Use of drought tolerance indices in corn breeding

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USE OF DROUGHT TOLERANCE INDICES IN CORN BREEDING

Chernobai L.M., Ponurenko S.G.

Abstract

The article describes application of method for evaluating and grouping test accessions according to their levels of drought tolerance (or, in a broader meaning, stress tolerance in general, if other stresses are detected) that is expedient to apply for preliminary description of breeding materials of different types. This method uses available data, does not require specific experiments, is based on the agronomic meaning of stress tolerance, and is simple enough with regard to calculations.

Keywords: maize hybrid, breeding, productivity, drought tolerance indices

A stable trend towards a global rise in temperature and an increase in carbon dioxide concentration in the atmosphere have been demonstrated in numerous surveys recently and aroused concern at all social levels [1, 2, c. 1]. The results of instrumental observations show that over the past 150 years the global surface air temperature increased, and warming accelerated from 0.35°C in 1910-1940 to 0.55°C in the 1970s and the present [3, c. 1]. A possible increase in climate risks and exacerbation of negative effects of environmental factors on all aspects of human lives and activities encourage scientists and international organizations to develop scenarios and forecasts of climate changes. Predictions based on mathematical simulations of atmospheric circulation, which can give expected values of climatic parameters for several decennia forward both in the global and regional scales have widely spread. There are several scenarios of climatic and agroclimatic conditions in Ukraine [4, 5, 6, c. 1]. Despite some discrepancies in quantitative simulation results, the authors believe that changes in season lengths, extension of frost-free period and crop vegetation duration, rise in heat provision during the growing season, some improvement in humidification in almost all areas of Ukraine will become general climatic features of 20112050 in Ukraine. At the same time, irregular rainfall during some periods of the year, which will make drought more probable, will become the main feature of warming. Droughts will often coincide with dry winds, damaging plants at different developmental stages and reducing their performance [7, c. 1].

Such climate scenarios are important because they allow one to lay out priorities in development of a particular sphere, to outline ways of adaptation to new conditions and to adjust the current course of development. In particular, prospects of increasing croppage due to a rise in the bioclimatic potential, provided effective counteraction to stressful events, open in plant production. It is noteworthy that utilization of the existing bioclimatic potential in Ukraine is far from being full.

Thus, [8, c. 1] assessed the utilization level of the bio- climatic potential in Sumy, Cherkasy, Poltava and Kharkiv regions related to corn cultivation as 60 - 70%.

The essential obstacle for obtaining consistently high yields is abiotic stresses, and drought is the main stress in the steppe and forest-steppe of Ukraine. In the last years of the XX century and in the last decennium, the share of arid events increased both in Ukraine and in Europe [3, c.1]; therefore, the issue of combating this negative phenomenon is becoming more important. According to the UN data, losses from droughts exceed 20% of the total losses from natural disasters. In particular, the annual drought-induced losses in corn crop are estimated as 15% or 120 million tons of grain [2, c.1]. Drought is often the primary cause of sharp fluctuations in yield, especially if agricultural technologies are not high enough. Thus, the yield of grain corn in the developed countries of North America and Europe is on average 8.7 t/ha versus 3.7 t/ha in less-developed tropical countries of Asia and Africa [9, c. 1]. Drought is the most important abiotic stress limiting and destabilizing the corn grain production in the both production environments. Its impact is eminently palpable in South and East Africa, where corn is predominantly grown under rainfed conditions. For example, in 1990-2009 the average yield of corn in South Africa was characterized by the variation coefficient of 23% versus 7% in the United States with the average yield of 4.1 and 9.8 t/ha, respectively [9, c. 1]. Irrigation is an important measure of yield stabilization under dry conditions. For example, the share of irrigated acreage of corn is about 14% in the US, 40% in China, and about 100% in Egypt, but it is often <10% in the majority of other countries, and the most of the world's corn acreage (160 million hectares) is naturally moisturized [2, c. 1].

