Comparative analysis of ultraviolet-B radiation and abscisic acid revealing distinct mechanistic approaches to drought tolerance in highland barley.

Drought tolerance in highland barley (qingke) is modulated by distinct responses to ultraviolet-B radiation (UVB) and abscisic acid (ABA). Plants were exposed to two drought levels (moderate: 50 % soil water content and severe: 30 % soil water content), alongside two UVB intensities (low: 5-6 kJ m-2 d-1 and high: 14-15 kJ m-2 d-1), and two ABA concentrations (low: 70 μM and high: 150 μM). Physiological analyses revealed that UVB exposure significantly reduced the photosynthetic rate by 46.5 % under severe drought conditions. In contrast, ABA treatment helped to maintain the rate and improved water use efficiency by 15 % compared to UVB treatment. ABA primarily affected the starch and sucrose biosynthesis. ABA upregulated genes involved in sucrose synthase and starch synthase, promoting enhanced starch and sugar accumulation. UVB, in contrast, showed weaker effects on these pathways, with only slight upregulation of genes involved in starch biosynthesis. The phenylpropanoid metabolism and flavonoid biosynthesis were notably upregulated under UVB, with significant enrichment in phenylalanine metabolism. On the other hand, ABA treatment enriched alpha-linolenic acid metabolism and amino acid biosynthesis. Hormonal analysis revealed significant shifts in ABA, jasmonic acid (JA), and salicylic acid (SA) signaling. Under ABA treatment, genes related to ABA signaling were upregulated, while UVB exposure suppressed ABA-related transcripts. JA and SA signaling were also affected, with the majority of JA-related genes being upregulated under ABA but downregulated under UVB conditions. UVB exposure also increased the content of flavonoids and fatty acids, while ABA treatment enhanced the accumulation of starch and sucrose. The findings established the distinct yet corresponding roles of UVB radiation and ABA in improving drought tolerance, with UVB inducing secondary metabolic responses and ABA regulating primary metabolic processes. Together, they contribute to the understanding of the molecular mechanisms underlying drought resistance in highland barley.