ARCHITECTURE OF THE ROOT SYSTEM OF POTATO CULTIVARS GROWN IN AEROPONICS
DOI:10.30825/5.EJPAU.14.2018.21.1

Krystyna Rykaczewska, Krystyna Zarzyńska, Dominika Boguszewska-Mańkowska
Plant Breeding and Acclimatization Institute – NRI, Potato Agronomy Department, Jadwisin, Poland

ABSTRACT

The root system architecture of potato plants can play a key role in breeding new cultivars with higher drought tolerance. The purpose of this work was to assess the variability of the root systems of selected potato cultivars in the periods of highest sensitivity to drought. The plants of fifteen cultivars from different maturity classes were tested by using aeroponics. The study of the aboveground part of plants, tuber formation and the root system features was conducted at the early stage of tuberization. The results of the experiment were analyzed using ANOVA. Means were separated with Tukey’s contrast analysis at significance level 0.05. Significant differences among maturity class of cultivars in terms of length of stems, fresh matter of the above-ground part of plants, rooting depth, and fresh and dry matter of roots were found. The root system differences between genotypes were related to maturity class. At the early stage of tuberization very early cultivars were characterized by shallower rooting depth and less fresh and dry matter of roots than early and medium early cultivars. It was found that cultivars Tetyda and Finezja are cultivars of special beneficial traits of the root system.

Key words: drought stress, potato, rooting depth, Solanum tuberosum, tuberization.

INTRODUCTION

Potato (Solanum tuberosum L) crop is the world’s number one non-grain food commodity and the fourth main food crop in the world after maize, rice and wheat, with 368 million tons produced on an area of 20 million hectares in 2013 [2]. It is grown in more than 100 countries, mainly in Asia and Europe and it is a plant typical mainly of temperate climate [3]. Drought, the most important abiotic stress in agriculture, is of particular importance in potato [6, 14]. Research on drought tolerance in this crop is still limited but in the last few years has increased in response to the expansion of its cultivation to drought prone areas [7, 8].  Drought is a severe issue particularly for non irrigated fields. Improvement of drought resistance in potato is important. The root system architecture of plants can play a key role in breeding new cultivars with higher drought tolerance.

Root studies, which were far behind other plant sciences, changed during the second half of the 20th century, but have accelerated since that time [1, 16]. It was not only the technological innovations that enabled this trend but it is mainly the notion that there is a missing gap in plant studies, which is incomplete without this link [15, 17]. Rooting at depths below the plough layer is a trait which can improve access to subsoil water and alleviate drought stress. The purpose of this work was to assess the variability of the root systems of selected potato cultivars in the periods of highest sensitivity to drought.

MATERIALS AND METHODS

The study was carried out in 2015 in the Potato Agronomy Department on the root systems of 15 cultivars from different maturity classes. Cedron, Denar, Justa, Lord, Miłek (very early), Aruba, Etola, Gwiazda, Hubal, Michalina (early), Etiuda, Finezja, Oberon, Stasia, Tetyda (medium early) were tested by using aeroponics.

Three devices for the production of potato minitubers in aeroponics (2.51 m length × 1.26 m width × 0.60 m depth) of our own design were used in the study. They were located in a greenhouse. Pre-sprouted minitubers of transversal diameter of about 1 cm were planted on 24th May into baskets with dimensions of 5 x 7 cm filled with mineral wool. They were placed into holes on top of the devices. In this way the foliage of the plants grew in the light while roots, stolons and tubers developed under total darkness inside the aeroponics chamber. Minitubers from a particular maturity group were planted in one of the three devices. The plant density was 20 per m². The total number of plants was 60 per aeroponic device with 12 of each cultivar. A modified nutritive solution (pH=5.5, EC=2.5 mS cm⁻¹, NO³⁻, SO₄²⁻, H₂PO₄⁻, Cl⁻, K⁺, Ca²⁺, Mg²⁺, NH⁴⁺, Na⁺ and micronutrients) from Rolot et al. [2002] was prepared and supplied by fog nozzles located at the bottom of the device. The fog nozzles sprayed a small quantity of nutrient solution every 3 min for 10 s. during the day and every 5 min for 10 s. during the night. Residual nutrition solution was recirculated. Plants were collected at the beginning of tuberization, consecutively on 3^rd, 9^th and 23^rd June (40, 47 and 61 days after planting) depending on the maturity class of cultivar. On these dates the studies of the aboveground part of plants, tubers forming and the root system features were conducted.

The length of stems, mass of the above-ground parts of the plant, leaf area, number and mass of tubers of transverse diameter greater than 1 cm and also the maximum depth range of roots and their fresh and dry weight were determined. Roots of tested cultivars were divided into layers of 10 cm up to 50 cm, and summed above 50 cm, and each layer was separately weighed (Fig.1). Dry matter of roots was determined after drying at 50°C for 24 hours and next at 105°C until no further weight reduction was noted.

The results of the 12 plants of each cultivar were divided into three groups and average values for 4 plants were calculated. Therefore the number of repetitions was 3. The results of the experiment were analyzed using ANOVA with Statistica 12 computer program. Means were separated with Tukey’s contrast analysis at significance level 0.05.

RESULTS

Significant differences among maturity class of cultivars in terms of length of stems, mass of the above-ground part of plants, number and mass of tubers, rooting depth, and fresh and dry matter of roots were found  (Tab. 1–3).

State of growth of the aboveground parts of plants and tubers at the start of the root system studies
It was found that the cultivars with the longest stems and the highest fresh mass of above-ground part of plants belong to two groups of maturity: early and medium early (Tab. 1). The differences in leaf area were not confirmed statistically. The number of tubers per plant and their fresh mass were significantly differentiated among the maturity classes of cultivars, but the results indicate that the tested cultivars were at the early stage of tuberization (Tab. 1).

