Evaluation of selection indices for heat tolerance and their correlation with yield in some chickpea (Cicer Arietinum L.) genotypes of sudan

Among the food legumes, chickpea occupies second position after beans. It is cultivated in more than fifty countries worldwide ¹². Chickpea grains are nutritious and a leading source of proteins, carbohydrates, vitamins and minerals for the masses in many developing and non-developing nations. World’s major chickpea production comes from India, Australia, Pakistan, Turkey, Burma, Ethiopia and Iran where the crop is largely grown on arid and semi arid tropics, dependent on rainfall as a source for soil moisture ¹.

In Sudan, chickpea is the third most economically important food legume crop after faba bean and cowpea, as a cash crop that generates income for farmers and rural communities, and as a significant source of protein for Sudanese people ³. It is traditionally grown as a winter crop in River Nile State, northern Sudan. However, chickpea production has recently expanded to the central clay plain of central Sudan. Nevertheless, a biotic stresses remain one of the leading constraints to the global chickpea productivity, causing serious yield loss ⁴.

Chickpea reproductive stages (flowering and podding) are vulnerable to external environmental changes and heat stress ⁵. Frequent decreases in the yields of chickpea seed were observed when plants were exposed to high (> 35°C) temperatures at flowering and pod development stages⁶. Most importantly, increasing evidences of heat stress (HS) is receiving serious attention and, is going to be an emerging threat to the cool season grown global food crops including chickpea⁷. A 1 C⁰ increase in seasonal temperature decreased chickpea yield by 53 kg ha^-18.

Several indices based on the yield under non-heat stress (Yp) and heat stress (Ys) have been introduced for the selection of heat tolerant genotypes. Among these, the indices employed in various stress conditions are stress tolerance (TOL) and mean productivity (MP) introduced by some researchers⁹. Stress susceptibility index (SSI) was also produced in 1978 ¹⁰. Moreover, stress tolerance index (STI) and geometric mean productivity (GMP) produced in 1992¹¹, while Harmonic mean of yield (HM) was released in 2009 ¹². Furthermore, yield index (YI) was introduced in 1997 ¹³, and the yield reduction ratio (YRR) in 1998¹⁴.

To date, limited genetic resources for heat stress tolerance in chickpea have been reported ¹⁵. In order to screen heat tolerant crop genotypes, several physiological and plant breeding based parameters are available ^{6, 17}Equally, yield based indices are also essential for comparing crop performance under non stress and stress condition and thus enable in selecting superior genotype ⁸.

Thus, achieving optimum grain yield underheat-tolerant heat stress remains prime criteria to chickpea breeders. Implication of various indices helps in measuring yield loss and screening heat stress tolerant genotype under stress condition ¹⁹. However, heat tolerant varieties/cultivars are needed for improving chickpea yields in warm season environments and late sowing conditions especially in central Sudan (Gezira State), to expand its cultivation to new areas and improving its resilience to the impacts of climate change. The genetic variability presents in the base population for desired characters play an important role in development of improve chickpea genotypes. Less information is available on chickpea genotypes tolerance to heat grown under Sudan conditions. Keeping in view the above research findings, the present study was carried out to investigate different heat tolerance indices as well as their correlation and indentifying the potential chickpea genotypes for heat stress and non-heat stress conditions.

The study area of this research was conducted over one consecutive winter season 2018/2019 in two locations in the Agricultural Research Corporation (ARC) of Sudan. Namely the Gezira Research Station Farm (GRSF), located in central clay plain of Sudan within the latitude 14°24' N, and longitude 33°29' E and altitude 407 meters above sea level with soil characterized by cracking heavy clay (vertisols), very low water permeability, pH of 8.3, organic matter (0.4%), nitrogen (0.04% ppm), and phosphorus (ESP, 4 ppm) and (2) farmers field in the Northern state of Sudan, Merowe locality ( latitude: 18° 27' 0" N, longitude: 31° 49' 59" E, elevation: 258 meters above sea level).

Forty three chickpea genotypes were selected from advanced materials of the national chickpea breeding program. In addition, five commercial chickpea cultivars which were released by the Agricultural Research Corporation of Sudan namely, Shiekh Mohamed, Merowe, Wad Hamid, Salwa and Hwata (Table 1).

