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Published online 27 March 2006
Published in Crop Sci 46:1156-1168 (2006)
© 2006 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Response of Stored Potato Seed Tubers from Contrasting Cultivars to Accumulated Day-Degrees

P. C. Struika,*, P. E. L. van der Puttena, D. O. Caldizb and K. Scholtea

a Crop and Weed Ecology (CWE) Group, Dep. of Plant Sciences, Wageningen Univ., Haarweg 333, 6709 RZ Wageningen, the Netherlands
b McCain Argentina, Balcarce, Argentina

* Corresponding author (paul.struik{at}wur.nl)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In potato (Solanum tuberosum L.), the accumulated day-degrees (temperature sum, calculated by accumulating the daily temperatures) from dormancy break until seed tuber use has been suggested as an indicator of the physiological status of the seed. We tested whether similar temperature sums differing in timing of a short period of high temperatures gave similar seed performance. Four field experiments were performed in which seed was used that had been exposed to different storage temperature regimes, differing in total temperature sum or in timing or duration of a warm period. Emergence, number of stems, number of tubers, and early and mature tuber yield were assessed. During the storage period, the onset of sprouting was recorded. Cultivars with a high rate of physiological degeneration ("ageing") were usually sensitive to warm storage during the second part of the storage period, especially if the first 12 to 18 wk of storage had also been warm. This was reflected in reduced emergence (≤10%), low densities of stems (≤0.5 stems m–2) and tubers (≤5 tubers m–2), and low yields, especially with early harvesting (≤20 g m–2). Specific phasing of the warm period could reduce yields to levels even below the yield of the seed tubers exposed to the highest accumulated temperature sum. A higher temperature sum after the end of dormancy advanced and accelerated the process of ageing of seed tubers. Cultivars with a high rate of ageing showed much greater difference between the same temperature sums built up over time in different ways than cultivars with a low rate of ageing. The resulting maximum differences in final fresh tuber yield between seed lots exposed to the same temperature sum could be 65 Mg ha–1 for Astarte (a cultivar with a high rate of ageing) compared with nil for Désirée (a cultivar with a low rate of ageing).

Abbreviations: dm, dry matter • H, high • L, low


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN POTATO, many factors determining crop yield are influenced by seed quality. Relevant seed quality characteristics include seed tuber size, other physical characteristics such as shape and presence of wounds, physiological age, and seed tuber health. The physiological status of seed potatoes has a great impact on the emergence, number of stems per plant, number of tubers per stem, tuber-size distribution, and tuber yield of the progeny crop (Van der Zaag and Van Loon, 1987; Reust, 1982; Van Ittersum, 1992 and papers therein; for an overview see Struik and Wiersema, 1999). The physiological status needs to be optimized to efficiently produce a specific crop structure that allows tuber production for specific outlets (Struik et al., 1990, 1991).

Physiological age can be defined as the stage of development of a seed tuber, which is modified progressively by increasing chronological age, depending on growth history and storage conditions (Reust, 1986; Struik and Wiersema, 1999). The definition emphasizes that physiological age also includes aspects other than chronological age. The development of a seed tuber starts with a phase of dormancy. Immediately after it is initiated, a tuber develops a certain degree of dormancy. Dormancy is the physiological state of the tuber in which autonomous sprout growth will not occur within a reasonable period of time (usually 2 wk), even when the tuber is kept in conditions ideal for sprout growth (Reust, 1986; Van Ittersum, 1992; Struik and Wiersema, 1999). During dormancy, biochemical and physiological processes occur that do not trigger immediate morphological changes but are relevant for the number of sprouts produced after breaking of the dormancy and for their growth vigor.

