The use of pulse amplitude modulated fluorometry to determine fine-scale temporal and spatial variation of in situ photosynthetic activity within an Isoetes-dominated canopy

Abstract

We used in situ pulse amplitude modulated (PAM) fluorometry to investigate photosynthetic activity of Isoetes alpinus (Kirk) in submerged meadows of Lake Wanaka, New Zealand. We examined diel and depth-related variability in the quantum yield of photosystem II (PSII) under varying ambient light, as well as variability due to self-shading effects within the I. alpinus canopy and inter-leaf differences. We also determined the utility of short-term (minutes) irradiance versus yield estimates in assessing longer-term responses to fluctuating light. Results showed that under natural lighting, PSII yield was highly variable within the plant canopy for a given irradiance incident to that canopy, ranging from 0.02 to 0.80. This variability could be explained by relating PSII yield to irradiance within the canopy measured using a miniature scalar irradiance sensor. For individual plants, yield and irradiance were closely related, and quantitatively similar yield–irradiance ( Y– E) curves were obtained for plants from a given depth by experiments using diel, short-term (seconds) and in-canopy irradiance changes. Maximum fluorescence yield consistently averaged 0.55–0.65 and the light saturation parameter ( E k( Y)) at 3 m depth ranged from 189 to 247. There were water depth-related differences between mean responses of the I. alpinus canopy; individual leaves from the maximum and minimum growth depths (7 and 3 m) showed ranges of ( E k( Y)) of 159–228 and 109–151 μmol m −2 s −1, respectively. Irradiance alone is the dominant determinant of PSII activity in I. alpinus in Lake Wanaka, and accurate description of light within a canopy is a pre-requisite to estimating community photosynthesis. This, combined with a comprehensive assessment of the variability of individual plant responses to irradiance, by depth, may provide a sound basis for modelling community activity. We conclude that, by its ability to obtain fine-scale activity measurements rapidly, and with no enclosure or disturbance effects, PAM fluorometry offers new insights to the activity of specific plants and parts of plants in complex submerged canopies. It remains to be determined, however, whether PAM-determined quantum yield of PSII in these plants can be directly related to carbon fixation.