High temperature stress on grain phytic acid and mineral bioavaility in rice

Abstract

The study entitled “High temperature stress on grain phytic acid and mineral bioavailability in rice (Oryza sativa L.)” was conducted at the Department of Plant Physiology, College of Agriculture, Vellayani, during 2023–2025. The present study examined the influence of high temperature stress (36 ± 2°C) imposed during the reproductive stage on grain phytic acid accumulation, mineral bioavailability, physiological performance, and yield traits in eleven rice genotypes differing in pericarp colour and stress tolerance. The experiment was conducted during the Rabi 2024 season using a Completely Randomized Design (CRD) with two temperature regimes (ambient and high temperature) and three replications. The plants were maintained under normal conditions until panicle initiation and after which they were exposed to high temperature condition in a polyhouse. Morphophysiological, biochemical, and molecular parameters, including the expression of the SPDT (SULTR-like Phosphorus Distribution Transporter) gene, were analysed to elucidate the molecular basis of genotypic variation in phytic acid content. High temperature stress significantly affected physiological, biochemical, and yield parameters, though the extent of response varied among genotypes. Exposure to elevated temperatures resulted in a 7–18 % reduction in plant height, a 21–51 % decrease in leaf area, and a 15–32 % decline in chlorophyll content. Yield related traits were also adversely affected, with spikelet fertility declining by 2–15 % and grain yield decreasing by 18–50 % due to elevated temperature. Pigmented rice genotypes such as Kalabath and Assam Black maintained higher chlorophyll retention, membrane stability, and yield indicating strong thermotolerance. In contrast, white pericarp genotypes such as Khira, White Jasmine, and Jeerakasala exhibited substantial reductions in photosynthetic efficiency and spikelet fertility, resulting in greater yield losses. Jyothi and Urunikaima demonstrated moderate tolerance, with intermediate stability under stress Under heat stress, a significant biochemical response was evident, reflected by a 2–34 % increase in grain phytic acid compared to plants grown under ambient conditions. This increase reflects a temperature-induced shift in phosphorus metabolism that helps

stabilize internal phosphorus reserves. However, it was accompanied by a 20–30% reduction in bioavailable iron and zinc, demonstrating a strong negative correlation between phytic acid accumulation and mineral bioavailability. Molecular analysis revealed that the SPDT gene, responsible for phosphorus transport to grains, was upregulated under stress, thereby promoting enhanced phytic acid biosynthesis. Bran colour exerted a strong influence on stress response and nutrient balance. Pigmented genotypes, particularly those with black and red pericarps, exhibited only a 10–15% reduction in antioxidant activity compared with 25–30% in white rice. Their higher levels of phenolics, anthocyanins, and flavonoids likely mitigated oxidative injury, preserving grain integrity and mineral stability. Conversely, white genotypes, lacking such antioxidant protection, experienced a stronger rise in phytic acid and greater yield decline. In conclusion, high temperature stress during the reproductive stage significantly influenced grain phytic acid content, mineral bioavailability, and yield parameters in rice, with the magnitude and nature of these effects varying among genotypes of different bran colours. The results demonstrate that the increase in phytic acid under heat stress is a protective yet nutritionally disadvantageous response associated with altered phosphorus metabolism and reduced mineral availability. The observed inverse relationship between phytic acid accumulation and mineral bioavailability underscores the importance of developing rice genotypes with optimized phosphorus transport and enhanced thermotolerance. The study thus fulfilled its objective by identifying pigmented genotypes such as Kalabath and Assam Black as promising donor lines combining superior antioxidant potential, balanced phytic acid regulation, and stable mineral bioavailability, providing valuable resources for breeding climate-resilient and nutritionally enriched rice cultivars.