In the physiological sense, germination begins with seed water uptake and ends with the initiation of elongation by the embryonic axis, usually the radicle The driving forces and constraints on expansion by the embryo are examined, particularly for seeds in which the embryo is surrounded by endosperm and testa tissues that restrict growth Models have been developed to predict germination based on thermal time, hydrotime and combined hydrothermal time These population-based models indicate that the timing of germination is closely tied to physiologically determined temperature and water potential thresholds for radicle emergence which vary among individual seeds in a population The restraint imposed by tissues surrounding the radicle is a major determinant of the threshold water potential Enzymatic weakening of these tissues is a key event regulating the timing of radicle emergence Considerable evidence suggests that endo-/3-mannanase is involved in this process in a number of species, although it is doubtful that it is the sole determinant of when radicle emergence occurs Molecular and biochemical studies are revealing the complexity of events occurring in endosperm and embryo cells associated with the completion of germination Unique permeability properties and the presence of enzymes associated with pathogen resistance suggest additional functional roles for the tissues enclosing the embryo The insights gained from physiology and modelling are being extended by the application of molecular techniques to identify and determine the function of genes expressed in association with germination Single-seed assay methods, in vivo reporters, specific modification of gene expression and mutagenesis will be critical technologies for advancing our understanding of germination "Correspondence Fax

The viability and vigor of osmotically primed (0.3 m KNO3, 6 days, 25C) and nonprimed `PMR 45' muskmelon (Cucumis melo L.) seeds were compared after storage for 9 years at <20C and 6% moisture content (MC, dry weight basis). Viability was compared at 20, 25, and 30C at water potentials of 0, -0.2, -0.4, -0.6, and -0.8 MPa and in soil. Additionally, stored primed and nonprimed seeds were either primed, aged (15% MC and 45C) for up to 8 days, or aged for 72 hours and primed. The force required to puncture 5-mm-long, micropylar seed pieces was measured using an Instron universal testing machine. Less force was required to puncture primed seed pieces at 0, 5, 15, 20, and 25 hours of imbibition, demonstrating that osmotic priming weakens the perisperm envelope tissue that the radicle must penetrate for germination to occur. In an earlier report, germination rate and final germination percentages were higher for osmotically primed seeds both in laboratory tests and field emergence studies conducted immediately after priming. After 9 years in storage, nonprimed seeds germinated to higher percentages in water at 30C and reduced water potential at all temperatures, while primed seeds germinated to higher percentages in water at 20 and 25C and exhibited a higher percentage of seedling emergence at a soil MC of 17%. Priming durations of ≤5 days had no effect on the viability, while longer durations decreased the viability of stored primed and nonprimed seeds. Priming generally decreased the log mean time to germination of stored nonprimed seeds but increased values for stored primed seeds. Controlled deterioration increased the log mean time to germination and decreased the viability of primed seeds faster than nonprimed seeds. Priming following controlled deterioration had no effect on nonprimed seeds and reduced the percent viability of primed seeds by 20%. Osmotic priming has a deleterious effect on the seed storage life of muskmelon seeds.

The production of high-quality seed is the basis for a durable a profitable agriculture. After production, seed is processed, conditioned, stored, shipped and germinated. For quality assurance, seed quality has to be controlled at all steps of the production chain. Seed functioning is accompanied by programmed transitions from cell proliferation to quiescence upon maturation and from quiescence to reinitiation of cellular metabolism upon imbibition. Despite the obvious importance of these control mechanisms, very little information is available at the molecular level concerning those elements that regulate seed germination. In the present study, the induction of cell cycle activity and the regulation of ß-tubulin expression is related to the water content and other physical properties of the seed.

Priming (controlled hydration followed by drying) has been shown to decrease seed storage life in some species The germinablity of primed (0.3 M KNO3, 6 d, 25°C) and unprimed muskmelon (Cucumis melo L., cv. PMR 45) seeds were compared after storage for 9 yrs at less than 20°C and 6% moisture content (MC) (dwt basis). Germination performance was compared at 30°C in water and polyethylene glycol solutions of -0.2, -0.4, -0.6, -0.8, and -1.0 MPa water potential or in water at 15, 20, and 25°C. Seeds were also germinated in field soils at 17, 19, and 21% (dwt. basis) MC in a greenhouse, Some seeds were subjected to controlled deterioration at 20% MC and 45°C for 72 hrs prior to testing. The germination percentage and rate of stored, primed seeds at 30°C and all water potentials was less than stored, unprimed seeds. At 30°C, stored, unprimed seeds germinated more rapidly and to higher percentages at -0.2 MPa than in water, while germination percentages and rates of stored, primed seeds were essentially the same. At 15, 24, and 25°C, stored, primed seeds outperformed unprimed seeds in all germination tests. In saturated soils at 21% MC, there was no germination of either stored, primed or unprimed seed. At 17% soil MC, stored, primed seeds germinated 73% compared to only 56% for unprimed seeds. The enhancement due to priming was retained after 9 yrs of storage at germination temperatures <30°C. At higher temperatures, the germination of unprimed seeds was superior to primed.