PLANT-GROWTH REGULATORS IN CITRICULTURE - FACTORS REGULATING ENDOGENOUS LEVELS IN CITRUS TISSUES

Since the review on endogenous growth substances of citrus tissues by Goldschmidt in 1976 (HortScience, 11: 95-99), much information regarding this topic has been published in a wide array of journals. The present review provides a comprehensive overview of published information on endogenous levels of the five classes of plant growth substances (i.e., auxins, cytokinins, gibberellins, ethylene, and abscisic acid), plus polyamines and other endogenous substances that appear to have a role in regulating citrus growth and development. It is the first in a three-part series that next examines hormonal regulation of physiological processes in citrus followed by review of current uses and commercial applications of plant growth regulators in citrus production. In this article, a brief history of the detection and characterization of each class of plant growth substances is given. Following this, variation in endogenous levels associated with different organs (and/or tissues), stages of development, species, cultivars, cultural practices, and environmental factors is reviewed. For each class, current knowledge regarding biosynthesis, metabolism and transport in citrus tissues is summarized. The concluding section deals with future research directions.