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 (gs ) is frequently coupled with a model of assimilation to estimate water and carbon exchanges in plant canopies. The empirical slope (m) and 'residual' gs (g0 ) 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 g0 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 for maize (m = 4.53 ± 0.65; g0  = 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 g0 , supporting the use of constant seasonal values, but water stress reduced both parameters. In sunflower, inter-plant variability of m and g0 was large (m = 8.84 ± 3.77; g0  = 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 g0 values trended downward, and m values trended downward with water stress while g0 values declined substantially.