Rice (Oryza sativa L.) is one of the major staple crops widely cultivated in Vietnam. Traditionally, rice is grown primarily for grain harvest or biomass use in animal feed, while rice leaves have rarely been utilized as a source of raw material for producing bioactive compounds. However, during the panicle initiation stage, rice leaves contain many nutrients beneficial to human health, such as chlorophyll and phenolic compounds, which aid digestion and boost the immune system. Utilizing rice leaf biomass to produce bioactive compounds offers a promising solution to increase the value of rice cultivation.

During leaf development, metabolic changes occur within plant tissues as the plant grows and accumulates bioactive compounds. The production and content of these compounds vary throughout leaf development. The concentrations of chlorophyll and polyphenols are affected by plant maturity, with younger leaves containing significantly higher amounts of polyphenols than mature or senescent leaves.

Light conditions significantly influence the quality, composition, and quantity of compounds in plants. Chlorophyll content is typically higher in plants grown under low light compared to high light conditions. In shaded environments, plants increase chlorophyll content and absorb light more efficiently in chloroplasts to optimize light capture. Variations in light intensity also affect the accumulation of secondary metabolites such as flavonoids and phenolic compounds, as well as their antioxidant activities.

Extracting health-beneficial compounds from rice leaves can add value to the rice production process, offering a new direction for enhancing the value of rice grown in Vietnam. Therefore, evaluating the effects of growth stages and light intensity on the accumulation of chlorophyll and polyphenols in IR50404 rice leaves is essential to determine optimal cultivation conditions and harvest timing.

To vary light intensity, the study implemented shading treatments using mesh covers. The experimental conditions included: no shading (Tn), single-layer mesh shading (S1), and double-layer mesh shading (S2). Light intensity measurements were taken from 6:00 AM to 6:00 PM under all three shading conditions. Light intensity increased significantly in the morning, peaked between 11:00 AM and 1:00 PM, and then gradually decreased toward the end of the day.

The study results showed that 3-week-old rice leaves grown under S1 conditions (average light intensity of 22,686 Lux, approximately 60% light reduction) had the highest chlorophyll accumulation, reaching 1,357.49 µg/g fresh leaf weight. Meanwhile, rice grown under natural light (Tn, average intensity of 60,791 Lux) had the highest polyphenol content (6.88 mg/g fresh leaf weight) at 5 weeks after sowing.

Furthermore, antioxidant activity analysis using DPPH free radical scavenging revealed that rice grown under S1 conditions exhibited higher DPPH activity (p < 0.05) compared to S2 conditions (average light intensity of 6,072 Lux, approximately 90% light reduction) and natural light (Tn). The findings demonstrate that varying light intensities influence the accumulation of bioactive compounds in rice leaves, including chlorophyll a, chlorophyll b, and polyphenols, at different growth stages of the IR50404 variety. These plant-based compounds likely play a key role in the antioxidant activity of rice leaves.

This study confirms that light intensity, controlled through shading treatments, significantly affects the accumulation of beneficial bioactive compounds such as chlorophyll and polyphenols, as well as antioxidant properties in IR50404 rice leaves. Leaves harvested at 3 weeks of age under single-layer shading conditions showed the highest chlorophyll accumulation, while those harvested at 5 weeks under natural light had the highest polyphenol content.