RELATIVE SENSITIVITY OF LEAF ELONGATION AND STOMATAL CONDUCTANCE OF CUCUMBER PLANTS TO CHANGES IN LEAF AND SOIL-WATER POTENTIALS

Identification of drought-resistant cucumber genotypes and optimizatio n of irrigation scheduling require an improved understanding of cucumb er plant water relations. Stomatal conductance (ks), relative leaf elo ngation rate (RLER), and leaf water relations were examined on greenho use-grown cucumber (Cucumis sativus L. 'Marketmore 80') over two droug ht cycles. Water was withheld from the entire root system of cucumber plants and ks, leaf length, leaf water (Psi(L)) and osmotic potential, and soil matric potential were measured periodically. In both drought cycles, ks decreased before, and RLER decreased after predawn Psi(L) began to decline. No osmotic adjustment was found after one drought cy cle and slight osmotic adjustment (- 0.14 MPa) was observed in expandi ng leaves after a second drought cycle. Evidence for a nonhydraulic ro ot signal in cucumber was examined in split-root experiments. Psi(L) w as Similar in plants which received water to one (stress treatment) or to both halves (control treatment) of the root system. Stomatal condu ctance and RLER remained at control levels as soil water potential (Ps i(L)) around half the root system declined to - 0.25 to - 0.30 MPa No convincing evidence for effects of root signals on ks and RLER at lowe r Psi(soil) was observed.