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Yield and canopy development of field grown potato plants derived from synthetic seeds
Europ. J. Agronomy 22 (2005) 175–184
Yield and canopy development of field grown potato plants derived from synthetic seeds Aggrey Bernard Nyende∗ , Siegfried Schittenhelm, Gunda Mix-Wagner, Jörg Michael Greef Federal Agricultural Research Centre (FAL), Institute of Crop and Grassland Science, Bundesallee 50, D-38116 Braunschweig, Germany Received 3 October 2003; received in revised form 30 January 2004; accepted 12 February 2004
Abstract Although detailed growth studies and yield analysis are common for the conventional seed tuber plants in potato, their application to synthetic seeds does not exist. Growth measurements of potato plants from two field grown cultivars of synthetic seeds in comparison to their respective seed tubers were conducted during two seasons in the town of Braunschweig, Germany. In the 2001 season, using pre-germinated synthetic seeds, maximum plant height was recorded at 98 days after planting (DAP), 21 days later than for seed tuber plants. Maximum intercepted photosynthetic active radiation (IPAR) for plants derived from synthetic seeds was 96.4% for the cv. Désirée and 93.2% for the cv. Tomensa, while for plants derived from the seed tubers was 98.4% for the cv. Désirée and 96.6% for the cv. Tomensa. On average, plants from the synthetic seeds matured 40 days later than plants from the seed tubers. The plants derived from the synthetic seeds had lower tubers yields of 103 and 105 dt ha−1 , while the plants derived from seed tubers yielded 152 and 147 dt ha−1 for the cvs. Désirée and Tomensa, respectively. In the 2002 season, using transplanted synthetic seed seedlings, maximum plant height was recorded at 84 DAP for both seed types. Maximum IPAR for plants derived from synthetic seeds was 92.5% for the cv. Désirée and 95.8% for the cv. Tomensa, while for plants derived from the seed tubers was 92.4% for the cv. Désirée and 96.5% for the cv. Tomensa. On average, the synthetic seed seedling plants matured approximately 12 days later than plants from the seed tubers. Tuber yields of 104 and 127 dt ha−1 were recorded for the plants derived from synthetic seeds, while 101 and 126 dt ha−1 for the plants from seed tubers of the cvs. Désirée and Tomensa, respectively. The results demonstrated that synthetic seeds can be an alternative propagating method. Their yield potential, similar to that of seed tuber plants is also dependent on fast canopy growth, early attainment of maximum leaf area for maximum light interception, and completing growth within the available growing season. © 2004 Elsevier B.V. All rights reserved. Keywords: Synthetic seeds; Yield; Canopy growth; Light interception (IPAR); Potato
1. Introduction
∗
Corresponding author. Present address: P.O. Box 31382, Nairobi, Kenya. E-mail address: [email protected] (A.B. Nyende).
Potato is the fourth most important food crop in the world, with annual production approaching 300 million t from a production area of 18 million ha (Manrique, 2000). Potato contains high quality
1161-0301/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.eja.2004.02.003
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protein, substantial amounts of essential vitamins, minerals, trace elements, and very low fat content (CIP, 2002). Due to its starch value it provides three to four times more calories per unit area as compared to cereals and is an ideal food for supplementing cereal-based diets (Khan, 1993). Conventionally, commercial important potato cultivars are clonally propagated by tubers. However, this propagation method has been one of the most important disadvantages of potato. As the crop is propagated vegetatively in the field, tissue borne viruses, fungi, and bacteria that have infected the crop during the previous seasons accumulate, this ultimately leads to significant losses in yield and tuber quality via seed degeneration. Approximately 50–70% (Roy, 1996) and 20–30% (Struik and Wiersema, 1999) of the total input cost is spent on seed tuber as a major input in the less developed and in developed countries, respectively. In addition to the high cost of the seed tuber, the potato propagation is characterised by a period of seed dormancy and low rates of seed multiplication. Under normal conditions, the rate of multiplication is 4–6 times and at best about 20 times under expert supervision in favourable growing areas. Moreover, the planting material is bulky, requiring a seeding rate of 2.0–2.5 t ha−1 and large storage space. Potato is plagued by field and seed storage problems, which limit sustainable and profitable potato production (Secor and Gudmestad, 1999). A continuous combination of planting resistant cultivars, clean healthy seeds, crop rotation, fungicides, and cultural practices are necessary for disease control. As such, the primary concern in any potato production system is to obtain initial propagating material, which is free of diseases, especially viruses (Georgakis et al., 1997). The use of synthetic seeds has been long considered as a promising alternative to conventional potato propagation but little has been accomplished to develop synthetic seeds in this crop (O’Hair et al., 1987; Sarkar and Naik, 1998). Based on the idea originally proposed by Murashige (1977, 1978), synthetic seeds are defined as artificially encapsulated somatic embryos, shoot buds, cell aggregates, or any other tissue that can be used for sowing as seeds. They should possess the ability to convert into plants under in vitro or ex vitro conditions and retain this potential even after storage (Capuano et al., 1998; Hussain et al., 2000). Due to the advantages of producing virus free, genetically
identical planting material for easy handling, transportation, and storage (∅ <1 cm), as well as offering a new option for maintenance of elite germplasm, there has been a growing interest in the production and use of synthetic seed technology (Redenbaugh et al., 1987; Ganapathi et al., 1992, 2001; Padmaja et al., 1995; Sarkar and Naik, 1998; Patel et al., 2000; Nyende et al., 2002). Sarkar and Naik (1998) showed for the first time that in vitro derived potato nodal segments encapsulated in whole bead alginate capsules could be used for potato propagule production. Later, Fiegert (2001) produced synthetic seeds of potato using encapsulated shoot tips in the hollow alginate beads. Nyende et al. (2003) also using potato shoot tips encapsulated in the hollow alginate beads recorded germination rates of 100% on non-sterile soil in the greenhouse and a 100% survival rate after 270 days storage. Despite the apparent usefulness of synthetic seeds, there has been no attempt to utilise the material for enhancement of potato production, especially in countries with poor seed tuber production capacities. In as much as growth characteristics of seed tuber as governed by genetic traits and management practices have already been modelled (Struik and Wiersema, 1999), there is very little information on production and field growth performance of synthetic seeds in potato. In this paper, results of yield and the growth characteristics of field grown plants derived from synthetic seeds of two potato cultivars vis-à-vis with that of their respective conventional seed tubers are outlined.
