Abstract
In field trials conducted in 2000–2002, we examined the influence of pre-planting treatments of seed tubers on the formation of leaf area index. The potato varieties used were Agrie Dzeltenie (early), Piret (middle-maturing) and Ants (late). The following treatments were used: untreated control, thermal shock and pre-sprouting. Pre-sprouting treatments of all varieties and thermal shock treatment of the variety Agrie Dzeltenie had a significant effect on the leaf area index. The value and timing of maximum leaf area index were: variety specific, 4.0 units of the early variety Agrie Dzeltenie, 3.7 units of the middle-maturing variety Piret and 3.9 units of the late variety Ants. The weight of the haulms of the plants developed from physiologically older seed tubers formed faster and remained smaller. Pre-planting treatment of seed tubers provided quicker field emergence. The slower the potato plants developed the haulms, the greater the maximum weight achieved. Pre-planting treatments influence the leaf area index. The importance of this influence lies in potato varieties with different maturity times since increases in quality and yield depend on the size of the photosynthetic area.
Introduction
Potato yield is strongly affected by the size of the leaf area and the duration of photosynthesis. The duration of photosynthesis is influenced by agro-climatic conditions and agrotechnics, e.g., night frosts can shorten the duration of photosynthesis. Rapid emergence (i.e., the formation of optimum-sized leaf area) and maintaining the plant's productivity for as long as possible are vital for obtaining high potato yields. However, early development is also associated with the earlier senescence of the leaves (Allen et al., Citation1979; O'Brien et al., Citation1983). Plants that have emerged earlier have the benefit of using more sunlight. It is known that the rate of photosynthesis decreases with senescence because the respiration rate in young leaves is higher than in older leaves. The rate of photosynthesis is highest in leaves that have just reached their maximum leaf area but this rate declines in leaves older than 50 days (van der Zaag, Citation1992) and the ageing process progresses more rapidly at higher temperatures. Plants from pre-sprouted tubers develop faster and reach their maximum leaf area earlier but their leaves also age more rapidly.
The rate of photosynthesis depends on the leaf area, which itself depends on the growing conditions and variety. These conditions and varieties may vary widely, and consequently the leaf areas of potato varieties differ by a magnitude of three. However, a larger mass of top leaves (canopy) may be an indicator of a larger leaf area, a higher rate of photosynthesis or a higher yield only when the leaves are not overshadowed and all the necessary components are provided. Unilateral nitrogen fertilization significantly increases the weight of the haulms of the potato plant as well as the leaf area (Grindlay, Citation1997) and also reduces the effects of diseases and weeds (Jornsgard et al., Citation1996; Möller et al., Citation1998). The vigorous growth of haulms or the density of the plants after canopy closure will cause overshadowing of many of the leaves, especially those on the lower section of the plant. Under sufficient light intensity, 320 µmol m−2 s−1 (Struik & Wiersema, Citation1999), the processes of photosynthesis and respiration in a plant are in balance. As light intensity decreases, a greater number of the lower leaves switch from net producers to net consumers of photosynthetic products. The production of organic matter from the whole plant therefore decreases and the tuber yield may be negatively affected.
Leaf area index (LAI) indicates the ratio of the assimilative area of the leaf and the surface area (Watson, Citation1947). LAI needs a value higher than 4 for as long a period as possible to achieve an optimum photosynthetic rate; otherwise, photosynthetically active radiation causes the production of organic matter to decrease (Scott & Wilcockson, Citation1978; Allen & Scott, Citation1980; Khurana & McLaren, Citation1982).
Certain pre-planting treatments of seed tubers can result in significantly higher potato yields through a combination of faster emergence, the formation of optimum-sized leaf area and the maintenance of productivity for as long a period as possible. Seed tubers that are stored at higher temperatures for a week before planting become physiologically older (Eremeev et al., Citation2003). Haulms of physiologically older seed reach their maximum weight and the LAI reaches maximum value faster than plants of younger seeds and thus an economically optimum tuber yield can be harvested much earlier (Eremeev et al., Citation2005). Research using seed tubers that were exposed to different temperatures at various time intervals before planting was undertaken to examine the effects of physiological age on LAI, the growth and development of potato haulms.
