Abstract
The effect of moisture on seed longevity during experimental storage was investigated in pepper (Capsicum annuum) cultivar Demre. Seeds were stored hermetically at 7.0%, 8.6%, 9.5%, 10.5%, and 12.1% moisture content (MC; percent fresh weight basis) and 35 °C for up to 306 d. Viability (normal germination) was assessed periodically and the seed viability equation moisture constants, KE and CW, estimated by regression of log σ (the sd of distribution of seed death in time) with log MC. KE and CW values were found to be 7.767 and 4.670, respectively. The newly found moisture constants were combined with the temperature constants that had already been proposed as universal for all orthodox seeds (CH = 0.0329, CQ = 0.000478) and their validity was tested by predicting the viability loss in another pepper cultivar, Carliston. The Ki of the Carliston cultivar was determined by controlled deterioration at 40 °C and 14% MC. The observed viability loss between 30 and 270 d at 25 °C and 10% seed MC was closely related to that predicted by the equation R2 = 0.982 (P < 0.001).
Seeds of pepper (Capsicum annuum L.) are considered to have a short lifespan in storage (Priestley, 1986). In subtropical regions where pepper seeds are produced, relative humidity is high (greater than 70%) and when associated with high temperatures (30 to 35 °C), the viability of seeds can decline within 6 to 24 months (George, 1985). In previous work, we reported that the germination of the majority of pepper seed lots declined to less than the acceptable commercial level (75%) in Turkey after 8 months, even when they were stored at 5 °C with 8% moisture content (Basak et al., 2006). Commercial producers of pepper seeds may save time and money by allowing the early sale of poor-longevity lots and the holding of better lots for the next growing season. The estimation of viability loss would avoid the use of poorer quality pepper seeds in production. Moreover, prediction of seed longevity in long-term storage enables effective management of gene bank accessions by setting appropriate regeneration periods.
The objective of this research was to estimate the species-specific constants (KE and CW) of the viability equation for pepper seeds and to test the predictive value of the derived equation over time in storage.
Material and Methods
Seed material.
Pepper seeds (Capsicum annuum L.) of cultivar Demre were obtained from the Beta Seed Company (Ankara, Turkey) and stored hermetically at −20 °C before the investigation. Initial seed MC was determined by the low constant temperature method (see subsequently) as 6.9%.
KE and CW determinations.
Five lots of seeds were humidified in a desiccator over deionized water (≈98% relative humidity) until the target MCs of 7.0%, 8.6%, 9.5%, 10.5%, and 12.1% were reached. During humidification, MCs were checked by frequent weighing and using the following formula:
To ensure even distribution of water within each of the five lots, seeds were sealed inside aluminum foil packets and placed at 5 °C for 3 d. Seed MC (percent fresh weight basis) was determined by the low constant-temperature-oven method prescribed by the International Seed Testing Association (ISTA, 2005) whereby two 4-g samples were dried in a mechanically ventilated oven at 105 °C for 17 h.
Eighteen subsamples of 200 seeds for each MC were sealed in laminated foil packets. All of the 90 subsamples were stored in incubators maintained at 35 ± 1 °C. Samples were removed at intervals varying from 1 to 30 d depending on MC for periods up to 306 d. Two samples for each MC treatment were taken to determine if seed MC changes occurred during storage. Samples were tested for germination by placing seeds between rolled 20 × 20 cm filter (BP) papers (Filtrak, Niederschlag, Germany) moistened with deionized water, rolled towels were then placed in polyethylene bags, and they were incubated at 25 °C in dark. Germination tests were performed by using four replicates of 50 seeds for each MC × storage period. Normal seedling development was used as the germination criterion and counts made after 7 and 14 d (ISTA, 2005).
Validation of the constants.
A separate storage experiment was conducted to test the validity of the constants determined in the previous experiment by comparing the relationship between by the experiment (actual) and predicted (by the equation) germination values.
Seeds of pepper cultivar Carliston, obtained from the Beta Seed Company (Ankara, Turkey), were hermetically stored (aluminum foil packets) at 10.0% MC and 25 °C. Samples of 200 seeds were used to determine germination percent after 30, 45, 60, 75, 90, 105, 120, 140, 200, 230, 240, 250, 260, and 270 d of storage. Seed humidification, storage procedure, and germination tests were conducting as described previously.
