Background: The time course of polyphenol oxidase (PPO) activity in the leaves of two olive cultivars (Picual and FS-17) irrigated with nutrient solutions differing in Mn concentration (0, 2 and 1280 micromol L(-1)) was studied under hydroponic conditions to determine whether PPO activity could be used as an early criterion of Mn status of olive plants, and to elucidate whether genotypic differences exist between the two olive cultivars studied, concerning the effect of Mn concentration on PPO activity.
Results: In all the Mn treatments, PPO activity was greater in Picual than in FS-17. Under excess Mn (1280 micromol L(-1)), PPO activity gradually increased with time, starting from day 30 of the experiment in both cultivars, and this increase preceded the appearance of Mn toxicity symptoms. In contrast, in the other two Mn treatments (0 and 2 micromol L(-1)) PPO activity increased and afterwards decreased during the experiment, but the trend was not clear. In the 1280 micromol L(-1) treatment, PPO activity linearly increased (R = 0.8836 for Picual and 0.943 for FS-17) with the increase of Mn concentration in the leaves of both cultivars. In the 1280 micromol L(-1) Mn treatment, PPO activity was negatively related with Fe and Zn concentrations in the leaves, and positively in the 0 and 2 micromol L(-1) Mn treatments with the Ca, Mg and K concentrations.
Conclusion: From the differential time course of PPO activity in the three Mn treatments (0, 2 and 1280 micromol L(-1)), it is concluded that periodic measurements of PPO activity in the leaves of the olive cultivars Picual and FS-17 can be used for the early detection of Mn toxicity (before the appearance of symptoms).