Leaf gas exchange and water relation characteristics of field quinoa (Chenopodium quinoa Willd.) during soil drying

The effects of soil drying on leaf water relations and gas exchange were studied in quinoa grown in pots with sandy soil and in lysimeter plots with sandy loam in the field. Midday values of leaf water potential (ψl), leaf osmotic potential (ψπ), relative water content (RWC), leaf conductance (gl), light saturated net photosynthesis (Asat), and specific leaf area (SLA) were determined in fully watered and droughted plants. At branching, flowering and grain filling gl in leaves of fully watered plants varied from 0.3 to 1.0, 0.3 to 0.6 and 0.2 to 0.7 mol m2 s−1 and Asat varied from 18 to 34, 14 to 24 and 8 to 26 μmol m2 s−1. In droughted plants stomatal closure began when leaf water potential (ψl) decreased below −1.2 to −1.6 MPa and Asat was reduced to 5–10 μmol m2 s−1 as a result of stomatal closure, when ψl decreased to −1.5 to −2.0 MPa. The osmotic potential at full turgor (ψπ100) decreased by age from −1.0 to −1.4 MPa. During severe water stress quinoa maintained positive turgor down to a zero turgor leaf water potential value (ψl0) of −1.8 MPa. Quinoa had a limited osmotic adjustment ψπ100 between fully watered and droughted plants being 0.3–0.4 MPa at the most. During branching the turgid weight/dry weight (TW/DW) ratio decreased from 9 to 5. At flowering and grain filling the TW/DW ratio was low (4–6). The bulk elastic modulus (εmax) determined at the beginning of the grain filling period was medium to high (18–22 MPa). SLA was high (23–21 m2 kg−1) during branching and decreased during the later growth stages. Conclusively, both high net photosynthesis rates and SLA values during early vegetative growth probably result in early vigour of quinoa supporting early water uptake and thus tolerance to a following drought. The stomatal response of quinoa was insensitive to drought induced decrease of leaf water status. The leaf water relations were characterised by low osmotic potentials and low TW/DW ratios during later growth stages sustaining a potential gradient for water uptake and turgor maintenance during soil drying.