Physiological characterization and proline route genes quantification under long-term cold stress in Carrizo citrange

Abstract Low temperature is one of the abiotic stresses that most limits crop production, growth and distribution. Currently climate change weather alters temperature worldwide, which produces both high and low temperatures. This work aimed to know how citrus responds to long-term low temperatures after being submitted to 1 °C day-night during six weeks. Physiological parameters were analyzed and it was obtained that photosynthesis and growth drastically decreased, water potential increased and osmotic potential did not change. Fv/Fm (maximum quantum yield of PSII) lowered, and degradation of chlorophylls a and b took place. The proline concentration in leaves moderately increased. The expression of two genes from the glutamate synthesis route (P5CS1 and P5CS2) and one gene from the ornithine route (dOAT) was quantified. The results reveal that the dOAT transcripts from the proline synthesis gene underwent greater induction than P5CS1 and P5CS2. A marked increase in the expression of proline degradative genes (PDH and P5CDH), a drop in starch, a decrease in the concentration of soluble sugars, and higher total nitrogen quantification in leaves were also observed. Citrus plants stopped growing, but mobilized reserve substances to survive cold stress conditions.