A Water Relations Analysis of Seed Germination Rates

Abstract

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...

Estresse hídrico e reguladores de crescimento na germinação de sementes de canafístula

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Single-seed oxygen consumption measurements and population-based threshold models link respiration and germination rates under diverse conditions

“…delay) with even small decreases in Ψ (Gummerson, 1986;Bradford, 1990). Unlike the T b , which in the simplest case is assumed to be constant among all seeds in a population, the Ψ b values differ among seeds, such that some can germinate to lower Ψ than others (Gummerson, 1986).…”

Germination of Acacia harpophylla (Brigalow) seeds in relation to soil water potential: Implications for rehabilitation of a threatened ecosystem

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“…However, given the unique pedological and climatic conditions of the Brigalow Belt, the question remains how Brigalow seeds respond to changes in water availability from soil. This quantitative information is crucial to simulate timing and success of seed emergence, for example when applying the hydrotime model (Bradford 1990;Gummerson 1986). …”