Physiological Control of Differentiation of Xylem Elements

The physiological control of xylem differentiation is considered from holistic and reductionist perspectives. Cell type determination is identified as a central problem, but cell quality is also noted as being especially important from a utilization standpoint. The review focuses on biochemical and genetic controls and on cytological events during differentiation. The concluding section deals with the molecular assembly of xylem cells and suggests that a systems theory approach might be a powerful tool in understanding the physiology of xylem cell differentiation

A Biometric Technique for Reaction Tissue Research

Images of sections of tissue taken from the upper side of leaning stems of Populus deltoides were projected on a paper grid system. Specimens were oriented so that the cambium and the last three annual increments were included in cross section. Counts of stimulated and non-stimulated cells in randomly selected grids were recorded. This procedure provides a means of subjecting the incidence of tension wood and other cytohistological phenomena to statistical analysis

Ultrastructural and Molecular Concepts of Cell-Wall Formation

Biochemical, physiological, and cytological aspects of cell-wall formation are discussed. The relationship between cell division, cell extension, and wall formation are analyzed and presented in a unified frame of reference. The cell is considered as a packet of energy—some of which is in the form of information and some of which is in the form of structure. The possibility of a self-assembly system for cell-wall synthesis is raised.Recent work in biochemistry and genetics has elucidated the shikimic acid and chorismic acid pathways leading to the aromatic amino acids. Phenylalanine ammonia-lyase has been suggested as a key enzyme in controlling lignification, and these results, together with the Freudenberg hypothesis, have provided at least a preliminary understanding of the lignification process. The future use of auxotrophic mutants of higher plants should greatly deepen our knowledge in this arca. The sugar nucleotides have been shown to be the primary agents in cellulose, hemicellulose, and pectin synthesis. Indole acetic acid directly promotes synthesis of the matrix polysaccharides of the wall but only indirectly controls the formation of the cellulose framework. Matrix polysaccharides have been shown to be intercalated throughout the pre-existing wall, while current evidence is that cellulose is added by apposition. The cytological level is probably the least known component of wall development. The role of the Golgi vesicles in the formation and orientation of the cell plate is clearly established, but whether such vesicles play a role in actual wall formation is open to question. Three types of paramural bodies found in pine and lentil seedlings are described, and their possible origins and functions in cell-wall formation are discussed. A new calculation for pore size in the hydrated and dehydrated cell walls is also presented and discussed

Physiological and Stuctural Mechanisms of Niche Differentiation for Three Sky Island Oaks in Relation to Light and Temperature

In an effort to identify the influence of light and temperature on the physiology and leaf structural characteristics of three species of Quercus from Coahuila, Mexico, we measured a comprehensive suite of plant traits as functions of light and temperature intensity. We tested the hypotheses that 1) species' physiological responses to light and temperature were related to their distributions in their native habitats; and 2) that species' physiological responses corresponded to similar variation in leaf anatomical and morphological traits. Quercus sideroxyla was adapted to high elevation forest over stories as evidenced by its high photosynthetic rate, transpiration rate, relative water content (RWC), leaf density (LD), and thick palisade and spongy parenchyma. Quercus rugosa displayed typical characteristics of a forest understory species including a low photosynthetic rate and light saturation point, thick spongy parenchyma tissue and high RWC, leaf density, and leaf mass per unit area. Quercus laceyi was adapted to hot, dry sites based on its lower RWC and LD, intermediate photosynthetic rate, thick cuticle and upper epidermis, and low transpiration rates at high temperatures. Our results suggest that the physiological and structural adaptations of Mexican oaks to changing environmental conditions across resource gradients are key regulators of plant community structure.Desert Plants is published by The University of Arizona for the Boyce Thompson Southwestern Arboretum. For more information about this unique botanical journal, please email the College of Agriculture and Life Sciences Publications Office at [email protected]