2. Materials and methods 2.1. Plant material and propagation The genetic materials used in this study were provided by Nordkartoffel Zuchtgesellschaft, Ebstorf, Germany (cv. Tomensa), and Saatzucht Fritz Lange, Bad Schwartau, Germany (cv. Désirée). The materials were transferred into in vitro and routinely propagated from their meristematic shoot tips and leaf buds, with each cycle lasting 3–4 weeks. The plantlets were raised on Murashige and Skoogs (MS) basal solid medium (Murashige and Skoog, 1962) containing 10 g l−l sucrose and 3.4 g l−l gelrite (Roth, Karlsruhe, Germany) maintained at pH 5.8, at 20 ± 2 ◦ C and a
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14 h photoperiod with 40 mol m−2 s−1 radiation provided by overhead white fluorescent 25 W PHILIPS lamps. 2.2. Preparation and encapsulation of shoot tips Using a stereo-microscope placed on a clean bench, the shoot tips of the in vitro propagated plantlets (≈5 cm long) were dissected and trimmed to 3–4 mm in size using hypodermic needles as described by Schäfer-Menuhr et al. (1996) and then precultured on MS basal solid medium for 2 days. Thereafter, the precultured shoot tips were mixed with 10 ml of a 5% calcium chloride solution containing 2.5% carboxymethylcellulose (Aqualon, Düsseldorf, Germany) under sterile conditions. Using a 10 ml hypodermic syringe, the suspension was dropped in 500 ml of a 0.5% sodium alginate (Protan, Norderstedt, Germany), solution stirred in a 1-l beaker. After 20 min of polymerisation (gelation), the alginate hollow beads were collected on a sterile sieve and washed twice with 500 ml deionised water by running the water quickly through the sieve. The hollow beads were then transferred into a stirred 400 ml 1% calcium chloride solution containing 0.4 g of the fungicide carbendazim (BASF, Limburgerhof, Germany). Thereafter the capsules were incubated in a MS basal liquid medium without gelrite for 1 h while continuous stirring. The synthetic seeds were then cultured on MS basal solid medium for 14 days before use. 2.3. Experimental site The field experiments were conducted in the 2001 and 2002 growing seasons on a sandy loam soil at the Federal Agricultural Research Centre, Braunschweig, Germany (52◦ 17 N latitude, 10◦ 26 E longitude, altitude 80 m). Climatic data were obtained from the Agro-meteorological Research Station of the German Weather Service located on the research site. The monthly daily mean temperature was 14.3 ◦ C in both seasons, mean relative humidity was 76.6% in 2001 season and 77.6% in 2002 season, while the total precipitation was 600 mm in the 2001 and 823 mm in the 2002 season. In each year, soil samples were taken in April, just before N-fertilization to determine the residual mineral N in the soil. An average of 64
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and 45 kg N ha−1 was measured in 2001 and 2002, respectively. 2.4. Experimental layout, treatments, and crop husbandry In both seasons, seed tubers (ranging in size from 53 to 67 g each) of the cultivars Désirée and Tomensa were pre-sprouted 2 weeks prior to planting. In the 2001 season, the synthetic seeds were either cultured on MS basal solid medium for 2 weeks, in a growth chamber at 20 ± 2 ◦ C and a 14 h light photoperiod provided by overhead white fluorescent Philips light, before planting or pre-germinated on MS basal solid medium for 4 weeks and directly sown into the field, after attaining an average shoot length of 2 cm. The seedbed was finely prepared using a rotovator and the soil fertilised with 100 kg N ha−1 , 70 kg P ha−1 , and 140 kg K ha−1 . Planting was done on the 17 May 2001. Each tuber plot consisted of four rows, 0.6 m apart and 0.3 m between plants in the rows giving a density of 5.6 plants m−2 . The synthetic seed plots had four rows planted at 0.6 m × 0.1 m giving a density of 16.7 plants m−2 , which after establishment were thinned to 5.6 plants m−2 . The synthetic seeds were covered with translucent plastic cups to prevent dehydration and to maintain high humidity for 1–2 weeks. The tubers were immediately ridged after planting, whereas the synthetic seeds were sown in furrows on flat beds and ridging done later when the plants were 15 cm tall. The field was irrigated on the planting day and then every second day to ensure a wet seed bed, until the synthetic seed plants had established. All the plots were weeded regularly by hand. For protection against late blight (Phythopthora infestans) the potato crop was sprayed alternatively with Manex (Pentathlon active ingredient) and Shirlan (Fluazinam active ingredient) every fortnight. In the 2002 season, the synthetic seeds were germinated in trays containing soil–compost mixture in the greenhouse. Each tray had 72 seedlings. The trays were covered with translucent boxes for 1 week to maintain humidity. The field seedbed was finely prepared for planting with a rotovator and the soil fertilised with 140 kg N ha−1 , 140 kg K ha−1 , 70 kg P ha−1 . Planting was done on the 22 May 2002. The seedlings after 2 weeks growth in the greenhouse
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(at ≈10 cm height) were transplanted to the field. Each plot consisted of eight rows, 0.625 m apart and 0.3 m between plants in the rows, having 64 tubers per plot giving a plant density of 5.3 plants m−2 . The synthetic seed seedlings were overseeded three times, and thinned to 5.3 plants m−2 after establishment. The tubers were ridged immediately after planting, while the synthetic seed seedlings were sown in furrows and ridged when the plants had attained 15 cm height. The field was irrigated on the planting day and twice weekly until establishment. All the plots were weeded regularly by hand. For protection against late blight, the potato crop was sprayed alternatively with Tattoo (Chlorothalonil and Propamocarb-HCl active ingredients) and a mixture of Mancozeb (Dithiocarbarnate active ingredient) with Propamocarb-HCl. 2.5. Data measurements and statistical analysis The following data were recorded for statistical analysis in the 2001 season. Emergence was scored daily until crop establishment and the germination percentage of each cultivar was calculated. The survival percentage of the pre-germinated synthetic seeds was also calculated. Plant height was measured weekly from the fourth week after planting. The IPAR through the established potato crop canopy was measured weekly from the third week (at around mid day) up to crop maturity, using a SunScan quantum line sensor connected to a point quantum sensor (Delta-T Devices Ltd, Cambridge, UK) and the IPAR percentage calculated as difference between the incident radiation and the radiation transmitted through the canopy expressed as a percentage of the incident radiation. For every weekly measurement, 12 readings were made per plot, by diagonally placing the SunScan ground level, across the plant rows in the plots. Time duration to 50% flowering (half the number of plants had flowered) and to 80% plant maturity (when the plants had 80% of their leaves yellow) was also recorded. At harvest, the tuber yield (dt ha−1 ) was calculated from eight competitive plants selected from the middle two rows, while the tuber dry matter was determined by oven drying at 65 ◦ C to constant weight. The nitrogen content of the tuber dry matter was determined using the Kjeldhal method. The tuber dry matter starch content was measured using the Ewers method, where 2.5 g of the oven dried sample
was mixed with 50 ml of 1.12% hydrochloric acid and submerged in an agitated hot water bath for 15 min, removed and 20–30 ml cold water quickly added followed by 10 ml of 4% tungstophosphoric acid sodium salt. This mixture was then filtered and the filtrate measured in a polarimeter for starch content calculation. In the 2002 season, the same parameters as described in the 2001 season were again measured. At harvest, the tuber yield (dt ha−1 ) was calculated from 12 competitive plants selected from the middle two rows. In all experiments, treatments were arranged in a completely randomised design. Two replications were maintained in the 2001 season and four replications in the 2002 season. Replications were treated as random factors whereas cultivar and seed type were regarded as fixed factors. The data were analysed by one or two way analysis of variance using the PLABSTAT computer program (Utz, 1991). The least significant difference at the P < 0.05 level was used for comparison of the treatment means. Sample means and standard deviations were also computed. Data for each year were analysed separately.
