Each perennial woody plant is a highly integrated system of competing carbohydrate sinks (utilization sites). Internal competition for carbohydrates is shown by changes in rates of carbohydrate movement from sources to sinks and reversals in direction of carbohydrate transport as the relative sink strengths of various organs change. Most carbohydrates are produced in foliage leaves but some are synthesized in cotyledons, hypocotyls, buds, twigs, stems, flowers, fruits, and strobili. Although the bulk of the carbohydrate pool moves to sinks through the phloem, some carbohydrates are obtained by sinks from the xylem sap. Sugars are actively accumulated in the phloem and move passively to sinks along a concentration gradient. The dry weight of a mature woody plant represents only a small proportion of the photosynthate it produced. This discrepancy results not only from consumption of plant tissues by herbivores and shedding of plant parts, but also from depletion of carbohydrates by respiration, leaching, exudation, secretion, translocation to other plants through root grafts and mycorrhizae and losses to parasites. Large spatial and temporal variations occur in the use of reserve- and currently produced carbohydrates in metabolism and growth of shoots, stems, roots, and reproductive structures. A portion of the carbohydrate pool is diverted for production of chemicals involved in defense against fungi, herbivores, and competing plants. Woody plants accumulate carbohydrates during periods of excess production and deplete carbohydrates when the rate of utilization exceeds the rate of production. Stored carbohydrates play an important role in metabolism, growth, defense, cold hardiness, and postponement or prevention of plant mortality.

Carbohydrate sources and sinks in woody plants

Phenylalanine ammonia lyase

INTRODUCTION ... ........ ...... 133 DEVELOPMENT OF EXPERIMENTAL METHODS TO STUDY ABSCISSION . 135 THE TIME COURSE OF ABSCISSION ....... 136 INFLUENCE OF HORMONES ON THE TIME COURSE 137 The Lag Phase 137 The Separation Phase 139 ANATOMY OF ABSCISSION ...... , , 140 Structural Features of Abscission Zones 140 Changes During the Lag Phase 143 Changes During the Separation Phase 145 BIOCHEMISTRY OF WALL BREAKDOWN 148 OTHER ENZYMIC CHANGES DURING CELL SEPARATION ,.... 152 ROLE OF LIGNIN, CALLOSE. AND TYLOSES , ,.... 153 MECHANICS OF SEPARATION . ........ ........ .. ..... ......... 153 ORGANIZATION OF ABSCISSION .... 155 SUMMARY AND CONCLUSIONS .... ........ 157

Cell Biology of Abscission

Post harvest biology and technology of citrus fruits is gaining importance as the therapeutic value of citrus fruits is realized and supported by the increase in health awareness among the general public This book is the most comprehensive reference on citrus fruit biology, biotechnology and quality Basic and applied scientific information is interwoven to serve the researcher, marketer, scientist, nutritionist, or dietician With discussions of fruit morphology, anatomy, physiology and biochemistry and chapters on growth phases, maturity standards, grades and physical and mechanical characteristics of citrus trees, this book provides the foundation for understanding growth, harvest and post harvest aspects of these important plants Insect-pests and diseases, irrigation, nutrition and rootstocks are also addressed * Provides practical tips for post harvest management * Includes all aspects of citrus fruit biology, technology and quality evaluation * Discusses biotechnological applications and potential fresh citrus fruit quality improvement * Evaluates medicinal and therapeutic applications and recent clinical findings * Exhaustive glossary included

Citrus Fruit: Biology, Technology and Evaluation

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Alternate Bearing in Fruit Trees

The activity of pectin esterase and cellulase in abscission of citrus explants was studied No relation was established between pectin esterase and abscission, while cellulase activity was markedly increased before abscission and for a certain period after excision IAA and cycloheximide delay abscission and cellulase activity, while ethylene and, to a lesser extent, GA3 accelerate them Application of cycloheximide during the lag period and before cellulase activity can be measured, inhibits to a certain extent the formation of cellulase An escape from the inhibitory effect of cycloheximide is detected when inhibitor is supplied at the end of the lag period

Activity of pectin esterase and cellulase in the abscission zone of citrus leaf explants.

A role for carbohydrate levels in the control of flowering in citrus

l-Phenylalanine ammonia-lyase (PAL) activity is low in the external layers (flavedo) of intact mature grapefruit peel. Flavedo discs evince upon incubation increasing PAL activity and ethylene production. Light has no effect in enhancing PAL activity in discs. Exogenous ethylene stimulates PAL activity in the flavedo of intact mature grapefruits (half maximum stimulation at 15 ppm); such activity rapidly decreases when fruit is removed from the ethylene containing atmosphere. Carbon dioxide inhibits both ethylene production and PAL activity of discs; exogenous ethylene only partly relieves PAL inhibition. Cycloheximide inhibits both PAL activity and ethylene production by flavedo discs. The same concentration of cycloheximide also inhibits PAL activity of discs in the presence of exogenous ethylene. Protein synthesis seems therefore to be needed at both levels of ethylene evolution and enhancement of PAL activity.

S.P. Monselise

Papers

Activity of pectin esterase and cellulase in the abscission zone of citrus leaf explants.

A role for carbohydrate levels in the control of flowering in citrus

Ethylene-controlled Induction of Phenylalanine Ammonia-lyase in Citrus Fruit Peel

The use of growth regulators in citriculture; a review

Growth abscission relationships in developing orange fruit