Impact of microsecond-pulsed plasma-activated water on papaya seed germination and seedling growth

The seed of Carica papaya consists of a hard shell-like testa with inhibitors in vivo causing slow, erratic and asynchronous germination. In this work, plasma-activated water prepared by microsecond-pulsed plasma jets (mu PAW) was applied to treat papaya seeds. The mu PAW after plasma activation of 30 min was about 40 degrees C. The reactive species such as NO2, NO3, and H2O2 in the mu PAW activated from deionized water were measured and correlated to the seed germination rate and the seedling growth performance. The mu PAW-treated papaya seed achieved a higher germination rate of 90%, which is 26% higher than the control group using deionized water. Comparing the results with a hot water (40 degrees C) reference group showed that the reactive species in mu PAW played primary roles in germination improvement, with little effect caused by the heat shock. The mu PAW also sterilized the treated seeds, reducing the germination stress. The morphological change in the seeds was observed by SEM, showing an effect of physical etching after treatment promoting seed imbibition. The biochemical mechanism of the seed germination was deduced with reference to the evolution of surface chemistry, functional groups, and ABA content. The accelerated seed metabolism observed was corresponded to the chemical modification pathway. Besides, early seedlings developed from treated seeds were observed to be healthy, grow more leaves, and have better root structures. The content of MDA in the treated papaya seedlings decreased along with increased SOD and higher ion concentration. The mu PAW that can be prepared at atmospheric pressure for bulk production offers a low-risk and cost-effective seed priming technology that may significantly increase the production of agricultural crops.