Stomatal diffusion resistances and leaf growth during droughting of Lolium perenne plants selected for contrasting epidermal ridging

Abstract

SUMMARYGroups of Lolium perenne plants selected for either deep or shallow adaxial epidermal ridging were grown in a 16 h day of 70 W m‐2 at 25°C, and either watered daily to 33% or allowed to dry to and then watered daily to 21% or to 16% soil moisture. During a 9 day experimental period, adaxial leaf resistances (r1) were measured with a diffusion porometer four times daily, transpiration was estimated gravimetrically, and daily rates of leaf extension were recorded.Measurements were also made of minimum abaxial resistances, stomatal frequencies and lengths, and relative leaf water content (RLWC). At 33%, 21% or 16% soil moisture, leaf extension rates of deep ridged leaves were, respectively, slower, the same, and more rapid than those with shallow ridges. At 21% or 16% soil moisture, the adaxial rl of deep‐ridged was much lower than that of shallow‐ridged leaves at all four sampling times. This difference was most marked on leaves below the youngest fully expanded, and was observed among older leaves even when plants were well watered. At low RLWC (< 85%), leaf resistance was greatest in leaves with shallow ridges. There was no significant difference between the leaf types in the calculated contributions of stomatal frequency or of morphology at any one pore opening, to rl but deep‐ridged leaves had more stomataonthe abaxial surface. Daily rate of plant water loss was directly correlated (r=+ 0.86, P < 0.01) with mean daily maximum stomatal conductance (1/rl), and rate of leaf extension negatively with maximum rl. It is suggested that stomata operating in the concavity formed by deep ridges open wider and are less responsive to internal changes in, for example, leaf water status, than those on shallow‐ridged leaves because of a more humid microenvironment at the epidermal surface. The results are discussed in relation to the concept of ‘water‐savers’ and ‘water‐spenders’ and its application to breeding for dry conditions.