General Reports

Young kamahi leaves prove remarkably tolerant of heat stress

Kevin S. Gould

School of Biological Sciences
Victoria University of Wellington

With growing concern over possible impacts of global climate change, there is concerted scientific effort worldwide to understand how plant communities will react both to elevated air temperatures and to altered rainfall patterns.  Professor John Choinski, a plant physiological ecologist from the University of Central Arkansas, recently visited my lab in the School of Biological Sciences to study these responses in kamahi (Weinmannia racemosa), one of New Zealand’s commonest broadleaf evergreen trees.  John brought with him an infrared gun, capable of recording leaf temperature accurately from considerable distance, and a thermoelectric stage, which allowed him to warm or cool leaves extremely rapidly and to monitor the effects of temperature changes on leaf physiology.

When John pointed his gun at kamahi in the Tararua ranges, he found that young leaves at the growing tips were several degrees warmer than were the older leaves further down the branches.  That did not surprise him; John had observed a similar trend among his own plants in Arkansas.  More remarkable, however, were differences in the abilities of immature and mature leaves to withstand temperature fluctuations.  When kamahi shoots were heated rapidly from 25 to 40°C, the photosynthesis of immature leaves barely altered, even though the temperatures extended well beyond the range these leaves would naturally experience.  In contrast, the fully mature leaves always showed a sharp decline in photosynthetic efficiency when warmed above 30°C.  Similar results were found among trees growing at Otari-Wilton’s Bush, and in kamahi saplings purchased from a local nursery; it appeared to be a widespread phenomenon.

Young kamahi leaves, therefore, are both warmer and more thermotolerant than the mature leaves. By way of providing a possible explanation for this, John examined the lipid composition of membranes inside cells of the two leaf types, and found that the younger leaves held far lower levels of trienoic fatty acids.  Low levels of these unsaturated fatty acids in chloroplasts are known to be associated with photosynthetic high temperature tolerance, possibly preventing membranes from becoming leaky as they warm up.

John’s work has been submitted for publication in Plant, Cell & Environment.  It provides tantalizing clues as to the complexities of the responses of plants to elevated temperatures, and underscores the urgency for further scientific research in this area.

Immature and fully expanded leaves of kamahi.