3. Results 3.1. 2001 growing season 3.1.1. Field growth The germination rates varied among the seeds types with the synthetic seeds having rates as low as 18% for both cultivars, while the seed tubers had 100% germination. The synthetic seeds due to poor germination rates were not further monitored for data collection. All the germinated seed tubers survived to maturity, while the survival frequency for the pre-germinated synthetic seeds sown into the field was 92% for the cv. Désirée and 86% for the cv. Tomensa, after which thinning was done to 5.6 plants m−2 . The plants derived from the seed tubers established fast and grew taller than the plants from the pre-germinated synthetic seeds (Fig. 1), clearly observed from the 21 days after planting (DAP). The maximum plant height for the plants derived from the synthetic seeds was recorded at 98 DAP and for the seed tubers plants at 77 DAP. Plants derived from the synthetic seeds flowered and matured late, with maturity recorded 32 days later
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Table 1 Mean period to flowering and maturity of plants derived from the pre-germinated synthetic seeds and seed tubers in the 2001 season Cultivar/seed type
Time to flowering (days)
Time to maturity (days)
D´esir´ee Tuber Synthetic
63 84
112 152
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
48 80 69 5
105 137 127 4
Comparison of means by least significant difference at P < 0.05 within the columns. a
Fig. 1. Height of plants derived from pre-germinated synthetic seeds and seed tubers of the cultivars D´esir´ee and Tomensa in the 2001 season.
in the cv. Tomensa and 40 days later in cv. Désirée (Table 1). 3.1.2. Canopy development The plants from the seed tubers developed and attained maximum IPAR earlier than the plants derived from the pre-germinated synthetic seeds (Fig. 2). Correspondingly, the IPAR measurements for the plants derived from the seed tubers began earlier at 25 DAP compared to 41 DAP for the plants derived from the synthetic seeds. Mean maximum IPAR was 98.4% for the seed tuber plants of cv. Désirée and 96.4% for the plants derived from the synthetic seed of the cv. Désirée. For the cv. Tomensa, maximum IPAR was
Fig. 2. Time courses for intercepted photosynthetic active radiation (%) of the plants derived from pre-germinated synthetic seeds and seed tubers in the 2001 season.
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Table 2 Tuber yield (dry matter), tubers per plant, tuber starch content, and tuber nitrogen content of plants derived from pre-germinated synthetic seeds and seed tubers in the 2001 season Cultivar/seed type
Tuber yield (dt ha−1 )
Tubers per plant (no.)
Tuber starch content (% in DM)
Tuber nitrogen content (% in DM)
D´esir´ee Tuber Synthetic
152.0 103.4
26.9 29.0
78.4 73.7
1.2 1.5
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
147.0 105.0 126.9 37.5
18.4 16.5 22.7 3.4
80.4 78.5 77.7 2.4
1.2 1.5 1.4 0.2
a
Comparison of means by least significant difference at P < 0.05 within the columns.
96.6% for the seed tuber plants and 93.2% for the plants derived from the synthetic seeds. 3.1.3. Yield Significant difference was observed in the tuber yield, with the plants derived from the synthetic seeds yielding less (Table 2). No significant differences were observed in the number of tubers per plant in both the cultivars and seed types. No significant difference was observed in the starch content within the cv. Tomensa for both seed types, while plants from the seed tuber cv. Désirée had a significantly higher starch content. For both cultivars, the tubers harvested from the plants derived from the synthetic seeds had significantly higher nitrogen content.
tubers and the transplanted synthetic seed seedlings survived to maturity. A higher number of primary branches were recorded for plants derived from seed tubers (Table 3). The plants derived from the synthetic seeds consistently produced only single stems. The plants derived from the synthetic seeds flowered 10 days later than those from the seed tubers. Correspondingly, plants from the synthetic seeds matured later than those from the seed tubers. This deviated from the 2001 season where the synthetic seed plants matured 32–40 days later. Further, unlike the 2001 season, the maximum plant height was recorded at the same time (84 DAP) for both seed types and cultivars (Fig. 3). Establishment of the synthetic seed seedlings was comparatively faster compared to the pre-germinated synthetic seeds (Figs. 1 and 3) in the previous season.
3.2. 2002 growing season 3.2.1. Field growth Mean germination among the seed tubers was 98% for Désirée and 100% for Tomensa. All the germinated
3.2.2. Canopy development The IPAR measurements began at the same time (28 DAP) for all seed types (Fig. 4), unlike in the 2001 season where by measurements for synthetic seed plants
Table 3 Number of branches per plant, time to flowering, and time to maturity of plants derived from synthetic seed seedlings and seed tubers in the 2002 season Cultivar/seed type
Branches per plant (no.)
Time to flowering (days)
Time to maturity (days)
Tomensa Tuber Synthetic
4.1 1.1
38 47
115 127
D´esir´ee Tuber Synthetic Mean L.S.D.0.05 a
4.2 1.1 2.6 0.5
51 61 49 3
121 134 124 2
a
Comparison of means by least significant difference at P < 0.05 within the columns.
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Fig. 3. Height of plants derived from synthetic seed seedlings and seed tubers of cultivars D´esir´ee and Tomensa in the 2002 season.
began at a later period. Maximum IPAR was 92.4% for the plants from the seed tubers and 92.5% for the plants derived from the synthetic seeds of the cv. Désirée. For the cv. Tomensa, maximum IPAR calculated was 96.5% for the plants derived from the seed tubers and 95.8% for the plants derived from the synthetic seeds. 3.2.3. Yield No significant differences were observed in the tuber yield per hectare and nitrogen content for all seed types within the cultivars (Table 4). No differences were observed in the number of tubers per plant for the cv. Désirée, while the plants from the synthetic
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Fig. 4. Time courses for the intercepted photosynthetic active radiation (%) of the synthetic seed seedlings and tubers in the 2002 season.
seeds of the cv. Tomensa had more tubers per plant than those from the seed tubers. While no significant difference was observed in the cv. Tomensa, synthetic seeds of the cv. Désirée had significantly higher starch content than the seed tuber of the cv. Désirée.
4. Discussion In young tuber grown potato plants the rate of growth and leaf expansion is closely related to available carbohydrate, initially sourced from the seed potato reserves, increasingly supplemented, and then
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Table 4 Tuber yield (dry matter), tubers per plant, tuber starch content, and tuber nitrogen content of plants derived from synthetic seed seedlings and seed tubers in the 2002 season Cultivar/seed type
Tuber yield (dt ha−1 )
Tubers per plant (no.)
