The effect of pretreatment temperature on the exudation of xylem sap by detached barley root systems

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Abstract
Roots of barley (Hordeum vulgare L.) plants which had been grown at 20° were cooled to temperatures of 12° C or lower for 1–5 days and then returned to 20° C when measurements were made of ion and water movement into the xylem after excision. Very marked increases in exudation were observed in cooled roots, sometimes as much as four times greater than in controls kept at 20° C throughout their life. There were no consistent increases in the concentration of K+, Ca2+ or H2PO4′ in the exudate from cooled roots and it was concluded that the extra water flow across the root occurred without any increase in the osmotic potential of the sap. Similar changes in ion absorption and translocation were found in intact plants transpiring either rapidly or slowly. Cooled roots contained appreciably more soluble carbohydrate than controls and when they were returned to 20° C their respiration was 50–120% greater. These changes were not, however, strictly correlated with differences in exudation in cooled and untreated roots. Cooled plants which had been heavily shaded respired at the same rate as controls but exuded sap at twice the rate. It is concluded that the effects of cooling on both exudation and the accumulation of roluble carbohydrate are consequences of reduced growth and the possible alteration of the selative amounts of growth substances in the root.