The initial quality of this seed lot, Ki, was determined by storing the seeds at 40 °C and 14% MC. Samples of 200 seeds were used for germination testing at intervals of 1 or 2 d up to 42 d of storage. Percentage germination values were converted to probits and linear regression analysis was carried out using SPSS to estimate Ki.
The values determined for KE and CW, together with the estimate for Ki, were used to predict the longevity of pepper seeds stored at 25 °C and 10% MC. Linear regression analysis using SPSS was then conducted between predicted and observed values to assess the validity of these species constants for pepper seeds.
Results
Longevity in hermetic storage gradually increased as MC decreased as expected in orthodox seeds. Seeds with 12.1% MC were dead after 53 d, whereas those with 7% MC did not all die until after more than 300 d (Fig. 1). The estimates of the sd of frequency distribution of seed deaths in time (σ, i.e., the time elapsed between e.g., 84% and 50%) ranged between 10.1 d and 102.1 d in corresponding seed MCs, respectively.
Survival curves (germination versus storage period) fitted by probit analysis of pepper cultivar Demre seeds stored at five different moisture contents. The curves shown were fitted through the data from the point of rapidly declining viability (i.e., through the data after germination had fallen to ≈85%).
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1679
The log σ values were regressed against the log of storage moisture content to estimate the values of the moisture viability constants, K and CW (Eq. 4) for pepper (Fig. 2). Log seed MC and log σ values were highly correlated (Fig. 2; R2 = 0.959, P < 0.01) and showed a negative linear relationship with K = 6.03 (se 0.541) and CW = 4.67 (se 0.555). KE was then calculated using the universal values for CH (0.0329) and CQ (0.000478; Dickie et al., 1990) in Eq. [5]:
The negative logarithmic relation between seed longevity (σ) in hermetic storage and seed moisture contents for pepper cultivar Demre.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1679
Thus, the estimates of the seed viability constants for pepper are:
To validate the use of these values for KE and CW in the prediction of pepper seed viability, seeds of another pepper cultivar, Carliston, were hermetically stored at 25 °C at 10.0% MC. A typical survival curve from a sequential sampling during storage revealed a slow decline in germination within the first 120 d of storage followed by a much faster reduction (Fig. 3). The initial seed quality constant (Ki) of this cultivar, Carliston, seed lot was determined by converting the percentage germination values during storage at 40 °C and 14% MC to probits and fitting a linear regression (Fig. 4). The probit intercept value was 2.151 (se 0.083).
Survival curve of a seed lot of cultivar Carliston pepper seeds stored under hermetic conditions at 25 °C with 10.0% seed moisture content.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1679
Probit values of germination percentages determined pepper cultivar Carliston seeds hermetically stored at 40 °C with 14% moisture content.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1679
Predictions of the viability of seed lot cultivar Carliston stored at 25 °C and 10.0% MC for the experimental storage periods shown were calculated. Predicted values were obtained by using the estimated Ki for the mentioned seed lot, the values determined for KE and CW cultivar Demre, and the universal values for CH and CQ. There was a close relationship between the actual and predicted values (Fig. 5; R2 = 0.982, P < 0.001, slope = 0.890).
The relationship between actual and predicted germination values for pepper cultivar Carliston seeds stored at 25 °C and 10% moisture content fitted by linear regression (dashed line). The solid line shows the relationship actual = predicted.
Citation: HortScience horts 44, 6; 10.21273/HORTSCI.44.6.1679
Discussion
In the present work, the viability equation moisture constants (KE and CW) for pepper cultivar Demre seeds were found to be KE = 7.767 and CW = 4.670. These constants were shown to be valid for the prediction of seed viability for a different pepper cultivar, Carliston, during experimental storage. Validity of moisture constants in a different cultivar than that of constructed one confirms the view that seed moisture constants are applicable to the storage of any cultivar within the same species as indicated by the basic philosophy of the viability equation (Ellis and Roberts, 1981). We did not intend to determine temperature constants for pepper, because universal values for temperature constants were published and proved that they are applicable for all orthodox seeds (Dickie et al., 1990). We used these values to calculate the predicted viability during storage. The highly significant (P < 0.001) relationship between predicted and actual germination values (Fig. 5) indicates that the values that we determined for the moisture constants (KE and CW) and these universal values for the temperature constants are applicable to pepper seeds.
