Seasonal variability of the parameters of the Ball-Berry model of stomatal conductance in maize (Zea mays L.) and sunflower (Helianthus annuus L.) under well-watered and water-stressed conditions

The Ball-Berry (BB) model of stomatal conductance (g(s)) is frequently coupled with a model of assimilation to estimate water and carbon exchanges in plant canopies. The empirical slope (m) and 'residual' g(s) (g(0)) parameters of the BB model influence transpiration estimates, but the time-intensive nature of measurement limits species-specific data on seasonal and stress responses. We measured m and g(0) seasonally and under different water availability for maize and sunflower. The statistical method used to estimate parameters impacted values nominally when inter-plant variability was low, but had substantial impact with larger inter-plant variability. Values formaize (m = 4.53 +/- 0.65; g(0) = 0.017 +/- 0.016 mol m(-2) s(-1)) were 40% higher than other published values. In maize, we found no seasonal changes in m or g(0), supporting the use of constant seasonal values, but water stress reduced both parameters. In sunflower, inter-plant variability of m and g(0) was large (m = 8.84 +/- 3.77; g(0) = 0.354 +/- 0.226 mol m(-2) s(-1)), presenting a challenge to clear interpretation of seasonal and water stress responses - m values were stable seasonally, even as g(0) values trended downward, and m values trended downward with water stress while g(0) values declined substantially.