Stomatal responses to alterations of soil and air humidity in grapevines

In 1985, during 31 diurnal cycles the assimilation and transpiration of unstressed (predawn Ψw -2 to -3 bar) and cyclically stressed (predawn Ψw -2 to -8 bar) Riesling and Silvaner vines were recorded: • In unstressed vines, the average stomatal conductance at daily maximum assimilation (g at Amax) was lower in Riesling than in Silvaner ( - 29 % ). The higher water use efficiency (assimilation to transpiration ratio, A/E at Amax) of Riesling was mainly due to its lower transpiration. At reduced water supply, g (at Amax) was reduced in both varieties causing a decline of A and E (at Amax); especially in Riesling the A/E ratio ( at Amax) was increased due to water stress. • g (at Amax) and Amax were always positively correlated. In Silvaner, water stress led to a steeper slope of the regression line which indicates an increase of adaptation to drought; in Riesling, water stress caused a higher correlation coefficient. This closer correlation at water stress, the high A/E ratio (at Amax) and the steep slope of the regression line suggest that Riesling, especially under stress conditions, has a more precise functioning of stomatal action and herewith a higher adaptability to drought conditions than Silvaner. • In unstressed vines, the A/E ratio (daily average values) declined at increasing leaf-air water vapor pressure difference (∆w), Riesling exhibiting generally higher A/E values than Silvaner. In 1986, Riesling leaves were subjected to increasing ∆w (from 13 to 18 mbar · bar-1) at constant light and temperature: • At high leaf water potentials ( -4 to - 6 bar), alterations of ∆w led to simultanous decreases of g, A and E. Related to stomatal conductance, transpiration increased in a linear, assimilation in a curvilinear manner. The optimal stomatal conductance was given where the marginal transpiration rate equaled the marginal assimilation rate; the A/E ratio and the correlation coefficient g (at Amax)-Amax were found tobe highest in that range of stomatal conductance.