Tuber starch content (% in DM)
Tuber nitrogen content (% in DM)
D´esir´ee Tuber Synthetic
101.0 104.4
16.7 14.5
66.1 68.7
1.1 1.0
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
126.0 127.2 114.7 23.3
13.2 18.2 15.7 2.3
61.6 63.3 64.9 2.4
1.1 1.1 1.1 NS
a
Comparison of means by least significant difference at P < 0.05 within the columns.
reduced by photosynthates (Spedding et al., 1981). After the carbohydrate reserve in the seed is depleted, the rate of growth is strongly influenced by the environment, especially temperature, water, and available nitrogen. For the synthetic seed plants, the young potato plant is totally initially dependent on the available little water and nutrients in the capsule, available nutrients in soil, and eventually on the photosynthates from the developing leaves. Moreover, plants from synthetic seeds require time to adapt to the new environmental conditions, to switch from the in vitro physiological behaviour, and to develop a strong-vibrant root system in order to absorb soil nutrients. This explains the observed slow initial growth (Figs. 1 and 3). The lag phase, especially in the 2001 season during the initial growth phase could be attributed to transplant shock and a long adaptation period. Coupled to this, the plants from the synthetic seeds having an initially low IPAR, less light interception, less photosynthates manufactured, hence resulting in slower plant development. The late establishment and slower development was reflected in delayed flowering, maturation (40 days later), and comparatively lower yields, especially in the 2001 growing season. Planting synthetic seed seedlings in the 2002 season and having a nursery acclimatisation stage reduced the adaptation period, increased field vigour, and reduced the delay in maturation period from 40 to 12 days. The seedlings were transplanted after having developed a strong root system, having comparatively large leaves for photosynthesis, thus minimising the transplant shock. The pre-germinated
seeds in the 2001 season were sown at 2 cm height, while the synthetic seedlings in the 2002 season were transplanted at 10 cm height. These variations in plant size at delivery time into the field, further explains the differences observed in the establishment and growth behaviour of the plants derived from synthetic seeds. According to Allen and Scott (1980) in the absence of disease and drought, the essential objective in the production of potato is to maximise radiation interception, since the crop growth rate should be proportional to the rate of photosynthesis and hence net assimilation which depends on the amount of light energy intercepted by the green foliage. Due to delayed crop establishment, slower canopy development, and delayed maturity, the IPAR measurements in the plants derived from the synthetic seeds began later (41 DAP) than for the plants derived from the seed tubers (25 DAP) in the 2001 season. The plants from the seed tubers had higher values for the IPAR up to the 67 DAP. This implies that tuber growth and bulking began earlier in the plants from the seed tubers with enough green foliage for light interception attained earlier in the growing season. Using transplanted synthetic seed seedlings in the 2002 season, the measurements began on the same day (28 DAP). An initial seedling nursery stage improves field establishment and growth of the synthetic seed plants as shown by the comparatively close to fit IPAR development (Fig. 4) in the 2002 season. Tuber bulking and growth began at more or less the same time with both seed types maximising on the available growth season.
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Low tuber yields recorded for the plants from the synthetic seeds (Table 2) in the 2001 season was due to the late establishment and slower development. As a result, the plants behaved as late instead of early maturing cultivars, thus not maximising production during the growing period, which is limited in the temperate zones especially since light interception needs to be maximised in the early growing season of May and June (Manrique et al., 1990). Yields similar to conventional tubers (Table 4) were obtained when using transplanted seedlings, which had the same developmental pattern as the plants derived from the seed tubers. As observed in the 2001 season, slow canopy development implies that the plants from the synthetic seeds could not complete their growth cycle. Consequently the opportunity to make full use of the available growing season was lost. This could account for the lower yielding ability when compared with plants from the seed tubers and the synthetic seed seedlings. Moreover, the lower tuber yield, in the 2001 season, could be attributed to late and lower maximum IPAR values of plants derived from the pre-germinated synthetic seeds, leading to comparatively less-complete interception of solar radiation. As all crops, the potato is closely related to the amount of radiation intercepted during the growing season which is dependent on the leaf area, its duration, and the efficiency with which the intercepted radiation is converted into dry matter (Struik and Wiersema, 1999). Therefore in principle, in a growing season that is limited in its duration, fast development to full canopy is required to obtain high yields. In both seasons, it was observed that the plants from synthetic seeds produced only single stems compared to multi-stems from plants from the seed tubers (Table 3). Single-stemmed plants tend to have lower yield ability under short growing season, due to a lower rate of canopy development and ultimately lower amount of intercepted radiation (Struik and Wiersema, 1999). But as observed in a greenhouse experiment (Nyende, 2003), plants from the synthetic seeds had a higher number of leaves per stem and this could compensate for having less number of stems. This ultimately, may explain the similarity in the maximum IPAR values between the seed types, irrespective of the differences observed in number of stems per plant, leaf duration, and canopy development.
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5. Conclusions Based on these results it is clear that synthetic seed capsules can be used as propagating materials to produce disease free materials especially for less developed countries in Sub-Saharan Africa, but that a seedling nursery stage for the synthetic seeds seems not only necessary to enhance field establishment, but to improve field growth which is necessary for high tuber yields. While the slow field establishment of the pre-germinated synthetic seeds in the 2001 season resulted in slow growth, delayed maturation, and hence low tuber yields (behaving as a late maturing crop in a limited growing season), using transplanted seedlings in the 2002 season improved field establishment and growth, giving tuber yields comparable to those of seed tubers. Their yield potential, similar to that of seed tuber plants is also dependent on fast canopy growth, early attainment of maximum leaf area, and completing growth within the available growing season. In addition, low field germination (less than 18% in the 2001 season) of directly sown synthetic seeds indicates that further germination improvement research needs to be done. Conclusively, the synthetic seed capsules can be used as an alternative propagating method for seed production, especially in countries where clean healthy certified seed tubers are not readily available for farmers to use. Lastly a cost analysis of synthetic seed production coupled with large scale field trials need to be done to asses their competitiveness compared to the seed tubers.
Acknowledgements The first author thanks the German Academic Exchange Service (DAAD) for financial support. Appreciation is extended to Nordkartoffel Zuchtgesellschaft and Saatzucht Fritz Lange for providing the seed tubers. The authors are also grateful for the field technical assistance of Sabine Peickert, Martina Schabanoski, and Bernd Kahlstorf.
