Tolerance to low phosphorus in rice varieties is conferred by regulation of root growth

Phosphorus use efficiency (PUE) can be improved through cultivation techniques and breeding. However, little is known about rice (Oryza sativa L.) agronomic and physiological traits associated with high PUE. We characterized the agronomic and physiological traits of rice varieties with different tolerances to low phosphorus in nutrient solution. Two varieties with strong tolerance to low phosphorus (STVs) and two with weak tolerance (WTVs) were grown at normal (NP, control) and low phosphorus (LP, 1/20 of NP) concentrations. Plants grown at LP produced significantly lower grain yield than those grown at NP. WTVs yields were lower than STVs yields. Compared to NP, LP significantly increased phosphorus translocation efficiency (PTE), internal phosphorus efficiency (IPE) and phosphorus harvest index (PHI). Under the LP condition, PTE and IPE were higher for STVs than for WTVs. LP also reduced tiller number, shoot biomass, leaf area index (LAI), leaf photosynthetic rate, and mean root diameter of both kinds of varieties at the main growth stages, but to a lower extent in STVs. LP significantly increased the number of productive tillers, root biomass, root-shoot ratio, root bleeding rate, and root acid phosphatase (RAP) activity. Total root length, root oxidation activity (ROA), and root total and active absorbing surface areas for STVs were significantly increased under LP, whereas the opposite responses were observed for WTVs. Total root length, ROA, root bleeding rate, root active absorbing surface area, and RAP activity were positively and significantly correlated with grain yield, PTE, and IPE. These results suggest that the tolerance of rice varieties to a low-phosphorus growth condition is closely associated with root growth with higher biomass and activity.