The breeding of drought-tolerant plant varieties is the most effective way to stabilize yields under dry conditions. Drought tolerance as a sphere of interest in physiology and plant breeding is more than a century old [10, c. 2]. Research on changes in physiological and biochemical status of plants under drought and elucidation of mechanisms of tolerance allowed physiologists to propose numerous methods of drought tolerance evaluation by determining water status of plants and efficiencies of photosynthetic apparatus, transport systems, redistribution of assimilates as well as nonspecific adaptive responses at suboptimal temperature and moisture modes [11, 12, 13, 14, c.2]. Extensive factual evidence and available generalizing concepts allow plant physiologist to predict impact of changes in climatic factors on productive processes in agricultural plants, to determine the most efficient physiological and biochemical indices as possible criteria for breeding [15, c. 2]. However, one should take into account that such a complex process as the impact of drought on plants can not be definitively determined using individual parameters and requires a system of assessments covering different aspects of plant vital functions [11, c. 2]. Advances of plant physiology in studies of adaptation mechanisms to stresses created foundation for biotechnological improvements of plants, including genetic engineering methods. Peculiarities of the regulation of gene transcription and protein synthesis were investigated for a number of gene families that seem promising for purposes of their involvement in adaptive response formation [16, c. 2].

Breeding for drought tolerance has a number of specific problems caused both by a common breeding scheme with existing methods and by biological characteristics of crops. Firstly, it is directly connected with understanding of the "drought tolerance" concept per se. If drought tolerance of wild species is often defined as the ability to survive, for crops it should be defined in terms related to performance [14, 17, c.2]. Richard thinks that selection for yield capacity by default combines all the known and unknown factors associated with drought tolerance, i.e. breeding for yield capacity is conducted under dry conditions. Secondly, phenotypic assessments of yield capacity per se both under stress and under other conditions in general are considerably influenced by paratypic variability, while for breeders the genotypic component of total variability is only of value. It can be considered proved drought tolerance, in the context of breeding, is a complex trait that shows a high level of genotype-environment interaction [18, c. 2]. Cooper, however, in the context of physiology, it can be simplified to more genetically determined traits [19, 20, c. 2]. Under stress, the shares of intra- and interplot variability increase accompanied by with a corresponding reduction in the genotypic variance, hence, to improve the phenotyping accuracy, one should also take into account, if possible, associated traits [21, c. 2]. In addition, the structure of phenotypic variability is compounded by genotype-environment variability, which is mostly epigenetic [22, c. 2]. These facts complicate the practical use of marker-assisted selection (MAS). It was shown that different QTLs of one trait express under different environmental conditions [23, 24, c. 2]. Genotype-phenotype associations, on which MAS principles are based, largely depend on the phenotyping accuracy, which remains a bottleneck for majority of broad associative research on genome [25, 26, 27, c. 2]. To assess effects of genotype-environment interaction, different statistical models based on analyses of variance and regression, which can already be considered traditional, are widely used. Simulations, which are not a mainstream yet, but developing rapidly, are also coming into common use. The complex of adaptive capacity assessments proposed by Kilchevsky and Hotyliova [28, c. 2], which combines different authors' approaches in a single system estimating genotype, environment and their interaction, can serve as an example of type I models. Simulations are based on the representation of interactions between different structural components using simultaneous differential equations. They enable conducting computer experiments within a wide range of initial conditions to test the response of a system to a set of external impacts [29, c. 2]. Using the potential of such simulations, DuPont Pioneer developed the software application «AquaMax» specifically to support breeding for drought tolerance Messina [30, c. 2], but field tests are still crucial.

Objective characterization of conditions of the environment, in which or for which breeding is conducted, is another important problem in breeding for drought tolerance. Highly instable weather conditions during the growing season in different years is an attribute of continental regions. It is clear that the importance of the "drought tolerance" trait will be only great under dry conditions, provided its sufficient strength and high repeatability from year to year. On better humidification, this trait does not provide competitive advantages to genotype. Therefore, creation of novel varieties should be focused on a specific area, under the environmental conditions of which the genotypic potential of a variety will be most fully unlocked, including due to genetic systems of adaptability.

Typification of weather conditions in the area of a breeding center and target environments of prospective dissemination of novel hybrids as well as detection of major factors limiting performance are essential prerequisite for building appropriate breeding strategies and choosing correct methods of breeding material assessments [31, c. 3]. Solving this problem requires reliable criteria characterizing environmental conditions in terms of their utility for crops. To characterize heat and water provision, including for description of drought phenomena, over a hundred parameters, mainly in the form of indices, were proposed, and the number of these parameters is increasing with evolving satellite monitoring. In domestic investigations, Selyaninov's hydrothermal coefficient (HTC) providing estimates comparable with the standardized precipitation index (SPI), which is commonly accepted in the world literature McKee [32, c. 3], is widely used to characterize hydrothermal conditions of the territory or the growing season.

