Abstract: Phosphorus (P) deficiency is a major constraint in rice production, causing significant growth and yield reductions. While P deficiency typically decreases chlorophyll content in many plant species, our previous studies revealed an unexpected increase in chlorophyll content in P-deficient rice seedlings. Here, we investigated this phenomenon in KDML105 rice under various P regimes and analyzed the physiological mechanisms involved. We found that P-deficient rice seedlings significantly increased chlorophyll a, chlorophyll b, and carotenoid contents in young leaves while reducing photosystem II quantum yield and enhancing non-photochemical quenching. This response was specific to P deficiency and was not observed under other stress conditions such as salinity or copper toxicity, which induced oxidative stress. Time-course experiments revealed that increased chlorophyll accumulation is an early adaptation response that occurs primarily in young leaves, while older leaves eventually develop chlorosis under prolonged P deficiency. The increased chlorophyll density may be attributed to reduced leaf width and altered leaf morphology under P-limited conditions. Furthermore, using custom hyperspectral imaging analysis coupled with machine learning classification, we successfully differentiated P status in rice leaves with 98.96% accuracy in older leaves. This study demonstrates that enhanced chlorophyll accumulation is an early response to P deficiency in rice, rather than a common stress response. Our findings caution against chlorophyll-based indices alone as indicators of plant health in precision agriculture, particularly in the context of P nutrition management, and provide a foundation for developing improved remote sensing technologies for accurate P status assessment in rice cultivation.

Key words: phosphorus deficiency, dark-green leaves, chlorophyll accumulation, quantum yield of photosystem II, non-photochemical quenching, hyperspectral imaging analysis