Materials and methods
Experimental site and design
The experiment was carried out during the growing seasons of 2000, 2001 and 2002 at the Department of Field Crop Husbandry at the Plant Biology experimental station (58°23′N, 26°44′E) of the Estonian University of Life Sciences (EMU). A randomized complete block with four replications was used (Hills & Little, Citation1972). The size of the test plot was 21 m2. Within-row spacing was 25 cm and the rows were 70 cm apart. Seed tubers with a diameter of 35–55 mm were used in the experiment.
Meteorological conditions
The Estonian climate is a transitional climate from maritime to continental. It is mainly influenced by the Baltic Sea and the north-eastern part of the Atlantic Ocean. Year-round intensive cyclonic action causes moist and continuously changing weather conditions. Summer is relatively short and cool, while autumn is long and winter is mild. The vegetation period (average diurnal temperature continuously above 5°C) usually begins in the second half of April, lasts 170–180 days and ends in September or October. The period of active plant growth, in which the average diurnal temperature is continuously above 10°C, usually ranges from 115 to 135 days (Tarand, Citation2003). The experimental area belongs to the south Estonian upland agroclimatic region where the annual sum of active temperatures greater than 10°C averages 1750–1800 degree-days and the total annual precipitation is 550–650 mm. The mean amounts of precipitation were, during the May–September vegetation period of the experimental years, greater than average in June and July, and less than average in May, August and September (). The air temperature (the mean of three years) was similar to that of the 32-year average (1966–1998); only July was significantly warmer.
Table I. Average monthly temperatures (°C) and precipitation (mm) in Estonia during the vegetation period.
Plant material
The late maturing variety Ants and the middle-maturing variety Piret, both bred at the J[otilde]geva Plant Breeding Institute in Estonia, and the early maturing variety Agrie Dzeltenie, bred at Latvia's Priekuli State Plant Breeding Station, were used in the experiments. These cultivars were chosen because local varieties are better adapted to Estonian climatic conditions and are, therefore, able to give reasonably high yields of good quality tubers. Also, the use and effect of fertilizers and plant protection products on these cultivars have been studied on EMÜ research fields since 1951.
Soil conditions and analysis
The soil of the experimental field was Stagnic Luvisol according to WRB classification (Deckers et al., Citation2002) with a texture of sandy loam and a humus layer of 20–30 cm (Reintam & Köster, Citation2006). Soil analyses were carried out 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 1 M KCl and in 0.01 M CaCl2 1:2.5), organic carbon by Tjurin, Ca and Mg in NH4OAc at pH 7 (Soil Survey Laboratory, Citation1996), P and K after the Mehlich-3 method (Handbook on Reference Methods for Soil Analysis, Citation1992). The Kjeldahl method was used to determine the content of total N of the soil (Procedures for Soil Analysis, Citation1995).
The humus layer of the experimental field had the following analysis: pHKCl ≈6.2; C 1.4%; Caavb 674 mg kg−1; Mgavb 101 mg kg−1; Pavb 183 mg kg−1; Kavb 164 mg kg−1; Ntot 0.11%; 56% sand; 35% silt and 9% clay.
Pre-planting treatments
All the dates of the particular stages of the pre-planting treatments were the same each year during the experiment. The seed tubers were kept in a storehouse, until 30 March, under equal storage conditions and at 4°C. All the potato tubers, irrespective of pre-planting treatment, were planted on the same day, 7 May. The sum of the temperatures above 0°C differed according to the treatment. The treatments were applied between 30 March and 6 May.
The effects of various pre-planting treatments of seed tubers on potato growing were analysed. The treatments were as follows:
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Untreated (0) treatment – 37 days. The seed tubers were kept from 1 April to 6 May, i.e., 37 days at 4°C. The total accumulated temperature was 148°C.