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Nyende, A.B., Schittenhelm, S., Mix-Wagner, G., Greef, J.-M., 2002. Synthetic potato seeds offer the potential to improve the Kenyan seed system. Landbauforschung Völkenrode 52, 141– 148. Nyende, A.B., Schittenhelm, S., Mix-Wagner, G., Greef, J.-M., 2003. Production, storability and regeneration of shoot tips of potato (Solanum tuberosum L.) encapsulated in calcium alginate hollow beads. In Vitro Cell. Dev. Biol. Plant 39, 540– 544. O’Hair, S.K., Baker, C.M., Bryan, H.H., 1987. Fluid drilling of embryos in potato improvement—a future possibility. In: Bajaj, Y.P.S. (Ed.), Biotechnology in Agriculture and Forestry: Potato, vol. 3. Springer, Berlin, pp. 487–498. Padmaja, G., Reddy, L.R., Reddy, G.M., 1995. Plant regeneration from synthetic seeds of groundnut, Arachis hypogaea L. Indian J. Exp. Biol. 33, 967–971. Patel, A.V., Pusch, I., Mix-Wagner, G., Vorlop, K.-D., 2000. A novel encapsulation technique for the production of artificial seeds. Plant Cell Rep. 19, 868–874. Redenbaugh, K., Slade, D., Viss, P., Fujii, J.A., 1987. Encapsulation of somatic embryos in synthetic seed coats. Hort. Sci. 22, 803–809. Roy, K., 1996. Low cost technology for potato production. J. Indian Potato Assoc. 23, 77–80. Sarkar, D., Naik, P.S., 1998. Synseeds in potato: an investigation using nutrient-encapsulated in vitro segments. Sci. Hort. 73, 179–184. Schäfer-Menuhr, A., Mueller, E., Mix-Wagner, G., 1996. Cryopreservation. An alternative for the long term storage of old potato varieties. Potato Res. 39, 507–513. Secor, G.A., Gudmestad, N.C., 1999. Managing fungal diseases of potato. Can. J. Plant Pathol. 21, 213–221. Spedding, C.R.W, Walsinhum, J.M., Hoxey, A.M., 1981. Biological Efficiency in Agriculture. Academic Press, London. Struik, P.C, Wiersema, S.G., 1999. Seed Potato Technology. Wageningen Pers, Wageningen, The Netherlands. Utz, H.F., 1991. PLABSTAT. A computer program for statistical analysis of plant breeding experiments, Version 2F. Institute of Plant Breeding, Seed Science and Population Genetics, University of Hohenheim, Stuttgurt, Germany.
Yield and canopy development of field grown potato plants derived from synthetic seeds Aggrey Bernard Nyende∗ , Siegfried Schittenhelm, Gunda Mix-Wagner, Jörg Michael Greef Federal Agricultural Research Centre (FAL), Institute of Crop and Grassland Science, Bundesallee 50, D-38116 Braunschweig, Germany Received 3 October 2003; received in revised form 30 January 2004; accepted 12 February 2004
Abstract Although detailed growth studies and yield analysis are common for the conventional seed tuber plants in potato, their application to synthetic seeds does not exist. Growth measurements of potato plants from two field grown cultivars of synthetic seeds in comparison to their respective seed tubers were conducted during two seasons in the town of Braunschweig, Germany. In the 2001 season, using pre-germinated synthetic seeds, maximum plant height was recorded at 98 days after planting (DAP), 21 days later than for seed tuber plants. Maximum intercepted photosynthetic active radiation (IPAR) for plants derived from synthetic seeds was 96.4% for the cv. Désirée and 93.2% for the cv. Tomensa, while for plants derived from the seed tubers was 98.4% for the cv. Désirée and 96.6% for the cv. Tomensa. On average, plants from the synthetic seeds matured 40 days later than plants from the seed tubers. The plants derived from the synthetic seeds had lower tubers yields of 103 and 105 dt ha−1 , while the plants derived from seed tubers yielded 152 and 147 dt ha−1 for the cvs. Désirée and Tomensa, respectively. In the 2002 season, using transplanted synthetic seed seedlings, maximum plant height was recorded at 84 DAP for both seed types. Maximum IPAR for plants derived from synthetic seeds was 92.5% for the cv. Désirée and 95.8% for the cv. Tomensa, while for plants derived from the seed tubers was 92.4% for the cv. Désirée and 96.5% for the cv. Tomensa. On average, the synthetic seed seedling plants matured approximately 12 days later than plants from the seed tubers. Tuber yields of 104 and 127 dt ha−1 were recorded for the plants derived from synthetic seeds, while 101 and 126 dt ha−1 for the plants from seed tubers of the cvs. Désirée and Tomensa, respectively. The results demonstrated that synthetic seeds can be an alternative propagating method. Their yield potential, similar to that of seed tuber plants is also dependent on fast canopy growth, early attainment of maximum leaf area for maximum light interception, and completing growth within the available growing season. © 2004 Elsevier B.V. All rights reserved. Keywords: Synthetic seeds; Yield; Canopy growth; Light interception (IPAR); Potato
1. Introduction
∗
Corresponding author. Present address: P.O. Box 31382, Nairobi, Kenya. E-mail address: [email protected] (A.B. Nyende).
Potato is the fourth most important food crop in the world, with annual production approaching 300 million t from a production area of 18 million ha (Manrique, 2000). Potato contains high quality
1161-0301/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.eja.2004.02.003
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protein, substantial amounts of essential vitamins, minerals, trace elements, and very low fat content (CIP, 2002). Due to its starch value it provides three to four times more calories per unit area as compared to cereals and is an ideal food for supplementing cereal-based diets (Khan, 1993). Conventionally, commercial important potato cultivars are clonally propagated by tubers. However, this propagation method has been one of the most important disadvantages of potato. As the crop is propagated vegetatively in the field, tissue borne viruses, fungi, and bacteria that have infected the crop during the previous seasons accumulate, this ultimately leads to significant losses in yield and tuber quality via seed degeneration. Approximately 50–70% (Roy, 1996) and 20–30% (Struik and Wiersema, 1999) of the total input cost is spent on seed tuber as a major input in the less developed and in developed countries, respectively. In addition to the high cost of the seed tuber, the potato propagation is characterised by a period of seed dormancy and low rates of seed multiplication. Under normal conditions, the rate of multiplication is 4–6 times and at best about 20 times under expert supervision in favourable growing areas. Moreover, the planting material is bulky, requiring a seeding rate of 2.0–2.5 t ha−1 and large storage space. Potato is plagued by field and seed storage problems, which limit sustainable and profitable potato production (Secor and Gudmestad, 1999). A continuous combination of planting resistant cultivars, clean healthy seeds, crop rotation, fungicides, and cultural practices are necessary for disease control. As such, the primary concern in any potato production system is to obtain initial propagating material, which is free of diseases, especially viruses (Georgakis et al., 1997). The use of synthetic seeds has been long considered as a promising alternative to conventional potato propagation but little has been accomplished to develop synthetic seeds in this crop (O’Hair et al., 1987; Sarkar and Naik, 1998). Based on the idea originally proposed by Murashige (1977, 1978), synthetic seeds are defined as artificially encapsulated somatic embryos, shoot buds, cell aggregates, or any other tissue that can be used for sowing as seeds. They should possess the ability to convert into plants under in vitro or ex vitro conditions and retain this potential even after storage (Capuano et al., 1998; Hussain et al., 2000). Due to the advantages of producing virus free, genetically
identical planting material for easy handling, transportation, and storage (∅ <1 cm), as well as offering a new option for maintenance of elite germplasm, there has been a growing interest in the production and use of synthetic seed technology (Redenbaugh et al., 1987; Ganapathi et al., 1992, 2001; Padmaja et al., 1995; Sarkar and Naik, 1998; Patel et al., 2000; Nyende et al., 2002). Sarkar and Naik (1998) showed for the first time that in vitro derived potato nodal segments encapsulated in whole bead alginate capsules could be used for potato propagule production. Later, Fiegert (2001) produced synthetic seeds of potato using encapsulated shoot tips in the hollow alginate beads. Nyende et al. (2003) also using potato shoot tips encapsulated in the hollow alginate beads recorded germination rates of 100% on non-sterile soil in the greenhouse and a 100% survival rate after 270 days storage. Despite the apparent usefulness of synthetic seeds, there has been no attempt to utilise the material for enhancement of potato production, especially in countries with poor seed tuber production capacities. In as much as growth characteristics of seed tuber as governed by genetic traits and management practices have already been modelled (Struik and Wiersema, 1999), there is very little information on production and field growth performance of synthetic seeds in potato. In this paper, results of yield and the growth characteristics of field grown plants derived from synthetic seeds of two potato cultivars vis-à-vis with that of their respective conventional seed tubers are outlined.