Krijthe (1962) and Van Ittersum (1992) already demonstrated that both mother tuber and sprout age affect the physiological age of a seed tuber. Until breaking of the dormancy, changes in the physiological status of the seed are only reflected by biochemical and physiological changes in the seed tuber itself and not by morphological changes. After dormancy breaking, the physiological age is still influenced by the age of the mother tuber but modified by the additional effects of conditions and treatments on the behavior of the sprouts (Caldiz et al., 2001). The evidence for these separate effects from experimentation is, however, still limited.

Conditions during dormancy and thereafter affect the progress of the physiological ageing and are therefore relevant for the performance of the seed tuber. An overview of the different stages of physiological age and their consequences for crop performance is provided by Ewing and Struik (1992) and Struik and Wiersema (1999). It is essential to control the conditions during storage to optimize seed tuber quality. Environmental factors during storage having an effect on physiological age include relative humidity, temperature, photoperiod, and diffuse light. The temperature effect especially is highly complex. As the metabolic processes and physiological events taking place before and after dormancy differ, the sensitivity toward environmental conditions, and especially toward temperature, during the different stages of physiological development of the seed tuber may also differ (Scholte, 1986; Struik and Wiersema, 1999). Heat shocks, cold shocks, and similar accumulated day-degrees built up in different ways may all have their specific effects, depending on cultivar (Van Ittersum, 1992; Struik and Wiersema, 1999).

It has been stressed by many authors that both for scientific and practical purpose a good indicator of dormancy and/or physiological age would be useful. Many different characteristics have been proposed as indicators, including physiological, (bio)chemical, molecular, and biophysical ones (see, e.g., Bachem et al., 2000; Caldiz et al., 2001). Such indicators are needed to quantify and explain differences in rate of ageing between seed lots as induced by differences in origin, storage conditions, cultivar and treatment, and to quantify and model the effects of seed age on crop growth and yield.

The most direct and simple way to indicate physiological age is on the basis of the accumulated day-degrees from dormancy break (O'Brien and Allen, 1981; O'Brien et al., 1983), storage temperature sum (Scholte, 1986; Struik and Wiersema, 1999), and relative growth vigor indices (Bodlaender et al., 1987; Van der Zaag and Van Loon, 1987; Van Ittersum et al., 1990; Van Ittersum, 1992). Developing a unifying concept that is applicable under all conditions of production and storage and to all cultivars, however, is difficult.

This study shows that the concept of accumulated day-degrees from dormancy break is not a good estimator for physiological age across temperature conditions during storage and across cultivars and, therefore, needs to be refined.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
General
Data sets presented in this paper are extracted from an unpublished set of large experiments containing many combinations of storage temperature treatments and cultivars and starting in four different years. These experiments included a seed storage phase (including the storage treatments), a phase in which incubation tests were performed to determine the time of dormancy breaking, and a field phase in which the effects of the different storage treatments were assessed on various crop physiological parameters throughout the growing season.

Starting Material
Experiments were performed with a set of cultivars differing in maturity type and rate of physiological ageing (Table 1). Five of the six cultivars used in this research were categorized into different classes of rate of physiological ageing by Van Ittersum et al. (1990) using different performance parameters and relative growth vigor indices. The sixth cultivar, Sirtema, was tested for its rate of ageing in comparison with many other varieties by Van Ittersum (1992) and in unpublished experiments by the Department of Agronomy, now part of the Crop and Weed Ecology Group, Wageningen University.


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  		Table 1. Relative rate of physiological ageing of the cultivars used (ranked on the basis of their maturity type) and their presence in the four experiments.

 
Seed tubers from these cultivars were produced under field conditions with known and strictly controlled agricultural practice and selected for uniformity in weight (Table 2) and absence of disorders and infections of pests or diseases.


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  		Table 2. Relevant details on materials and methods of the four experiments carried out in four different years.

 
Storage Facilities and Treatments
Seed tubers were stored in trays under darkness. Storage treatments were performed in growth cabinets of the former Department of Agronomy of Wageningen University with precise temperature and relative humidity control.