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Thermal shock (TS) treatment – 37 days. The seed tubers were initially removed from storage and kept for 30 days, from 1 April to 30 April, at 4°C (accumulated temperature of 120°C); then for two days, from 31 April to 1 May, at 30°C (accumulated temperature 60°C) and lastly for five days, from 2 May to 6 May, at 12°C (accumulated temperature 60°C). These seed tubers received during the thermal shock treatment an accumulated total of 240°C, which is 92°C more than in 0 treatment.
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Pre-sprouted (PS) treatment – 37 days. The seed tubers were removed from storage and kept for 37 days, from 1 April to 6 May, at 12°C. These seed tubers received during the pre-sprouting treatment an accumulated total of 444°C, which is 204°C more than TS and 296°C more than in 0 treatment.
Experimental field agrotechnics
The agrotechnic inputs included composted manure (60 t ha−1) used as an organic fertilizer before the autumn ploughing and a compound mineral fertilizer (78 kg N: 72 kg P: 117 kg K ha−1) was applied at planting. For plant protection, the insecticide Fastac and fungicides Ridomil Gold, Acrobat Plus and Shirlan were used. All active ingredients for plant protection were used with 400 litres of water per ha.
Measurements
LAI was determined at five-day intervals. Each sample consisted of four plants from the test plot. The time of sample collection and the number of samples collected varied among treatments and years due to differences in emergence dates and durations of the vegetative period (e.g., in 2001 the vegetation period was long, lasting 118 days).
Leaf area can be measured in various ways including portable devices (Gordon et al., Citation1994; Lusk, Citation2002; Veteli et al., Citation2002) or manual methods (Haverkort et al., Citation1991; Burstall & Harris, Citation1983; Khurana & McLaren, Citation1982). The so-called ‘copy method’ was selected for determining the leaf area in this research experiment. The haulm and leaves of the sample plants were weighed together. Randomly selected leaf segments were then scanned at 100 dpi resolution and saved as black-and-white Windows Bitmap images (*.bmp). A special PC program written by Department of Botany EMU was used to calculate the areas of the scanned leaves.
Statistical analysis
Regression analysis was conducted on the compiled data (see Mead et al., Citation1993; Lauk, Citation1995, Citation1996; Lauk et al., Citation1996) using the following formula: y=a+bx+cx 2 ; where y is the argument function – LAI, a is the constant term of the equation, b and c are regression coefficients and x is the argument, the number of days after planting (DAP).
Standard errors (SE) were calculated using Lauk and Lauk's method (Citation2000) and Student's theoretical criterion (see Mead et al., Citation1993) to determine confidence (CL) limits (CL05–statistical significance at p=0.05).
To assess the probability of differences between the three treatments, the three potato varieties and the three-year average, the least significant differences (LSD05) were calculated according to the corresponding methodology (Lauk et al., Citation2004).
The results are expressed not only as a three-year average but also for separate seasons to indicate the effect of different years.
Results and discussion
The dynamics of LAI depending on the year
At the start of the growing cycle of the potato plant, the increase in LAI was highest in 2002 when the increase was statistically significant (LSD05 0.3) until 55 DAP. Evidently, the weather of this period of the growing season was most favourable for haulm growth. The LAI started to decrease slowly, after 55 DAP, because the soil water supply decreased and nearly reached wilting point. The haulms turned yellow and dried up. The LAI was lower (LSD05 0.4) during the 2000 vegetation period, in the periods 45–50 DAP and 80–100 DAP, than the average LAI of the three experiment years. This was due to the relatively cool weather conditions during the vegetation period. The good weather in 2001 was a significant factor in the highest LAI of the experiment's years, from the period of 65 DAP (LSD05 0.3) until the harvest. Moisture evaporation through the plant foliage in the field is somewhat lower (41%) than from the soil (59%). The value of LAI should be at least 3.0, in order to considerably reduce the evaporation from the soil surface, and this level of shading also reduces the amount of radiation into the soil by 95–98% (MacKerron & Waister, Citation1985).
