-bound and soluble acid invertases were differentially
affected by UV-B exposure (Fig. 3). The cell wall invertase
activity showed at 1min time–dose a significant increase compared
with the control in both flavedo and albedo, but the
highest activity was observed in flavedo tissue. Maxima increases
were 59.6% and 56.0% that correspond to activities of 1.94 and
0.072mol reducing sugars min−1 g−1 FW in flavedo and albedo,
respectively. The pattern of changes of cell wall-bound invertase
was approximately similar in both tissues, excepting at 5 min
time–dose because at this time–dose the invertase activity of irradiated
albedo strongly decreased compared with untreated albedo
(Fig. 3a). The soluble acid invertase was also increased by UVB
treatment, but it showed a different activity pattern in both
flavedo and albedo. Although the highest increasesweredifferent in
albedo and flavedo (54.0% and 35.2%, respectively), maxima activities
(0.032 and 0.034mol reducing sugars min−1 g−1 FW) were
not significantly different. In flavedo of UVB-treated lemons, the
soluble acid invertase showed between 1 and 3 min time–dose
similar activity values. While in albedo tissue, the maximum activity
was observed at 1 and 5min time–dose, respectively (Fig. 3b).
Contrary to cell wall invertase, the activity of soluble acid invertase
of UVB-treated albedo at 5min time–dose was increased
compared with the control. Although a soluble alkaline invertase
activity was also detected in albedo, its level was lower than
those cell wall-bound and soluble acid invertases. The maximum
activity of alkaline invertase was detected at 5 min time–dose
reaching a value of 0.01mol reducing sugars min−1 g−1 FW (data
not shown). The alkaline invertase activity was not detected