Storage treatments reported in this paper consisted of different combinations of phases of low temperature (4°C) and warm temperatures (16 or 20°C) during storage. Storage temperatures during a specific phase were constant, without diurnal cycle. Temperature sums accumulated during a specific phase can therefore be calculated by the product of the number of days of that phase and the set temperature, without considering a base temperature or a maximum temperature. Experiments 1 through 3 were similar in set up; storage treatments and their codes and temperature sums (T-sum in °C d) are illustrated in Fig. 1A . In Exp. 4, not only the timing but also the duration of the warm temperature was varied; these storage treatments and their codes and T-sums are indicated in Fig. 1B. The caption to Fig. 1 also contains an explanation of the coding of the treatments. L4 or L5 are considered control treatments as they reflect common practice in seed tuber storage.


Figure 1
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  		Fig. 1. Schematic presentation of storage treatments, their codes and accumulated temperatures (T-sums) of Exp. 1 through 3 (Fig. 1A) and Exp. 4 (Fig. 1B). The storage period of each experiment was subdivided into different phases of 6 (Exp. 4), 7.5 (Exp. 2 and 3), or 8 (Exp. 1) wk, giving four phases in the first three experiments and five phases in Exp. 4. Different treatments were created by assigning a low or a high temperature to the different phases. For Exp. 1 through 3, we only selected treatments with constant temperature during the first two phases and the last two phases. Treatments were coded by indicating the temperature (L for low and H for high) followed by the number of phases that such a temperature was maintained. Treatments without a temperature switch are indicated by L4 or H4 (Exp. 1–3) or L5 or H5 (Exp. 4) for a low storage temperature throughout the storage period and high storage temperature throughout the storage period, respectively. In Exp. 1 through 3, the treatments with a switch in storage temperature from low to high are indicated by L2H2, treatments with a switch in storage temperature from high to low are indicated by H2L2. Similarly, in Exp. 4, the letters L and H in the treatments with temperature switches are followed by the numbers 1, 2, 3, or 4 indicating the number of phases that a specific temperature was maintained. These codes are listed behind the schematic representation of the treatments and followed by the temperature sum over the entire storage period in the different experiments.

 
Observations on Sprouting and Incubation Tests
Observations on sprouting during storage and special incubation tests under controlled conditions were performed for all experiments. Here, we will only report on the observations on sprouting performed on the seed lots stored for Exp. 4, as the split-up of the storage period into different phases was most refined in this experiment and as the data from these observations in Exp. 4 will be used to interpret the field data of that experiment. Thermotime needed to reach onset of sprouting (sprouts >3 mm in 80% of the seed tubers) and the thermotime from the onset of sprouting until the end of the storage phase were assessed. Detailed data on the incubation tests can be made available by the corresponding author on request.

Field Experiments
General Methodology
Storage treatments started at the end of August to end of September depending on year (Table 2). RH was high (at least 80%) to avoid any effect of RH on treatment effects. Tubers that already had produced sprouts were desprouted some time before planting (Table 2). Tubers were planted in the field to test the rate of emergence, growth vigor, and yield potential at different harvesting dates. The experimental design depended on the other treatments included in the experiments not reported here. Agronomic and experimental details are provided in Table 2.

Data Collected
Timing of early and late harvests depended on cultivar maturity type. Field data collected included number of plants emerged, number of stems m–2, number of tubers harvested m–2 and per stem, fresh tuber yield, tuber dry matter content, and fraction of tubers >55 mm in diameter. On the basis of the analysis over time of these parameters and the statistical interaction between harvest and treatment effects, we report on the final or maximum number of stems and tubers on the basis of the data of the very early harvest or the early harvest.

Data Processing and Statistical Analysis
Standard ANOVA with a randomized complete block design was performed followed by LSD tests using GENSTAT PC version (GenStat, 2000). Although in some cases raw data were transformed before analysis to obtain normality or uniformity of standard deviations, data and LSD values in the tables and figures represent untransformed values.

