Pospíšilová, Jana; Dodd, I.C.
In Handbook of Photosynthesis (Second Edition; Revised and Expanded). New York : Marcel Dekker : 811-825, 2004

Keywords: auxins; brasinosteroids; cytokinins
Abstract: Although prolonged water stress can decrease Yleaf and close stomata via hydraulic influences, in many cases, plants use chemical signals travelling in the xylem to initiate stomatal closure thus preventing any decrease in Yleaf. Such signals can operate prior to any increase in bulk leaf hormone concentrations. Most interest has centred on ABA as the most probable root-to-shoot chemical signal regulating stomatal aperture. Apoplastic ABA concentration around the guard cells is crucial in determining stomatal responses, and this will depend on many factors including xylem ABA delivery to the leaf, leaf mesophyll ABA catabolism, leaf ABA synthesis and apoplastic pH. Even when apoplastic ABA concentration is constant, environmental and physiological variables such as CO2 concentration, temperature and current leaf water status can alter stomatal response to ABA. Other plant hormones are also important in modifying stomatal response to a given ABA concentration. The most probable candidates for alleviating ABA effects seem to be CKs and auxins, and for stimulating ABA effects, JA and MeJA. Although some non-stomatal effects of hormone application on photosynthesis have been demonstrated, in the majority of cases changes in endogenous hormone concentrations during water stress affect gs thus modifying intercellular CO2 concentration and then photosynthesis. Application of synthetic or natural plant growth regulators may modify stomatal response in vivo, but the effects of a given application can vary according to uptake and degradation of the compound of interest, and the effect of the compound on endogenous phytohormone contents. Transgenic technologies give considerable scope for manipulating endogenous phytohormone contents, and may provide a way of reproducibly modifying stomatal responses to ABA. Under dryland agriculture where soil moisture is depleted as the crop nears maturity, enhancement of ABA-induced stomatal closure may allow the crop to survive for a sufficient period to produce some yield from stored photosynthate. Alternatively, under irrigated environments where water supply is assured, suppression of ABA-induced stomatal closure may minimise photosynthetic limitations, and maximise crop yield. Given that much of the photosynthetic limitation that occurs under managed agriculture is mostly stomatal in nature, understanding how variation in plant hormone status affects photosyntesis seems important.
DOI: IEB authors: Jana Pospíšilová