For annual crops, the time from planting to seedling establishment is a crucial phase in the production cycle. Uniformity and percentage of emergence of direct-seeded crops can have a major impact on final yield and quality, particularly for vegetables (84). Techniques of precision planting to achieve desired plant populations depend on a high probability of successful establishment for each seed planted (26). Furthermore, increasing use of expensive hybrid seed has placed additional emphasis on the performance of each seed planted. The soil environment often, however, is not conducive to rapid germination and seedling growth. Physical stresses, such as extreme temperatures, excess or deficit of water, salinity, or soil crusting; and biological stresses, including pathogens and insects, can all adversely affect germination and seedling growth. These problems are compounded by the increased susceptibility of plants to many of these stresses during germination and emergence. It is not surprising, therefore, that there have been many attempts to devise presowing treatments to improve seed performance in the field. The wide range of such treatments has been thoroughly reviewed and categorized by Heydecker and Coolbear (38).
In 1981, R.H. Ellis and E.H. Roberts published a classic paper on the quantification of aging and survival in seeds (Seed Sci. & Technol. 9:373). This paper and subsequent refinements described a model of seed aging in storage that was based on the fact that to a good approximation, deaths over time in a seed population are normally distributed. The model provides a quantitative description of seed longevity across a wide range of storage temperatures and moisture contents. Despite its theoretical importance and practical success, the Ellis–Roberts approach has not been widely adopted by the seed industry to assess seed quality and predict longevity in storage. This may be due, in part, to the rather unfamiliar statistics (probit analysis) used in the model and the apparent complexity of the equations. It will be the argument of this presentation, however, that the precise quantification of seed longevity that this model affords is less significant than the insight that it provides into the nature of seed populations and how to think about them. The objective of this presentation will be to demystify the Ellis–Roberts model and illustrate with concrete examples how the application of population-based thinking is advantageous in many aspects of seed storage and quality assessment.
Lettuce seeds were germinated at 20 C in the dark in water and sampled at various intervals during the first 18h of germination to determine quantitative and qualitative differences in proteins. The soluble protein fraction was partitioned into albumins and globulins by dialysis and the proteins of the globulin fraction were visualized by SDS-PAGE. Heat stable proteins were obtained by boiling the proteins, cooling on ice, and resuspending in buffer.
The soluble protein content remained constant during the first 8h of germination. Thereafter protein content decreased and was 6% of the amount present in unimbibed seed in 21 day old seedlings. The ratio of heat stable to heat unstable proteins decreased during the germination process. No differences in banding patterns were observed when the soluble protein fraction were run on SDS-PAGE. However, on gels run with the globulin fraction a 57 kD protein appeared 4 and 8 h after imbibition and had disappeared by 12 h after imbibition. The role of proteins and heat-stable proteins during germination and prevention of dessication during early seedling growth is discussed.
Cell growth models were applied to characterize the response of seed germination, based upon the timing of radicle emergence, to y and ABA. Using probit analysis, three basic parameters can be derived to describe the population characteristics of seed lots. In the response of seed germination to osmotic stress, these three parameters are the “hydrotime constant” (q H), the mean base water potential (y b), and the standard deviation (s b) population. In the response to ABA, they are the “ABA-time constant” (qABA), the mean base ABA concentration (ABAb), and the standard deviation (sABAb) of the seed population. Using only these three parameters, germination time courses can be predicted at any corresponding medium y or ABA concentration. In the presence of both ABA and osmotic stress, the same parameters can be used to predict seed germination time courses with any combination of y and ABA concentration. The water relations model and the ABA model were additive and it appeared that the two factors slowed down germination independently. Effects of osmotic stress and ABA on the parameters in Lockhart equation are also discussed.
