Agronomy Journal 93:1054-1058 (2001)
© 2001 American Society of Agronomy
CROPPING SYSTEMS
Physiological and Harvest Maturity of Canola in Relation to Seed Quality
Sabry G. Elias*,a and
Larry O. Copelandb
a Dep. of Crop and Soil Sciences, The Seed Lab., Oregon State Univ., Corvallis, OR 97331
b Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824
* Corresponding author (sabry.elias{at}orst.edu)
Received for publication October 18, 2000.
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ABSTRACT
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The ability to identify physiological (PM) and harvest maturity (HM) of canola (Brassica napus L.) by phenological characteristics would permit timely harvest to avoid problems of both under- and overripe pods. This study was conducted to identify PM and HM of six winter and spring canola cultivars using morphological and physiological markers including seed and pod color, seed dry weight (DWT), seed moisture content (SMC), and to measure seed quality (germination and vigor) at PM and HM. Pods and seeds from each cultivar were sampled at weekly intervals from pod formation until HM. Standard germination, accelerated aging, and cold tests were conducted to assess seed quality. Canola attained PM when pods turned from green to greenish-yellow or light brown, and contained seeds ranging from brownish green to greenish brown and light brown. The seeds were firm. Seed DWT did not change significantly from PM to HM. Seed MC at maximum DWT ranged from 203 to 360 g kg-1. Pods at HM were yellow to brown and the seeds were brown or dark brown to black, depending on the cultivar. The SMC was near 100 g kg-1 and the seeds were hard and rattled inside the pod. Seeds of all cultivars had greater germination and vigor at HM than at PM. Canola can be harvested 2 wk before reaching HM without affecting yield; however, SMC at this stage is not suitable for direct harvest and seed quality is not at the highest level.
Abbreviations: PM, physiological maturity • HM, harvest maturity • SMC, seed moisture content • DWT, dry weight, SGT, standard germination test • CT, cold test • AAT, accelerated aging test
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INTRODUCTION
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STAGE of seed development at harvest influences both canola yield and seed quality. Harvesting too early may result in low yield and poor seed quality, whereas harvesting too late may result in shattering and reduced seed yield (Oplinger et al., 1989). Harvesting at full (i.e., harvest) maturity (when seed moisture content is near 100 g kg-1) is preferred for better threshing and storability because of the suitable moisture content of both pods and seeds. However, it may be advisable to harvest the crop at PM than at HM if the crop is excessively weedy (Salunkhe and Desai, 1986; Fenwick, 1988) or to avoid excessive bird damage or unfavorable weather conditions during late maturation and harvest (e.g., possible frost damage or excessive rain). Therefore, it is important to determine when PM is reached in canola.
Many studies have focused on determining PM on crops other than canola. Seed shrinkage and loss of green color from the pod have been suggested as indicators of PM for soybean [Glycine max (L.) Merr.] by Crookston and Hill (1978), TeKrony et al. (1979), and Gibkpi and Crookston (1981). Fraser et al. (1982) reported that moisture content represents an accurate indicator of PM for soybean. Formation of a black layer at the base of kernels (Daynard and Duncan, 1969; Carter and Poneleit, 1973) and/or progression of the milk line (Afuakwa and Crookston, 1984) have been suggested as reliable indicators of PM in corn (Zea mays L.). Black layer formation in the placental area of the seed was reported to be a good indicator of PM in sorghum [Sorghum bicolor (L.) Moench] (Eastin et al., 1973). Because quick field estimation of PM from physiological measurements such as seed dry weight or moisture content is somewhat difficult, methods for determining PM in canola based on morphological indicators are needed.
Identification of HM is important because the proper period for harvesting canola is short. If canola is harvested past the appropriate time, crop loss can be expected as a result of overripening, which causes pods to shatter easily, especially under adverse weather conditions (Oplinger et al., 1989; Salunkhe and Desai, 1986). Delouche (1980) defined HM as when seed moisture content is low enough to allow effective threshing with a mechanical harvester. Fenwick (1988) and Oplinger et al. (1989) suggested hard, dark seeds with 100 to 140 g kg-1 moisture content as an indicator of HM in canola. However, we felt it would be useful to confirm this indicator and provide comprehensive methods for determining HM based on other morphological traits.
