The effect of planting time on nutrients, phenolics, and antioxidant activities of rice grown in different soil salinities

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Rice (Oryza sativa) is a staple food crop with a significant role in nutrition and health. Its production yield mainly depends on climatic conditions. Soil salinity, an abiotic factor as a resulting from drought, also impacts rice growth, production, and quality. Researchers have widely investigated newly developed salt–tolerant rice varieties and suitable fertilizers to enhance rice yield and quality. However, the effects of planting times of in– season and off–season rice varieties grown in soils with varying salinity levels on rice qualities, such as nutrients, bioactive compounds, and health–related properties remain unexamined. This study evaluated the qualities (nutrients, phenolics, and antioxidant activities) of thirty-eight in season– and off–season rice samples grown in soils with different physicochemical properties: electrical conductivity extract (ECe), pH (pHH2O), organic matter (OM), available minerals (potassium and phosphorus), and soluble salts (calcium, magnesium, sodium, and potassium). Results indicated that rice nutrients, total phenolic contents (TPCs), and antioxidant activities were influenced by planting seasons rather than rice varieties or the soil salinity marker ECe. The in–season rice samples exhibited higher levels of protein, iron (Fe), TPCs, and antioxidant activities compared to the off–season rice samples, which had higher vitamin B3 content. Principal Component Analysis (PCA) and Pearson’s correlation were used to determine the relationships among rice qualities and soil physicochemical properties. The rice samples were divided into four groups: Group 1 consisted of rice samples grown in soil with high ECe and soluble salts, showing unrelated rice nutrients, TPCs, and antioxidant activities; Group 2 included off–season rice samples grown in soil with high OM that contained high vitamin B3 content; Group 3 contained in–season rice samples grown in soil with high pHH2O that had high energy and fat content, and Group 4 comprised in–season rice samples with high protein, Fe, TPCs, and antioxidant activities, demonstrating moderate to very weak correlations with soil properties. This information can be used to determine the appropriate utilization of fertilizer in soil with different salinity levels and assist in the selection of applicable planting seasons for specific rice qualities.