Across all growing locations, the land was prepared by disc ploughing, disc- harrowing, leveling and ridging. In two locations the genotypes were evaluated in two environments i.e., normal sown (15 November) and late sown (5 December). The experiment was conducted in Alpha Lattice design with three replications under normal sown and late sown conditions. Each replicate consisted of twelve incomplete blocks and four plots in each block. Each genotype was planted in a separate plotwhich was consisted of one ridge /row of 4 meter length, witha plant-to-plant and row-to-row distance of 10 cm and 60 cm,respectively. The plots were separated by a distance of 60cm. Irrigation was carried out at 12-14 days intervals to avoid any water stress. A starter dose of nitrogen in the form of urea urea was applied at a rate of 43 kg N/ha with the third irrigation. Weeds were kept to the minimum by hand removal during the first month after sowing. Seed yield was assessed from a net area of 2.4 m² (one row x 4 m long x 0.6m).

The results of the analysis of variance for different traits under non-heat stress and heat stress conditions are presented in Table 2. There were highly significant differences among the genotypes for all the various Agro-morphological traits were recorded under both normal sown (non-heat stress) and late sown (heat stress) conditions, indicating the presence of variability that can be exploited through selection. This result was inconformity with the results reported by many authors, ²⁵.

Similarly, previous studies on chickpea landraces also reported by ^{26, 27}The interaction between genotype (G) x location (L) were not significantly for some traits under heat stress and non-heat stress field conditions.

Different heat tolerance indices were calculated on the basis of seed yield of the chickpea genotypes under normal sowing date (non-heat stress) (Ypi) and late sowing date (heat stress) (Ysi) conditions are presented in (Table 3). The highest seed yield (3.93 t ha^-1) under non-heat stress (Ypi) condition was obtained by two genotypes no. 1 and, while the lowest (1.84 t ha^-1) was produced by genotype no. 41. On the other hand, under heat stress condition (Ysi), the highest seed yield (2.52 t ha^-1) was recorded for genotype no. 11 and the lowest (0.83 t ha^-1) for the genotype no. 28 (Table 3).

Comparison of genotypes based on reduction of yield and yield related traits by growing them under non-heat stress and heat stress environments could be an important parameter for selection of tolerant genotype under drought, heat and other a biotic stresses in crop plants^{28, 29} In the present study, Reduction in yield as affected by heat varied among chickpea genotypes and ranged between 8.9% and 66.7%.Similarly, several kinds of stress indices based on seed yield are available that help in comparing crop performance under non-stress and stress condition and thus, enable selection of superior genotypes¹⁸. Likewise, heat tolerance indices viz, geometric mean productivity (GMP), yield index (YI), mean productivity (MP), stress susceptibility index (SSI), stress tolerance index (STI) and tolerance index (TOL) could be employed for selecting superior genotypes relying on yield performance under heat stress in field condition.

The genotype no. 11 and the check Wad Hamid (no. 46) exhibited excellent seed yield in heat stress (2.52 and 2.35 t ha^-1 as well as in non - heat stress condition (3.29 and 2.58 t ha^-1 (Table 3). Further, these two genotypes have the least stress susceptibility index (0.48 and 0.18) and minimum reduction in seed yield (23.3% and 8.9%) respectively, due to heat stress. It isreported that heat resistant genotype had the highest heat tolerance efficiency, minimum heat susceptible index and minimum reduction in seed yield due to heat stress³⁰.

Considering YI as an important selection index for heat tolerance, genotypes no. 11(1.65), 47 (1.57), 48 (1.49), 4 (1.39), 44 (1.36), 43 (1.30), 34 (1.27), 30 (1.25) and 26 (1.22), showed higher YI value than the check Merowe (1.15) (Table 3). While, considering MP as an important selection index for heat tolerance, eleven genotypes viz., 47 (2953), 11 (2907), 48 (2902), 45 (2835), 43 (2787), 4 (2786), 1 (2725), 40 (2616), 34 (2581), 39 (2571) and 30 (2524) showed higher MP value than the check Wad Hamid (2462). Importantly, considering GMP as an important selection index for heat tolerance, genotypes no. 47, 48, 11, 4, 45, 43, 44 and 34 showed higher value than the check Wad Hamid (no. 46) (Table 3). Importantly, YI index as an important selection parameter for drought tolerance in rice have been reported^31. Similarly based on this parameter both drought and heat tolerant genotypes were identified in common bean ¹⁸

