Applications of hydrotime analysis in seed testing (original) (raw)

2004

Abstract

Hydrotime is a way to describe the relationship between water potential (\|/) and seed germination rates and percentages. The minimum \|/ that allows germination to be completed is called the base water potential (\|/j,). The ger mination rate (inverse of time to radicle emergence) increases linearly as the seed y increases above i|/i, to its maximum rate in water (\|/ = 0 MPa). As t|q, values vary among individual seeds, germination rates also vary, resulting in lack of uniformity that can be quantified by the standard deviation in t|q, val ues (<3yb)The hydrotime constant (0h) indicates the inherent speed of ger mination in a seed lot. Thus, hydrotime analysis quantifies the speed of ger mination (0h), the stress tolerance of germination (\|/i,) and the uniformity of germination for a seed lot (Cfyj,), which are all useful indicators of seed vigor. Hydrotime analysis of seed lots under diverse conditions allows them to be ranked according to their potential for successful emergence. It is also a valuable tool for developing and assessing seed enhancement treatments such as pelleting and priming. Hydrotime analysis can be simplified into an endpoint test that could be useful for ranking seed lots according to vigor and for diagnosing seed lot potential under stressful conditions. The Hydrotime Concept in Seed Germination The hydrotime concept is a unifying model to describe the patterns of germi nation that occur in response to the water potential (\|f) of the seed's environ ment. It is similar to thermal time, or degree-days, in which the degrees in excess of a base or threshold temperature (Ty), multiplied by the time to a develop mental event (for example, radicle emergence) is a constant. Gummerson (1986) proposed that in analogy with thermal time or degree-days responses in rela tion to temperature, the time to germination is related to the magnitude of the difference between the \p of the seed or environment and the physiological \|f threshold for radicle emergence (\|/&). Interestingly, Gummerson (1986) showed that in the case of germination responses to reduced t\f, the total hydrotime (MPa-hours or MPa-days) to radicle emergence was the same for all seeds in the population, but that individual seeds varied in their threshold \|/ at which radicle emergence would be prevented. The following equation describes the basis of the hydrotime model (Bradford, 1990; 1995): 0H=[V|f-yb(g)]tg (1) where 0h is the hydrotime constant (MPa h), \|/ is the actual seed water potential Kent J. Bradford* Department of Vegetable Crops, University of California, Davis, CA 95616-8631; David W. Still, Department of Plant Science, California State Polytechnic University, Pomona, CA 97168. Original results of D.W.S. supported in part by California Agriculture Research Initiative. ''Cor responding author. Received 19 September 2003.

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