Errors associated with the measurement of nocturnal stomatal conductance.
Barbour, Margaret*,1, Whitehead, David1, Griffin, Kevin2, Turnbull, Matthew3, Tissue, David4, 1 Landcare Research, Lincoln, New Zealand2 Columbia University, Palisades, NY3 University of Canterbury, Christchurch, New Zealand4 Texas Tech University, Lubbock, TX
ABSTRACT- Fluxes of water vapour and CO2 from leaves at night are commonly close to the detection limits of portable photosynthesis systems, like the LiCor 6400, when fitted with a standard leaf chamber (6 cm2). It has been recognised for some time that accurate measurement of leaf temperature is imperative for calculation of transpiration rate (E) and stomatal conductance (gs). However, instrument error in the measurement of the flow rate and water mole fraction of air entering (wo) and leaving the leaf chamber (wi), total atmospheric pressure, chamber fan speed, and leaf area enclosed within the chamber could also generate errors in gs calculations which may lead to incorrect conclusions regarding significance (or non-significance) of nocturnal gs. To assess the effect of instrument noise on calculation of gs, we conducted a Monte Carlo analysis (10,000 individual calculations) on gs estimated under average nocturnal conditions for leaves of Quercus rubra growing in Black Rock forest, with a leaf chamber area of 6 cm2. We included typical instrument error, as outlined in the LI-6400 manual. As expected, the coefficient of variation (cv) increased as the error in leaf temperature increased, and as wi − wo decreased. A coefficient of variation as high as 37%, at wi − wo = 0.1 mmol mol-1, suggests that under these conditions calculated gs should be interpreted with extreme caution. As errors in gs calculation are reduced at higher wi − wo, larger leaf chambers and lower flow rates are recommended. Cuticular conductance is generally thought to be in the order of 0.4 to 5.0 mmol m-2 s-1. We suggest significant water loss through stomata to be when gs minus one standard deviation (s) is greater than 7.0 mmol m-2 s-1, where s is estimated from Monte Carlo analysis at the measured wi − wo.
Key words: stomatal conductance
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