Stomatal behaviour, leaf water status and photosynthetic response in field-grown olive trees under water deficit

Stomatal behaviour, leaf water status and photosynthetic response in relation to long-term water deficit were investigated in southern Italy on young trees of Olive (Olea europaea) to clarify mechanisms of stomatal control. Trees were subjected to three irrigation treatments, T0, T33 and T66 that received 0, 33 and 66%, respectively, of crop evapotranspiration by a drip irrigation system. The prolonged drought during the summer significantly affected soil and leaf water status and gas exchange. In the unirrigated treatment, the drought decreased volumetric soil water content from 30 to 21%, midday leaf water potential from −1.5 to −3.4 MPa, relative water content from 84 to 74% and stomatal conductance to water vapour from 0.190 to 0.023 mol m−2 s−1. Similar responses to milder water deficit were observed for the irrigated treatments. Good positive relationships were found between stomatal conductance and both leaf water potential and soil moisture. This indicates that both hydraulic feedback and feed-forward mechanisms could be invoked in the response of stomata to soil drying. In late summer, a significant re-increase in both leaf water potential and relative water content was observed in the absence of significant rainfall. Conversely, stomatal conductance remained at quite a low value as did soil moisture. The disruption of the positive relationship between stomatal conductance and leaf water potential at constant soil moisture clearly indicates that soil or root water status directly affected stomatal conductance, minimising the possibility of a feedback mechanism through leaf water status. Photosynthetic CO2 assimilation in T0 decreased during the season from 14 to 3.3 μmol m−2 s−1, the minimum value that was found in correspondence with minimum leaf water potential and soil moisture. The response curve of assimilation to intercellular CO2 partial pressure showed that photosynthetic metabolism was greatly depressed by long-term water deficit, with a reduction of 48% for the slope at low CO2 and of 67% for photosynthetic assimilation at high CO2, and that it played a greater role than stomata in limiting photosynthesi