Optimality explains photosynthetic responses to elevation. By Wang Han

Alpine plants have long fascinated ecologists, and provide a rich variety of adaptations to extremes of low temperature, wind strength, and others stressors. But just one aspect of high-elevation environments is unique, namely the low atmospheric pressure. There has been speculation about how low pressure might influence plant physiology. For example, it has been suggested that low carbon dioxide (CO2) partial pressure at high elevation might make plants more sensitive to changes in the mixing ratio of CO2 to dry air, whether natural lower as in past ice ages, or higher, as we see currently due to human activity. But this view is oversimplified, as the low partial pressure of oxygen (O­2) at high elevation reduces the photorespiratory burden as well. It has also been shown both by field measurements, and indirectly using stable carbon isotope ratios (δ13C), that ratios of leaf-internal to ambient CO2 partial pressures (pi:pa) are consistently lower at high elevations, whereas photosynthetic capacity (Vcmax) tends to be higher at high elevations. This phenomenon has been discussed extensively, but no clear explanation has emerged.

The least-cost theory now provides a coherent explain on this well documented phenomenon. The least-cost theory states that plants minimize the combined unit costs of maintaining water transport and carbon fixation capacities, and has been tested with field observations. With this theory, we argue that alpine plants adapt to the low air pressure (the unique elevation effect) by investing more on photosynthesis capacity and less on water transpiration because the first cost is cheaper (enhanced affinity of Rubisco for CO2 under low O2 environments) while the latter is more expensive (higher leaf to air vapor pressure deficit as actual vapor pressure declines with elevation). So the ecological explanation is quite simple and clear, but can never be achieved unless considering the two competing costs of photosynthesis at the same time, whereas previous hypothesis only focus on minimizing the transpiration cost.

With the help of partial derivative, theoretical quantification of the elevation effects (i.e. air pressure changes) on pi:pa and Vcmax could also be achieved, and shows consistent results with previous observation. A stronger CO2 fertilization for alpine plants compared to the low land plants is also predicted.

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