MPPT aka Maximum Power Point Tracking is an active mechanism to collect maximum power a PV cell may deliver. This is typically achieved by varying the resistive load connected to a cell.
Juno will perhaps be the most distant robot in interplanetary yet to rely upon Sol for it's power. Appropinquity to, nay, immersion in Jupiter's magnetosphere, and remoteness from Sol - are two factors (there may be others) that could serve to limit her mission due to loss of power. This may partially be mitigated by an on-board MPPT implementation, which brings us to the question.
Does Juno use MPPT?