Investigations into the Stornatal Physiology of Upland Cotton

Stomatal behaviour was studied under field conditions using an infiltration method with solutions of iso-butanol diluted with ethylene glycol. For mature leaves not suffering water strain, stomata in both epidermes commence opening at sunrise (6.30 a.m. local time) and have a maximum aperture between 10 a.m. and 3 p.m.; subsequently a slow closure commences and this is complete by 7 pjn. For leaves suffering marked water stress, i.e. with relative turgidity values falling below 85 per cent., the period of maximum aperture is curtailed by a varying amount dependent upon the extent of water stress; closure occurs earlier, and no evidence has been found for a second opening later in the day. For leaves not suffering water strain it is shown by means of a multiple correlation analysis that stomatal aperture is significantly correlated with solar radiation, temperature, and hour of day. It is suggested that the hour-of-day effect may be a true diurnal rhythm controlled by factors within the leaf. The correlations noted above are not found for leaves with low values of relative turgidity, since the effects of water strain tend to over-ride those due to the other factors.