Chemical Composition and Expression Analysis of Oil-Related Genes in Upland Cottonseeds

Cottonseed is an important resource for edible oil and protein. Here, we evaluated cottonseed oil, protein, and gossypol contents using traditional chemical methods and near-infrared reflectance spectroscopy (NIRS) in diverse upland cotton (n = 456) and sea island cotton (n = 205) germplasm. In upland cotton, oil content averaged 21.23 ± 3.98% (12.74–43.56%), protein averaged 23.63 ± 4.63% (15.53–49.79%), and gossypol averaged 1.47 ± 0.21 mg/g (0.06–2.07). Correlation analysis showed a significant negative association between oil and protein (r = −0.125, p < 0.01; confirmed by NIRS: r = −0.171, p < 0.01), a significant negative association between protein and gossypol (r = −0.375, p < 0.01), and a significant positive association between oil and gossypol (r = 0.409, p < 0.01). In sea island cotton, oil, protein, and gossypol contents averaged 24.82 ± 6.15% (14.64–41.43%), 25.75 ± 2.04% (18.84–39.00%), and 1.60 ± 0.15 mg/g (1.22–2.08), respectively. The oil–protein association was strongly negative by NIRS (r = −0.744, p < 0.01), whereas correlations with gossypol were weak and not significant by the traditional method. After screening and evaluation, high oil and protein varieties were identified in upland cotton (n = 15) and sea island cotton (n = 6). Fourteen extreme-oil upland materials were further used to examine flowering-date effects on oil accumulation and physiological indicators, showing rapid oil accumulation and a flowering-date-dependent maximum. Finally, qRT-PCR analysis of lipid-metabolism-related candidate genes showed that seven genes were expressed significantly higher in high-oil than in low-oil materials (p < 0.05), peaking at the late stage of oil accumulation. GhDGAT1 and GhDGAT2 showed positive regulatory effects on oil accumulation, whereas GhFAD3 and GhKCR2 showed negative regulatory effects. Collectively, these findings provide compositional benchmarks, clarify trait relationships, and identify candidate genes useful for breeding cotton cultivars with improved seed oil/protein traits.