Knowledge and prediction of seasonal weed seedling emergence patterns is useful in weed management programs. Seed dormancy is a major factor influencing the timing of seedling emergence, and once dormancy is broken, environmental conditions determine the rate of germination and seedling emergence. Seed dormancy is a population-based phenomenon, because individual seeds are independently sensing their environment and responding physiologically to the signals they perceive. Mathematical models based on characterizing the variation that occurs in germination times among individual seeds in a population can describe and quantify environmental and after-ripening effects on seed dormancy. In particular, the hydrothermal time model can describe and quantify the effects of temperature and water potential on seed germination. This model states that the time to germination of a given seed fraction is inversely proportional to the amount by which a given germination factor (e.g., temperature or water potential) exceeds a threshold level for that factor. The hydrothermal time model provides a robust method for understanding how environmental factors interact to result in the germination phenotype (i.e., germination pattern over time) of a seed population. In addition, other factors that influence seed dormancy and germination act by causing the water potential thresholds of the seed population to shift to higher or lower values. This relatively simple model can describe and quantify the germination behavior of seeds across a wide array of environmental conditions and dormancy states, and can be used as an input to more general models of seed germination and seedling emergence in the field.

Seed germination culminates in the initiation of embryo growth and the resumption of water uptake after imbibition. Previous applications of cell growth models to describe seed germination have focused on the inhibition of radicle growth rates at reduced water potential (4,). An alternative approach is presented, based upon the timing of radicle emergence, to characterize the relationship of seed germination rates to 4A. Using only three parameters, a 'hydrotime constant' and the mean and standard deviation in minimum or base A' among seeds in the population, germination time courses can be predicted at any A, or normalized to a common time scale equal to that of seeds germinating in water. The rate of germination of lettuce (Lactuca sativa L. cv Empire) seeds, either intact or with the endosperm envelope cut, increased linearly with embryo turgor. The endosperm presented little physical resistance to radicle growth at the time of radicle emergence, but its presence markedly delayed germination. The length of the lag period after imbibition before radicle emergence is related to the time required for weakening of the endosperm, and not to the generation of additional turgor in the embryo. The rate of endosperm weakening is sensitive to # or turgor.Seed germination is the process of initiating growth of a previously quiescent or dormant embryo. For most seeds, it begins with imbibition of water. Imbibition is generally a triphasic process, with rapid initial water uptake (phase I), followed by a plateau phase with little change in water content (phase II), and a subsequent increase in water content coincident with radicle growth (phase III) (3). In terms of the regulation of germination, phase II is of primary interest, since germination in the physiological sense can be considered to be completed when embryo growth is initiated. It is the length of phase II that is generally extended by dormancy, low or high temperatures, water deficit, or abscisic acid, while factors which promote germination do so by shortening this lag phase. Once the radicle has penetrated any enclosing tissues and is growing, germination is complete and seedling growth has begun.A number of attempts have been made to apply the Lockhart (19) model for plant cell growth to the initiation of radicle growth during seed germination (e.g., 5, 21, 23, 29). The Lockhart model describes cell growth by the empirical equation where dV/ Vdt (Table I) is the rate of volume increase relative to the total volume, m is an extensibility coefficient relating growth rate to Ap, Ap is the turgor pressure, and Y is the minimum or threshold turgor that must be exceeded for growth to occur. Since water uptake is required for the volume increase during growth, growth rate is also described bywhere L is the hydraulic conductance of the tissue (incorporating pathway geometry, usually unknown), + is the water potential of the external medium, and 4,j is the water potential of the growing cell. (2,4,6). In the case of seed germination, where growth is initiat...

Temperature ( T ) and water potential ( y ) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination ( T b ), an optimum T at which germination is most rapid ( T o ), and a maximum or ceiling T at which germination is prevented ( T c ). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of T b multiplied by the time to a given germination percentage ( t g ). Similarly, germination at reduced y can be characterized on a hydrotime basis, or t g multiplied by the y in excess of a base or threshold y that just prevents germination ( y b ). Within a seed population, the variation in thermal times to germination among different seed fractions ( g ) is based on a normal distribution of y b values among seeds ( y b ( g )). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds T o . We report here that supra-optimal temperatures shift the ψ ψ ψ ψ b ( g ) distribution of a potato ( Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above T o . A modified hydrothermal time model incorporating changes in ψ ψ ψ ψ b ( g ) at T > T o describes germination timing and percentage across all T and ψ ψ ψ ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.

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Seed enhancements may be defined as post-harvest treatments that improve germination or seedling growth, or facilitate the delivery of seeds and other materials required at the time of sowing. This definition includes three general areas of enhancements: pre-sowing hydration treatments (priming), coating technologies and seed conditioning. Pre-sowing hydration treatments include non-controlled water uptake systems (methods in which water is freely available and not restricted by the environment) and controlled systems (methods that regulate seed moisture content preventing the completion of germination). Three techniques are used for controlled water uptake: priming with solutions or with solid particulate systems or by controlled hydration with water. These priming techniques will be discussed in this paper with reference to methodology, protocol optimization, drying and storage. Coating technologies include pelleting and film coating, and coatings may serve as delivery systems. Seed conditioning equipment upgrades seed quality by physical criteria. Integration of these methods can be performed, and a system is described to upgrade seed quality in Brassica that combines hydration, coating and conditioning. Upgrading is achieved by detecting sinapine leakage from nonviable seeds in a coating material surrounding the seeds. Seed-coat permeability directly influences leakage rate, and seeds of many species have a semipermeable layer. The semipermeable layer restricts solute diffusion through the seed coat, while water movement is not impeded. Opportunities for future seed enhancement research and development are highlighted.