Brassica crops have an extended flowering period due to both progressive development within a given raceme and variability among multiple racemes. Early harvest can result in poor seed quality due to immaturity, while delayed harvest may sacrifice yield due to shattering. To characterize the development of seed quality, we measured maturity indices and conducted vigor tests on hybrid red cabbage (Brassica oleracea var capitata) seed harvested weekly starting 33 days after full bloom (DAF). Viability and germination rate increased from the top to the bottom of the raceme, and were maximal by 40 and 48 DAF, respectively. After 48 DAF, there was little difference in seed quality due to position on the raceme. Seed dry weight also reached a plateau by 48 DAF, when rapid dehydration began. Sensitivity of germination to inhibition by reduced water potential or abscisic acid (ABA) was assessed using a threshold model based upon germination rates. Germination became less sensitive to both factors and more uniform during maturation, with -1.0 MPa or 50 μM ABA being required to inhibit germination by 50% after about 48 DAF Seed ABA content reached a peak of 10 μg/g dry wt. by 40 DAF, then declined linearly to 1.5 μg/g dry wt. by 68 DAF Overall, optimal seed quality was attained at 54 DAF
Seed priming (controlled hydration followed by drying) is used to alleviate high temperature inhibition of germination and improve seedling emergence of lettuce (Lactuca sativa L.) and other species. However, seed priming can also reduce the longevity of seeds during dry storage. Alternative drying methods [i.e., slow drying or moisture content reduction (MCR) before drying] can extend seed longevity compared to conventional rapid drying procedures after priming. Three postpriming drying treatments were tested on `Conquistador' and `Genecorp Green' lettuce seeds: rapid drying, slow drying and MCR (10% fresh weight loss, then held at 100% relative humidity (RH) for 6 hours, followed by rapid drying). The effects of the postpriming treatments on seed quality and longevity were compared based upon standard germination tests, germination rates, thermogradient table tests, controlled deterioration (CD) tests, and headspace volatiles analysis. The latter may be correlated with seed longevity as release of volatiles (e.g., acetaldehyde, ethanol) is associated with lipid peroxidation. While neither slow drying nor MCR before drying restored lettuce seed longevity to that of the control (not primed) seeds, the MCR method generally gave better results in both cultivars compared to rapid drying. Among the CD test conditions used, 50 °C and 75% RH gave the most consistent results for estimating potential longevity. Headspace volatile emissions from both control and primed lettuce seeds were very low and were not well correlated with seed longevity. Alternative postpriming drying regimes can extend seed longevity while retaining the beneficial effects of priming.
With many seed crops, the most difficult production decision is when to harvest. In indeterminate crops such as Brassica species, early harvests result in immature seed of low vigor while late harvests risk seed deterioration and seed loss due to shattering. To provide a biological basis on which to determine harvest timing, we have characterized seed development in rape seed (Brassica napus L. `Weststar') and red cabbage (Brassica oleracea L. Group Capitata) using population-based hydrotime and ABA-time models. These models provide information relevant to assessing physiological maturity, and therefore, seed quality. The hydrotime and ABA-time models quantify germination rate, the uniformity of germination, viability, and the sensitivity of germination to water potential and ABA. Indices derived from these models, along with maximum germination and t50 values, were used to determine physiological maturity (maximum seed quality) of the seeds during development. The overall trends in seed development were similar in both species: as seeds matured, germination became more uniform and less sensitive to low Ψ and externally applied ABA. The models accurately described germination time courses and final germination percentages except for seeds imbibed at very high concentrations of ABA. In rape seed, physiological maturity was attained several days after maximum seed dry mass, while in red cabbage physiological maturity occurred at or after maximum seed dry mass. Vigor indices were correlated with easily discerned traits such as moisture content and silique phenotypic characteristics. The results of these experiments suggest that hydrotime and ABA-time models can be successfully used to provide a biological basis on which to determine harvest in brassicas.
Waterlogging of the soil rapidly and dramatically alters both the physical and biological environment of plant roots. In response to environmental stimuli, physiological events occur within the plant which affect its growth and development. The purpose of this paper is to review certain aspects of the physiological responses of plants to waterlogging with respect to the response mechanisms and the subsequent adaptations in the growth and development of the whole plant. Many important aspects of the subject must be only briefly mentioned here, such as the effects of waterlogging on soil chemistry, nutrient availability and uptake, microbiology, pathology, and senescence. The reader is referred to recent literature for information on these topics (3, 13, 19, 21). The overriding effect of soil flooding is to limit the diffusion of oxygen to the root zone. The focus here, therefore, will be on the responses of plants to root anaerobiosis and some initial, rapid mechanisms of adjustment. Further information on long-term adaptation, especially in woody plants, can be found elsewhere (76).
Plants of two cultivars of processing tomato (Lycopersicon esculentum Mill, cvs. UC82B and VF145B-7879) were sprayed with ethephon solutions 10 days prior to harvest. Seeds were extracted from fruits that had been tagged at anthesis to allow separation into age classes ranging from 31 to 70 days at harvest. Seed dry weight increased rapidly until ≈40 days after anthesis, then remained relatively constant for 10 to 15 days before increasing again. During the plateau phase, the initial (5-day) germination percentages increased by 20% to 40% and final (10-day) germination increased by 6% to 10%. Germination percentages then fell slightly with increasing maturity and dry weight accumulation. Preharvest ethephon treatment caused younger fruit to ripen and shifted the development of maximum germinability toward young seed ages, without influencing dry weight accumulation. Seeds ≤53 days old at harvest increased in germination due to ethephon treatment, whereas seeds older than 53 days showed decreased germination. Since the bulk of harvested seeds will be in the older classes, overall seed quality may be affected adversely by preharvest ethephon applications. Although seed lots from ethephon-treated plants still had acceptable germination, there would appear to be no benefit from ethephon applications to tomatoes destined solely for seed production. Chemical names used: (2-chloroethyl)phosphonic acid (ethephon).