Cold-Inhibited Phloem Translocation in Sugar Beet: II. CHARACTERIZATION AND LOCALIZATION OF THE SLOW-COOLING RESPONSE

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Abstract
Experiments were undertaken on a simplified sugar beet system to characterize the phloem translocation response to slow cooling treatments that were applied to the source leaf petiole. In these experiments the temperature was decreased by 4°C every 16 min, such that the tissue temperature was lowered from 25°C to 1°C over a period of 80 min. Our results indicated that an initial slow cooling treatment, on a given test plant, caused no change in the rate of translocation. However, all subsequent slow cooling regimes that were applied to the same petiole position elicited a characteristic step-type inhibition. This inhibition averaged about 10% of the original translocation rate in all cases with no recovery being observed. The data suggest that the initial cooling treatment induced an alteration in the petiole tissue which facilitated the inhibition phenomenon during subsequent slow coolings. This alteration was shown to be localized within the upstream region of the chilled petiole segment, following an initial slow cooling, or throughout the chilled petiole segment after an initial quick cooling from 25°C to 1°C. Results also show that the alteration is a long-lived phenomenon that has no detectable influence on the quick-cooling induced transient inhibition of translocation.