Data Presentation
After presenting the data on onset of sprouting of Exp. 4, we will then show data of the Exp. 1 through 4, in which potato tubers stored at different temperature regimes were then planted out in the field. Detailed information for Exp. 4 on the relation between early yield and temperature sum after onset of sprouting will be given.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Observations on Onset of Sprouting in Experiment 4
Prolonged cold storage (L5) delayed the end of the dormancy period measured in day-degrees compared with prolonged warm storage (H5) in cultivars Sirtema and Astarte (Table 3; for treatment codes see explanation in Fig. 1). In cultivars Jaerla, Kennebec, Bintje and Désirée, seed tubers stored cold were still dormant after 30 wk of storage, so precise comparison between these two treatments for end of dormancy was not possible, but also for these cultivars, L5 prolonged dormancy compared with H5. When seed tubers were stored at 4°C during the first phase of storage (LH treatments), dormancy was broken at increasingly lower thermotime when warm storage was initiated earlier (and thus when warm storage lasted longer and the final temperature sum was higher), except for Désirée, the cultivar with a very low rate of physiological ageing. When seed tubers were initially stored at 16°C (HL treatments), dormancy was not affected by the timing of the low temperatures for cultivars Sirtema and Bintje, whereas for the other cultivars only a difference was recorded between H1L4 and the other three HL treatments. Cultivar differences in thermotime needed to break dormancy were larger for HL treatments than for LH treatments, but the ranking of the cultivars remained more or less consistent within the group of LH or the group of HL treatments. Ranking of cultivars was not the same for LH and HL treatments.


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  		Table 3. Thermal time in °C d until onset of sprouting (i.e. thermal time needed to break dormancy; see Materials and Methods; value before slash) and during the period from onset of sprouting until the end of the controlled storage period (value after slash) for several cultivars differing in relative rate of physiological ageing and different storage treatments (Exp. 4). For storage treatment codes see Fig. 1B. Values between parentheses after the codes are temperature sums during controlled storage (in °C d). Data based on assessments on 5 dates starting on 13.10 and ending on 30.03.

 
Field Experiments
At the early assessments of number of plants emerged, number of stems and number of tubers, and of early yields, all interactions between storage treatment and cultivar were statistically significant. For final yields and quality assessments, this was also true except for the tuber dry matter content.

Fraction of Plants Emerged and Number of Stems m–2
Cold storage (L4 or L5) gave 91 to 100% (usually 99 or 100%) emergence in all cultivars and experiments (Table 4). The cultivar Jaerla (ageing rapidly) proved to be sensitive to prolonged warm storage (H4 or H5): emergence rates varied between 1% (Exp. 1) and 81% (Exp. 2) and were always lowest for this cultivar. Also the emergence of cv. Bintje was often seriously reduced by H4 or H5 storage treatments, whereas the effects on Kennebec and especially Astarte were inconsistent over experiments. Treatments L2H2 or L2H3 gave low emergence rates for Astarte (1–10%) in all experiments and in Jaerla for Exp. 1 (55%) (Table 4). In all other experiments or cultivars, rates of emergence were well above 80% for these treatments. Treatments H2L2 or H3L2 gave better or similar emergence rates compared with L2H2 or L2H3 (with the same thermotime), with most values being 98 to 100%. However, emergence rates of Jaerla in Exp. 1 (75%) and Exp. 4 (68%) were significantly reduced (Table 4). So, in general, the cultivars with a high rate of physiological ageing proved to be most sensitive to high storage temperatures during the entire storage period or during the last 12 to 18 wk of the storage period and much less sensitive to warm storage during the first 12 to 18 wk of storage. Sirtema, another cultivar with a very high rate of ageing, performed relatively well.


