Photosynthesis — Thick Leaves
Leaf level gas exchange measurements are often made using clamp style leaf chambers, which are small in volume and often enclose only a portion of the leaf of interest. These chambers consist of two hinged sections that are clamped around a portion of a leaf. The seal between each chamber half and the leaf must be gas tight to prevent carbon dioxide and water vapor from leaking into or out of the chamber. To achieve a gas tight seal, closed cell foam gaskets are often used.
The gasket thickness and clamp configuration are optimized for thin leaves (e.g. Glycine or Phaselous) and work well for a wide variety of species. These gaskets, however, do not work well for thick, fleshy or cylindrical leaves, or leaves with thick vasculature. Historically, achieving an adequate seal around these leaves required higher chamber clamping pressure and/or the use of additional sealants, which resulted in erroneous measurements due to chamber leaks, damaged leaf material, and considerable extra hassle when working with these types of leaves. By increasing the gasket thickness and changing how the two chamber halves hinge relative to each other, the LI-6400XT leaf chamber can accommodate thick and fleshy leaves.
Because of the conformability of thick gaskets, gas-exchange measurements of narrow diameter stems and petioles also can be made. For many species these organs contain chlorophyll and are known to assimilate carbon, however, little is known about their overall role in plants' carbon budgets.
For thick leaved succulent species, which use Crassulacean Acid Metabolism (CAM) for carbon assimilation (e.g. Agave), the thick gaskets and wider chamber opening are particularly useful. In CAM species, carbon assimilation and carbon fixation are often separated temporally. In the classic CAM model, the plant opens its stomata at night and fixes carbon in the dark as a four carbon (C4) organic acid using phosphoenolpyruvate carboxylase (PEPC). During the day the stomates remain closed and CO2 is liberated from the previously fixed C4 acid pool for use in photosynthesis. However, for most CAM species the actual mode of carbon uptake exists somewhere on a continuous grade between C3 photosynthesis and CAM. To fully characterize carbon assimiliation, it is often necessary to perform measurements that track gas exchange over a 24-hour period. When combined with an automated gas exchange system, thick gaskets provide a powerful tool for this type of physiological exploration.
LI-COR Instruments for Measuring Photosynthesis on Thick Leaves:
- Compact, rugged, field portable instrument
- Accurate and precise
- Measures fluorescence and gas exchange simultaneously
- Used with the LI-6400XT Portable Photosynthesis System
- Allows the leaf chamber to be used with thick or irregularly shaped leaves
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