The use of high-quality seed is essential for good stand establishment and yield in any crop. Consequently, germination and selected vigor tests were used in this study to determine the quality of canola seed at PM and HM. Although researchers have reported that seed of some crops attain maximum viability and vigor at PM (Delouche, 1974; Knittle and Burris, 1976), no reports of seed quality of canola seed at both PM and HM have been published. Therefore, the objectives of this research were to: (i) identify PM and HM for different canola cultivars using morphological and physiological markers, and (ii) evaluate the seed quality of canola at physiological and harvest maturity.
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MATERIALS AND METHODS
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Cultivars Planting and Management
Four winter canola cultivars, Cascade, Crystal, Glacier, and Lirabon, and two spring cultivars, Topas and Westar, were planted in 0.18-m rows on a clay loam soil of the Capac (fine-loamy, mixed, mesic Aeric Endoaqualfs) series at East Lansing, MI, at a rate of 5.6 kg ha-1 in a randomized complete block design with four replications. Plot size was 6.08 m long and 0.91 m wide. One hundred and forty kg ha-1 of N (urea, 46–0–0) was applied in the spring to all plots. In the first year, the winter cultivars were planted on 7 Sept. 1988, and harvested for yield on 24 July 1989; the spring cultivars were planted on 26 April and harvested on 26 Aug. 1989. In the second year, the winter cultivars were planted on 5 Sept. 1989, and harvested on 22 July 1990; and the spring cultivars were planted on 26 April and harvested on 16 Aug. 1990.
Sampling Procedure
Pod sampling was started when approximately 90% of the flowers in a plant formed pods, i.e., when the flower petals began to fall. Twenty-five plants from each replication were randomly chosen and tagged for subsequent sampling. An area between branches three and six of each selected plant was marked with red ribbons and pods were sampled at weekly intervals starting from the time of pod formation until HM. Harvest maturity was determined when seed moisture content dropped to near 100 to 120 g kg-1, i.e., suitable for direct mechanical harvest, and when approximately 90% of pods and seeds of plants in a plot had turned to brown. Physiological maturity was determined when seeds reached maximum dry weight. The color of pods and seeds was monitored at PM to provide some morphological indicators for determining time to harvest. Starting from Week 5 until HM, morphological changes in pods and seeds were noted every other day. On each sampling date, about 50 randomly selected pods from the marked areas were measured for the selected morphological and physiological parameters. The sampled pods were immediately placed in plastic Ziploc bags and kept at 5°C until processing.
Morphological and Physiological Assessments
Ten randomly selected pods from each sample in 1989 and five in 1990 were collected and the pedicels removed. Pod and seed color and texture, fresh seed mass, dry mass, and moisture content were measured. Munsell color charts (Munsell, 1977) for plant tissues were used to describe seed color at different development stages. The remaining pods in each sample were carefully opened, the seeds removed, and the seeds allowed to air-dry in the laboratory for a week.
Seed Quality Determination
The standard germination test (AOSA, 1988) was conducted on 100-seed samples of each cultivar at 20°C for 7 d on moistened blotter papers. Tests were replicated four times. Only normal seedlings were counted. The cold test (Elias and Copeland, 1997) was performed on four 100-seed replications of each cultivar by exposing the samples to 5°C for 5 d on moistened blotter papers and then transferred to 22°C for 5 d for germination. The accelerated aging test was conducted by aging seeds at 42°C for 48 h (Elias and Copeland, 1997) using the wire-mesh tray method (McDonald and Phaneendranath, 1978). A single layer of seeds from each sampling date of each cultivar was placed on 10 by 10 by 3 cm copper wire mesh tray inside a 11 by 11 by 3.5 cm plastic box containing 2 cm water (about 100 mL) above the bottom of the box. Following incubation, the seeds were germinated at 22°C for 7 d as described above. The percent of normal seedlings was recorded.
Experimental Design and Statistical Analyses
All data were subjected to analysis of variance (ANOVA) appropriate to a randomized complete block design. The experimental model used to analyze seed dry weight, moisture content, and the seed quality (germination, accelerated aging, and cold tests) studies was a two-factor (cultivar and sampling date) randomized complete block design combined over years (1989 and 1990) for both winter and spring canola cultivars. The least significant difference (LSD) test was used when mean differences were significant. The data were analyzed using the statistical package MSTAT (Michigan State Univ., East Lansing, MI). Both field and laboratory tests were repeated for two seasons.
