Plant self-defense mechanisms against oxidative injury and protection of the forest by planting trees of triploids and tetraploids

The depletion of the ozone layer, and the resulting substantial increase in incident ultraviolet (UV) irradiation and subsequent oxygen radical formation on the Earth, have caused an extensive variety of damage to the worldʹs forests. Superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px), which scavenge harmful oxygen radicals and inhibit lipid peroxides, were examined in two types of Japanese cedars, black pines, and cypresses, namely those with leaves showing premature withering, shedding, or dying and those with leaves not showing these effects prematurely. The effect of homogenates from these trees on lipid peroxide formation in a reaction system which UV light induces was also studied. The results indicate that strong black pines have significantly higher SOD activities than ordinary black pines, the leaves of which prematurely wither or die. Remarkably, trees that had triploid or tetraploid chromosomes showed higher SOD levels than diploid trees and markedly inhibited lipid peroxide formation since the SOD gene resides on a chromosome. This was especially true of plus trees of Japanese cypress, some of which had five times higher SOD activities than common Japanese cypresses although GSH-Px appears to play less of a role in this regard. Rice leaves and osmunda which are resistant to UV damage showed markedly higher SOD and GSH-Px activity. Our experiments suggest that the trees that have high SOD can protect themselves by scavenging oxygen radicals induced by UV irradiation and inhibit harmful lipid peroxide formation. In order to protect forests from oxidative damage by UV light, we should plant trees of natural mutants and artificially crossed triploids and tetraploids.