We analyzed hydrothermal conditions of the corn growing season for a 65-year period (1951-2015) basing on data of Kharkiv weather station and using HTC. Aridity levels were categorized as recommended by Zoydze [33, c. 3]. The analysis (Table 1) showed that the general water provision in May - July was adequate: there were no droughts in these months in 60% of years. The category of medium droughts with the drought frequency in May - July of 15-17% and slightly more frequent droughts in August (21.5%) is the next most common. Generally, arid phenomena dominated in August both in terms of frequency and in terms of intensity as compared to previous months. The frequency of severe droughts gradually increased from 3.1% in May to 12.3% in August. Severe droughts were recorded at the frequency of 10-11% in June-August and of 6.2% in May. During an individual growing season, drought phenomena occurred at different stages and had different duration and intensity, therefore, we can assume that the number of years when corn plants suffered from droughts during growth and development will be much greater. There is a need for other, more sensitive criteria to describe water provision during the growing season that would quickly reflect the impact of drought on plants, depending on their developmental phase and crop status, and take into account previous reserves of available soil moisture. It is expected that these criteria will be found due to the rapid development of the remote sensing technology.

Table 1 - Droughts during the Corn Growing Season (Kharkiv, 1951-2015)

The deficit of moisture available to plants associated with drought is not the only abiotic factor reducing corn yields in the forest-steppe and steppe zones of Ukraine. A significant reduction in yields due incomplete seed set in cobs is caused by long-term effects of high temperature during corn anthesis and pollination, which is usually observed in the 2^nd - 3^rd 10 days of July. Having analyzed extremality of air temperatures

in July (by the maximum daily air temperatures) and intensity (duration) of the factor in Kharkiv in 19512015 (Table 2), we found that 42% of all the years did not have high air temperatures in July. However, 25% of the years were noticeable for high air temperatures in July, and 6 years had strong thermal stressed in July (Table 3).

Table 2 - Frequency and Duration of Thermal Stresses in July (Kharkiv, 1951-2015)

Analyzing the weather of years with high temperatures in July (Table 3), we paid attention to the fact that the long-term effect of high temperatures usually took place simultaneously with severe drought (4 of 6 years), but in 2002 and 2010 the HTC did not indicate drought. It seems rather strange for 2010, since the drought in that year is known to be one of the harshest for the entire period of meteorological observations [34, c. 4]. The high HTC in July of 2010 is attributed to two rain showers with 18 and 24 mm of rain 17 days apart. A similar situation occurred in 2002, when 18 and 65 mm of rain fell 9 days apart. What is more, there were no rains before and after July's precipitation amidst high air temperature in these two years. This indicates that drought is a too complicated natural phenomenon, and one parameter is not enough to characterize it.

Table 3 - Characteristics of Years with Extreme Air Te: eratures in July, (Kharkiv, 1951-2015)

When Table 3 being analyzed, it should also be noted that five of six years with high strength of the thermal factor in July fell into the period of 1999-2015, while the previous 48-year period contained only one year (1959) with similar conditions in July. If this trend continues in the future, and this is what most of the climate scenarios say, the relevance of breeding for heat tolerance will augment, especially this concerns heat tolerance of gametophyte.

The complexity and ambiguity of the environment characterization for the purpose of breeding using meteorological parameters as well as impossibility to define the overall impact of weather factors on individual genotypes under uncontrolled field conditions, which are associated with individual phenorythms, led to the development of characteristics of stressful conditions, including drought, and adaptability of genotypes based on analysis of variations in yields of varieties in stressful and favorable environments, which are presented as so-called "yield indices". This approach is consistent with the agronomic understanding of stress as mentioned above.

Yield indices are suitable to determine the effect of any stress and are often used to evaluate drought tolerance. The essence of this method consists in assessment of the degree of drought-induced reduction in yield in comparison with wet backdrop presented as percentage or in other units, which is a drought index. Literature describes a large number of drought indices widely tested in different crops [35, 36, 37, 38, 39, 40, c. 4]. The drought indices that we used in our work as well as their design are summarized in Table 4.

Table 4 - List the Major Drought Indices

It is important that all the indices use <4 parameters, namely variety yields under dry and sufficiently wet conditions and the average yield of several varieties under these conditions. Moreover, the average yield of several varieties is used to describe the environmental conditions, drought intensity, and yields of individual varieties - to characterize responses of individual genotypes to drought. The advantages of methods evaluating drought tolerance by indices are as follows: availability of necessary data, possibility to investigate a large number of varieties, simple calculations, presentation of the most important agronomic characteristic, yield, in units. Of drawbacks, we should admit that drought indices do not provide information on antistress mechanisms, which may vary depending on genotypes.

