Seed Dormancy and Germination
Plant life begins with seed formation and is renewed when seed germinates. Seeds are highly dehydrated, quiescent or resting structures, and carriers of the next generation in the life cycle of plants. Seeds have different shapes and sizes, ranging from the smallest orchid seed (10−6 g) to the huge seed of the double coconut palm (30 kg). Some seeds are short-lived, e.g., willow seeds are viable for less than 1 week. Mimosa seeds can live up to ~200 years. Seeds of Canna compacta have been reported to remain viable up to 600 years, and seeds of lotus can survive up to 1000 years. To accomplish the remarkable feat of next generation, seed contains an embryo with reserve food. During seed germination, the embryo divides and differentiates into shoot-root axis. The tissue containing reserve food material (depending on the species) may persist and get reabsorbed later. Depending on the species, the reserve food material is stored either in the embryo or in the endosperm, and in some cases, it is present in both, embryo and endosperm. The degree of arrest is variable in different species. It may be true sleeping (dormancy) or quiescent stage, which only requires water for resuming the growth. A viable and nondormant seed is capable of germination after all the necessary environmental conditions are met, but dormant seeds do not germinate even when provided with favorable conditions. Different types of mechanisms impose seed dormancy in diverse climates and habitats. Abscisic acid (ABA) present in seeds induces dormancy, but the intensity of dormancy induction also depends on the genetic makeup of the plant and the environment in which it grows. Temperature, relative humidity, and day length also interact to modulate dormancy, thereby making it a complex phenomenon. Germination in different seeds is not synchronous, and stimuli required to promote germination vary widely. Prior to germination, seeds need to undergo imbibition, i.e., uptake of water by dry seed, followed by reactivation of metabolic activity and redifferentiation of embryonic tissue to mobilize the reserve food material stored in the seed and initiate meristematic activity. The transition from dry seed to seedling is highly sensitive to different environmental conditions, especially light, temperature, and availability of water. This response to environmental signals is mediated by one or more hormones whose signaling events lead to activation or de novo synthesis of hydrolytic enzymes. The emergence of radicle is the first visible step and indicates that seed is viable. If germination occurs in the dark, then root growth is slow. On the other hand, shoot growth accelerates. This behavior increases chances of seedling to obtain light so that it can turn green and start photosynthesizing. Once the seedling comes out of the soil, it turns green and starts producing new leaves.