Water relations under root chilling in a sensitive and tolerant tomato species

The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling tempera- tures of around 5 ∞C. Under the same treatment, a chilling- tolerant congener (Lycopersicon hirsutum LA 1778) main- tains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure cham- bers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, how- ever, in stomatal behaviour: in L. hirsutum, stomatal con- ductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 ∞C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differen- tial stomatal behaviour during root chilling has distinct shoot and root components.