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RESULTS AND DISCUSSION
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Seed Physiological Characteristics
Because of the vernalization requirement, winter cultivars required 267 d from planting to pod formation compared with 70 d for the spring cultivars. The long growing season of winter cultivars explained their higher yield compared with the spring cultivars (data not shown). However, the time from pod formation to PM was similar in both spring and winter cultivars. Spring cultivars reached PM 36 and 37 d after pod formation, whereas the winter cultivars reached PM after 39 and 36 d in the first and second year, respectively. Weather conditions, especially precipitation, affected both the initiation of pod formation and the duration between pod formation and PM. The amount of precipitation during May and June in the first year was 252 mm compared with 139 mm in the same period of the second year. This, in part, resulted in a 7-d difference in the initiation of pod formation of spring cultivars between years and 3-d difference in the period between pod formation and PM of winter cultivars between years. The range at which spring and winter cultivars reached HM was 9 to 16 d after PM. The time to reach PM, and from PM to HM varied among cultivars and between years, to a large extent because of the change in weather conditions during seed maturity. For example, the amount of precipitation in August of the first year was 175 mm, compared with 61 mm in the second year. Consequently, spring cultivars in the first year were harvested 10 d later than the second year. The variability in the length of the time between PM and HM is not unusual. Crookston and Hill (1978) and TeKrony et al. (1979) reported such variation among soybean cultivars and between years.
Seed dry weight of all cultivars increased gradually following seed formation and remained without significant change after PM until harvest with few exceptions (Fig. 1 and 2). The ANOVA results showed that year and stage of seed development (seed age) as well as the interaction between them had significant effects on seed dry weight of the spring cultivars. However, the interaction between year and stage of seed development was not significant for winter cultivars, indicating that all cultivars developed similarly in both years (Table 1). Gradual change in seed color was observed with progressive development and maturity. At the very early stages of pod growth, the seeds were colorless and transparent. With further development, the seeds turned to light green and then darker green. At PM, the pod contained seeds with colors ranging from dark green or brownish green to light brown (Table 2). The seeds were firm but not hard and could be marked with a fingernail, probably because the SMC level averaged approximately 280 g kg-1. As seed and pod color changed throughout the period of seed development and maturation, the seed dry matter also changed (Fig. 1 and 2). Therefore, the change in seed color and accompanying changes in pod color can be dependable indicators of PM in canola. Visual indicators of PM have also been suggested for soybean (Crookston and Hill, 1978; Te-Krony et al., 1979; Gibkpi and Crookston, 1981).
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Fig. 1. Seed dry weight and moisture content of two spring canola cultivars during seed development in 1989 and 1990. Error bars indicate the standard deviation from means of seed dry weight (DWT) and moisture content (MC) at P = 0.05. Moisture content curve represents the average of two cultivars and 2 yr. Seed dry weight 89 and 90 curves are average of two cultivars.
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Fig. 2. Seed dry weight and moisture content of four winter canola cultivars during seed development in 1989 and 1990. Error bars indicate the standard deviation from means of seed dry weight (DWT) and moisture content (MC) at P = 0.05. Moisture content and seed dry weight curves represent the average of four cultivars.
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Table 1. Analysis of variance of seed dry weight (DWT) and moisture content (MC) of two spring and four winter canola cultivars combined over years.
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Table 2. Seed color at different stages of seed development of winter and spring canola cultivars averaged over 2 yr.
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The change in seed moisture content of spring cultivars was significantly affected by sampling date (time from initial pod development). Neither cultivar, nor year, nor any of the interactions involving those two main effects were significant (Table 1). However, year, sampling date, and the interaction between them significantly affected seed moisture content of winter cultivars (Table 1). Variation in dry matter and moisture content among cultivars and between years may be due to the difference in temperatures and/or precipitation during the two growing seasons. Generally, canola cultivars used in this study reached PM when seed moisture ranged between 203 to 360 g kg-1 (data not shown). The weather conditions during seed development and maturation can cause significant change in the moisture content level at PM. Thus, it is difficult to accurately estimate the date of PM from MC measurements alone. This contrasts with the situation in soybean, where MC represented a more accurate indicator of PM than seed dry weight (Fraser et al., 1982). At the very early stages of pod growth, the moisture content of seeds was so high that it was difficult to separate them from the pod without damage (Fig. 1 and 2). The greatest gain in dry weight occurred during the third and fourth weeks after pod formation with little change during the last 2 wk of seed maturity (Fig. 1 and 2). This suggested that maximum translocation of assimilates from the source (e.g., leaves) to the sink (i.e., seeds) occurred 21 to 28 d after the beginning of seed formation in spring cultivars. In 1989, seeds of the winter cultivar Cascade reached maximum dry weight approximately 6 wk after pod formation with seed moisture content of 306 g kg-1. This may be due to the longer growing season of the winter cultivars. The dry weight remained unchanged during the last 2 wk. All winter cultivars behaved similarly in this respect.
