With a distinct flavor, plum fruit is rich in proteins, potassium, calcium, phosphorus, and nitrogen and has high nutritional and medical value (Crisosto & Kader, 2000;Ertekin et al., 2006). During harvesting, transportation and storage, however, mechanical injury is frequently caused by external vibrations and bumps (Lien et al., 2009). Consequently, browning gradually occurs in the injured tissues and spreads around afterwards, resulting in a massive decay (Ceponis et al., 1987), which shortens the plum's shelf life, weakens its sales value, and causes huge economic losses (Martinez-Romero et al., 2004). A fast non-destructive discriminate method, which can help to detect the mechanical injury in the epidermal tissues, turns to be particularly significant. Refrigeration is usually applied in normal storage to decelerate the decay, which, on the other hand, restrains the obvious browning in plum's injured tissues that have intact epidermis. Hence, detecting plum's early mechanical injury, which has undamaged epidermis, is always a challenge (Li et al., 2017).