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Published online 11 June 2008 | Nature | doi:10.1038/news.2008.884
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Leaves keep their cool
Leaves resist temperature extremes regardless of the weather.
Whether growing in the heat of Puerto Rico or in the icy chill of northern Canada, tree leaves are able to buffer against the outside temperature, new research has found.
A survey of 39 North American tree species over an area spanning 50° of latitude has shown that plants protect one of their most important functions – photosynthesis – by maintaining average leaf temperatures at around 21 °C, regardless of the weather.
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There is no doubt that plants are able to moderate leaf temperature by a few degrees C in the long term by leaf shape, angle and clumping and in the short term by adjusting leaf angle or drooping, stomata opening/closing. I have not read the Nature article so I maybe there is a point I have not read, but the best way to determine leaf temperature is not to use isotopes, but simple to measure it directly. Of course this has been done many, many times, (I've done it myself on crops and trees) using thermocouples and other means. Leaf temperature varies a great deal with weather conditions between days and over the course of the day. On a sunny day with an air temperature of 30C, not at all atypical temperature in the Eastern US, sunlit leaves on tree species in the Southeastern U.S. almost always exceeeds 30C, much greater than 21C. I think it is more likely that leaves adjust photosynthetic machinery to adapt to temperatures, and this has been found. I'll have to find the whole article, but this research suggests to me that O isotopes are discriminated by something other than temperature and may not be a good indication of leaf temperature, not that leaf temperature is moderated to such a great extent.
The ratio of O16 to O18 varies in naturally occurring molecules subject to evaporation like water. This ratio is otherwise constant. The ratio of O16 to O18 in cellulose in tree rings will necessary match that of the source from which the oxygen atoms came from but it isn't water who supplies the oxygen atoms in photosynthesis, it is CO2. The ratio of O16 to O18 in plant-breathable CO2 is constant and most certainly independent of the regional temperature therefore it bears no relation to the leaf temperature.
Hi, you may have a better understanding as to how water oxygen isotope ratio is relevant to cellulose oxygen isotope ratio after reading the following paper: DeNiro MJ, Cooper LW (1989) Post photosynthetic modification of oxygen isotope ratios of carbohydrates in the potato: implications for paleoclimatic reconstruction based upon isotopic analysis of wood cellulose. Geochim Cosmochim Acta 53:2573–2580
This news suggests that the coming article will show how Nature's editors can be fooled in accepting a paper that contradicts the laws of thermodynamics.
After reading Helliker & Richter, I believe that the first section and title of this news piece somewhat distort the letter, especially in the use of the word "weather" as opposed to "climate." Ledford only gets to the true conclusions of Helliker & Richter in the second section, with: "the isotope content of the tree rings should represent the average temperature of the leaves when they incorporated carbon from CO2 into sugars." Thus 21 C is not necessarily the time-weighted average of leaf temperature, but instead the photosynthesis-weighted average of leaf temperature. Helliker & Richter's conclusions could be a result governed more by the diurnal and seasonal timing of maximum photosynthesis than to by biophysical particularities of tree canopies.
I am not sure if I understand how the second law of thermodynamics is broken here.
I am not sure if I understand how the second law of thermodynamics is broken here.
Glad to see your discussion on this paper. All of you know this paper has already been published in paper megazine on 24 July. Acturally I have lots of thoughts for this paper... Firstly, unequal leaf and air temperature is absolutely correct if considering from energy balance, however, A different from ambient atmosphere but also constant (21.4) leaf temperature is a considerably new proposition. I think, future studies in this field really can test this conclusion. Hopefully this day will be not too long ...