Earlier experiments (Eremeev et al., Citation2001) with late potato varieties have shown that LAI reaches the maximum value (average 3.7 units) by 72 DAP, and then starts to decrease. The Norwegian scientist Eltun (Citation1996) concluded that the maximum LAI was formed at 68 DAP. Our experiment records that the highest LAIs were reached at 68–81 DAP; in 2002 the maximum LAI (3.7 units) was attained at 68 DAP, in 2000 at 73 DAP (3.6 units) and in 2001 at 81 DAP (4.5 units). The weather conditions were most favourable for the growth of potato plants and development of the leaf area in 2001. The water supplies were sufficient in the period of intensive growth and tuber formation, and were also efficient protection against late blight, providing effective photosynthesizing foliage until the harvest.
The dynamics of LAI depending on the seed tuber treatment
The pre-planting treatment of the seed tubers had a different effect on the leaf area formation of the different potato varieties. The initial slower development of the 0 variant caused the prolongation of its growing period. The LAI in untreated variants could be determined until 110 DAP, while the assimilative leaf area of treated variants was destroyed by 105 DAP (). J[otilde]udu et al. (Citation2002) indicated that plants developed from pre-sprouted tubers have a better ability to assimilate nutrients from the mother tuber. The pre-planting treatment of seed tubers (PS and TS) stimulated the development of the haulms and their weight exceeded the same characteristic of 0 variants until 60 DAP. Later, the weight of the haulms of the plants developed from the 0 variant exceeded both TS and PS until the leaves perished. In the current experiment both of the pre-planting treatments of potato seed resulted in larger LAI.
Table II. The effect of seed tuber pre-planting treatment on leaf area index (average of 2000–2002).
The LAI maximum was reached, in all variants, at 72–76 DAP. The maximum LAI in the PS variants (3.7 units) was reached at 72 DAP, in TS variants (3.8 units) at 73 DAP and in the 0 variant (4.1 units) at 76 DAP. The LAI of the PS variant plants, starting from 75 DAP, was significantly (LSD05 0.4) lower than in the 0 variants and from 80 DAP in TS variants (LSD05 0.4).
Dynamics of LAI depending on the variety
Comparison of different potato varieties indicated that from the start of the haulm development until 90 DAP, the LAI was quite stable in all variants (see ). The earliest leaf development was observed in the early variety Agrie Dzeltenie: its LAI exceeded the middle-maturing variety Piret until 85 DAP (0.2–0.4 units) and the late variety Ants until 70 DAP (0.1–0.2 units). The LAIs of Piret, from 95 DAP, and of Agrie Dzeltenie, from 85 DAP, were, compared with Ants, statistically lower (LSD05 0.4) (see ).
Table III. The effect of potato variety on leaf area index (average of 2000–2002).
Maximum LAI was attained by all varieties at 72–75 DAP. Agrie Dzeltenie reached its maximum LAI (4.0 units) at 72 DAP, followed by the middle-maturing variety Piret (3.7 units) and the late variety Ants (3.9 units), at 74 DAP and 75 DAP, respectively. Thus, the difference between varieties in attaining the maximum LAI was 1–3 days and 0.1–0.3 units.
Dynamics of the LAI in Ants
According to Putz (Citation1986), rapid development and growth of assimilative leaf area occurs from emergence to the start of tuber formation. The pre-planting treatments of Ants seed tubers had a different effect on the leaf area development. The 0 variant of Ants had a slow initial development which meant that the vegetation period was longer. The LAI at 40 DAP, of the 0 variant and the TS variant, was 0.5–0.6 units lower than that of the PS variant ().
Table IV. The effect of seed tuber pre-planting treatment on leaf area index of variety Ants (the average of 2000–2002).
