Characterization of qualitative trait variability in gamma-irradiated Chinese potato tubers (Plectranthus rotundifolius)

Introduction

Tuber crops constitute a major component of global food systems, ranking next to cereals and legumes as important sources of dietary carbohydrates [7,5] . These crops produce underground storage organs rich in starch that develop from modified stems or roots. Although their energy yield is comparatively lower than that of cereals due to high moisture content, tuber crops play a crucial role in food and nutritional security, particularly in developing countries [12] . In addition to their nutritional importance, many tuber crops possess medicinal properties and are traditionally used in the preparation of stimulants, tonics, carminatives, and expectorants [6,16] . Chinese potato (Plectranthus rotundifolius (Poir.)), synonymously known as Solenostemon rotundifolius (Poir.) J.K. Morton is an underutilized tuber crop belonging to the family Lamiaceae [2,11] . It is widely cultivated in parts of Africa and Asia and is well adapted to marginal environments and low-input agricultural systems [17,3] . The crop produces small, potato-like tubers that are rich in carbohydrates, minerals, and essential nutrients required for normal physiological functions [10,15] . Despite its nutritional and agronomic potential, Chinese potato remains underexploited, and its genetic improvement has been limited due to a narrow genetic base and lack of systematic breeding efforts. The assessment of genetic variability is fundamental to crop improvement, and morphological characterization is a primary tool for evaluating diversity, especially in less-studied crops [19] . Among morphological descriptors, qualitative traits are particularly valuable as they are usually governed by simple genetic mechanisms, exhibit minimal environmental influence, and show stable and distinct expression across generations [8,14] . These traits are therefore widely used for varietal identification, classification, and the detection of phenotypic variations in breeding populations. Several studies have reported the importance of morphoagronomic characterization in Chinese potato. Evaluations of germplasm collections have utilized a combination of qualitative and quantitative descriptors to assess variability in foliage, growth habit, and tuber characteristics [15,18] . The results indicated that qualitative traits serve as reliable markers for distinguishing cultivars due to their consistent expression [14] . Additionally, significant associations between growth traits and tuber yield have been reported, emphasizing their usefulness as selection criteria in crop improvement programs [13] . However, the limited genetic variability in Chinese potato necessitates the use of alternative approaches to broaden its genetic base. Mutation breeding, particularly through gamma irradiation, has proven to be an effective method for inducing heritable variations and generating novel phenotypes in vegetatively propagated crops [4,9] . Therefore, the present investigation aims to characterize qualitative trait variability in mutagenized populations of Chinese potato, with a focus on identifying stable and distinguishable traits for future breeding and selection programs.

Materials and Methods

The Chinese potato cv. Sree Dhara was used as the experimental material. Breeder seed tubers were subjected to gamma irradiation at ICAR-National Research Centre for Banana (NRCB), Tiruchirappalli. A total of 21 treatment doses ranging from 0 Gy (control) to 100 Gy were applied to induce variability. It was observed that tubers treated beyond 65 Gy did not survive and were excluded from further evaluation. Morphological observations were recorded following standard descriptors of the National Bureau of Plant Genetic Resources (NBPGR Descriptors) [1] .

Qualitative Trait Characterization

Qualitative morphological traits were recorded to assess the extent of induced variability in the mutagenized plants.

Observations were taken at 60 and 120 days after planting (DAP), as well as at harvest for tuber-related traits. The characterization was carried out using the minimum descriptor guidelines developed by the National Bureau of Plant Genetic Resources (2006). These descriptors are based on visually observable, discrete traits that are largely governed by simple genetic factors and are minimally influenced by environmental conditions.

The following qualitative traits were recorded

The observed qualitative traits included plant vigour (PV), pigmentation of stem (PS), pigmentation of leaves (PL), tuber shape (TS), tuber skin colour (TSC), tuber flesh colour (TFC), largest tuber shape (LTS), medium tuber shape (MTS), smallest tuber shape (STS), presence of aroma (PA), and biotic/abiotic stress response (BSR). These descriptors were used to assess the morphological and physiological variation among the evaluated samples.

Results and Discussion

The qualitative characterization of gamma-irradiated populations of Chinese potato showed noticeable variation across treatments, as defined by the descriptor classes (Table 1) and their corresponding scoring patterns (Table 2). The expression of these traits based on standard descriptors is given in Table 3 and their frequency distribution in Table 4 (Fig. 1). Plant vigour was predominantly classified as good (78.57%), followed by very good (14.29%) and poor (7.14%), with reduced performance observed at 65 Gy indicating the adverse effect of higher irradiation doses, which may be attributed to radiation-induced physiological and cellular damage as reported in mutation breeding studies [4, 9] . Stem pigmentation was largely green (85.71%), with occasional expression of purple and purple at the base in specific treatments, suggesting induced variation in pigment expression. Leaf pigmentation remained uniform (green) across all treatments, indicating stability under mutagenic conditions, as has also been reported for strongly inherited qualitative traits [19,8] . Tuber shape was mainly globose (78.57%), while oblong types (21.43%) appeared in certain treatments, particularly at higher doses, reflecting induced morphological variability similar to earlier findings in Chinese potato and related crops [15,18] . Similarly, the largest tubers were predominantly globose (85.71%) with limited occurrence of oblong forms, whereas medium tubers showed comparatively higher variation with both globose and oblong shapes. A greater degree of variability was observed in the smallest tubers, which exhibited globose, oblong, and long shapes, indicating that mutagenic effects were more evident in secondary tuber development, a pattern commonly reported in mutation breeding experiments [4] . Tuber skin colour varied between dark brown (57.14%) and light brown (42.86%), with a shift towards lighter shades in higher dose treatments, possibly due to alterations in pigment-related biochemical pathways [5] , while tuber flesh colour remained relatively stable with white (71.43%) as the dominant class and only slight variation towards light yellow (28.57%), indicating strong genetic control [10] . Aroma intensity was generally low to medium, with no high aroma types recorded, and a slight reduction was observed at higher irradiation levels, which may be due to disruption of metabolic pathways involved in volatile compound synthesis [9] . The response to biotic stress showed variation from very low to intermediate levels, with some treatments exhibiting nematode incidence while others remained unaffected, highlighting the dual effect of mutagenesis in inducing both susceptibility and tolerance [14] . Overall, the results indicate that gamma irradiation was effective in inducing variability in several qualitative traits, particularly at moderate doses, thereby providing useful scope for selection and genetic improvement in Chinese potato [4,9] .

