Cold plasma treatment of seeds: deciphering the role of contact surfaces through multiple exposures, randomizing and stirring

Cold plasma technologies are an efficient approach to improve the germination properties of seeds, especially in a stacking configuration within a dielectric barrier device (DBD). In such dry atmospheric plasma priming process, we show that a helium-nitrogen plasma treatment of 20 min can reduce the median germination time of lentil seeds from 1420 min to 1145 min, i.e. 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 DBD. 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). It turns out that 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 min plasma treatment by a ‘plasma sequence’, i.e. 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). As an example, a sequence of ten plasma treatments (upon 2 min) separated by short stirring procedures leads to a gain in vigor as high as 405 min (+28.5%) vs 275 min (+19.4%) 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 (increase from 29% to 55%) and the wettability state of their coating (decrease of contact angle from 113.5^◦ to about 38^◦).