Abstract: Conifer forests occur around the world and contain about a third of the earth’s trees (Blinkley and Fisher, 2019). Understanding the underlying physiological responses of conifers to environmental drivers is important for making accurate and reliable predictions of forest productivity in the face of climate change scenarios. Two important physiological measurements useful for forest health assessment are the effective quantum efficiency of Photosystem II (ФPSII) and the rate of electron transport from chlorophyll fluorescence measurements (Genty et al., 1989) and stomatal conductance (gsw) assessed by gas exchange (Farquhar and Sharkey, 1982, Lawson and Blatt, 2014). Traditionally, multiple conifer needles or branches are measured together to provide enough signal to make accurate and simultaneous measurements of gsw and ФPSII.
In this study, we introduce a new solution for making rapid, accurate gsw and ФPSII measurements on a single conifer needle using the LI-600 Porometer/Fluorometer. Survey measurements of ФPSII and gsw using the LI-600 were made on single needles of well-watered and water-stressed, greenhouse-grown loblolly pine (Pinus taeda) seedlings at growth irradiances of 150-200 µmol m-2 s-1 PPFD. In addition, the LI-6800 was used for traditional measurements of multiple needles as a comparison for survey measurements as well as adding additional eco-physiological insight through light response curves of the pine seedlings. LI-600 and LI-6800 survey measurements revealed significant differences in ФPSII and gsw between the well-watered and water-stressed seedlings. The mean gsw measured in the water-stressed seedlings using the LI-6800 was not different from 0 (± 0.002 SE) mol m-2 s-1, while the LI-600 measured a mean gsw of 0.01 (± .003 SE) mol m-2 s-1. Significant differences of about 0.1 in ФPSII measurements were seen between treatments in both the LI-600 and LI-6800 at the growth light level surveyed. The light response curves of the well-watered seedlings showed that the survey measurements were made at a light level at the top of the linear portion of the light curve, while light-saturated photosynthesis of these seedlings was at PPFD of 300 µmol m-2 s-1 and above. The water-stressed seedlings exhibited net respiration at all light levels measured by LI-6800.
In summary, the LI-600 is optimized for making accurate measurements on small samples such that it allows rapid measurements of gsw and ФPSII on a single conifer needle or narrow leaf that are much more difficult to measure by traditional methods.
