Seed phytic acid concentration affects rice seedling vigor irrespective of soil phosphorus bioavailability

Rice with a black-colored pericarp (hereafter, black rice) is well-known as an antioxidant-rich food, but a high grain phytic acid (PA) concentration affects its nutritional quality. However, phytic acid helps improve seedling vigor, which is crucial for enhancing subsequent plant growth. This study investigated the effect of seed phytic acid concentration in black rice on seedling vigor compared to the effects on white rice. In the first experiment, three phytic acid concentrations in the seeds of black rice, low (LPA, 15.5 mg g^-1 per seed), medium (MPA, 24.7 mg g^-1 per seed), and high (HPA, 35.4 mg g^-1 per seed) were tested for seedling vigor in phosphorus-deficient soils. The HPA seedlings showed substantially increased seedling vigor and shoot P uptake due to early root development and enhanced physiological processes. LPA grown seedlings showed increased ethylene production in response to P stress, which is the main physiological mechanism modulating seedling growth under P stress conditions. In the second experiment, the three phytic acid concentrations in black and white rice seeds were tested under low and high soil P conditions. Again, LPA seedlings showed significantly reduced seedling vigor in both rice varieties in P-deficient soils. Interestingly, seed phytic acid and external P application had an additive effect on seedling vigor, suggesting that the combined effect further improved seedling growth. Our results reveal that black rice seeds with a HPA concentration can be used as a seed source for planting in P-deficient ecosystems for rice plants as they can increase seedling vigor and subsequent growth, thus reducing dependence on finite P resources.