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  		Table 4. Fraction of plants emerged (maximum number of plants divided by number of seeds planted), number of stems per m2 and number of tubers per m2 at early (Exp. 1, 2, and 4) or very early (Exp. 3) harvest and tuber fresh yields (g m–2) at very early (Exp. 1 and 3) or early (Exp. 2 and 4) harvest. Cultivars (CV) are abbreviated by the first four letters of their full names and are listed in the order of their maturity type. Sirt = Sirtema; Jaer = Jaerla; Kenn = Kennebec; Bint = Bintje; Dési = Désirée; Asta = Astarte.

 
Cold storage (L4 or L5) resulted in high stem densities (Table 4). Depending on cultivar, these densities ranged from 9.2 to 30.5 stems m–2 (Table 4). Jaerla produced fewer stems per square meter than the other cultivars. At H5, cultivars Jaerla and Astarte produced low stem densities, except in the case of Astarte in Exp. 4 (see also fraction of plants emerged), but the effects depended on the experiment (Table 4). In some cases hardly any stems were produced at all. The same two cultivars also produced lowest stem densities after storage treatments L2H2 or L2H3, although the differences with the other cultivars were in some experiments smaller than for treatments H4 or H5. On average, stem densities were higher after storage at L2H2 or L2H3 than after H4 or H5 (Table 4). Numbers of stems per square meter were usually high for H2L2 or H3L2 (on average even higher than for the treatments L4 or L5), although again comparatively low for Jaerla.

So, for stem density, especially the temperature during the last 12 to 18 wk of the storage period was the determining factor. When the temperature during this period was warm, stem densities were low, especially when the temperature during the first 12 to 18 wk of storage had also been warm.

Number of Tubers per Square Meter and Fresh Tuber Yield on Early Observation Dates
For the crops from seed tubers stored at L4 or L5, the number of tubers per square meter at early observation dates differed considerably among experiments. These differences were mainly associated with differences in stem density (Table 4). Bintje generally had the highest number of tubers, whereas Kennebec had the lowest. In Exp. 1 through 3, early tuber densities were considerably lower for H4 or H5 than for L4 or L5 in almost all cases. Largest differences were obtained for the rapidly ageing cv. Astarte and the abundantly tuberizing Bintje; differences were smallest for Désirée. For Exp. 4, the differences in early tuber densities were much smaller, but the pattern was similar.

Tuber densities varied greatly for treatments L2H2 and L2H3. Values of particular cultivars within certain experiments could be both significantly lower (e.g., for Astarte in Exp. 4) or significantly higher (e.g., for all cultivars but Astarte in Exp. 3 and Jaerla in Exp. 4) than those for H4 or H5 and significantly lower (Exp. 1; Jaerla in Exp. 2; Astarte in Exp. 3; Bintje and Astarte in Exp. 4) or statistically not different compared with those for L4 or L5 (Bintje and Désirée in Exp. 2; all cultivars but Astarte in Exp. 3; all cultivars but Bintje and Astarte in Exp. 4).

Early numbers of tubers per square meter for H2L2 or H3L2 were usually similar with those for the control treatment L4 or L5, except in Exp. 2, where the three cultivars showed inconsistent effects and in Exp. 4, where Désirée showed more tubers at H2L2.

So, for the early number of tubers per square meter especially the temperature during the last 12 to 18 wk of storage was the determining factor. High temperature during the last 12 to 18 wk of storage usually resulted in fewer tubers, especially after warm storage during the first 12 to 18 wk of storage. Effects were not the same for all cultivars. For example, for Désirée warm storage during the last 12 to 18 wk of storage did not reduce tuber density.

