The Relative Magnitude of the Partial Resistances to Transpirational Water Movement in Sunflower (Helianthus annuusL.)

Abstract

The effects on leaf water status of cooling entire root systems or stem and petiolar segments were examined using hydroponic sunflower plants. Leaf water potential (Ψ^I) decreased by up to 7 × 10⁵ Pa when root temperature was reduced to 10 °C or below; complete recovery occurred subsequently provided freezing was avoided. Leaf water status was unaffected by cooling stem or petiolar segments unless freezing occurred, when severe irreversible water stress developed above the cooled zone. The leaves below the cooled zone were unaffected, demonstrating that the stress did not originate from transmitted effects on the roots. Subsequent measurements using small heads of water applied to excised petioles demonstrated that petiolar resistance was low (c. 0.04 Pa s cm₋₄) except in immature and senescent petioles where resistance was up to 10 times larger. This trend reflected the developmental stage and condition of the xylem. Abrupt increases in evaporative demand, obtained by rapid reduction of relative humidity from 100% to 60 or 40%, induced transient decreases in Ψ of approximately 4.5 and 2.5 × 10⁵ Pa, respectively, which were accompanied by simultaneous large increases in stomatal resistance. No simultaneous transient effects were observed in the stem xylem, demonstrating that the factor responsible for the formation of the transient foliar stress resides within the lamina. The results are discussed in relation to the relative magnitude of the various partial resistances to transpirational water movement.