Influence of the thermal shock and pre-sprouting on potato tuber yield

Thermal shock and pre-sprouting increase the initial development of potato (Solanum tuberosum L.) plants, allowing for earlier harvests. Growing early potatoes without pre-sprouting is not economically viable under Nordic climatic conditions. The aim of this research was to determine how seed tuber pre-planting treatments (untreated, thermal shock and pre-sprouting) influence time to emergence, mean tuber weight, the number of tubers per plant, and yield (including yield dynamics). The main findings of this work were that thermal shock shortened the time to emergence by 2-5 days, while pre-sprouting shortened it by 7-12 days. In addition, thermal shock significantly increased the number of tubers produced, although their mean weight was lower than that of the tubers produced by the pre-sprouted plants. The presprouted tubers provided a very early harvest of large tubers; the harvest time for the thermal shock-treated tubers was a little later. The untreated seed tubers were the last to produce harvestable plants. Additional key words: day after planting, emergence, growth rate, Solanum tuberosum, tuber formation.


Introduction 1
Potato (Solanum tuberosum L.) consumption trends around the world show that consumers prefer locallygrown tubers, which tend to be fresher and therefore taste better.To satisfy the demand for locally grown potatoes, research is underway to determine the agrotechnical measures that help obtain the best yields of high quality tubers that retain their nutritional quality during storage.To be economically competitive, potato varieties must be able to provide high quality yields as early as possible.The pre-sprouting of seed tubers of early and late potato varieties is widely used in the Netherlands as a pre-planting, yield-increasing
technique (Struik and Wiersema, 1999).Thermal (or thermal shock) treatment is also used to encourage an early harvest; this increases the physiological age of seed tubers and shortens the time needed for the formation of a harvestable crop (Allen et al., 1992;Van der Zaag, 1992a).
If seed tubers are kept at higher temperatures for certain lengths of time during spring, physiologically older tubers are obtained (Van Loon, 1987).This is vital when growing early as well as late potato varieties since the maximum haulm weight and leaf area index (LAI) are obtained earlier; it therefore becomes possible to harvest the economically optimum tuber yield at an earlier date.Physiologically younger plants can be more vigorous, however, and may produce larger yields somewhat later in the growing period (Wurr, 1979).
Temperatures over 30-35°C are rarely used in thermal shock treatment, even for a short time, since potato albumins curdle at 40°C (Kulaeva, 1997).While thermal shock is recognised as a good alternative to pre-sprouting the literature contains little in-depth information on this technique.
The pre-treatment of seed tubers is known to influence plant development and yield structure (Eremeev et al., 2008).The working hypothesis of the present investigation was that a high temperature treatment for a short period has a positive influence on seed tuber maturation contributing to an earlier yield.The aims of this experiment were to analyse the influence of seed tuber thermal shock and pre-sprouting on time to emergence, mean tuber weight, the number of tubers per plant, and yield maturation.

Experimental site and design
This work was performed during the growing seasons of 2000, 2001 and 2002 at the Plant Biology Experimental Station (58°23'N, 26°44'E), Department of Field Crop Husbandry, Estonian University of Life Sciences (EMU), Kreutzwaldi.A randomised complete block design with four replicates was used (Hills and Little, 1972).The size of the test plot was 21 m 2 .The distance between seed tubers was 25 cm and the distance between rows was 70 cm.All experimental seed tubers had a diameter of 35-55 mm; the planting depth was 8 cm.The dynamics of the tuber yield, the number of tubers per plant and mean tuber weight were determined at intervals of 3-5 days; each sampling involved four plants.

Pre-planting treatments
All the seed tubers used in this work were kept in a storehouse at 4°C until the 30 th March of each experimental year.Irrespective of the pre-planting treatment to which they were subjected, all were planted on 7 th May of each year.Treatments were applied between 30 th March and 6 th May; the sum of the temperatures above 0°C differed according to each treatment.
The treatments applied were as follows: 1. Untreated seed tubers (T o ) (n = 126).These tubers were kept from 1 st April to 6 th May (i.e., 37 days) at 4°C.The total accumulated temperature was 148°C.
2. Thermal shock treatment (T S ) (n = 130).The seed tubers were removed from initial storage but, between 1 st April and 30 th April, were still kept at 4°C (accumulated temperature 120°C).They then spent two days, from 31 st April to 1 st May, at 30°C (accumulated temperature 60°C), and then another five days, from 2 nd May to 6 th May, at 12°C (accumulated temperature 60°C).These seed tubers therefore accumulated a total of 240 º C (92°C more than in the T o treatment).
3. Pre-sprouting (P S ) (n = 137).The seed tubers were removed from storage and kept for 37 days, from 1 April to 6 May, at 12°C.During this pre-sprouting treatment they accumulated a total of 444°C (204°C more than in the T S treatment, and 296°C more than in the T o treatment).

