Light-induced spin-correlated radical pairs in plant photosystem I are studied by high time resolution transient EPR following pulsed laser excitation. The time evolution of the transverse magnetization is monitored at various static magnetic fields. This implies a two-dimensional variation of the signal intensity with respect to both the magnetic field and time axis. Zero quantum coherence between two of the four eigenstates of the radical pair is observed at early times after the laser pulse. Model calculations, based on the stochastic Liouville equation, provide detailed information about the spin dynamics of correlated radical pairs.