2. Materials and methods 2.1. Plant material and propagation The genetic materials used in this study were provided by Nordkartoffel Zuchtgesellschaft, Ebstorf, Germany (cv. Tomensa), and Saatzucht Fritz Lange, Bad Schwartau, Germany (cv. Désirée). The materials were transferred into in vitro and routinely propagated from their meristematic shoot tips and leaf buds, with each cycle lasting 3–4 weeks. The plantlets were raised on Murashige and Skoogs (MS) basal solid medium (Murashige and Skoog, 1962) containing 10 g l−l sucrose and 3.4 g l−l gelrite (Roth, Karlsruhe, Germany) maintained at pH 5.8, at 20 ± 2 ◦ C and a
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14 h photoperiod with 40 mol m−2 s−1 radiation provided by overhead white fluorescent 25 W PHILIPS lamps. 2.2. Preparation and encapsulation of shoot tips Using a stereo-microscope placed on a clean bench, the shoot tips of the in vitro propagated plantlets (≈5 cm long) were dissected and trimmed to 3–4 mm in size using hypodermic needles as described by Schäfer-Menuhr et al. (1996) and then precultured on MS basal solid medium for 2 days. Thereafter, the precultured shoot tips were mixed with 10 ml of a 5% calcium chloride solution containing 2.5% carboxymethylcellulose (Aqualon, Düsseldorf, Germany) under sterile conditions. Using a 10 ml hypodermic syringe, the suspension was dropped in 500 ml of a 0.5% sodium alginate (Protan, Norderstedt, Germany), solution stirred in a 1-l beaker. After 20 min of polymerisation (gelation), the alginate hollow beads were collected on a sterile sieve and washed twice with 500 ml deionised water by running the water quickly through the sieve. The hollow beads were then transferred into a stirred 400 ml 1% calcium chloride solution containing 0.4 g of the fungicide carbendazim (BASF, Limburgerhof, Germany). Thereafter the capsules were incubated in a MS basal liquid medium without gelrite for 1 h while continuous stirring. The synthetic seeds were then cultured on MS basal solid medium for 14 days before use. 2.3. Experimental site The field experiments were conducted in the 2001 and 2002 growing seasons on a sandy loam soil at the Federal Agricultural Research Centre, Braunschweig, Germany (52◦ 17 N latitude, 10◦ 26 E longitude, altitude 80 m). Climatic data were obtained from the Agro-meteorological Research Station of the German Weather Service located on the research site. The monthly daily mean temperature was 14.3 ◦ C in both seasons, mean relative humidity was 76.6% in 2001 season and 77.6% in 2002 season, while the total precipitation was 600 mm in the 2001 and 823 mm in the 2002 season. In each year, soil samples were taken in April, just before N-fertilization to determine the residual mineral N in the soil. An average of 64
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and 45 kg N ha−1 was measured in 2001 and 2002, respectively. 2.4. Experimental layout, treatments, and crop husbandry In both seasons, seed tubers (ranging in size from 53 to 67 g each) of the cultivars Désirée and Tomensa were pre-sprouted 2 weeks prior to planting. In the 2001 season, the synthetic seeds were either cultured on MS basal solid medium for 2 weeks, in a growth chamber at 20 ± 2 ◦ C and a 14 h light photoperiod provided by overhead white fluorescent Philips light, before planting or pre-germinated on MS basal solid medium for 4 weeks and directly sown into the field, after attaining an average shoot length of 2 cm. The seedbed was finely prepared using a rotovator and the soil fertilised with 100 kg N ha−1 , 70 kg P ha−1 , and 140 kg K ha−1 . Planting was done on the 17 May 2001. Each tuber plot consisted of four rows, 0.6 m apart and 0.3 m between plants in the rows giving a density of 5.6 plants m−2 . The synthetic seed plots had four rows planted at 0.6 m × 0.1 m giving a density of 16.7 plants m−2 , which after establishment were thinned to 5.6 plants m−2 . The synthetic seeds were covered with translucent plastic cups to prevent dehydration and to maintain high humidity for 1–2 weeks. The tubers were immediately ridged after planting, whereas the synthetic seeds were sown in furrows on flat beds and ridging done later when the plants were 15 cm tall. The field was irrigated on the planting day and then every second day to ensure a wet seed bed, until the synthetic seed plants had established. All the plots were weeded regularly by hand. For protection against late blight (Phythopthora infestans) the potato crop was sprayed alternatively with Manex (Pentathlon active ingredient) and Shirlan (Fluazinam active ingredient) every fortnight. In the 2002 season, the synthetic seeds were germinated in trays containing soil–compost mixture in the greenhouse. Each tray had 72 seedlings. The trays were covered with translucent boxes for 1 week to maintain humidity. The field seedbed was finely prepared for planting with a rotovator and the soil fertilised with 140 kg N ha−1 , 140 kg K ha−1 , 70 kg P ha−1 . Planting was done on the 22 May 2002. The seedlings after 2 weeks growth in the greenhouse
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(at ≈10 cm height) were transplanted to the field. Each plot consisted of eight rows, 0.625 m apart and 0.3 m between plants in the rows, having 64 tubers per plot giving a plant density of 5.3 plants m−2 . The synthetic seed seedlings were overseeded three times, and thinned to 5.3 plants m−2 after establishment. The tubers were ridged immediately after planting, while the synthetic seed seedlings were sown in furrows and ridged when the plants had attained 15 cm height. The field was irrigated on the planting day and twice weekly until establishment. All the plots were weeded regularly by hand. For protection against late blight, the potato crop was sprayed alternatively with Tattoo (Chlorothalonil and Propamocarb-HCl active ingredients) and a mixture of Mancozeb (Dithiocarbarnate active ingredient) with Propamocarb-HCl. 2.5. Data measurements and statistical analysis The following data were recorded for statistical analysis in the 2001 season. Emergence was scored daily until crop establishment and the germination percentage of each cultivar was calculated. The survival percentage of the pre-germinated synthetic seeds was also calculated. Plant height was measured weekly from the fourth week after planting. The IPAR through the established potato crop canopy was measured weekly from the third week (at around mid day) up to crop maturity, using a SunScan quantum line sensor connected to a point quantum sensor (Delta-T Devices Ltd, Cambridge, UK) and the IPAR percentage calculated as difference between the incident radiation and the radiation transmitted through the canopy expressed as a percentage of the incident radiation. For every weekly measurement, 12 readings were made per plot, by diagonally placing the SunScan ground level, across the plant rows in the plots. Time duration to 50% flowering (half the number of plants had flowered) and to 80% plant maturity (when the plants had 80% of their leaves yellow) was also recorded. At harvest, the tuber yield (dt ha−1 ) was calculated from eight competitive plants selected from the middle two rows, while the tuber dry matter was determined by oven drying at 65 ◦ C to constant weight. The nitrogen content of the tuber dry matter was determined using the Kjeldhal method. The tuber dry matter starch content was measured using the Ewers method, where 2.5 g of the oven dried sample
was mixed with 50 ml of 1.12% hydrochloric acid and submerged in an agitated hot water bath for 15 min, removed and 20–30 ml cold water quickly added followed by 10 ml of 4% tungstophosphoric acid sodium salt. This mixture was then filtered and the filtrate measured in a polarimeter for starch content calculation. In the 2002 season, the same parameters as described in the 2001 season were again measured. At harvest, the tuber yield (dt ha−1 ) was calculated from 12 competitive plants selected from the middle two rows. In all experiments, treatments were arranged in a completely randomised design. Two replications were maintained in the 2001 season and four replications in the 2002 season. Replications were treated as random factors whereas cultivar and seed type were regarded as fixed factors. The data were analysed by one or two way analysis of variance using the PLABSTAT computer program (Utz, 1991). The least significant difference at the P < 0.05 level was used for comparison of the treatment means. Sample means and standard deviations were also computed. Data for each year were analysed separately.
