TropAg Notes

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Vol. 4, No. 1, 1998

Potentials and Realities

INTRODUCTION
True potato seed (TPS) technology is one viable alternative of potato propagation for the tropics. It was initially developed in China in the late fifties primarily to overcome serious constraints associated to the use of conventional seed. Since then, several features of western greenhouse technology (sowing seeds in plugs, use of growing media other than soil and sand, use of disposable containers and movable benches, and outdoor production) have been added to the current TPS technology.
ADVANTAGES AND DISADVANTAGES
There are several advantages linked to TPS such as low cost of planting materials, reduced disease and pest infestation, ease of transportation and long-term storage, and ready adaptation to new areas. But a major disadvantage of TPS is the segregation found in progenies either of cultivar crosses or following self-pollination. Also TPS plants have a longer growing season, the seedlings have limited tolerance to environmental stresses, and the resulting yields are generally low and highly variable with a high proportion of small tubers. Evidently, there are some attributes which TPS cultivars must have to minimize these adverse effects. These attributes are: (i) a yielding capacity comparable to that of locally available tuber propagated cultivars, (ii) little segregation for tuber color, shape, size, and quality, (iii) high transplant survivability, and (iv) high resistance to major diseases and pests.
MAIN CHARACTERISTICS
TPS, which are very small, are obtained from potato berries that look like small green tomato fruits and which grow on the above-portion of the potato plant. Two types of TPS are available: (i) hybrid seeds, which result in higher and more uniform plants, are relatively expensive because of hand pollination, controlled conditions, and more sophisticated breeding techniques required for seed production, and (ii) open pollinated seeds, are less expensive to produce, do not require special skills for seed production but the resulting tuber yields are much lower than those of hybrid seed or virus-free seed tubers. TPS seeds can be planted using three different planting techniques: (i) direct sowing into the field, (ii) sowing into nurseries to produce seedlings for further transplanting, and (iii) and planting of seedling tubers or tuberlets originally produced from TPS. The first method is, from a practical standpoint, unfeasible due to the high variability in germination and poor survivability of seedlings. The second method gives better results but the last method appears at present to be the most widely used. It requires two growing seasons but allows the prospect of producing seedling tubers in a more suitable environment under sound crop management.
REQUIREMENTS
Conventional soils often provide a poor environment for germination and initial plant growth of TPS seedlings. This often stems from water stress, low fertility, high soil temperatures, soil compaction and crusting, reduced aeration, and weed competition. Growing media (composed of soil, sand, and organic and/or inorganic materials), in turn, are ideal for rapid germination but are expensive and labor intensive. Several locally available media are currently used with various degrees of success, i.e. in Peru media containing sand and organic materials (peatmoss or compost) in a ratio 1:1 plus fertilizers produce excellent results; in Egypt, media containing 70% (by volume) of soil, 15% of sand, and 15% of peatmoss are often used by small farmers.
TPS seedlings have similar external nutrient requirements than tuber propagated plants. They require a steady supply of N, P, and K. The standard recommendation is that 1,000 seedlings/m2 can be produced in 35 days by using a growing media of sand and peat moss fertilized with 10, 30, and 10 g of N, P, and K, respectively per 100 kg of media.
CROP MANAGEMENT
TPS production strongly depends upon flowering and fruit set. But flowering and fruit set are reproductive characteristics which are cultivar specific, i.e., some cultivars seldom flower and few do not flower at all. Furthermore, flowering strongly depends upon environment, i.e., flowering and fruit set are favored by long days (15 to 17 hours) and cool temperatures (15 to 16 oC). Several chemical treatments are available to promote flower ing and reduce flower abscission.
Manipulating the environment is also critical for flowering and fruit set. Long days, for example, strongly promote flowering and fruit set. Temperature also affects flower production but the effects are not always consistent. In the tropics, high temperatures often reduce pollen fertility, increase flower abscission, and result in TPS of poor quality. TPS produced in cool environments, in turn, have strong dormancy, produce high seed yields, but are more sensitive to high temperatures during germination than TPS produced in warm environments.
