Responses of chlorophyll fluorescence of alfalfa with various phosphorus utilization efficiencies to phosphorus deficiency

Objective This study was aimed to identify the effects of P deficiency on photosynthetic performance of alfalfa (Medicago sativa L.) and reveal the differences in light energy conversion and utilization among different varieties.

Methods Sand culture method was used, two high-P-efficient varieties, Gannon 6 (G6), and Aurora (JG); and two low-P-efficient alfalfa varieties, Bara 310SC (B310) and Gannon 4 (G4) were selected as test materials. On the basis of 1/4 Hoagland solution, normal P control (0.5 mmol/L) and low P stress (0.05 mmol/L) solution were prepared. At the 30 days of culture, the chlorophyll fluorescence parameters of alfalfa leaves were measured, then the whole seedlings were harvested for the determination of biomass and P content.

Results Compared with the control group, P deficiency decreased the biomass and P utilization efficiency of four alfalfa varieties, increased fluorescence intensity at points J and I of the relative variable fluorescence induction curve, increased the maximum rate of Q_A reduction (M_o) significantly, parameters such as receptor pool capacity (S_m) decreased and impaired the PSII receptor side; the number of active reaction centers (RC/CS_o) decreased, light energy absorption (ABS/RC, ABS/CS_o), capture (TR_o/RC, TR_o/CS_o), transfer (ET_o/RC) and dissipation (DI_o/RC, DI_o/CS_o) increased significantly, performance index (PI_abs and PI_total) decreased, photosynthetic electron transfer was hindered, and the overall performance of PSⅠ and PSⅡ decreased. In terms of the differences among varieties, the high-P-efficient varieties G6 and JG showed less variation in these parameters than the low-P-efficient varieties B310 and G4, and had higher biomass and P utilization efficiency under the same P treatment.

Conclusion P deficiency mainly inhibits the performance of the electron transport chain on the acceptor side and reaction center of alfalfa leaves, which decreases the activity of PSⅠ and PSⅡ and hinders photosynthetic electron transport. These responses result in a reduction of biomass and P utilization efficiency. Compared to P-inefficient varieties, P-efficient varieties have higher photosynthetic electron transfer activity and more stable photosynthetic system. Among rapid chlorophyll fluorescence parameters, PI_abs is more sensitive to P deficiency, and shows a significant correlation with P utilization efficiency, which could effectively reflect the status of PSII light energy conversion efficiency, active reaction center and receptor side. All these indicate that PI_abs is an effective indicator for evaluating alfalfa photosynthetic performance under P deficiency.