Synergistic adaptation of rice root phosphorus uptake kinetics and leaf carbon-nitrogen metabolism under...

Phosphorus (P) deficiency is a major constraint on rice growth and productivity. Clarifying the physiological basis of root-shoot coordination under phosphorus limitation is essential for improving phosphorus-use efficiency in rice.

Following a preliminary phenotypic screening of 156 rice germplasm resources, four rice cultivars representing two contrasting low-P tolerance groups were selected, including two tolerant cultivars (J873 and J705) and two sensitive cultivars (T35 and L20). The physiological responses of these four cultivars to low-P stress were systematically compared using the Claassen-Barber nutrient depletion method, gas exchange measurements, and enzymatic assays.

Phosphorus deficiency was associated with marked changes in root phosphorus uptake kinetics. The tolerant cultivars developed an uptake pattern characterized by the coexistence of high uptake capacity (high I _max) and high affinity (low K _m), improving phosphorus acquisition efficiency. In leaves, these cultivars maintained higher stomatal conductance, supporting transpiration-driven nutrient transport, and exhibited greater acid phosphatase activity, consistent with enhanced intracellular phosphorus recycling. Along with delayed leaf senescence, these responses alleviated the biochemical limitations on photosynthesis, with the net photosynthetic rate at day 21 remaining 39.47-45.09% higher than that of the sensitive cultivars. Metabolic analysis indicated that enhanced sucrose phosphate synthase activity was accompanied by a 42.18-43.10% increase in the root-to-shoot ratio, reflecting greater carbon allocation to roots, while the accumulation of free amino acids contributed to the maintenance of carbon-nitrogen metabolic balance. Structural equation modeling indicated that under low-P conditions, biomass accumulation was jointly regulated by photosynthetic carbon assimilation and coordinated carbon-nitrogen metabolic processes.

Coordinated regulation of root phosphorus uptake kinetics and SPS-mediated carbon partitioning is a key physiological strategy underlying rice adaptation to low-P conditions.