Thus genotypes exhibiting higher MP, GMP and YI could be efficiently used in selecting superior genotypes under heat stress. While emphasizing SSI as another important selection parameter for heat tolerance, genotypes showing SSI<1 were considered as higher heat tolerance ¹⁸ The result indicated that the genotypes no. 11 and 36 showed lower SSI value than the four checks Shiekh Mohamed, Merowe , Salwa and Hwata (Table 3). Similarly this selection index was employed for selecting drought tolerant genotypes in chickpea^{32, 33} Additionally, selection of superior genotypes based on lower value of SSI and TOL under drought tolerance was discussed in chickpea³⁴ According to stress tolerance index (STI), genotypes no. 11, 47, 48, 4 and 43 exhibited the most and 1, 41 and 28 the least relative tolerance, respectively³⁵

Study ³⁴ indicated that the genotypes with high STI usually have high difference in yield in two different conditions. They reported in general, similar ranks for the genotypes were observed by GMP and MP parameters as well as STI, which suggests that these three parameters are equal for screening heat tolerant genotypes. For sensitivity heat index (SHI) and relative heat index (RHI), the genotypes no. 11, 46, 4, 12, 26, 30, 31, 44, 47, 48 and 36 were the most relative tolerant genotypes. The genotypes no. 11. 4, 34, 26 and 30 plus the commercial cultivars, Wad Hamid (no. 46), Shiekh Mohamed (no.44), Salwa (no. 47) and Hwata (no.48) recorded least stress susceptibility index and minimum reduction in seed yield, maximum geometric mean productivity, highest yield index, maximum mean productivity, maximum stress tolerance index, minimum tolerance index, least sensitivity heat index and maximum relative heat index.

To determine the most desirable heat tolerant criteria, the correlation coefficients between Yp, Ys and other quantitative indices of heat tolerance were calculated (Table 4). In other words, correlation analysis between seed yield and heat tolerance indices can be a good criterion for screening the best cultivars and indices used. Seed yield in stress condition (Ys) was positively and significantly corrected with geometric mean productivity, yield index, stress tolerance index and relative heat index and negatively and highly significant correlated with stress susceptibility index and sensitivity heat index. This indicates that these six factors were more effective in identifying high-yielding cultivars when heat stress was present.

 It was found that seed yield under heat stress condition (Ys) showed negative and highly significant correlation with SSI ³⁶. It is reported) that GMP and STI were significantly and positively correlated with stress seed yield ³⁷. On the other hand, Yield in non-heat stress condition (Yp) was positive and highly significant correlated with geometric mean productivity, yield index, stress tolerance index and tolerance index. A researcher ³⁸ found the seed yield under non-heat stress (Yp) was positively and highly significantly correlated with GMP and STI. It is believed that the most appropriate index for selecting stress tolerant cultivars is an index which has partly high correlation with seed yield under stress and non-stress conditions. Seed yield in non-stress (Yp) condition was positive and non-significant correlated with stress susceptibility index ³⁹. This result was comparable to result obtained by Ehdaie and Shakiba 1996. found in wheat that there was no correlation between stress susceptibility index and yield under optimum condition.

Considering MP, it had high and positive correlation with GMP, YI, STI, RHI and Yp but it had negative correlation with TOL and SHI. STI showed high and positive correlation with GMP, YI, RHI, Yp and Ys, but it indicated negative association with SSI and RHI. Both SSI and TOL showed high significant negative association with RHI, whereas SSI and TOL exhibited high positive correlation.

Based on results of this study, it could be concluded that there was a wide range of genetic variability detected among the seed chickpea genotypes for heat tolerance. This variability can be exploited in the improvement for heat tolerance in this crop.

The environmental stress (heat stress) has been confirmed reduce seed yield of all chickpea genotypes.

However, GMP, YI, STI and RHI show highly significant and positive correlation with seed yield under stress only when stress is too severe. These four indices can be considered as suitable criteria for selecting heat tolerant and highly efficient genotypes in environments where heat is predominant. While the SHI and SSI showed negative and highly significant correlation with yields under heat stress condition. These two indices can be also used as the most suitable indicators for selecting heat tolerant chickpea genotypes.

In consideration to all indices, the cultivars, no. 44 (Shiekh Mohamed), 46 (Wad Hamid), 47 (Salwa) and 48 (Hwata) and genotypes, no. 11, 4, 30, 34 and 26 were found to be high yielding and hence they were defined as the most heat tolerant genotypes. These genotypes are intended for further evaluation for varietal approval to recommend for general cultivation on farmer fields in heat affected areas. While the genotypes no. 28, 41, 42, and 33 were the most sensitive for heat stress.

The authors gratefully acknowledge the International Center of Agricultural Research and Dry Areas (ICARDA) for making available the plant materials. We also thank the staff of Merowe and Gezira Research Stations.

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