Chlorophyll fluorescence imaging

Chlorophyll a fluorescence measurements using modulated light (to distinguish from ambient light) have been growing in popularity since the late 1980's. This technique estimates the rate of electron transport by photosystem II (PSII) in vivo which can then be used to predict CO2 consumption rates. This is possible because for every photon absorbed by a leaf we know its energy is either lost as heat, re-emitted as fluorescence, or used in electron transport. Since we can saturate PSII with a short pulse bright light, we are able to compare the maximal fluorescence during saturation with other conditions to estimate the portion of light energy used in electron transport. One very attractive feature of this method is that it can be measured without touching the organism. A more recent advance allows imaging of fluorescence over areas up to 10cm x 10cm (practical, not theoretical limit) thereby generating a picture of photosynthetic rates. We use this method for three sorts of projects in my lab.

Variation in photosynthesis across leaves

Dogma suggests that under most circumstances, photosynthetic rates are fairly uniform across leaves. However, tools to measure this variation have only recently become accessible for a wide range of studies. Patchiness of photosynthetic function is beginning to get more attention and though early work has documented examples of it, the conditions that cause patchiness are not yet clear. My lab, in a collaboration with Dr. Pockman's lab here at UNM, is using chlorophyll fluorescence imaging (the primary tool for assessing patchiness) to examine photosynthetic patchiness under a range of conditions that alter plant hydraulic conductance. This collaboration was initiated as a class project on plant stress and is now a major project for one of our Ph.D. students.

Variation of photosynthesis across bryophyte canopies/populations

Bryophyte photosynthesis is generally measured using a petri-dish size sample in a gas exchange system. This has two major problems. First, the photosynthetic rate measured is the average rate of a population of plants that are almost always experiencing different micro-environments (especially regarding wind and light). Second, most systems require detaching samples from their substrate which significantly alters their structure, including intricate connections between the moss and its substrate (see my bryophyte research page for more info). Photosynthetic rates of bryophytes are highly dependent on water content and the disruptions caused by pulling up samples has an enormous affect on the way the samples dry. Chlorophyll fluorescence imaging solves both of these problems. Images of photosynthesis can provide small scale detail only limited by the resolution of your camera and the size of the area to be imaged. This means that variation between plants and even between branches and leaves on a single moss can be distinguished (see my fluorescence page). In addition, these data can be collected without even touching the bryophytes, thereby minimizing artifacts from disrupting community structure. I have been collaborating with Dr. Steven Rice from Union College on a project where we are assessing the affects of Sphagnum architecture on photosynthetic function, light attenuation, and boundary layer conductance by combining his 3-D imaging techniques with gas exchange and fluoresence imaging.

Soil crust photosynthesis

Cyanobacterial dominated desert soil crusts are some of the most difficult samples to analyze for photosynthetic function. First, they are ephemeral in nature - potentially only active for hours at a time. Second, they are extremely fragile, so they cannot be touched without damaging them. Third, they have low photosynthetic rates per unit area, and finally they are difficult to maintain in culture. Fluorescence imaging has great promise for measuring photosynthetic function in the field since the measurements are quick, the crust doesn't need to be touched, and it gives information spatial variation. However, standard fluorescence tools for plants have to be altered for good cyanobacterial measurements due to interference from the phycobilisome pigments. Also, cyanobacterial photosynthetic electron transport and respiratory electron transport are integrated which greatly complicates and limits interpretation of the data. Still, much can be learned from this technique and no other methods are better. We currently have an NSF funded project (a collaboration with Dr. Robert Sinsabaugh of UNM, Dr. Scott Collins of UNM, and Dr. Michael Allen of UC-Riverside) to examine nitrogen cycling in desert grasslands at the Sevilleta Long Term Ecological Research Station and we are using chlorophyll fluorescence imaging to assess photosynthetic activity of the crusts.