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 st udied in quinoa grown in pots with sandy soil and in lysimeter plots with s andy loam in the field. Midday values of leaf water potential (psi(1)), lea f osmotic potential (psi(pi),), relative water content (RWC), leaf conducta nce (g(1)), light saturated net photosynthesis (A(sat)), and specific leaf area (SLA) were determined in fully watered and droughted plants. At branch ing, flowering and grain filling g(1) in leaves of fully watered plants var ied from 0.3 to 1.0, 0.3 to 0.6 and 0.2 to 0.7 mol m(2) s(-1) and A(sat) va ried from 18 to 34, 14 to 24 and 8 to 26 mu mol m(2) s(-1). In droughted pl ants stomatal closure began when leaf water potential (psi(1)) decreased be low - 1.2 to - 1.6 MPa and A(sat) was reduced to 5-10 mu mol m(2) s(-1) as a result of stomatal closure, when psi(1) decreased to - 1.5 to - 2.0 MPa. The osmotic potential at full turgor (psi(pi)(100)) decreased by age from - 1.0 to - 1.4 MPa. During severe water stress quinoa maintained positive tu rgor down to a zero turgor leaf water potential Value (psi(1)(0)) of -1.8 M Pa. Quinoa had a limited osmotic adjustment psi(pi)(100) between fully wate red and droughted plants being 0.3-0.4 MPa at the most. During branching th e turgid weight/dry weight (TW/DW) ratio decreased from 9 to 5. At flowerin g and grain filling the TW/DW ratio was low (4-6). The bulk elastic modulus (epsilon(max)) determined at the beginning of the grain filling period was medium to high (18-22 MPa). SLA was high (23-21 m(2) kg(-1)) during branch ing and decreased during the later growth stages. Conclusively, both high n et photosynthesis rates and SLA values during early vegetative growth proba bly result in early vigour of quinoa supporting early water uptake and thus tolerance to a following drought. The stomatal response of quinoa was inse nsitive to drought induced decrease of leaf water status. The leaf water re lations were characterised by low osmotic potentials and low TW/DW ratios d uring later growth stages sustaining a potential gradient for water uptake and turgor maintenance during soil drying. (C) 2000 Elsevier Science B.V. A ll rights reserved.