The nutritional status of mother plants also strongly affects the viability and potential vigor of TPS. As a general rule, mothers plants require a high and steady nutrient supply for maximum seed production. The environmental conditions during storage also strongly affect seed quality. TPS stored dry germinate faster than TSP stored in humid ambient air, the germination rate increases as the length of storage increases, and percent germination is always high in seed stored dry and produced in high N regimes. Overall, seeds stored at low relative humidity and room temperature will remain viable from several months to about two years. Breaking seed dormancy inTSP is one prerequisite for rapid and uniform germination. TPS generally have a dormancy period of 4 to 6 months. Under moist conditions and room temperatures (i.e., 20 oC), diploids and tetraploids lose dormancy in 2 and 8 months, respectively, while S. tuberosum x S. phureja hybrids lose dormancy after 7 and 10 months at 20 oC and 4 oC, respectively. Several chemicals are available for breaking dormancy (KNO3 and K3PO4, cystein and gibberellic acid (GA), and activated charcoal). For a given chemical, wide differences in doses and procedures are often used. Accatino and Malagamba (1982) report that dormancy can be broken immediately after harvest by immersing seed in GA at 1500 ppm for 24 hours. D'Antonio and McHale (1988), in turn, indicate that GA applied at 1,000 ppm as a 24-hour soak to freshly extracted seeds eliminates dormancy as effectively as GA applied during germination and produces no adverse effects on seed viability up to two years in storage. Pallais et al. (1990) found that GA applied at 1,500 ppm 3 months after storage increases emergence from less than 38% to 73%.
Once dormancy is broken, germination generally occurs in 8 to 10 days after sowing. Germination is highly variable with variability occurring within families of the same genotype as well as across genotypes. In addition, temperature strongly affects germination. High germination rates usually occur between 12 to 15 oC while low rates occur at extreme temperatures, i.e., 5 oC and 25 oC. In tropical environments, shading is a good practice to reduce the adverse effects of high temperatures on germination. In San Ramon (Peru), for example, shading at 75-80% reduced temperatures of seedbeds by 7 oC and resulted in faster emergence and enhanced initial growth.
POTENTIALS AND CONSTRAINTS
Despite of recent advances in TPS technology, the use of TPS at the farmer level has so far produced mixed results. In China, lack of available seed, poor germination, susceptibility of seedlings to late blight, and excessive labor requirements heavily constrain TPS production. In Vietnam, lack of vigorous growth, low tuber yields, and excessive proliferation of small tubers plague TPS production. Overall, except for high elevation environments, where seedlings are grown under optimum temperatures for rapid plant growth and tuberization, potato production using TPS in the lowlands is poor at best with tuber yields generally being lower than yields of tuber propagated plants. Evidently, several factors simultaneously affect TPS production in the tropics and the mechanisms for overcoming them are either inefficient or are not fully implemented. Among these factors are: (i) lack of access to TPS technology, (ii) unfavorable environmental conditions during storage and initial plant growth, and (iii) low quality and poor survivability of transplants.
In the assessment of potential benefits of TPS, it is often postulated that plantlets from TPS can be successfully grown to produce tubers which can then be used as seed for the next planting. The premise is to produce tubers large enough to sustain initial plant growth, ensure optimum tuber initiation and enlargement, and ultimately produce high tuber yields. Yet, most evidences indicate that tuberlets often produce low tuber yields with an abundance of small tubers. Evidently, the technology to manage tuberlets is far from being fully implemented. For example, one question that needs to be answered here is what could be the best plant density to produce large size tuberlets (i.e., > 25 g).
SUMMARY AND CONCLUSIONS
Overall, TPS technology is a promising alternative for rapid seed propagation in the tropics. But it has to overcome one major characteristic of TPS cultivars which is the production of low tuber yields with a high proportion of small tubers and extreme variability in tuber quality. In addition, the field portion of TPS-based potato production systems suffers from the same soil and environmental constraints associated to routine seed tuber production systems.
The use of dual environments for TPS production, that is the relatively high elevation environments for seedling production and the lowland environments for field production, provides with great flexibility for potato production. But the shift in scenarios, from the virtually stress-free settings and high technology of the highlands to the highly unfavorable environments and subsistence technology of the lowlands, greatly increases the risk of crop failure and is at present the largest source of uncertainty in TPS production. Likewise, the use of tuberlets for field production is perhaps the most innovative attribute of the TPS technology. But again tuberlet-based potato production is beset by the same constraints associated to potato production using ordinary seed tubers plus two added disadvantages, (i) the intrinsic inability of tuberlets to produce plants of sufficient size to attain economic yields, and (ii) the long time span (two growing seasons) required to obtain yields equivalent to those of an ordinary tuber propagated growing season. In addition, there are other requisites for successful TPS production in the tropics which have not yet been fully developed and/or implemented. These are: (i) the use of high yielding, stress-tolerant TPS cultivars, (ii) steady water supply, (iii) adequate growing media, (iv) good plant protection, and v) farmers knowledgeable of TPS production.
REFERENCES
Accatino, P., and P. Malagamba. 1982. Potato production from true potato seed. International Potato Center, Lima, Peru.
D'Antonio, V.L., and N.A. McHale. 1988. Effect of storage temperature and extraction methods on dormancy and germination of true potato seed. Am. Potato J. 65:573-581.
Malagamba, P. 1988. Potato production from true seed in tropical climates. HortSci. 23:495-500.
Pallais, N., N. Fong, R. Garcia, and J. Santos-Rojas. 1990. Factors affecting seedling vigor in potatoes. II. Genotype, dormancy, and pre-sowing treatments. Am. Potato J. 67:109-119.

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