Our estimate of KE for pepper seeds is within the range of values published for other species. In a previous summary of the viability constants for 53 diverse species (Hong et al., 1996), the estimates of KE ranged from 3.278 for a tree species, Pinus taeda (Bonner, 1994), to 10.858 for the pulse, Vigna radiata (Ellis et al., 1989). Some of the crop seeds had reasonably high KE values, i.e., 8.037 in potato, 8.988 in beet root, and 9.993 in maize (Ellis and Hong, 2007). KE has been found to vary with seed maturity (Hay et al., 1997; Zanakis et al., 1993) and between ecotypes (Hay et al., 2003), mutants (Lyall et al., 2003) and subspecies within a species (Ellis et al., 1992). Nonetheless, the successful use of the derived KE for pepper to predict germination of a seed lot from another cultivar within the same species is compatible with the view that KE is a species constant (Ellis and Roberts, 1980).
Like with KE, the close fit between our predicted and observed values for viability during storage (Fig. 5) supports the view that CW is generally a constant specific for a species (Ellis and Roberts, 1980). Estimates of CW are known to vary among species, largely as a result of differences in seed oil content (Dickie et al., 1990; Ellis et al., 1988; Roberts and Ellis, 1989). Among 53 diverse species (Hong et al., 1996), estimates of CW varied from 0.983 for the tree Pinus elliotti (Bonner, 1994) to 6.305 in sorghum (Sorghum bicolor; Kuo et al., 1990). Seed oil content in pepper has been seen to range between 18.2% and 22% (Demir et al., 2008). Our derived CW value of 4.498 for pepper seeds is close to those for crop seeds that have similar oil contents such as soybean, which has CW = 3.979 and 17% oil content (Ellis et al., 1982). On the contrary, more recent work in tree seeds showed that there were poor correlations (P > 0.05) among seed oil content, CW, and longevity (de Lima and Ellis, 2005). Such insignificant correlation indicates that a generalization regarding differences in seed longevity among orthodox species in relation to oil content is not possible.
The negative logarithmic relation between seed longevity and MC in hermetic air-dry storage is well described in various crop seeds (Fig. 2) but is subjected to low moisture limit (Ellis et al., 1988, 1989, 1990, 1992). Reducing the seed MC lower than this critical MC does not provide an increase in longevity. In this experiment, the lowest MC used in pepper seeds was 7.0%. This is well above the low critical MC found in some crop seeds, e.g., 4.47% in sugar beet (Ellis et al., 1990), 2.04% in sunflower (Ellis et al., 1988), and 6% in pea (Ellis et al., 1989). Storage at lower MCs than those investigated in this work would have been required determining the low critical MC for pepper seeds.
This work may be of practical significance for the commercial storage of pepper seeds, which are sensitive to adverse conditions and may lose viability within a short time if conditions are suboptimal (Priestley, 1986). The seed viability constants estimated here could provide a sound approach to estimate declines in viability in commercial storage. Although the relationship between predicted and actual values was highly significant (Fig. 5; R2 = 0.982, P < 0.001), there is inevitably some modeling error and, in our case, there was some systematic deviation. The fitted regression (Fig. 5) suggested that as viability declined during storage, observed germination became increasingly better than that predicted. This would occur if our estimated Ki was close to the actual value, but σ was being underestimated by our species constants. The actual data indicated that although there was departure over the middle part of the survival curves when viability was declining the most rapidly (between 30% and 80%; Fig. 5) and where the error associated with percentage values would be expected to be greater anyway, observed and predicted values were close at high and low percentage values. Usberti et al. (2006) reported very similar R2 like in our case in the relationship between predicted and actual seed germination in cotton. If we could have determined moisture constants on more than one seed lot as was done in some other crop seeds (Ellis and Hong, 2007) and/or a range of temperatures (Hay et al., 2003), the equality between observed and predicted values may have been higher. We intend to go on to control the present constants and their validity with other pepper cultivars and for a larger number of seed lots. The equation could also be used to determine gene bank regeneration intervals after investigations into the effects of lower moisture and temperatures than covered by the present work.
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