A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C₁₅H₂₀O₄ (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various aspects of plant physiological function, including development, growth, and stress responses. Abscisic acid is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. In addition, abscisic acid is produced by mosses, certain algae, several phytopathogenic fungi, and various animals, ranging from sea sponges to mammals. Some abscisic acid signaling mechanisms appear to be common to plants and animals. See also: Plant; Plant development; Plant growth; Plant hormone; Plant metabolism; Plant physiology

The process whereby a plant sheds one of its parts. Leaves, flowers, seeds, and fruits are parts commonly abscised. Almost any plant part, such as very small buds and bracts to branches several inches in diameter, and scales or sheets of bark, may be abscised by some species. However, other species, including many annual plants, may show little abscission, especially of leaves.

A natural process in which harmful or beneficial effects are caused by secondary metabolites that spread from a donor organism to a recipient and are produced by plants, algae, bacteria, and fungi. Allelopathy is a key ecological process and has been studied predominantly in plants (Fig. 1). The chemical compounds involved in allelopathy are referred to as allelochemicals and comprise almost all classes of organic chemical substances. Hans Molisch (1937) coined the term allelopathy from the Greek words allelon for mutual and pathos for harm or affection, based on his observation of the premature ripening of apples and pears that were stored together with fruits from early ripening varieties. Depending on the purpose, the early ripening effect could be regarded as beneficial or harmful. Historically, detrimental effects have made botanists aware of allelopathy. One of the first accounts of an allelopathic effect that is commonly observed, namely the zone of growth inhibition around walnut trees (Juglans species), was reported by Pliny (23–79 CE). Even earlier, Theophrastus (372–287 BCE) described allelopathic effects of weeds on crop plants, including the inhibition of growth of alfalfa (Medicago sativa) by pigweed (Amaranthus retroflexus). See also: Agricultural science (plant); Agricultural soil and crop practices; Agriculture; Botany; Chemical ecology; Plant growth

Correlative inhibition of the growth of lateral (axillary) shoots exerted by the growing apical bud of the plant stem. Competition for growth occurs between plant parts. Strongly growing fruits and branches (organs) act as sinks that divert energy and nutrients from other nearby plant organs, reinforcing repressed growth in those weaker organs. This effect is called correlative inhibition (or correlative dominance) and is thought to involve hormonal signals flowing from and to the dominant organs. Simple pruning studies suggest that growing shoot tips repress the growth of tiny buds elsewhere by transmitting repressive signals. This is a specific type of correlative inhibition called apical dominance and is an important factor in controlling final plant shape. See also: Apical meristem; Bud; Lateral meristem; Plant growth; Plant organs; Plant physiology; Stem

Permanently embryonic tissue involved in cell division at the apices of roots and stems, and forming dynamic regions of growth. These apical meri­stems, usually consisting of small, densely cytoplasmic cells, become established during embryo development. Thereafter they divide, producing the primary plant body of root and shoot. Below the apical meristems, tissue differentiation begins; the protoderm gives rise to the epidermal system, the procambium to the primary vascular system, and the ground meristem to the pith and cortex (Figs. 1 and 2). Plant apical meristems have been the object of experiments on development similar to those carried out on animal embryos. See also: Apical dominance

Any of a class of plant growth substances (phytohormones or plant hormones) that have an essential role in coordination of many growth and developmental processes during the life cycle of plants, especially the ability to increase the rate of shoot elongation. Auxins are important phytohormones (plant hormones). They have been defined collectively as a group of organic compounds (Fig. 1) that, when applied in low concentration, are able to promote elongation growth of plant shoots excised from a growing region of a young seedling. These substances were the first plant hormones to be studied, and they were given the generic name auxins in 1931 by Fritz Kögl and Arie Jan Haagen-Smit because they increase growth (Greek auxein, meaning to increase or grow). From the very early research, it was known that the role for this signal in plant development was profound and included regulation and coordination of organ formation, development, directional growth in response to light and gravity, and normal cell elongation and growth. A unique aspect of auxin is its ability to move directionally from the apical meristem, thus establishing a shoot–root polarity that is maintained throughout the life of the plant. Findings have shown that auxin signaling involves a soluble receptor that initiates the degradation of proteins that otherwise inhibit expression of auxin-responsive genes. See also: Apical meristem; Plant development; Plant growth; Plant hormone; Plant hormone receptors; Plant movement; Plant physiology

A word generally referring to the surface region of a stem or a root. The term has a variable meaning and sometimes part or all of the bark is called rind. Occasionally the word is used as a substitute for periderm or for cork only. Most commonly, however, bark refers to all tissues external to the cambium (Fig. 1). If this definition is applied to stems having only primary tissues, it includes phloem, cortex and epidermis; the bark of roots of corresponding age would contain cortex and epidermis. More generally, the term bark is restricted to woody plants with secondary growth.

An embryonic shoot containing the growing stem tip surrounded by young leaves or flowers or both and the whole frequently enclosed by special protective leaves, the bud scales. See also: Stem

A member of a class of common pigments, typically of yellow, orange, or red hues, that serve in both light absorption and the protection against too much light in photosynthesis as well as in the vision process. Carotenoids comprise a class of labile, easily oxidizable, naturally occurring pigments that are distributed widely in plants (Fig. 1) and animals, as well as being preferentially soluble in fats and fat solvents. In addition, dietary carotenoids function as important gene regulators, impacting, for example, the risk for major human diseases, and as signals affecting reproductive success in many plants and animals. See also: Photoprotection (plants); Photoreception; Photosynthesis; Pigmentation; Plant pigment; Vision

A specialized structure primarily found in plant cells and algae. Cell plastids are diverse in distribution, size, shape, composition, structure, function, and mode of development. Various types of cell plastids are recognized. Chloroplasts occur in the green parts of plants and are responsible for the green coloration, for they contain the chlorophyll pigments. These pigments, along with certain others, absorb the light energy that drives the processes of photosynthesis, by which sugars, starch, and other organic materials are synthesized. Amyloplasts, nearly or entirely colorless, are packed with starch grains and occur in cells of storage tissue. Proteoplasts are less common and contain crystalline, fibrillar, or amorphous masses of protein, sometimes along with starch grains. In chromoplasts, the green pigment is masked or replaced by others, notably carotenoids, as in the cells of carrot roots and many flowers and fruits. See also: Carotenoids; Chlorophyll; Starch