  Cold plasma technologies are an efficient approach to improve the germination
properties of seeds, especially in a stacking configuration within a dielectric
barrier device. In such dry atmospheric plasma priming process, we show that a
He-N2 plasma treatment of 20 min can reduce the median germination time of
lentil seeds from 1420 min to 1145 minutes, ie a gain in vigor of 275 min (or
+19.4 %). Considering that this result depends on the plasma-seed interaction
and therefore on the contact surfaces between the seeds and the plasma, a
topographic modeling of a 100 seeds-stack is performed in the dielectric
barrier device. This model drives to the distinction between the seed-seed
contact surfaces (276 contacts standing for a total area of 230.6 mm2) and the
seed-wall contact surfaces (134 contacts standing for a total area of 105.9
mm2). After a single plasma treatment, the outer envelope of each seed is 92%
exposed to plasma: a value high enough to support the relevance of the plasma
process but which also opens the way to process optimizations. In this outlook,
we propose to replace the single 20-minute plasma treatment by a "plasma
sequence", ie a succession of shorter plasma treatments whose total duration
remains 20 min. Between two successive plasma treatments, the seeds can follow
either a trapping procedure (seeds in same positions and orientations) or a
randomizing procedure (seeds in same positions but reoriented) or a stirring
procedure (seeds vigorously shaken). A sequence of 10 plasma treatments (upon 2
min) separated by short stirring procedures leads to a gain in vigor as high as
405 min vs 275 min for a single plasma treatment of 20 min. We propose to
understand these results by correlating the gain in vigor with the water uptake
of the seeds and the wettability state of their coating.

Plasma Physics