Immature leaves of Weinmannia racemosa are more heat tolerant than mature leaves based on differences in chlorophyll a fluorescence and solute leakage

Numerous studies have examined how leaf temperature affects photosynthesis, but few have investigated photosynthetic temperature sensitivity during leaf expansion. Here, we show that immature leaves (similar to 18% of final leaf area) of the common New Zealand tree, Weinmannia racemosa L. f. (Cunoniaceae), are less sensitive than mature leaves to rapid increases in leaf temperature (from 25 to 35 degrees C) as measured by chlorophyll a fluorescence. Relative differences in the tolerance of photosystem II to heat were supported by solute leakage experiments, in which disks from immature leaves incubated at 47 degrees C lost significantly fewer solutes than did disks from mature leaves. In addition, levels of trienoic (18:3) fatty acids were far lower in immature than in mature leaves (low levels are known to be associated with photosynthetic thermotolerance). The developmental changes in photosynthetic tolerance to heat stress likely correspond to compositional differences of the thylakoid membranes during leaf expansion. This result may be useful in understanding photosynthetic responses to temperature in young leaves which lack functioning stomates, or under extreme conditions when leaf temperatures are high or fluctuate rapidly.