Drought Resistance of Two Soybean (Glycine max (L.) Merr.) Varieties on Rainfed Soil Inoculated with Biofertilizer and Their Genetic Closeness

Drought tolerance testing of various soybean varieties under specific regional conditions is important, but it has never been conducted on the rainfed soils of Gunungkidul, Indonesia. This study aims to assess the drought-tolerance responses of Demas 1 and Dering 2 soybeans, evaluate the response of root nodule formation to biofertilizer application, and determine the relationship between genetic closeness and phenotypic responses. The study was conducted using a semi-field pot experiment with a factorial design of 2 (drought levels) × 3 (biofertilizer variations) × 2 (variety types). The response of Demas 1 and Dering 2 was assessed based on growth rate, chlorophyll, proline, malondialdehyde (MDA) content, and the number of root nodules. Genetic closeness was analyzed using NTSys ver. 2.11a based on RAPD profiles. The results showed that drought and variety factors suppressed growth, chlorophyll, and proline content, whereas biofertilizer factors had no effect on most parameters. Both varieties showed drought tolerance, characterized by increased proline and MDA, and a decrease in growth of only 10–30%. Dering 2 is genetically close enough to Demas 1 (similarity >82%) and Dering 1 (similarity >95%). The RAPD-based DNA similarity aligned with physiological phenotypic similarity for Demas 1 and Dering 2, especially in drought tolerance.