FACTORS AFFECTING CITRUS TREE ISOZYME-GENE EXPRESSION

Ten enzymatic systems of Citrus species and cultivars have been evalua ted for identification purposes and for genetic variability studies. T he following factors that could affect their expression were studied: season of sampling, location, rootstock, position of the branch, infec tion, and age of the tree. Differences involving the presence-absence of the Cu/Zn SOD within the same tree were found. This difference is m ainly related to the position of the leaf relative to the sunlight. No change was observed at any of the ten enzymatic systems assayed regar ding the location, the rootstock, the growing condition, the season, o r the infection with most virus and virus-like pathogens. Viroids indu ced noticeable changes on 6PG and PRXa zymograms in C. medica. A new p eroxidase (not present in healthy plants) was detected that could be r elated to appearance of symptoms. This may induce errors when trees wi thout sanitary control are characterized by this enzymatic system. On the other hand, it provides a new possibility for studying the plant r esponse to the presence of viroids. An effect of age, from 3 months up to 12 years, was observed on citrange Troyer and mandarin Cleopatra P RX, MDH and 6PG patterns. An important change occurs around the first year, most likely related to the end of the seedling stage. This is fo llowed by a long transition phase, the end of which (around 9 years la ter) coincides with a change in the PRX pattern. These age-related cha nges seem to involve post-translational mo difications of pre-existing isozymes.