This study investigated the role of silicon (Si) and manganese (Mn) rates on the activity of peroxidases (POX), polyphenoloxidases (PPO) and phenylalanine ammonia-lyases (PAL) on rice plants non-inoculated and inoculated with Bipolaris orzyae. Rice plants (cultivar Metica 1) were grown in nutrient solution with 0 (-Si) or 2 mmol Si L-1 (+Si) and with 0.5, 2.5 and 10 μmol Mn L-1. Plants were inoculated with B. oryzae and leaf samples were collected at 6, 12, 18, 24, 36 and 48 h after inoculation (hai). Leaf samples were also collected from non-inoculated plants at these same time-points. POX activity increased for -Si plants inoculated with B. oryzae at both rates of 0.5 and 2.5 µmol Mn L-1 with one peak at 48 hai at 10 µmol Mn L-1. For plants supplied with Si, POX activity also increased, but seemed to be low at the rates of 2.5 and 10 µmol Mn L-1. POX activity did not show to be greatly influenced by the Mn rates in the presence of Si. However, in the absence of Si, POX activity increased. PPO activity on -Si plants increased at the rate of 0.5 µmol Mn L-1 and was not affected by the other two Mn rates, except a peak at 18 hai at the rate of 10 µmol Mn L-1. In the presence of Si, PPO activity increased on plants supplied with 2.5 µmol Mn L-1 in contrast to the other two Mn rates for which the PPO activity decreased. By comparing the -Si and +Si treatments at the rate of 2.5 µmol Mn L-1, PPO activity increased from 18 to 36 hai in the presence of Si than on its absence. PAL activity on -Si plants decreased regardless of the Mn rates. On plants supplied with Si and inoculated with B. oryzae, PAL showed low activity at the highest Mn rate and also at the rate of 0.5 µmol Mn L-1, besides its activity been very low compared to the non-inoculated plants. PAL did not increase in activity in the presence of Si, but on its absence, high levels of activity were achieved. Results from this study showed that the activities of POX, PPO and PAL were not boosted by Si at any Mn rate.
Bipolaris oryzae; Oryza sativa; foliar disease; mineral nutrition
