Plants respond to light by changes in morphology and activity. Irradiation by red light converts the photoreceptor phytochrome to its far-red light-absorbing form—Pfr. This form controls germination and flowering in response to length of day, as well as triggers photosynthesis in dormant plants or those that just emerged from the soil. Blue-light receptors, cryptochromes, and phototropins are responsible for phototropism.
Amyloplasts, which contain heavy starch granules, sense gravity. Shoots exhibit negative gravitropism, whereas roots exhibit positive gravitropism. Plant hormones—naturally occurring compounds synthesized in small amounts—can act both in the cells that produce them and in distant tissues and organs.
Auxins are responsible for apical dominance, root growth, directional growth toward light, and many other growth responses. Cytokinins stimulate cell division and counter apical dominance in shoots. Gibberellins inhibit dormancy of seeds and promote stem growth. Abscisic acid induces dormancy in seeds and buds, and protects plants from excessive water loss by promoting stomatal closure. Ethylene gas speeds up fruit ripening and dropping of leaves.
Plants respond to touch by rapid movements (thigmotropy and thigmonasty) and slow differential growth (thigmomorphogenesis). Plants have evolved defense mechanisms against predators and pathogens. Physical barriers like bark and spines protect tender tissues. Plants also have chemical defenses, including toxic secondary metabolites and hormones, which elicit additional defense mechanisms.
abscisic acid (ABA)
plant hormone that induces dormancy in seeds and other organs
physiological process that leads to the fall of a plant organ (such as leaf or petal drop)
plant hormone that influences cell elongation (in phototropism), gravitropism, apical dominance and root growth
molecule that absorbs light
protein that absorbs light in the blue and ultraviolet regions of the light spectrum
plant hormone that promotes cell division
volatile plant hormone that is associated with fruit ripening, flower wilting, and leaf fall
plant hormone that stimulates shoot elongation, seed germination, and the maturation and dropping of fruit and flowers
small family of compounds derived from the fatty acid linoleic acid
growth away from Earth’s gravity
hormone important in plant defenses against bacterial and fungal infections
growth and development of plants in response to light
occurrence of plant processes, such as germination and flowering, according to the time of year
blue-light receptor that promotes phototropism, stomatal opening and closing, and other responses that promote photosynthesis
directional bending of a plant toward a light source
plant pigment protein that exists in two reversible forms (Pr and Pfr) and mediates morphologic changes in response to red light
growth toward Earth’s gravitational center
(also, amyloplast) plant organelle that contains heavy starch granules
hormone that promotes seed germination in some species and inhibits lateral apical development in the absence of auxins
developmental response to touch
directional growth of a plant independent of the direction in which contact is applied
directional growth of a plant in response to constant contact