Fig. 8. Model for the regulation of dormancy and germination by abscisic acid (ABA) and gibberellins (GA) in response to the environment. According to this model, ambient environmental factors (e.g. temperature) affect the ABA:GA balance and the sensitivity to these hormones. ABA synthesis and signalling (GA catabolism) dominate the dormant state, whereas GA synthesis and signalling (ABA catabolism) dominate the transition to germination. The complex interplay among hormone synthesis, degradation and sensitivities in response to ambient environmental conditions can result in dormancy cycling. Change in the depth of dormancy alters the requirements for germination (sensitivity to the germination environment); when these overlap with changing ambient conditions, germination will proceed to completion. The model is based on work with Arabidopsis thaliana ecotype Cvi, modified from Cadman et al. (2006) and reprinted with permission from Blackwell Publishing. Key target genes are in parentheses (see text for definitions).

Figure 1. Time course of dormancy induction and release, including partial summary of regulatory factors.Width of abscisic acid (ABA) and gibberellin (GA) symbols represents relative hormone levels due to action of indicated biosynthetic and catabolic loci. Induction depends on combination of ABA- independent maternal and embryonic factors and ABA-dependent signaling. Release is promoted by many environmental factors, largely integrated through changes in ABA:GA signaling balance, eventually resulting in wall expansion to permit radicle emergence. Positive regulation is indicated by (+) and arrows, negative regulation by (−) and bars. NCED, 9-cis-epoxycarotenoid dioxygenase; CYP707A, Cytochrome P A; ZEP, zeaxanthin epoxidase; NO, nitric oxide; AOS, active oxygen species; BME3, BLUE MICROPYLAR END 3; PIL5, PHYTOCHROME-INTERACTING FACTOR 3-LIKE 5; SPT, SPATULA; GA20ox, GIBBERELLIN 20 OXIDASE; GA3ox, GIBBERELLIN 3 OXIDASE; GA2ox, GIBBERELLIIN 2 OXIDASE; SLY, SLEEPY; XTH, xyloglucan endotransferase/hydrolase; EXP, expansins; bGLU, β-1,3 glucanase; PME, pectin methylesterase; DAG, DOF (DNA-binding with one zinc finger) AFFECTING GERMINATION; LEC, LEAFY COTYLEDON; FUS3, FUSCA 3; PP2C, protein phosphatase 2C; ERA, ENHANCED RESPONSE TO ABA; ABI, ABA INSENSITIVE; DOG, DELAY OF GERMINATION; SnRK, SNF1- related protein kinase; ABAR, ABA receptor; GCR, G protein–coupled receptor.

Fig Stages in germination of garden bean. Fig Germi-nation in pea. Fig Seed germination. (Left): corn. (Right): onion.

Figure 4-8. The stages of seed development. The stages include histodifferentiation (rapid increase in seed size due predominantly to cell division), cell expansion (largest increase in seed size for deposition of food reserves), and maturation drying (dramatic loss in seed fresh weight due to water loss). (Redrawn from Bewley and Black, 1994.)

Figure 7-1. Seed morphology in typical monocot, dicot, and gymnosperm species.

Figure 7-3. Phases of water uptake during germination.

Figure 7-7. The balance of forces involved in germination. In many seeds, the seed coverings provide a physical resistance to radicle emergence. The ability of the radicle to penetrate the seed coverings determines the speed of germination and can be an important mechanism for controlling germination in dormant seeds. (Adapted from Bradford and Ni, 1993).

Figure Pawpaw (Asimina triloba) is typical of species that require chilling stratification. It shows the typical population effect, where some seeds in a seed lot require only a few weeks of chilling, while others require longer times to be released from dormancy.

Figure Phytochrome controls the dormancy condition of photodormant seeds. Lettuce seeds are the model to study the photoreversibility of phytochrome. The last quality of light the seeds are exposed to determines the dormancy state. Far-red light or darkness keeps seeds dormant, while red light (natural sunlight) will relieve dormancy.