Plant material
The late maturing variety 'Ants' and the middlematuring variety 'Piret', both bred at the Jõgeva Plant Breeding Institute in Estonia, plus the early maturing variety 'Agrie Dzeltenie', bred at the Latvian Priekuli State Plant Breeding Station, were used as the experimental plant material.'Bintje', a middle-maturing variety of Dutch origin widely grown in Europe (Wolf and Van Oijen, 2003), has for years been used as a standard variety in comparative trials under Estonian climatic conditions at the Jõgeva Plant Breeding Institute.It was therefore chosen as the standard variety in the present work for the comparison of total yields.

Soil conditions and analysis
The soil of the experimental f ield was a Stagnic Luvisol according to the World Reference Base for Soil Resources (1998 classification); the texture was that of a sandy loam with a humus layer of 20-30 cm (Reintam and Köster, 2006).
Soil analyses were performed at the laboratories of the Department of Soil Science and Agrochemistry, EMU.Air-dried soil samples were passed through a 2 mm sieve.The following characteristics were determined: pH (in 1M KCl and in 0.01M CaCl 2 , 1:2.5 w:v), organic carbon (using the standard Tjurin method), and Ca and Mg in NH 4 OAc at pH 7 (Soil Survey Laboratory Staff, 1996).Available P and K were analysed according to the Mehlich-3 method (Soil and Plant Analysis Council, 1992).The Kjeldahl method was used to determine the soil's total N content.
The humus layer of the experimental field had a pH KCl of ≈6.2 and a C content of 1.4%.The plantavailable elements in the soil were: Ca 674 mg kg -1 , Mg 101 mg kg -1 , P 183 mg kg -1 and K 164 mg kg -1 .The soil total N content was 0.11%.The soil texture was 56% sand, 35% silt and 9% clay.

Weather conditions
During the growth period (May to September), the rainfall in all experimental years was above average for June and July, and below average in May, August and September (Table 1).The air temperature remained similar to the mean for the 32 preceding years , although July was significantly warmer.

Experimental field techniques
The agrotechnical measures employed were typical for potato experiments.Composted manure (60 Mg ha -1 ) was used as an organic fertilizer before autumn ploughing.The tubers in all treatments were planted on the 7th May each year; inorganic fertilizer (78 kg N, 72 kg P, 117 kg K ha -1 ) was applied locally at the same time.
The dynamics of the tuber yield, the number of tubers per plant and the mean tuber weight were determined at intervals of 3-5 days.Four plants were harvested by hand from the test plot at each sampling point.The experiment was terminated at 120 days after planting (DAP).

Statistical analysis
The results were subjected to regression analysis (Mead et al., 1993;Lauk et al., 1996), using the formula: y = a + bx + cx 2 ; where «y» is the argument function, i.e., tuber yield, number of tubers per plant or mean tuber weight, «a» is a constant term, «b» and «c» are regression coefficients, and 'x' is the number of days after planting.The derivative of the function (b -2c) indicates the increase in the growth rate of the mean tuber weight or potato yield (Figs. 1 and 2) per day, calculated according to the regression formula (y = a + + bx + cx 2 ).Separate regression formulae were found for every variant and the average formulae for multiple years were calculated.Standard errors (SE) and confi- dence limits (CL 05 , P = 0.05) were calculated using the method of Lauk and Lauk (2000).The calculation of confidence limits was based on Student's theoretical criterion (Mead et al., 1993).All the data in Tables 2, 3 and 4 were calculated according to the regression formula (y = a + bx + cx 2 ).
To determine the probability of differences between the treatment programmes, least significant differences (LSD 05 ) were calculated according to Lauk et al. (2004).Statistica 7 software (Statsoft, 2005) was used for all statistical analyses.

