Exogenous spermidine delays chlorophyll metabolism in cucumber leaves (Cucumis sativus L.) under high temperature stress

Abstract

This study evaluated the effects of exogenous spermidine (Spd) on chlorophyll (Chl) biosynthesis and catabolism in the leaves of cucumber (Cucumis sativus L.) seedlings under high temperature stress. Substrate culture experiments were performed using the high temperature-sensitive variety ‘Jinchun No. 2’ in an artificial climate chamber at 42/32 °C with foliar applications of 1.0 mmol L−1 Spd. The results suggested that high temperature stress markedly reduced the leaf Chl concentrations and inhibited plant growth; the harmful effect of high temperature on the cucumber seedlings was mitigated by exogenous Spd, which increased the leaf Chl concentration and promoted plant growth. Under high temperature stress, the conversion of porphobilinogen (PBG) into uroporphyrinogen III (UroIII) in the Chl biosynthetic pathway and the catabolic process of Chl were accelerated. Following the application of exogenous Spd, the conversion of PBG into UroIII was suppressed, and the accumulation of certain intermediates, e.g., protoporphyrin IX (ProtoIX) and Mg-protoporphyrin IX (Mg-ProtoIX), was decreased in the Chl biosynthetic pathway. Additionally, exogenous Spd reduced chlorophyllase (Chlase) and Mg-dechelatase (MDCase) activity and transcript levels and markedly downregulated pheophorbide A oxygenase (PaO), red Chl catabolite reductase (RCCR), Chl b reductase 1 (CBR1) and stay-green reductase 1 (SGR1) transcript levels. These results indicate that although high temperature stress accelerated Chl biosynthesis, it concurrently facilitated Chl catabolism in cucumber leaves. Exogenous Spd delayed the conversion of PBG into UroIII in the Chl biosynthetic pathway, effectively preventing oxidative bleaching of Chl in cucumber leaves. Meanwhile, Spd clearly decreased PaO pathway-related enzyme activity and transcript levels, thereby slowing Chl catabolism and increasing Chl concentrations.