Dark Stomatal Movement in Sunflowers in Response to Illumination under Nitrogen

Experiments were performed on intact sunflowers (Helianthus annuus) placed in a specially designed experimental chamber which allows instantaneous modifications of the atmospheric composition without changing any other conditions. After one night in normal conditions, the plant was illuminated under pure nitrogen atmosphere; the opening stomatal movement, measured as a transpiration rate variation, was inhibited. After an anoxia time period not exceeding one hour, the light was turned off and normal air restored. The stomatal movement was no longer inhibited, and a transient increase in the transpiration rate, referred to here as the postillumination transpiration peak (PITP), was observed.The quantity of transpired water during the PITP can be related to the total incident light energy supplied during the light-nitrogen period. Furthermore, the addition of a dark-nitrogen period between the light-nitrogen and dark-air periods caused the PITP to decrease. The PITP is almost suppressed after a 20-minute dark-nitrogen period.It is shown that the PITP does not result from a hydropassive mechanism but is metabolically controlled. Moreover, it seems that the PITP is not due to a CO(2)-suppression effect during the light-nitrogen period.The results are interpreted in terms of stomatal mechanism. The metabolites leading to PITP originate from the reducing equivalents created during the light-nitrogen period. They could be synthesized at the beginning of the PITP period (darkness under normal air) or during the dark-nitrogen plus CO(2) period between the light-nitrogen and PITP periods. The results obtained are related to the first steps of classical photoactive stomatal opening.