Conclusion

The present study demonstrated that gamma irradiation is an effective tool for inducing qualitative variability in Chinese potato. Noticeable variation was observed in key traits such as plant vigour, stem pigmentation, tuber shape, skin colour, and stress response, while traits like leaf pigmentation and tuber flesh colour remained relatively stable. Moderate irradiation doses (25-55 Gy) were found to be most effective in generating useful variability without severely affecting plant growth. The variability observed, particularly in tuber morphology and stress response, offers valuable opportunities for selection and future crop improvement. Overall, mutation breeding can serve as a promising approach for broadening the genetic base and enhancing desirable traits in Chinese potato.

Acknowledgement

AcknowledgementThe authors gratefully acknowledge the support and facilities provided by ICAR-National Research Centre for Banana for carrying out the gamma irradiation treatment of breeder seed tubers. Their technical assistance and cooperation were instrumental in successfully conducting the mutagenesis experiment.

References

1. Abraham Z, et al. Minimal descriptors for agrihorticultural crops: root and tuber crops. New Delhi: NBPGR; 2006.
2. Aculey PC, Quainoo AK, Mahami T. Nutritional and anti-nutritional composition of Plectranthus rotundifolius. Food Chemistry. 2011;125(1):279-283.
3. Adebola PO, et al. Evaluation of Chinese potato under low-input conditions. African Journal of Agricultural Research. 2015;10(12):1452-1458.
4. Ahloowalia BS, Maluszynski M, Nichterlein K. Global impact of mutation-derived varieties. Euphytica. 2004;135(2):187-204.
5. Chandrasekara A, Kumar TJ. Roots and tuber crops as functional foods: A review on phytochemical constituents and their potential health benefits. Journal of Food Science and Technology. 2016;53(1):1-14.
6. Dwivedi S, et al. Medicinal properties of tropical tuber crops: A review. Journal of Medicinal Plants Research. 2014;8(12):456-463.
7. Gregory PJ, Wojciechowski T. Impact of climate change on root and tuber crops. In: Advances in Agronomy. 2020;160:1-47.
8. IPGRI. Descriptors for root and tuber crops. Rome: International Plant Genetic Resources Institute; 1991.
9. Jain SM. Mutagenesis in crop improvement under the climate change. Romanian Biotechnological Letters. 2010;15(2):88-106.
10. Kana JR, Gnonlonfin GJB, Harvey J, Wainaina J, Sanni A. Nutritional evaluation of some tropical tuber crops. Livestock Research for Rural Development. 2012;24(3).
11. Laxminarayana K, Jha AK, Reddy KR. Genetic variability in Chinese potato. Indian Journal of Agricultural Sciences. 2016;86(5):654-658.
12. Mishra S, Singh P, Singh R. Role of tuber crops in food and nutritional security. Agricultural Reviews. 2022;43(2):123-130.
13. Namo OAT, Opaleye MA. Growth and yield relationships in Chinese potato. Journal of Root Crops. 2018;44(1):45-50.
14. Nanema RK, Traore RE, Bationo-Kando P, Zongo JD. Morphological characterization of Plectranthus rotundifolius accessions in Burkina Faso. Genetic Resources and Crop Evolution. 2018;65:145-156.
15. Prematilake DP. Characterization and evaluation of Chinese potato (Plectranthus rotundifolius) germplasm. Annals of the Sri Lanka Department of Agriculture. 2005;7:123-130.
16. Ravindran PN, Nirmal Babu K. Underutilized medicinal tuber crops. Indian Journal of Traditional Knowledge. 2007;6(1):123-129.
17. Schippers RR. African indigenous vegetables: An overview of the cultivated species. Chatham, UK: Natural Resources Institute; 2000.
18. Sugri I, Maalekuu BK, Gaveh E, Kusi F. Morphological characterization of Chinese potato (Plectranthus rotundifolius) in Ghana. International Journal of Agricultural Sciences. 2013;3(3):220-228.
19. Upadhyaya HD, Reddy KN, Gowda CLL, Singh S. Identification of diverse germplasm lines for agronomic traits in crop plants. Genetic Resources and Crop Evolution. 2007;54:1307-1316.