Heat-induced F0-fluorescence rise is not an indicator of severe tissue necrosis in thermotolerance assays of young and mature leaves of a tropical tree species, Calophyllum inophyllum.

Abstract

In heating experiments with leaves, the temperature at which dark-level F₀ chlorophyll a fluorescence begins to rise, T_crit, is widely used as an indicator of photosystem II thermotolerance. However, little is known about how T_crit correlates with irreversible leaf tissue damage. Young and mature leaves of the tropical tree species Calophyllum inophyllum were heated stepwise from 30 to 55°C, at 1°C min^-1. T_crit was 47°C in young leaves and 49°C in mature leaves. Contrary to the higher T_crit in mature leaves, heating to 55°C elicited greater tissue damage in mature than in young leaves. Young and mature leaves heated to their respective T_crit or T_crit + 2°C exhibited no or little tissue necrosis after 14 d of post-culture. It is concluded that measurements of the temperature-dependent F₀ fluorescence rise underestimate the thermal thresholds above which significant irreversible leaf damage occurs.