SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY Survival Characteristics of Corn Seed during Storage: I. Normal Distribution of Seed Survival

scribed by a normal distribution Understanding the survival characteristics of hybrid corn (Zea y 5 [1/s√2p]exp[2(p 2 p)2/2s2] [1] mays L.) seed during storage is necessary to predict seed deterioration. This investigation tested one key assumption of the Ellis and Roberts where y is the relative frequency of deaths occurring at viability equation, namely, that seed survival is normally distributed. time p, p is the mean viability period, and s is the Eleven corn seed lots (six hybrids) with little mechanical injury and standard deviation of the distribution of deaths in time. a wide range in initial vigor were stored in various combinations of Assuming that seed survival follows a normal distribuconstant temperatures (20, 30, 40, and 508C) and seed moisture contion, Ellis and Roberts (1980a) developed a viability tents (100, 120, 140, and 160 g kg21, fresh weight basis). Seed-survival model (Eq. [2]) to predict declines in seed germination curves were constructed by conducting successive germination tests during storage as a function of storage temperature, at frequent intervals during storage. The x2 goodness-of-fit test was seed moisture and initial seed quality. used to evaluate the normality of survival curves constructed from either full or truncated (germination between 95 and 5%) data sets. v 5 Ki 2 p/10KE2Cwlog10m2CHt2CQt 2 [2] When the data were truncated, the majority (79%) of the 187 survival curves analyzed were classified as normal (P . 0.05) or near-normal In this equation, v is the probit or normal equivalent [P , 0.05 but relatively small x2, heterogeneity factor (H 5 x2/df) , deviate of germination (%), Ki is an initial seed quality 10]. Only 57% of the curves from the full data set followed a normal constant (on the probit scale), p is the storage period or near-normal distribution. Seed moisture and storage temperature (days), m is seed moisture content (%, fresh weight had no consistent effect on the shape of the survival curves. Survival basis), t is temperature (8C), and KE, CW, CH, and CQ of low-vigor seed lots was more likely to be normally or near-normally are constants whose values are assumed to be the same distributed than was survival of high vigor seed lots. The assumption for all seed lots of a species. that seed survival is normally distributed was generally valid for trunA reliable application of the viability equation is decated data sets of hybrid corn seed in constant storage environments. pendent on several assumptions. One key assumption is a normal distribution of seed deaths. Visual examination of seed-survival curves in many grain crop species, M approaches to evaluating the rate such as barley (Hordeum vulgare L.), wheat (Triticum of seed deterioration have been extensively inaestivum L.), and rice (Oryza sativa L.), suggests that vestigated (Roberts, 1972, 1973; Ellis and Roberts, they follow a normal distribution (Roberts, 1972, 1973; 1980a, 1980b, 1981; Moore and Roos, 1982; Moore and Ellis and Roberts, 1980b, 1981; Ellis et al., 1982, 1990). Jolliffe, 1987). The goal was to develop a means of However, there are several reports in which x2 tests for predicting seed germination in germplasm repositories normality indicated that seed-survival curves deviated or during commercial seed storage. From a quantitative from the normal distribution (Moore and Roos, 1982; viewpoint, seed deterioration can best be defined as an Moore and Jolliffe, 1987; Wilson et al., 1989; Fabrizius increased probability of death of an individual seed as et al., 1999). Wilson et al. (1989) reported that analysis deterioration proceeds. Seed death is indicated by the of survival curves of field bean (Phaseolus vulgaris L.) failure to germinate and seed longevity is the period seed lots with low initial germination always resulted in until seed death occurs. significant x2. They suggested that adjustment for the Seed-survival curves (germination percentage plotinitial proportion of nongerminable seeds would result ted against time) have reverse sigmoid shapes resemin seed-survival curves that were more likely to follow bling a negative cumulative normal distribution (Roba normal distribution (Wilson et al., 1989), but Ellis et erts, 1972, 1973). Such a shape is expected if (i) under al. (1990) refuted the need for correction, stating that any given set of storage conditions, a seed lot has a correction was unnecessary because of the arbitrary particular mean viability period and a random distribustart of seed storage, and can be misleading by distorting tion of the viability period of the individual seeds around the estimate of Ki. this mean value and (ii) the distribution of the viability Two factors may be responsible for a significantly period of individual seeds in a population can be delarge x2 (Finney, 1971). First, individual seeds in the seed lot may not deteriorate wholly independently of Dep. of Agronomy, University of Kentucky, Lexington, KY 40546one another, resulting in random heterogeneity. Sec0091. Contribution from Kentucky Agric. Exp. Stn. No. 98-06-119. Received 8 August 1998. *Corresponding author (dtekrony@ca. ondly, a significant x2 may occur because the underlying uky.edu). Abbreviations: ISTA, International Seed Testing Association. Published in Crop Sci. 39:1394–1400 (1999). 1394 Published August 15, 2014