Discussion
The influence of pre-planting treatment on potato emergence and number of tubers Physiological ageing advances sprout growth, crop emergence, crop establishment and usually improves tuber yields (Burke and O'Donovan, 1998).However, the onset of the different developmental stages and their duration can be quite different depending on the biological characteristics of the potato variety in question, the quality of the seed tuber, climatic and soil conditions, and the agrotechnical measures employed.Some authors report that physiologically older seed tubers allow for faster emergence than their younger counterparts (Iritani, 1968;O'Brien et al., 1983), while others have found no difference (Bus and Schepers, 1978).The experiments of Van Loon (1987), however, showed that physiologically older seed tubers emerge more slowly.
A potato plant usually takes 20-35 days to emerge under Estonian climatic conditions.The time from planting to emergence depends on the treatment of the seed tubers, i.e., the physiological age with which they are invested (Struik and Wiersema, 1999;Jõudu et al., 2002).In this study the T S and P S treatments accelerated the emergence of plants by 2-5 days, and by 7-12 days respectively compared to the T o treatment.
Intensive tuber growth begins when the aboveground parts of the plant have fully developed (when the LAI is at least 4), although different varieties show significant variations (Putz, 1986).Tuber formation in early varieties usually takes place earlier and growth is much quicker than in late varieties.In addition, plants derived from physiologically older tubers of late varieties begin their tuber formation slightly earlier (Van der Zaag, 1992b).The T S treatment increased the number of tubers compared to the P S treatment (see Table 2), therefore, the former might be of interest to seed tuber-growing enterprises, whose main purpose is to obtain the maximum number of tubers from one plant.Similar findings were reported by Van der Zaag andVan Loon (1987) andMoll (1985).

The influence of pre-planting treatment on tuber weight
The tuber weight achieved depends on the weather conditions and the available nutrients during the period of tuber formation.It also depends on the growth and development of the leaves and branches, the formation of assimilation products and their distribution between different parts of the plant, the rate of tuber formation, and the perishing time of the haulms (Panelo and Caldiz, 1989).According to Burke (1997), the average weight of tubers increases with their physiological age, but in the present experiment this effect was made manifest only in the P S treatment (Table 3).This treatment had the strongest influence on tuber weight, returning the highest value (77.6 g) at 120 DAP.According to Putz (1986), after the death of the haulms the growth of the tubers ceases, and the skin hardens and starts to suberize.Decisions taken while planning the harvesting period should not be based on data for years with optimum weather conditions but on average years.At the end of growth period the increase in mean tuber weight occurred mainly at the expense of mean tuber size (35-55 mm) and the production of large tubers (over 55 mm).

The influence of pre-planting treatment and variety on yield
According to Möller et al. (2001), the duration of yield maturation can be shortened by 8-14 days by the pre-sprouting of seed tubers.With this treatment the time of maximum yield is shifted to about two weeks earlier; yield losses due to potato late blight are consequently reduced.The present work established that the T S treatment had a positive effect until mid-August; therefore if harvesting is planned in September, there is no need to thermally treat the seed tubers and bear the extra costs involved, especially if cultivation starts early in the growing season with medium to early varieties.Under certain conditions the T S treatment could be less labour and energy-consuming than presprouting, e.g., when using thermoregulated storehouses (Jõudu et al., 2002).The present results show that physiologically older tubers have a higher yield potential, with plants reaching their harvesting point more quickly.With the P S treatment the tubers accumulated more temperature than with the T S treatment, therefore the P S seed tubers can be considered physiologically older.The T S seed tubers were physiologically older than the T o tubers, allowing them to show a higher growth rate and earlier yield maturation than those of the latter treatment.The gradual maturation of the potato yield helps to lengthen the harvesting period even when growing just one variety.The average Estonian potato yield has recently been as low as 10-18 Mg ha -1 (ESA, 2004).The T S and P S treatments in the current experiment, however, produced maximum yields by 120 DAP of about 50 Mg ha -1 .Yields per hectare ultimately depend on radiation levels, the agrotechnical measures taken by the grower and the potential of the potato variety.Given favourable conditions and using affordable measures, the estimate is that potential yields in Estonia could reach as high as 67-78 Mg ha -1 .
In conclusion, for very early potato yields the P S treatment should be used.Thermal shock would be more efficient in seed tuber production since, while it produces more tubers than in the P S treatment, their mean weight is smaller.If the purpose is to grow potatoes for consumption then the T S treatment is a useful tool for achieving early to mid-period yields.The P S treatment may consume a lot of time and energy, but a very early yield is obtained and the full yield potential can be realised.
Different seed tuber pre-planting treatments allow variations in potato harvest times.A very early yield is possible with P S -treated tubers while the harvest period for T S -treated tubers starts in the second half of July or the beginning of August.

Figure 1 .Figure 2 .
Figure 1.The effect of the different pre-planting treatments on the growth rate of mean tuber weight (mean for 2000-2002 results).Data point values defined according to regression analysis.

Table 1 .
Average monthly temperatures and precipitation in Estonia during the growth period

Table 2 .
Effect of the different pre-planting treatments on the number of tubers produced per plant(mean of 2000-2002 results)

Table 3 .
Effect of the different pre-planting treatments on mean tuber weight (g).Mean for2000-2002 results

Table 5 .
Number of tubers per plant, weight of tubers per plant and yield produced by the different potato varieties.Means for 2000-2002 results