Temperature induced tissue alterations in the sweet potato root

Abstract

An investigation was made to define the temperature, course of time, and anatomical zones involved in alteration of sweet potato root tissue due to chilling temperature treatments. Enlarged fleshy roots of the sweet potato were chilled at 1°C and 6° for 6 weeks. Similar roots were assigned a 15° control temperature. Tissue was sampled from the root interior and exterior. Samples were withdrawn initially, and at 2, 4, and 6 weeks to determine intact tissue respiration, mitochondrial activity, water absorption, and concentration of chlorogenic acid (CA). Following 4 weeks of 1°, 6°, and 15° temperature treatments, additional root samples were exposed to a 1-week 30° recure then assayed the same as for tissue under continuous chilling. Chilling at 6°C resulted in high rates of respiration for both intact, tissue and mitochondria in addition to an increase in CA when compared with the control. During 6 weeks at 1° intact tissue respiration, mitochondrial activity, and CA concentration decreased. At the end of 6 weeks at either 1° or 6° oxidation was uncoupled from phosphorylation. The terms "severe" and "moderate" accurately describe the injury of tissue chilled in the ranges of 6°C to 7.5° and 0° to 1°, respectively. Oxygen uptake rate of intact tissue was higher in exterior than interior tissue. Chlorogenic acid in 6°C tissue increased to high concentrations in exterior tissue only. The 30°C recure resulted in extremely high concentrations of CA in tissue previously chilled at 6°. The high CA content of the intact root following the heat treatment failed to uncouple oxidation from phosphorylation in the mitochondrial system. Although tissue was degenerated after the recure, respiratory control and ADP:O ratios indicated the presence of mitochondrial activity. The primary site for chilling injury in the root then was thought to be the cell membrane.