There is a growing interest in studying plant biology using nuclear imaging technology. We are currently developing a novel imaging system that permits β imaging of an intake leaf. Positrons originated from a leaf may escape the leaf and can be imaged by a β imager. However, different parts of a leaf may have different thickness, which renders quantitative β imaging difficult. Coincident γ imaging provides more quantification and lower sensitivity image than imaging for positron emitting tracer. The proposed system combined with 11C labeled carbon dioxide will allow us to quantify carbon assimilation in plant and study the interplay between photosynthesis and photorespiration under different environments. The system will include two planar detectors, one is a typical PET detector and the other is a phoswich detector that detects both β and γ. The prototype phoswich detector consists of a plastic scintillator with a LSO array and a position-sensitive photomultiplier tube (PS-PMT). The plastic scintillator serves as a β detector, while the LSO array serves as a γ detector and a light guide that couples scintillation light from the plastic detector to PMT. PMT signal was feed into the Siemens QuickSilver electronics that provides shaping and waveform sampling. With the decay time of the plastic and LSO being 2.1 and 40 ns, respectively, pulse-shape discrimination was implemented to differentiate β events detected by the plastic from γ events detected by the LSO from the common PMT signals. Using this prototype phoswich detector, we simultaneously measured a β image γ and events (in singles mode) and obtained 1.6-1.8mm FWHM resolution for the β image using F-18 line sources.