Elevated temperature altered the reaction norms of stomatal conductance in field-grown grapevine

We measured the effect of elevated temperature, and its interaction with fruit load (exp. 1) and water supply (exp. 2), on the stomatal conductance (g s) of Vitis vinifera, cv. Shiraz. Thermal regimes (control vs. elevated temperature using open-top chambers) were initiated in spring, thus affecting leaf development, and were maintained during the whole growing season. We used a top-down approach where reaction norms were derived that relate g s for each treatment and the mean g s across treatments; the slopes of reaction norms quantify phenotypic plasticity.Stomatal conductance responded to neither the interaction between temperature and fruit load (P=0.37) nor the interaction between temperature and water regime (P=0.33). We therefore dealt with each factor separately. Reaction norms of g s under elevated temperature and control treatments diverged, i.e. elevated temperature had no effect on g s under conditions conducive to low conductance (below ∼100mmolm -2s -1) but increased g s in relation to controls under conditions favouring high conductance. Stomata density was 159±6.7mm -2, and was unaffected by temperature. Stomata length increased from 20.6μm in controls to 23.2μm in the heated treatment (P<0.0001) and width increased from 14.4μm in controls to 16.8μm in the heated treatment (P