Characteristics of the root system at the early stage of tuberization
Rooting depth of tested cultivars reached mean values significantly higher in cultivars of early and medium early maturity class than in cultivars of very early maturity class (Tab. 2). However, differences in rooting depth also exist amongst genotypes of the same maturity class, in the group of medium early cultivars. ‘Finezja’, ‘Stasia’ and ‘Tetyda’ were characterized by the greatest rooting depth while ‘Etiuda’ the smallest (Fig. 2A). Rooting depth of tested cultivars was significantly positively correlated with stem length, the above-ground part of plants, leaf area and fresh and dry matter of the roots (Tab. 4).

Fresh matter of the roots was significantly differentiated  among  the maturity classes of cultivars (Tab. 2). The lowest was in the group of very early cultivars. However, among cultivars of early and medium early maturity groups significant differences were found (Fig. 2B). The cultivars Aruba and Tetyda were characterized by the greatest fresh matter of the roots. The fresh matter of the roots of tested cultivars was significantly positively correlated with the above-ground part of plants, leaf area, rooting depth and fresh and dry matter of the roots (Tab. 4).

Dry matter of the roots was significantly higher in early and medium early cultivars than in very early cultivars (Tab. 2). It was particularly positively correlated with fresh matter of roots (Tab. 4).

Percentage share of fresh matter of roots with different rooting depth was significantly  differentiated among maturity classes of cultivars in all tested zones of depth with the exception of layer 0–10 (Tab. 3). In the deepest layer below 50 cm, the cultivars from medium early and early maturity classes produced the largest mass of roots, significantly higher than the cultivars from the very early maturity class (Fig. 2C).  The percentage share of fresh matter of roots of tested cultivars in the deepest layer was significantly positively correlated with the above-ground part of plants and was highly significantly positively correlated with rooting depth, and root fresh and dry matter (Tab. 4).

Correlation among tested features of potato roots and tuber number and tuber fresh matter
None of the tested features of the potato root system were correlated with the number and fresh matter of tubers at early stage of tuberization (Tab. 4).

DISCUSSION

It is becoming increasingly evident that optimization of root architecture of potato plants is vital for creating new cultivars more tolerant to drought. Observation of root systems under field conditions requires a lot of time and labor [4]. In our laboratory we utilized the devices for the production of potato minitubers in aeroponics of our own design [12]. They allow the study of the root system, growing under optimal conditions, in its entirety without damaging fragments thereof, which can happen in both field trials and pot experiments.

In carrying out studies on the potato root system, the time for analysis is important. In our experience, we decided on the period of early stage of tuberization, because it is the period of greatest sensitivity of potato plants to drought and high temperature, which was demonstrated in our earlier studies  [10, 11, 13]. Due to start up of tuberization at different times, cultivars  from each maturity class were planted in separate devices. During subsequent collections of plants, the roots are well developed and do not show signs of ageing. Period analysis of the roots in other studies varied and was performed on several dates [4, 5, 18] or fell to the beginning of plant maturity [4, 19].

 In our study, the important observation was a significant positive correlation between rooting depth of tested cultivars and the stem length and the above-ground part of plants. Similar results were obtained by Iwama [4] on Japanese cultivars. The maximum rooting depth of studied cultivars in our experiment reached 115 cm. Another important feature of the size of the root system is its mass. In our experiment, the highest value during early stage of tuberization was 120 g per plant and significant positive correlation between fresh matter of roots and the above-ground part of plants was found. Some authors express the size of the root system in relation to the unit area, or as the total length of all roots of a single plant. Iwama [4] showed that the variation in root length per unit area varied from 0.38 to 4.86 km m^-2 in wild potato relatives, while it varied from 1.12 to 1.69 km m^-2 in cultivars and from 0.63 to 1.40 km m^-2 in breeding lines. Subsequently Wishart [19] calculated that total root length of twenty-eight genotypes varied from 40 to 112 m per plant.

 An often used measure of potato root system is the dry matter of roots. In our study, the dry matter of roots was highly positively correlated  with the fresh matter of roots and was on average 3.12 g per plant. In studies by Lahlou and Ledent [19] in pot experiments in a greenhouse the dry matter of roots was similar and ranged from 0.84 to 3.80 per plant, but in the field it was higher, ranging from 1.1 to 11.8 g per plant. Cited authors have not studied the correlation of dry matter of roots with the fresh matter of the above-ground part of plants.

In our research we put emphasis on the rooting at depth of tested cultivars. The fresh matter of roots below 50 cm varied over a wide range of 0.6 to 34.3 g per plant. Cultivars of the highest weight of roots growing below 50 cm, may be of particular importance due to the improved access to subsoil water and alleviation of drought stress. According to Iwama [4] potato roots under field conditions are concentrated mostly in the plow layer up to 30 cm in soil depth but some roots extend up to 100 cm depth. In our studies there are large differences in root mass in the layer below 50 cm not only among the maturity class of cultivars but also among genotypes of the same maturity class. It was found that cultivars Tetyda and Finezja are cultivars of special beneficial traits of the root system. The root system differences between genotypes were generally related to maturity class. At the early stage of tuberization very early cultivars were characterized by shallower rooting depth and less fresh and dry matter of roots than early and medium early cultivars.

CONCLUSIONS

Under global climate change, potato tolerance to drought stress will become more important for gaining stable yields in the future. Breeding of new cultivars with excellent root system features will contribute to more efficient utilization of water for potato production and will improve tolerance to drought during the growing season. The results of the studies presented in this paper make it possible to demonstrate the high variability of tested cultivars in terms of the features of roots. The study of root architecture system should be continued in the future.

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