3. Results 3.1. 2001 growing season 3.1.1. Field growth The germination rates varied among the seeds types with the synthetic seeds having rates as low as 18% for both cultivars, while the seed tubers had 100% germination. The synthetic seeds due to poor germination rates were not further monitored for data collection. All the germinated seed tubers survived to maturity, while the survival frequency for the pre-germinated synthetic seeds sown into the field was 92% for the cv. Désirée and 86% for the cv. Tomensa, after which thinning was done to 5.6 plants m−2 . The plants derived from the seed tubers established fast and grew taller than the plants from the pre-germinated synthetic seeds (Fig. 1), clearly observed from the 21 days after planting (DAP). The maximum plant height for the plants derived from the synthetic seeds was recorded at 98 DAP and for the seed tubers plants at 77 DAP. Plants derived from the synthetic seeds flowered and matured late, with maturity recorded 32 days later
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Table 1 Mean period to flowering and maturity of plants derived from the pre-germinated synthetic seeds and seed tubers in the 2001 season Cultivar/seed type
Time to flowering (days)
Time to maturity (days)
D´esir´ee Tuber Synthetic
63 84
112 152
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
48 80 69 5
105 137 127 4
Comparison of means by least significant difference at P < 0.05 within the columns. a
Fig. 1. Height of plants derived from pre-germinated synthetic seeds and seed tubers of the cultivars D´esir´ee and Tomensa in the 2001 season.
in the cv. Tomensa and 40 days later in cv. Désirée (Table 1). 3.1.2. Canopy development The plants from the seed tubers developed and attained maximum IPAR earlier than the plants derived from the pre-germinated synthetic seeds (Fig. 2). Correspondingly, the IPAR measurements for the plants derived from the seed tubers began earlier at 25 DAP compared to 41 DAP for the plants derived from the synthetic seeds. Mean maximum IPAR was 98.4% for the seed tuber plants of cv. Désirée and 96.4% for the plants derived from the synthetic seed of the cv. Désirée. For the cv. Tomensa, maximum IPAR was
Fig. 2. Time courses for intercepted photosynthetic active radiation (%) of the plants derived from pre-germinated synthetic seeds and seed tubers in the 2001 season.
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Table 2 Tuber yield (dry matter), tubers per plant, tuber starch content, and tuber nitrogen content of plants derived from pre-germinated synthetic seeds and seed tubers in the 2001 season Cultivar/seed type
Tuber yield (dt ha−1 )
Tubers per plant (no.)
Tuber starch content (% in DM)
Tuber nitrogen content (% in DM)
D´esir´ee Tuber Synthetic
152.0 103.4
26.9 29.0
78.4 73.7
1.2 1.5
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
147.0 105.0 126.9 37.5
18.4 16.5 22.7 3.4
80.4 78.5 77.7 2.4
1.2 1.5 1.4 0.2
a
Comparison of means by least significant difference at P < 0.05 within the columns.
96.6% for the seed tuber plants and 93.2% for the plants derived from the synthetic seeds. 3.1.3. Yield Significant difference was observed in the tuber yield, with the plants derived from the synthetic seeds yielding less (Table 2). No significant differences were observed in the number of tubers per plant in both the cultivars and seed types. No significant difference was observed in the starch content within the cv. Tomensa for both seed types, while plants from the seed tuber cv. Désirée had a significantly higher starch content. For both cultivars, the tubers harvested from the plants derived from the synthetic seeds had significantly higher nitrogen content.
tubers and the transplanted synthetic seed seedlings survived to maturity. A higher number of primary branches were recorded for plants derived from seed tubers (Table 3). The plants derived from the synthetic seeds consistently produced only single stems. The plants derived from the synthetic seeds flowered 10 days later than those from the seed tubers. Correspondingly, plants from the synthetic seeds matured later than those from the seed tubers. This deviated from the 2001 season where the synthetic seed plants matured 32–40 days later. Further, unlike the 2001 season, the maximum plant height was recorded at the same time (84 DAP) for both seed types and cultivars (Fig. 3). Establishment of the synthetic seed seedlings was comparatively faster compared to the pre-germinated synthetic seeds (Figs. 1 and 3) in the previous season.
3.2. 2002 growing season 3.2.1. Field growth Mean germination among the seed tubers was 98% for Désirée and 100% for Tomensa. All the germinated
3.2.2. Canopy development The IPAR measurements began at the same time (28 DAP) for all seed types (Fig. 4), unlike in the 2001 season where by measurements for synthetic seed plants
Table 3 Number of branches per plant, time to flowering, and time to maturity of plants derived from synthetic seed seedlings and seed tubers in the 2002 season Cultivar/seed type
Branches per plant (no.)
Time to flowering (days)
Time to maturity (days)
Tomensa Tuber Synthetic
4.1 1.1
38 47
115 127
D´esir´ee Tuber Synthetic Mean L.S.D.0.05 a
4.2 1.1 2.6 0.5
51 61 49 3
121 134 124 2
a
Comparison of means by least significant difference at P < 0.05 within the columns.
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Fig. 3. Height of plants derived from synthetic seed seedlings and seed tubers of cultivars D´esir´ee and Tomensa in the 2002 season.
began at a later period. Maximum IPAR was 92.4% for the plants from the seed tubers and 92.5% for the plants derived from the synthetic seeds of the cv. Désirée. For the cv. Tomensa, maximum IPAR calculated was 96.5% for the plants derived from the seed tubers and 95.8% for the plants derived from the synthetic seeds. 3.2.3. Yield No significant differences were observed in the tuber yield per hectare and nitrogen content for all seed types within the cultivars (Table 4). No differences were observed in the number of tubers per plant for the cv. Désirée, while the plants from the synthetic
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Fig. 4. Time courses for the intercepted photosynthetic active radiation (%) of the synthetic seed seedlings and tubers in the 2002 season.
seeds of the cv. Tomensa had more tubers per plant than those from the seed tubers. While no significant difference was observed in the cv. Tomensa, synthetic seeds of the cv. Désirée had significantly higher starch content than the seed tuber of the cv. Désirée.
