Physiological aspects of shoot growth regulation in juvenile and adult Citrus sinensis (L.) Osbeck.

Abstract

As the requirements for more efficient and economical crop production emerge, so the need for a better understanding of plant growth and development is felt. It is especially the need to understand means by which man may influence growth and development in order to achieve greater productivity, that requires attention. Observation and analysis of plant growth and the concomitant study of plant components, organs and chemicals, provides one of the research oriented approaches to gaining an understanding of the mechanism of growth control. Ultimately, the extent of reproductive growth (fruiting) or the bearing capacity in all plants is governed by the volume and area of vegetative growth and the supply of substances required for growth produced by those vegetative organs. Productivity then, is governed by the efficiency of conversion of solar energy to chemical energy and by the relative ability to store this energy. Plant shoots play a fundamental role in both assimilation and storage of carbohydrates. Given the same growing conditions, similar plants sometimes exhibit different rates of growth. For example, ontogenetically juvenile plants exhibit differences in form and vigour in comparison with ontogenetically adult plants. Although the genotype of such adult and juvenile plants may vary only in expression and not in constitution, there does appear to be some property in juvenile plants which induces the characteristic enhanced vegetative vigour. It is this property, amongst many others, which needs to be better understood, for it is a potential means of gaining greater yields from plants without alterations being made to the environment. A high degree of organisation of growth processes must exist to account for the orderly development of plants. A few organic compounds that influence certain biochemical and physiological reactions have been isolated from the As the requirements for more efficient and economical crop production emerge, so the need for a better understanding of plant growth and development is felt. It is especially the need to understand means by which man may influence growth and development in order to achieve greater productivity, that requires attention. Observation and analysis of plant growth and the concomitant study of plant components, organs and chemicals, provides one of the research oriented approaches to gaining an understanding of the mechanism of growth control. Ultimately, the extent of reproductive growth (fruiting) or the bearing capacity in all plants is governed by the volume and area of vegetative growth and the supply of substances required for growth produced by those vegetative organs. Productivity then, is governed by the efficiency of conversion of solar energy to chemical energy and by the relative ability to store this energy. Plant shoots playa fundamental role in both assimilation and storage of carbohydrates. Given the same growing conditions, similar plants sometimes exhibit different rates of growth. For example, ontogenetically juvenile plants exhibit differences in form and vigour in comparison with ontogenetically adult plants. Although the genotype of such adult and juvenile plants may vary only in expression and not in constitution, there does appear to be some property in juvenile plants which induces the characteristic enhanced vegetative vigour. It is this property, amongst many others, which needs to be better understood, for it is a potential means of gaining greater yields from plants without alterations being made to the environment. A high degree of organisation of growth processes must exist to account for the orderly development of plants. A few organic compounds that influence certain biochemical and physiological reactions have been isolated from the wide array of organic and inorganic substances found in plant extracts. A growing body of evidence suggests that such growth regulatory compounds are ubiquitous in plants; are active at exceedingly low concentrations and are under genetic control. Plant hormones are almost certainly involved in directing or regulating plant growth and development. The subject of the present research concerns the characterization of differences in shoot growth of juvenile and adult phases of a commercial Citrus cultivar and an investigation of the possible role which plant hormones may play in such differences. In add i tion to a comprehensive literature survey, three separate sets of experiments were conducted. In the first, an analysis of the components and parameters of the difference in vegetative shoot growth of adult and juvenile scions was carried out. Anatomical and ultrastructural comparisons were also made. In the second set of experiments the endogenous levels of gibberellins, cytokini ns and inhibitors of buds and other tissues, which might playa role in the regulation of shoot flush growth, were examined. Specific extractions were performed and appropriate bioassay techniques were employed for the three hormone classes investigated. In the final set of experiments the effects of synthetic plant growth regulators on shoot flush growth were monitored. The growth regulators were applied by means of trunk injections or as droplets on shoot terminal buds. These experiments were planned and executed with the above mentioned concepts and ideals in mind. The objective of the project was to contribute in some measure to the knowledge which will allow better use to be made of available natural resources.