SYMBIOTIC PERFORMANCE, C ASSIMILATION AND PLANT WATER-USE EFFICIENCY IN FOUR GROUNDNUT VARIETIES GROWN IN NORTH WEST PROVINCE FIELD, SOUTH AFRICA, MEASURED USING ¹⁵N AND ¹³C NATURAL ABUNDANCE TECHNIQUES

Groundnut is a crucial legume crop that contributes significantly to worldwide food security, particularly in regions with limited agricultural resources. However, crop plants growth in Africa is generally poor due to soil infertility and climate change, leading to low grain yield, especially on smallholder farms. This study evaluated root nodulation, N₂ fixation, N contribution, and C accumulation, as well as plant water-use efficiency of four groundnut varieties grown in the field during the 2023 and 2024 cropping seasons using the ¹⁵N and ¹³C natural abundance techniques. The results revealed significantly marked differences in plant growth, N content, %Ndfa, N-fixed, soil N uptake, and grain yield in both the 2023 and 2024 cropping seasons. Groundnut varieties obtained 35.51 to 85.77 % of their N nutrition from symbiotic fixation and contributed 22.13 to 168.48 kg of N-fixed ha^-1 in both the 2023 and 2024 cropping seasons. The shoot ¹³C values were similar for groundnut varieties in 2023 and 2024 cropping seasons, ranging from -28.57 ‰ to -28.19 ‰ for 2023, and -30.17 ‰ to -28.34 ‰ for 2024. The data from this study show that the groundnut varieties that fixed the most N also produced the greatest amounts of biomass and provided markedly higher grain yields. In conclusion, we found marked differences in symbiotic performance between and among the four groundnut varieties tested over the two cropping seasons, with varieties Kwarts and PC474 exhibiting greater symbiotic dependency on N₂ fixation for their N nutrition and higher amounts of N-fixed and greater grain yield. These two varieties showed greater shoot δ 13C values and hence higher water-use efficiency. These two varieties have the potential for use in breeding programs.