Gain of the feedback loop involving carbon dioxide and stomata: theory and measurement.

The physiological and physical components of the feedback loop involving intercellular CO(2) concentration (c(i)) and stomata are identified. The loop gain (G) is a measure of the degree of homeostasis in a negative feedback loop [the expression 1/(1-G) represents the fraction to which feedback reduces a perturbance]. Estimates are given for the effects of G on responses of stomata and c(i) to changes in ambient CO(2) concentration, light intensity, and perturbations in the water relations of a leaf. At normal ambient CO(2) concentration, the gain of the loop involving stomatal conductance and c(i) was found to be -2.2 in field-grown Zea mays, -3.6 if plants of this species were grown in a growth chamber, and zero in well watered Xanthium strumarium in the vegetative state.