DISTRIBUTION OF ENZYMATIC BROWNING OF YOUNG SHOOT HOMOGENATES IN THE AURANTIOIDEAE

Distribution of polyphenol oxidase-catalyzed browning of young shoot homogenates was studied in 428 accessions of the orange subfamily Aurantioideae. Browning and nonbrowning phenotypes were found in both tribes, namely Clauseneae and Citreae. The browning taxa had sufficient polyphenol oxidase activity and a presently unknown phenolic substrate, while the nonbrowning taxa were devoid of both. A third group of taxa contained the substrate only and they too were designated as browning since their homogenates turned brown when the enzyme from a known source was added. The color of homogenates was taxon-specific in most browning and nonbrowning taxa. Within the genus Citrus, 4 out of the 16 species recognized by Swingle were browning and 2 were nonbrowning. Exceptions noted within either group of taxa were presumably hybrids or mutants. The significance of enzymatic browning as an additional taxonomic criterion in studying species relationships and as a genetic marker in identificaton of hybrid taxa was discussed.