Although physiological markers such as seed dry weight and moisture content are reliable indicators of PM in many crops, the loss of the green colors in the pod and seed, along with change in seed texture are practical and rapid field indicators of PM for canola. At HM, the seed moisture content was near 100 g kg-1 and the seeds were hard and rattled inside the pod. The pod color of the six canola cultivars at HM was completely yellow to light brown and the seed color was brown or dark brown to black, depending on the cultivar. Canola can thus be directly combined (without swathing) when SMC is approximately 100 g kg-1.
Seed dry matter results suggest that canola can be harvested 2 wk before reaching HM without significant reduction in dry weight. However, seed moisture content at this stage is too high for direct harvesting, threshing, or storage without further drying. Swathing can be a suitable practice in this situation. According to Loof (1972), threshing can be done when the average seed MC reaches about 200 g kg-1. However, a yield reduction may be expected at 200 g kg-1 MC if the crop is mechanically combined without swathing because of difficulties in separating seeds from pods during threshing (data not shown).
Seed Quality Characteristics
Spring cultivars behaved similarly in standard germination, accelerated aging, and cold tests in both years (P > 0.05). Seed development stage (i.e., seed age), and the interactions between seed age and cultivar were significant for all tests (Table 3). This reflected the difference between cultivars in seed quality at different stages of maturity. The influence of environmental conditions on seed quality was reflected by the significant year effect in germination and accelerated aging test results in both spring and winter cultivars (Table 3). Year, cultivar, seed age, and the interactions had significant effects on seed quality of winter cultivars as measured by the three tests (Table 3). Generally, the seed quality of all canola cultivars improved from PM to HM (Fig. 3 and Table 4), contrary to the general reports that seeds attain maximum quality (germination and vigor) at PM (Miles et al., 1983). This may be explained by the physiological changes (e.g., hormonal mechanism) that occur after PM, which can promote germination (Khan, 1971). Both germination capacity and seed vigor as indicated by the SGT, CT, and AAT (Fig. 3 and Table 4) support the premise that seeds develop germination capacity ahead of vigor (Delouche, 1980). For example, the germination percentage of Topas at PM was 79%, whereas the vigor test results were 69 and 70% for CT and AAT, respectively. Similar trends were observed for Westar (Fig. 3) and the winter cultivars (Table 4). However, as the seeds reached HM, the gap between germination capacity and vigor was narrowed (Fig. 3 and Table 4). Apparently, the temperature and relative humidity stresses in the CT and the AAT tests were not enough to affect seed vigor at HM, suggesting that seeds at HM reached maximum potential quality (viability and vigor). However, as the initial quality decreases (e.g., because of adverse storage or growing conditions, etc.), seeds become more susceptible to stress, such as low–high temperatures, drought, soil microorganism, etc. (Elias and Copeland, 1994). Differences among cultivars and conditions at which seeds developed affected the level of seed vigor (Tables 3, 4, and Fig. 3). For example, before seeds of spring cultivars reached PM, the AAT results of Topas and Westar were 19 and 25%, respectively, whereas they were 64 and 72% for the SGT for the same cultivars, respectively. This may be because seeds were immature, which make them sensitive to any stress such as high temperature and relative humidity in the AAT. However, Westar had higher germination and vigor than Topas (Fig. 3). These results confirmed the correlation between immature seeds and low quality and that immature seeds can be easily affected by stress conditions. The results also showed that the seed quality tests used in this study were suitable in assessing the viability and vigor of canola seeds (Elias and Copeland, 1997).
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Table 3. Analysis of variance of seed quality of two spring and four winter canola cultivars as measured by standard germination (SGT), cold (CT), and accelerated aging (AAT) tests.**
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Fig. 3. Germination of two spring canola cultivars using the standard germination test (SGT), cold test (CT), and accelerated aging test (AAT) during seed development averaged over 2 yr. Error bars indicate the standard deviation from means of the three tests at P = 0.05.
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Table 4. Standard germination test, cold test, and accelerated aging test results of four winter canola cultivars during seed development in 1989 and 1990.