4. Discussion In young tuber grown potato plants the rate of growth and leaf expansion is closely related to available carbohydrate, initially sourced from the seed potato reserves, increasingly supplemented, and then
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Table 4 Tuber yield (dry matter), tubers per plant, tuber starch content, and tuber nitrogen content of plants derived from synthetic seed seedlings and seed tubers in the 2002 season Cultivar/seed type
Tuber yield (dt ha−1 )
Tubers per plant (no.)
Tuber starch content (% in DM)
Tuber nitrogen content (% in DM)
D´esir´ee Tuber Synthetic
101.0 104.4
16.7 14.5
66.1 68.7
1.1 1.0
Tomensa Tuber Synthetic Mean L.S.D.0.05 a
126.0 127.2 114.7 23.3
13.2 18.2 15.7 2.3
61.6 63.3 64.9 2.4
1.1 1.1 1.1 NS
a
Comparison of means by least significant difference at P < 0.05 within the columns.
reduced by photosynthates (Spedding et al., 1981). After the carbohydrate reserve in the seed is depleted, the rate of growth is strongly influenced by the environment, especially temperature, water, and available nitrogen. For the synthetic seed plants, the young potato plant is totally initially dependent on the available little water and nutrients in the capsule, available nutrients in soil, and eventually on the photosynthates from the developing leaves. Moreover, plants from synthetic seeds require time to adapt to the new environmental conditions, to switch from the in vitro physiological behaviour, and to develop a strong-vibrant root system in order to absorb soil nutrients. This explains the observed slow initial growth (Figs. 1 and 3). The lag phase, especially in the 2001 season during the initial growth phase could be attributed to transplant shock and a long adaptation period. Coupled to this, the plants from the synthetic seeds having an initially low IPAR, less light interception, less photosynthates manufactured, hence resulting in slower plant development. The late establishment and slower development was reflected in delayed flowering, maturation (40 days later), and comparatively lower yields, especially in the 2001 growing season. Planting synthetic seed seedlings in the 2002 season and having a nursery acclimatisation stage reduced the adaptation period, increased field vigour, and reduced the delay in maturation period from 40 to 12 days. The seedlings were transplanted after having developed a strong root system, having comparatively large leaves for photosynthesis, thus minimising the transplant shock. The pre-germinated
seeds in the 2001 season were sown at 2 cm height, while the synthetic seedlings in the 2002 season were transplanted at 10 cm height. These variations in plant size at delivery time into the field, further explains the differences observed in the establishment and growth behaviour of the plants derived from synthetic seeds. According to Allen and Scott (1980) in the absence of disease and drought, the essential objective in the production of potato is to maximise radiation interception, since the crop growth rate should be proportional to the rate of photosynthesis and hence net assimilation which depends on the amount of light energy intercepted by the green foliage. Due to delayed crop establishment, slower canopy development, and delayed maturity, the IPAR measurements in the plants derived from the synthetic seeds began later (41 DAP) than for the plants derived from the seed tubers (25 DAP) in the 2001 season. The plants from the seed tubers had higher values for the IPAR up to the 67 DAP. This implies that tuber growth and bulking began earlier in the plants from the seed tubers with enough green foliage for light interception attained earlier in the growing season. Using transplanted synthetic seed seedlings in the 2002 season, the measurements began on the same day (28 DAP). An initial seedling nursery stage improves field establishment and growth of the synthetic seed plants as shown by the comparatively close to fit IPAR development (Fig. 4) in the 2002 season. Tuber bulking and growth began at more or less the same time with both seed types maximising on the available growth season.
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Low tuber yields recorded for the plants from the synthetic seeds (Table 2) in the 2001 season was due to the late establishment and slower development. As a result, the plants behaved as late instead of early maturing cultivars, thus not maximising production during the growing period, which is limited in the temperate zones especially since light interception needs to be maximised in the early growing season of May and June (Manrique et al., 1990). Yields similar to conventional tubers (Table 4) were obtained when using transplanted seedlings, which had the same developmental pattern as the plants derived from the seed tubers. As observed in the 2001 season, slow canopy development implies that the plants from the synthetic seeds could not complete their growth cycle. Consequently the opportunity to make full use of the available growing season was lost. This could account for the lower yielding ability when compared with plants from the seed tubers and the synthetic seed seedlings. Moreover, the lower tuber yield, in the 2001 season, could be attributed to late and lower maximum IPAR values of plants derived from the pre-germinated synthetic seeds, leading to comparatively less-complete interception of solar radiation. As all crops, the potato is closely related to the amount of radiation intercepted during the growing season which is dependent on the leaf area, its duration, and the efficiency with which the intercepted radiation is converted into dry matter (Struik and Wiersema, 1999). Therefore in principle, in a growing season that is limited in its duration, fast development to full canopy is required to obtain high yields. In both seasons, it was observed that the plants from synthetic seeds produced only single stems compared to multi-stems from plants from the seed tubers (Table 3). Single-stemmed plants tend to have lower yield ability under short growing season, due to a lower rate of canopy development and ultimately lower amount of intercepted radiation (Struik and Wiersema, 1999). But as observed in a greenhouse experiment (Nyende, 2003), plants from the synthetic seeds had a higher number of leaves per stem and this could compensate for having less number of stems. This ultimately, may explain the similarity in the maximum IPAR values between the seed types, irrespective of the differences observed in number of stems per plant, leaf duration, and canopy development.
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5. Conclusions Based on these results it is clear that synthetic seed capsules can be used as propagating materials to produce disease free materials especially for less developed countries in Sub-Saharan Africa, but that a seedling nursery stage for the synthetic seeds seems not only necessary to enhance field establishment, but to improve field growth which is necessary for high tuber yields. While the slow field establishment of the pre-germinated synthetic seeds in the 2001 season resulted in slow growth, delayed maturation, and hence low tuber yields (behaving as a late maturing crop in a limited growing season), using transplanted seedlings in the 2002 season improved field establishment and growth, giving tuber yields comparable to those of seed tubers. Their yield potential, similar to that of seed tuber plants is also dependent on fast canopy growth, early attainment of maximum leaf area, and completing growth within the available growing season. In addition, low field germination (less than 18% in the 2001 season) of directly sown synthetic seeds indicates that further germination improvement research needs to be done. Conclusively, the synthetic seed capsules can be used as an alternative propagating method for seed production, especially in countries where clean healthy certified seed tubers are not readily available for farmers to use. Lastly a cost analysis of synthetic seed production coupled with large scale field trials need to be done to asses their competitiveness compared to the seed tubers.
Acknowledgements The first author thanks the German Academic Exchange Service (DAAD) for financial support. Appreciation is extended to Nordkartoffel Zuchtgesellschaft and Saatzucht Fritz Lange for providing the seed tubers. The authors are also grateful for the field technical assistance of Sabine Peickert, Martina Schabanoski, and Bernd Kahlstorf.
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