Germination and growth enhancement of agricultural crops using cold plasma treatment

Abstract

The increased food demand caused by the world population growth necessitates massive improvement in agricultural production. Cold plasma (low-pressure or atmospheric pressure) technology has been used to increase the germination and growth of various crops with different degrees of success in the recent decades. In this study, three different crops (chilli, papaya, and Bambara groundnut) that were facing issues, such as low germination and growth, were subjected to cold plasma treatment for the first time to observe the germination and growth improvement in terms of phenotype and genotype. Initially, low-pressure plasma treatment was used, and significant improvements in germination and growth were observed for all three crops. Physical and chemical changes were studied using water contact angle, water uptake, electrical conductivity, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) to understand the mechanism of germination and growth improvement. The results from these physico-chemical techniques show that plasma micro-etched the seed surface to be porous to leach out electrolytes while oxidising the surface to increase the wettability of the seeds. At the cellular-biological level, changes in phytohormones and antioxidant activity were also detected and examined for one crop (papaya) to show the reduction of hormone abscisic acid (ABA). After obtaining significant positive results with lowpressure plasma treatment, the study was expanded to include atmospheric pressure plasma treatment for only two crops (chilli and papaya). Atmospheric pressure argon plasma treatment improves germination and growth as well for chilli and papaya. Following similar preliminary tests as low-pressure plasma, an atmospheric pressure plasma study was conducted on a genetic level, and the effect of cold plasma treatment on gene expressions was explored using next-generation sequencing (NGS) for only papaya. Transcriptomic changes were evaluated from the treated papaya seeds using two RNA sequencing approaches; 1) de novo assembly from the seed tissues, 2) highly fragmented genome (ASGPBv0.4). The generated transcriptomic assembly consisted of 56,137 transcripts with 34,571 being unigenes; among these unigenes, the majority are involved in the metabolism under the kyoto encyclopedia of genes and genomes (KEGG) classification. Different monomers were reported in the literature, such as air, nitrogen, oxygen, argon, or water (H2O) during cold plasma treatment. In this current study, oxygen was used as a monomer for chilli and papaya seeds, and H2O was used for Bambara groundnut during low-pressure plasma treatment, whereas Ar gas was used for chilli and papaya during atmospheric pressure plasma treatment because of their ability to etch and oxidise, and low ignition voltage for the atmospheric plasma. Optical emission spectroscopy confirmed that oxygen-related ion species are the main reactive ions to be responsible for this germination and growth in all three crops. Ageing theories were proposed to provide a time frame for the plasma-treated seeds to maintain the plasma effects. Compared to the plasma ion effect, the vacuum effect alone has some minor effects in germination and is not durable during the ageing/storage of the treated seeds. The success of this method has the potential to be scaled up to solve food security with seeds otherwise facing germination-related issues.