Sap flow within a leaf is a critical indicator of plant vitality and health. This paper introduces an easy-to-use, non-invasive, and real-time imaging method for sap microcirculation imaging. Utilizing the coherent backscattering of light on a leaf, the study demonstrates that the acquisition frequency of dynamic speckle can be linked to the microcirculation speed inside the leaf.

Unlike conventional methods based on speckle contrast, which require long integration times to observe temporal decorrelation within a single image, this approach operates in a regime where speckle patterns appear ‘frozen’ in each frame of a given sequence. This ‘frozen’ state indicates that any decorrelation of the speckle pattern within a frame is negligible. However, between successive frames, decorrelation becomes significant, and it is this inter-frame decorrelation that allows for the extraction of dynamic information.

In this context, the integration time primarily influences the radiometric levels, while the frame acquisition rate emerges as the key parameter for generating activity index maps. By accessing different ranges of sap flow activity levels through varying the frame acquisition rate, the authors reveal the anatomy of the leaf’s circulatory network in a non-invasive manner with unprecedented detail.

The method's ability to characterize the vitality of a fig leaf in real time is experimentally validated by observing a continuous decrease in sap circulation, first in the smaller vessels and subsequently in the larger ones, following the cutting of the leaf over a 48-hour period.