Autumn cold acclimation and freezing tolerance of three oak species in semi-Mediterranean Zagros forests

Abstract

Aim of study: To identify and compare the early frost resistance mechanisms in three oak species (Quercus brantii, Quercus libani and Quercus infectoria). Area of study: Zagros forests of Iran. Material and methods: The physiological and biochemical variables such as chlorophyll fluorescence, relative water content (RWC), electrolyte leakage, and osmotic metabolite content, such as proline, glucose, and potassium of three oak species seedlings with varying altitudinal and latitudinal ranges were measured under various treatments, including cold treatments (4°C: control, -20°C (1 hour) and -20°C (2 hours)) and four levels of hardening steps with decreasing temperature and photoperiod. Main results: Results showed that decreasing photoperiod and temperature during cold hardening was associated with decreasing Fv/Fm, ΦPSII and electron transport rate, increasing (NPQ) measured from chlorophyll fluorescence, as well as increasing osmotic metabolite content and decline of RWC, except the glucose content decreased in Q. brantii from lower altitudes and south aspect. On the other hand, Q. libanii, which originates from higher altitudes and north aspect showed the strongest cold-resistance and faster developing cold-acclimation capacity using earlier accumulation of osmotic metabolites, diminishing RWC and subsequently lowest EL compared to the other oak species. Research highlights: The physiological and biochemical responses of oak species differed based on origin and there was a positive relation between osmotic metabolite content, NPQ, altitude, and cold stress resistance. These physiological responses, especially NPQ (as a fast and non-invasive tool) provide a quantitative assessment of the risks associated with autumn freezing in different oak species and ecotypes relevant to conservation and reforestation projects of the Zagros forests under changing climatic conditions.