Fernandez (1992) divided genotypes into four groups depending on their performance under stressful and no stress conditions: genotypes that are superior in both environments (Group A); genotypes having high rates only under favorable conditions (Group B); genotypes that are better only under stressful conditions (Group C); and genotypes with negative characteristics both under stressful and under favorable conditions (Group D). It is believed that optimum selection criteria should differentiate Group A from the others [45, c. 5].

We used a set of drought tolerance indices (Table 4) to characterize corn hybrids of Kharkiv breeding. The study material was field study results on 20 accessions in the hybrid nursery for the period of 2010-2015. The year condition index [28, c. 5] showed that the study years were arranged from the most to the least favorable as follows: 2015-2014-2012-2013-20112010. It should be noted that yields of all the hybrids were lower than those under favorable conditions only in 2010 and 2011. For further analysis, the yields in 2010 and 2015 were chosen as the most contrastive ones.

Since drought indices are based on one set of variables, we performed correlation analysis to determine the degree of concordance between estimates based on different indices. The results are summarized in Table 5.

Table 5 Correlation Coefficients between Drought Tolerance Indices

The correlation analysis revealed that some indices had correlation coefficients close to 1 (or equal to 1 for a given data set), i.e. they are interchangeable. These are the following groups of indices: YI -K2STI- SNPI, TOL - SSPI, YSI - SSI - RDI, MP - STI and HM- GMP-STI. Occurrence medium or weak relationships in the correlation table suggests that these indices reflect different properties of accessions in relation to drought tolerance.

To reduce the number of variables, we conducted factor analysis by principal component method with varimax rotation of normalized data. Two factors that account for 99.8% of the data variation were highlighted. Factor 1 accounts for 59.4% of the total variance; factor 2 - 40.4%. Analysis of factor loadings of drought indices (Table 6) showed that factor 1 with high positive factor loadings includes indices, high values of which correspond to high yields both under stressful and under favorable conditions. This factor can be referred as a "tolerance factor." In the structure of factor 2, high positive values of factor loadings are inherent to indices that reflect susceptibility to stress - TOL, SSI, SSPI, and high negative values of factor loadings - to indices associated with high yields in a favorable environment - YSI, RDI, therefore, this factor can be referred as “susceptibility factor."

Table 6 - Factor Loadings of Drought Indices

Distribution of accessions into four groups in principal component space (Fig. 1), according to the adopted classification [41, c. 7], selected hybrids with high parameters both under favorable and under stressful conditions (Sector A), notably, the average yield in this group in a favorable year is 8.33 t/lia with 43% 6 yield reduction under unfavorable conditions. Hybrids in Sector A are inferior to Sector B hybrids with the average group yield of 9.15 t/ha in a favorable year, but have advantages under unfavorable conditions because of a conspicuous decrease (58%) in yields of hybrids of the latter group under stressful conditions.

Fig. 1 Distribution of Corn Hybrids in Principal Component Space

Hybrids in Sector C show advantages in terms of yield in an unfavorable year, which is manifested in a relative parameter - the minimal depression in yields (38%) in an unfavorable year. This ensured the maximum average grain yield of 4.98 t/ha in this group in a stressful year, with the lower yield of 8.04 t/ha as compared to Groups A and B in favorable 2015. Hybrids labeled as Sector D gave low yields (in the test sample) both in favorable and in stressful years - 7.43 and 4.04 t/ha, respectively, though their yields were higher by 0.2 t/ha than those of Sector B hybrids in an unfavorable year.

Thus, occurrence of hybrids with various responses to drought proves the theoretical possibility of selecting assortment of hybrids from appropriate environmental groups (A, B and C) for environments with varying recurrence of droughts in order to ensure maximally sustainable yields of corn. Use of several indices narrowed down to minimally correlating principal components to analyze drought tolerance (as a function of yield) allows one to largely dispose of paratypic "noise", which is present in any phenotypic evaluations, and to obtain fairly clear classification of the test material. The proposed method of evaluating and grouping test accessions according to their levels of drought tolerance (or, in a broader meaning, stress tolerance in general, if other stresses are detected) is expedient to apply for preliminary description of breeding materials of different types, since this method uses available data, does require specific experiments, is based on the agronomic meaning of stress tolerance, and is simple enough with regard to calculations. More detailed studies of drought tolerance mechanisms and identification of valuable accessions should be conducted using physiological and biochemical methods followed by hy- bridological analysis to determine breeding value and donor properties.

drought stress tolerance breeding

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