Leaf patch clamp pressure probe measurements on olive leaves in a nearly turgorless state

The non‐invasive leaf patch clamp pressure (LPCP) probe measures the attenuated pressure of a leaf patch, P_p, in response to an externally applied magnetic force. P_p is inversely coupled with leaf turgor pressure, P_c, i.e. at high P_c values the P_p values are small and at low P_c values the P_p values are high. This relationship between P_c and P_p could also be verified for 2‐m tall olive trees under laboratory conditions using the cell turgor pressure probe. When the laboratory plants were subjected to severe water stress (P_c dropped below ca. 50 kPa), P_p curves show reverse diurnal changes, i.e. during the light regime (high transpiration) a minimum P_p value, and during darkness a peak P_p value is recorded. This reversal of the P_p curves was completely reversible. Upon watering, the original diurnal P_p changes were re‐established within 2–3 days. Olive trees in the field showed a similar turnover of the shape of the P_p curves upon drought, despite pronounced fluctuations in microclimate. The reversal of the P_p curves is most likely due to accumulation of air in the leaves. This assumption was supported with cross‐sections through leaves subjected to prolonged drought. In contrast to well‐watered leaves, microscopic inspection of leaves exhibiting inverse diurnal P_p curves revealed large air‐filled areas in parenchyma tissue. Significantly larger amounts of air could also be extracted from water‐stressed leaves than from well‐watered leaves using the cell turgor pressure probe. Furthermore, theoretical analysis of the experimental P_p curves shows that the propagation of pressure through the nearly turgorless leaf must be exclusively dictated by air. Equations are derived that provide valuable information about the water status of olive leaves close to zero P_c.