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In summary, harvesting soon after physiological maturity but before harvest maturity may help avoid the effects of unfavorable weather and possible frosts that can occur before complete maturation and permit timely planting of the next crop. However, if canola is harvested at an earlier stage than full maturity (HM), swathing and/or chemical desiccation might be used to even out and speed up the maturation of seeds. Otherwise, difficulties in threshing and storage can be expected. Swathing is a common practice in Canada and North Dakota. Although canola can be harvested 2 to 3 wk before HM without significant reduction in dry weight and yield, it is better to leave the crop until seeds reach HM for maximum potential quality (i.e., germination and vigor), if the purpose of planting is for seed (e.g., foundation or certified seed).
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ACKNOWLEDGMENTS
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The authors thank Dr. G.L. Hosfield for helpful suggestions on the manuscript.
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REFERENCES
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- Afuakwa, J.J., and R.K. Crookston. 1984. Using the kernel milk line to visually monitor grain maturity in maize. Crop Sci. 24:687–691.[Abstract/Free Full Text]
- Association of Official Seed Analysts. 1988. Rules for testing seeds. J. Seed Technol. 6:1–126.
- Carter, M.W., and C.G. Poneleit. 1973. Black layer maturity and filling period variation among inbred lines of corn (Zea mays L.). Crop Sci. 13:436–439.
- Crookston, R.K., and D.S. Hill. 1978. A visual indicator of the physiological maturity of soybean seed. Crop Sci. 18:867–870.[Abstract/Free Full Text]
- Daynard, T.B., and W.G. Duncan. 1969. The black layer and grain maturity in corn. Crop Sci. 9:473–476.[Abstract/Free Full Text]
- Delouche, J.C. 1974. Maintaining soybean seed quality. p. 46–62. In Soybean: Production, marketing, and use. Bull. Y-69. NFDC, TVA, Muscle Shoals, AL.
- Delouche, J.C. 1980. Environmental effects on seed development and seed quality. HortScience 15:775–780.
- Eastin, J.D., J.H. Hultquist, and C.Y. Sullivan. 1973. Physiologic maturity in grain sorghum. Crop Sci. 13:175–178.[Abstract/Free Full Text]
- Elias, S.G., and L.O. Copeland. 1994. The effect of storage conditions on canola seed quality. Seed Technol. 18:21–29.
- Elias, S.G., and L.O. Copeland. 1997. Evaluation of seed vigor tests for canola. Seed Technol. 19:78–87.
- Fenwick, K.A. 1988. Seed production of agricultural crops. John Wiley & Sons, New York, NY.
- Fraser, J., D.B. Egli, and J.E. Leggett. 1982. Pod and seed development in soybean cultivars with differences in seed size. Agron. J. 74:81–85.
- Gibkpi, P.J., and R.K. Crookston. 1981. A whole-plant indicator of soybean physiological maturity. Crop Sci. 21:469–471.[Abstract/Free Full Text]
- Khan, A.A. 1971. Cytokinins: Permissive role in seed germination. Science (Washington, DC) 171:853–859.[Free Full Text]
- Knittle, K.H., and J.S. Burris. 1976. Effect of kernel maturation on subsequent seedling vigor in maize. Crop Sci. 16:851–855.[Abstract/Free Full Text]
- Loof, B. 1972. Cultivation of rapeseed. p. 49. In L.A. Appleqvist and R. Ohlson (ed.) Rapeseed: Cultivation, composition, processing and utilization. Elsevier, Amsterdam.
- McDonald, M.B., Jr., and B.R. Phaneendranath. 1978. A modified accelerated aging seed vigor test for soybean. J. Seed Technol. 3:27–37.
- Miles, D.F., D.M. TeKrony, and D.B. Egli. 1983. Effect of the desiccation environment and seed maturation on soybean seed quality. p. 119. In Agronomy abstracts. ASA, Madison, WI.
- Munsell, A.H. 1977. Munsell color charts for plant tissues. 2nd ed. Macbeth Div. of Kollmorgan Corp., Baltimore, MD.
- Oplinger, E.S., L.L. Hardman, E.T. Gritton, J.D. Doll, and K.A. Kelling. 1989. Alternative field crops manual, canola (rapeseed). Ext. Bull. Nov. 1989. Univ. of Wisconsin, Madison, WI.
- Salunkhe, D.K., and B.B. Desai. 1986. Postharvest biotechnology of oilseeds. CRC Lewis Publ., Boca Raton, FL.
- TeKrony, D.M., D.B. Egli, J. Balles, T. Pfeiffer, and R.J. Fellows. 1979. Physiological maturity in soybean. Agron. J. 71:771–775.[Abstract/Free Full Text]
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TOP
ABSTRACT
INTRODUCTION
