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

The state of our knowledge of plant pathology is reflected by the terms we use (120). Consider some of the terms used to describe wood altered by pro­ cesses associated with aging and injury of living trees: heartwood, wound heartwood, pathological heartwood, traumatic heartwood, false heartwood, precocious heartwood, blackheart, brownheart, red heart, blue butt, mineral streak, mineral stain, woundwood, discolored wood, wound-initiated discol­ ored wood, wetwood, ripewood, reaction zone, protection wood, and even true wood. Indeed, there is confusion! The main visible change observed in wood of trees is change of color. This can be the result of processes associated with aging (heartwood), injury (dis­ colored wood), or both. However there are more important characteristics than color for the sapwood altered by these processes. The factors that ini­ tiate the formation of heartwood, discolored wood, and extractives (which are largely responsible for color) are different. This is the major reason for the confusion in understanding a situation when color alone is the basis for distinguishing the type of tissue under study. When injury-altered tissues are considered as age-altered tissues, and the role of microorganisms in the processes are not considered, it is impossible to in­ terpret the situation accurately. The confusion is further compounded when the injury processes occur in tissues already altered by aging. Clarification of these processes obviously is needed. We will discuss in this review those processes in living trees that are associ­ ated with colored wood, in the hope of clarifying them so that future re­ search will be more accurately oriented and the opportunity to bring these changes under our control will be improved. We will consider and contrast two types of wood, which we will refer to as "heartwood" and "discolored wood."

Heartwood, Discolored Wood, and Microorganisms in Living Trees

Chapter 3 – The Infection Process

Primary leaves of barley were detached, infiltrated with various buffers, and centrifuged to yield 'intercellular washing fluid' (IWF). Effective pH control of the IWF was obtained only with Tris, among all buffers tried. In these liquids, up to 30 proteins were detected by gradient gel electrophoresis. Intracellular protein from injured cells at the cut ends of leaves was present in IWF but did not contribute significantly to the total protein recovered in this liquid. The yield of protein in the IWF depended on the buffer used for infiltration and on the concentration of the buffer. Higher concentrations of buffer yielded more protein. In other experiments leaves were infiltrated with Tris, centrifuged, and then infiltrated a second time with this buffer containing various concentrations of the zwitterionic detergent CHAPS, a sulphobetaine derivative of cholate. Gel electrophoresis of the IWF obtained after the second centrifugation revealed protein 'bands' not detected when the detergent had been omitted from the infiltration buffer. The electrophoretic patterns of protein 'bands' in the gels differed depending on the CHAPS concentration used for infiltration. The effect of CHAPS on plasmalemma integrity was studied by observing infiltrated tissue with the electron microscope and by treating isolated protoplasts with the detergent. After infiltration with CHAPS at 0-6 mM or 2-0 mM no plasmalemma breaks were detected in leaves, and isolated proto plasts survived exposure to CHAPS at these concentrations for 2 h without bursting. Evidently, CHAPS at these low concentrations did not destroy the integrity of the plasmalemma; the additional protein recovered in the IWF under these conditions probably originated in the cell wall. Infiltration of leaves with 6-0 mM CHAPS resulted.in breaks of the plasmalemma, in tissue collapse and leaf tip necrosis. Isolated protoplasts burst within minutes after being exposed to CHAPS at this concentration.

Proteins in Intercellular Washing Fluids from Leaves of Barley (Hordeum vulgare L.)

The cell wall is a polysaccharide-rich, extracellular structure which overlays and encloses the protoplast of higher plant cells. In meristematic cells, the wall is thin and consists of microfibrils embedded in a gel-like matrix. During maturation and specialization, the wall may be thickened and additional wall layers, also composed of microfibrils embedded in a matrix, are often deposited.

Metabolism of Noncellulosic Polysaccharides

Ethylene was found to be required for the release or secretion of cellulase from the cytoplasm to the cell wall in bean-petiole abscission-zone explants. This is an addition to its previously known action in accelerating senescence and cellulase synthesis in abscission.

F. B. Abeles

Papers

Abscission: Control of cellulase secretion by ethylene.