A comparison between pulsed electric field and moderate electric field for their effectiveness in improving the freezing tolerance of rocket leaves

Two electrotechnologies: pulsed electric fields (PEF) and moderate electric field (MEF) in combination with vacuum impregnation of glycerol as cryoprotectant were used to increase the freezing tolerance of rocket leaves. Rocket leaves were treated with PEF using a nominal field strength of 1200 V/cm or MEF at different combinations of voltage and frequency. Leaves were then immersed in a glycerol solution at 32, 36 and 40% (w/v) under vacuum for 26 min. After this treatment, the leaves were allowed to rest for 3 days before they were frozen and thawed. Leaf survival was assessed at different time points after thawing with microscopic observations and wilting tests. When the viability of the leaves was assessed 5 min after thawing, 60-68% of the leaves in the batch survived. There was no difference in the levels of surviving leaves when PEF and the lowest-tested voltage used in MEF were tested. However, from the leaves surviving 5 min after thawing, approximately half of them die over a 24 h period after thawing.