PARENT SESSION
Organized Oral Session 52: Casting light on nocturnal stomatal and canopy conductance
Organizer(s): NG Phillips and M Barbour
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 510a, Level 5, Palais des congrès de Montréal

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 (g_s). However, instrument error in the measurement of the flow rate and water mole fraction of air entering (w_o) and leaving the leaf chamber (w_i), total atmospheric pressure, chamber fan speed, and leaf area enclosed within the chamber could also generate errors in g_s calculations which may lead to incorrect conclusions regarding significance (or non-significance) of nocturnal g_s. To assess the effect of instrument noise on calculation of g_s, we conducted a Monte Carlo analysis (10,000 individual calculations) on g_s 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 w_i − w_o decreased. A coefficient of variation as high as 37%, at w_i − w_o = 0.1 mmol mol^-1, suggests that under these conditions calculated g_s should be interpreted with extreme caution. As errors in g_s calculation are reduced at higher w_i − w_o, 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 g_s 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 w_i − w_o.

Key words: stomatal conductance