Early Fresh Tuber Yields
Early fresh tuber yields varied greatly, depending on the harvest date, the earliness of the cultivar, and the physiological age of the seed tubers (Table 4). The seed tubers which were stored at warm temperatures throughout the storage phase (treatments H4 or H5) often performed poorly, especially in the cultivars Jaerla and Bintje. Désirée was much less sensitive to this treatment. Compared with the low temperature control, treatments L2H2 or L2H3 always gave much smaller yield reductions than H4 or H5 in all cultivars but Astarte, in which effects were similar or worse. Treatments H2L2 or H3L2 gave yields similar to L4 or L5. In fact, only in two cases were the yields significantly different; in Exp. 3 treatment H2L2 of Désirée yielded significantly more than L4, whereas in Exp. 4 treatment H3L2 of Bintje yielded significantly less than L5.

Differences in performance of seed lots differing in physiological age are usually visible in early tuber yields. When these early yields are plotted against the temperature sum after the onset of sprouting the differences between different types of storage treatments and cultivars become clear (Fig. 2 ). In general, the early tuber dry matter yield strongly declined with an increase in temperature sum after sprouting. With equal temperature sums but different sequences of cold and warm storage, the treatments with an early warm period were higher than or equal to the treatments with an early cool period. The differences between the lines for LH and HL were large for Sirtema and Astarte, two cultivars with a very high rate of physiological ageing (Table 1). At the storage temperature sum of 2856°C d, the H4L1 treatment of Sirtema gave an early tuber dry matter yield of 695 kg ha–1 compared with 540 kg ha–1 for L1H4. For Astarte these yields were 756 kg ha–1 for H4L1 and 56 kg ha–1 for L1H4. The two curves were almost coinciding for Kennebec (low rate of physiological ageing) and Désirée (very low rate of physiological ageing) over the entire range of temperature sums. Jaerla and Astarte showed a drastic decline in early performance with an increase in temperature sum after onset of sprouting but for different treatment types. Jaerla seemed to be an outlier in the set of cultivars tested, which might have been associated with the fact that this cultivar showed the largest effects of storage treatments on emergence.


Figure 2
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  		Fig. 2. The effect of the thermal time accumulated after the onset of sprouting (see Table 3) on the performance of six cultivars differing in relative rate of physiological ageing (see Table 1) when harvested between 13 and 20 July, depending on maturity type (Exp. 4). Open symbols for the seed lots exposed to low (L) temperature during the entire storage period or for the treatments starting with low temperature; black symbols for the seed lots exposed to high (H) temperature during the entire storage period or for treatments starting with warm storage. Dm = dry matter.

 
Final Number of Tubers per Square Meter and per Stem at Late Harvest
Effects of storage treatments on final numbers of tubers per unit area and per stem were assessed. Table 5 provides these data for Exp. 1 and 4. Numbers of tubers per square meter at late harvest were closely and linearly correlated with the tuber densities of the early harvest (Exp. 1: R2 = 0.953; N = 20; Exp. 4: R2 = 0.920; N = 24). In Exp. 1, about 11% of the tubers disappeared during the growing season; in Exp. 4, this figure was about 18%. In both experiments, early warm storage (treatment H2L2 or H3L2) did not affect tuber number per square meter compared with L4 or L5, except in Bintje of Exp. 4, where it increased the tuber number. Warm storage during the last 16 to 18 wk of storage (treatment L2H2 or L2H3) reduced tuber number per square meter in Sirtema, Bintje and Astarte in Exp. 1 and in Astarte in Exp. 4 compared with the control treatment (L4 or L5). Warm storage throughout the storage period (treatment H4 or H5) reduced the tuber number per square meter compared with L4 or L5 in all cases, although the effect was not significant in Kennebec and Désirée of Exp. 4. Differences between storage treatments in number of tubers per stem were usually not statistically significant within a cultivar. Only in Jaerla and Bintje low stem densities were consistently but only partly compensated by more tubers per stem. Compare for example the values for number of tubers per stem for treatments H4 or H5 with those of L4 or L5.


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  		Table 5. Final number of tubers per m2 or per stem at late harvest (Exp. 1 and 4). Sirt = Sirtema; Jaer = Jaerla; Kenn = Kennebec; Bint = Bintje; Dési = Désirée; Asta = Astarte.

