Critical temperature derived from the selected chlorophyll a fluorescence parameters of indigenous vegetable species of South Africa treated with high temperature

Abstract

In our experiments we aimed at testing of photosynthetic thermostability using chlorophyll fluorescence measurements in non-stressed and in drought-stressed conditions. All measurements were realized at 6 species (Vigna unguiculata, Corchorus olitorius, Cucurbita pepo, Amaranthus sp., Brassica rapa, and Hordeum vulgare as a reference). Plants were grown in pots in climatized growth chamber in moderate conditions. The main practice was temperature test with the chlorophyll fluorescence measurements realized after exposure at 38 °C to 50 °C in circulation water bath. We determined critical temperature values for basal fluorescence — F0 increase, maximum quantum yield of photochemistry Fv/Fm and we identified also temperature inducing K-step appearance in rapid fluorescence kinetic indicating impairment of oxygen evolving complex (OEC). Our results show significant differences among species in critical temperature. As the most susceptible species was found Brassica and Amaranthus substantially affected by temperatures about 40 °C with almost negligible increase of critical temperature in drought-stressed conditions, moderate susceptibility in Barley with increase of thermostability caused by moderate drought and high heat tolerance in Cucurbita, Vigna and Corchorus. If we focus on individual florescence parameters used for testing, the K-steps appeared at temperature 1–2 °C higher than initial Fv/Fm decrease, the antennae deconnection indicated by F0 increase appeared generally at 3–4 °C higher temperature, in some cases was not evident even at 50 °C.