Physiological effects of peracetic acid on hydroponic tomato plants

Peracetic acid (PAA) has potential as a disinfectant of low environmental impact for glasshouse hydroponic systems and other horticultural applications, but can have phytotoxic effects. This study examined the physiological effects of PAA when applied hydroponically to tomato plants. Plants treated with 0.5-5 pg ml(-1) PAA over several weeks exhibited a reduction in size of all vegetative organs. During the first 2 h of PAA treatment, plants also exhibited a transient wilting, with increased stomatal resistance, and reductions in transpiration and CO2 assimilation. The toxicity of PAA to roots was apparent from increased leakage of root electrolytes, reduced oxygen consumption, death of root tips, and collapse of the internal tissues. The shrivelling of PAA-treated roots resulted from loss of water to the shoot in the transpiration stream, as the effect could be eliminated by removal of the shoot and sealing of the cut stump. HgCl2, a reagent known to reduce the hydraulic conductivity of root systems, caused the same root shrivelling effects as PAA. Long-term growth of PAA-treated plants was dependent upon the replacement of taproot systems by adventitious roots, which, initially at least, displayed greater tolerance of PAA. In aqueous solution, PAA exists in equilibrium with H2O2 and acetic acid, both of which were individually toxic, but acetic acid exhibited a syndrome of effects distinct from those of PAA, while the effects of H2O2 paralleled those of PAA more closely, suggesting that oxidative rather than acidic mechanisms were primarily responsible for the phytotoxicity of PAA solutions.