 
Final Fresh Tuber Yields
Table 6 shows the final tuber yields for all treatments in Exp. 1 and 4. In Exp. 1, the final yields for Jaerla, Bintje, and Astarte were much lower for the warm storage than for the cool storage (H4 versus L4). The seed of Désirée did not age much, resulting in similar yields for all treatments. Sirtema, Bintje, and (especially) Astarte showed significant yield reductions when seed was stored warm during the second phase of the storage period. Warm storage during the first part of the storage period did not affect yield compared with L4, although the temperature sum was the same as for warm storage during the second part of the storage period. In Exp. 4, treatments with the same accumulated temperature sum during storage gave similar yields except for Astarte. For some treatments in this cultivar, yields could drop well below the yields of the seed tubers exposed to the highest temperature sum (i.e., H5). This was the case in treatments L1H4 and L2H3.


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  		Table 6. Tuber fresh weight (g m–2), dry matter content in tuber (%) and fraction of tubers > 55 mm (%), at late harvests of different cultivars (CV) in Exp. 1 and 4. Sirt = Sirtema; Jaer = Jaerla; Kenn = Kennebec; Bint = Bintje; Dési = Désirée; Asta = Astarte.

 
Tuber Dry Matter Content
Table 6 also shows that storage regime affected tuber dry matter content at final harvesting. In Exp. 1, warm storage resulted in lower tuber dry matter content, but this was not observed in Exp. 4. The trends over storage temperature sum were not consistent in Exp. 4. In both experiments, largest differences between storage regimes were observed in Astarte, where dry matter contents were also highest. In this cultivar, the treatments with low tuber densities also showed much reduced tuber dry matter content.

Grading at Final Harvest
In Exp. 1, treatment effects on percentage of tubers >55 mm in diameter were significant in three cultivars (Sirtema, Jaerla, and Astarte), but inconsistent. In Exp. 4, treatments with the same storage temperature sum often showed different proportions of large tubers, especially in cultivars Désirée and Astarte. Late warm storage periods shifted tuber-size distribution toward the larger sizes.

Relations between Yield Components in Experiment 4
Figure 3 shows for all cultivars in Exp. 4 the relations between temperature sum and yield components. The stem density usually increased when seed was used that was stored warm for a short period of time (Quadrant I). But when this period was prolonged the stem density decreased again. The temperature sum at which the maximum number of stems was reached was different for different cultivars but also differed for the two types of treatments: whether a warm period was followed by a cool period or a cool period was followed by a warm period. In most cultivars, there was a close relationship between number of stems square meter and number of tubers square meter (Quadrant II). There were no consistent effects of the sequence of warm and cool periods on this relationship. The same was true for the relation between number of tubers per square meter and the late tuber dry matter yield (Quadrant III). The overall response reflected by the relationship between thermal time during storage and late tuber dry matter yield (Quadrant IV) shows that the yields were usually slightly higher when the warm periods started earlier during the storage period (HL versus LH).


Figure 3
Figure 3
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  		Fig. 3. Four quadrant figures indicating the relationships between thermal time during controlled storage and number of stems or number of tubers produced per m2 and tuber dry matter (dm) yield at late harvest (28 August–23 October, depending on maturity type), for six cultivars differing in relative rate of physiological ageing (Exp. 4). Open symbols for the seed lots exposed to low (L) temperature during the entire storage period or for the treatments starting with low temperature; black symbols for the seed lots exposed to high (H) temperature during the entire storage period or for treatments starting with warm storage.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The present results show that a cultivar specific response to accumulated temperature sum during storage exists. Some cultivars (e.g., Jaerla) age much more rapidly than other cultivars (such as Désirée). This difference in cultivar specific behavior was consistent over the four experiments, when L4 or L5 treatments were compared with H4 or H5 treatments, with the exception of the behavior of Désirée in Exp. 4. Moreover, cultivars differ in their response to the timing of warm periods. Late periods of high storage temperatures result in poorer performance of the seed than early periods of warmth except in cultivars Bintje (inconsistent difference) and Kennebec (no difference). The most detailed experiment, Exp. 4, showed that this effect of the timing of the warm period depends on its duration.

