Enhanced Chlorophyll Accumulation is Early Response of Rice to Phosphorus Deficiency

Phosphorus (P) deficiency is a major constraint in rice production, causing significant reductions in growth and yield. 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 was an early adaptive response that occurred primarily in young leaves, while older leaves eventually developed chlorosis under prolonged P deficiency. The increased chlorophyll content 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 a characteristic early response to P deficiency in rice, rather than a typical general stress response observed in other conditions. Our findings highlight the limitations of relying solely on chlorophyll-based indices as indicators of plant health in precision agriculture, especially regarding phosphorus (P) nutrition management. This underscores the need for a more comprehensive approach and lays the groundwork for developing advanced remote sensing technologies aimed at accurately assessing P status in rice cultivation.