The LAI of all the 0 variants could be determined at 115 DAP, whereas in those for TS and PS variants the assimilative area had perished by 105 DAP. Plants developed from tubers treated with TS had a smaller leaf area during the vegetation period compared with the 0 variant. The PS variant of Ants had a higher LAI (LSD05 0.4) until 45 DAP and then, from 70 DAP until the end of vegetation, its LAI was lower than for the 0 variant.
All the variants of Ants reached their maximum LAI, 72–79 DAP. The PS variant reached maximum LAI (3.7 units) at 72 DAP, followed by the TS variant (3.9 units) and the untreated variant (4.3 units), at 74 DAP and 79 DAP, respectively. Consequently, the optimum leaf area needed for the photosynthesis of Ants in the TS variant perished two days later than in the PS variant and five days earlier than in the 0 variant.
Dynamics of LAI in Piret
The LAI of TS (0.3 units) and PS (0.7 units) in Piret exceeded the 0 variant at 40 DAP (). Due to the fast initial development, the LAI of treated variants exceeded that of untreated variants, from the beginning of the vegetation until 60 DAP for TS treatment and until 55 DAP for PS.
Table V. The effect of seed tuber pre-planting treatment on leaf area index of variety Piret (average of 2000–2002).
All the variants of Piret reached their maximum LAI value, 72–77 DAP. The PS variant reached 3.5 units at 72 DAP, followed by the TS variant's 3.7 units at 73 DAP, and the 0 variant's 4.0 units at 77 DAP. The LAI subsequently decreased gradually on a daily basis due to the loss of leaves. This decrease was clearly observed in the TS and PS variants since they reached their maximum LAI earlier, by four and five days, respectively, than the 0 variants. The LAI of the TS variant started to decrease from 85 DAP (LSD05 0.6) and that of the PS variant from 65 DAP (LSD05 0.5) until the end of the vegetation period.
Dynamics of LAI in Agrie Dzeltenie
As with other varieties, the TS and PS variants of Agrie Dzeltenie emerged earlier. The LAI of these two variants proved higher than the 0 variant at the start of the vegetation period. The LAI of the TS variant increased until 40 DAP (LSD05 0.5) and the PS variant until 45 DAP (LSD05 0.6) (see ). Moreover, PS variant plants reached their LAI maximum (3.7 units) at 72 DAP, followed by the TS variant (3.8 units) at 73 DAP and the 0 variant (4.1 units) at 76 DAP. The LAI of the thermally treated variants was subsequently lower than in the untreated variants from 60 DAP in the TS variants and from 65 DAP in the PS variants. The lower LAI in the thermally treated variants could be explained by the fact that a plant uses a larger amount of energy for better development of its underground organs. The LAI, in all varieties, was fairly stable from the start of haulm development until 90 DAP. As a three-years’ average, maximum LAI (3.9 units) formed at 74 DAP and on the 50th day after emergence. In all variants of Agrie Dzeltenie, the LAI formed relatively steadily during the whole vegetation period, reaching its maximum at 72–73 DAP. The maximum LAI of TS variant (3.9 units) and PS variant (4.0 units) were attained by 72 DAP and in the 0 variant (4.1 units) by 73 DAP.
Table VI. The effect of seed tuber pre-planting treatment on leaf area index of variety Agrie Dzeltenie (average of 2000–2002).
Conclusions
It should be noted that the weight of the haulms of plants developed from physiologically older seed tubers formed faster and remained smaller. Pre-planting treatment of seed tubers (i.e., increasing the physiological age) provided quicker field emergence. The slower the potato plants attained the maximum weight of the haulms, the bigger they formed. On physiologically older plants, obvious signs of senescence started to appear earlier (partial wilting and yellowing of the lower leaves). The value and timing of maximum LAI were, variety specific, 4.0 units at 72 DAP of the early variety Agrie Dzeltenie, 3.7 units at 74 DAP of the middle-maturing variety Piret and 3.9 units at 75 DAP of the late variety Ants.
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