Seed tubers were desprouted before planting when they had sprouts at the end of the storage period (see Materials and Methods). This means that it is likely that most of the ageing effects were associated with effects on the seed tubers themselves. However, ageing treatments were given when sprouts were still attached to the tubers and therefore the storage temperature treatments might have affected the tubers partly through effects on the sprouts. Desprouting in and of itself might also have contributed to the effects observed since not all storage temperature treatments allowed sprouting before planting (see Table 4).

There were large differences between treatments with a different order of low and warm storage temperatures (the LH versus the HL treatments). When the low storage temperature lasted no longer than 6 to 16 wk and preceded the high storage temperature, the fraction of the plants emerging, the number of stems per unit area, the number of tubers per unit area, the early tuber yield, and the final tuber yield were generally lower than when warm storage preceded cool storage. These parameters usually showed lower values for L2H2 than for H2L2 (Exp. 1), for L1H4 than for H4L1 (Exp. 4) and for L2H3 than for H3L2 (Exp. 4) (Tables 4, 5, and 6). When the low storage temperature lasted longer in Exp. 4, differences were much smaller (compare fresh tuber yields for L3H2 with those of H2L3 and fresh tuber yields of L4H1 with those of H1L4 [Table 6]).

With an increase in duration of the warm period (whether the storage season started or ended with a warm period) the early tuber yield and the final tuber yield declined, but this effect was dependent on cultivar. Cultivars Désirée and Astarte are representatives of two extremes: Désirée showed relatively little effect of temperature sum on crop performance and Astarte showed a large effect. The extreme behavior in Astarte was associated with a low number of stems per unit area.

The main effect of the ageing is through stem number. However, with a wide range of temperature sum, the stem number seems adequate to reach a relatively high yield. Over that same range, there usually was a close relationship between number of stems and number of tubers both per unit area and per plant (see also Fig. 3, Quadrant II). This indicates that the physiological age can be used to manipulate tuber number per unit area and average tuber weight, thus defining tuber-size distribution. Physiological age might have an additional effect on the coefficient of variation in tuber size. Such an effect is suggested by the data on proportion of tubers >55 mm (Table 6) but also by the data in lower size classes (data not shown). Also this effect was strongly dependent on cultivar. We did not record data on tuber numbers in classes with a much larger minimum grade as such tubers were rare in our research.

The large differences in behavior among cultivars calls for a separation of seed lots during storage whenever possible since each might need a special storage temperature regime dependent on when and how it will be used. Moreover, the yield reductions caused by warm storage might be drastic. Results also show that risk of early presprouting might be larger in some cultivars than in other cultivars.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The temperature sum before the end of dormancy has only a limited effect on the process of physiological ageing. The temperature sum after the end of dormancy is crucial for the process of ageing of seed tubers. Cultivars respond differently on the latter temperature sum and on the sequence of high or low temperatures. Cultivars with a high rate of ageing show much greater difference between the same temperature sums built up over time in different ways. In those cases, later warm storage was detrimental.


   ACKNOWLEDGMENTS
 
The authors thank the following students who have contributed to this research as partial fulfillment of the requirements for their MSc degrees: T. Biemond, M. Calon, H. Hoek, L. de Jong, F.M. Koops, P.L. Kooman, H. Marring, A. Nieuwhuijse, D.A.M. Risseeuw, H. Scholten, and C.J. van der Wekken. We also thank the staff of the research farm of the former Department of Agronomy, Wageningen University, and especially Ing. L. Mol and L. Haalstra, for their role in managing the experiments.

Received for publication August 23, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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