Freezing injury in cold-acclimated and unhardened spinach leaves

Abstract

Spinach plants (Spinacia oleracea L.) were frost-hardened by cold-acclimation to 1° C or kept in an unhardy state at 20°/14° C in phytotrons. Detached leaves were exposed to temperatures below 0°C. Rates of photosynthetic CO2 uptake by the leaves, recorded after frost treatment, served as a measure of freezing injury. Thylakoid membranes were isolated from frost-injured leaves and their photosynthetic activities tested. Ice formation occurred at about-4° to-5° C, both in unhardened and cold-acclimated leaves. After thawing, unhardened leaves appeared severely damaged when they had been exposed to-5° to-8° C. Acclimated leaves were damaged by freezing at temperatures between-10° to-14° C. The pattern of freezing damage was complex and appeared to be identical in hardened and unhardened leaves: 1. Inactivation of photosynthesis and respiration of the leaves occurred almost simultaneously. 2. When the leaves were partly damaged, the rates of photosynthetic electron transport and noncyclic photophosphorylation and the extent of light-induced H(+) uptake by the isolated thylakoids were lowered at about the same degree. The dark decay of the proton gradient was, however, not stimulated, indicating that the permeability of the membrane to-ward protons and metal cations had not increased. 3. As shown by partial reactions of the electron transport system, freezing of leaves predominantly inhibited the oxygen evolution, but photosystem II and photosystem I-dependent electron transport were also impaired. 4. Damage of the chloroplast envelope was indicated by a decline in the percentage of intact chloroplasts found in preparations from injured leaves. The results are discussed in relation to earlier studies on freezing damage of thylakoid membranes occurring in vitro.

Fluorescence as a tool in photosynthesis research: application in studies of photoinhibition, cold acclimation and freezing stress

“…Spinach plants ( Spinaciaoleracea L.), five weeks old, were acclimated for ten days to 18 ° (unhardened plants) or were hardened by decreasing the tem perature in two-day intervals: 18-15 °C, 15-10 °C, 10-3 °C, 3-1 °C, 1-1 °C (day-night) as described by Klosson & Krause (1981). In both cases, the photon flux density (p f d ) was 260-300 pmol quanta m-2 s-1 and the light period 8 h.…”

“…Previous studies on the effects of freezing stress (in the absence of additional light stress) showed that C O a assimilation is more readily affected than the electron-transport system in the thylakoids (Klosson & Krause 1981;Strand & Oquist 1985;Rumich-Bayer & Krause 1986;Bauer & Kofler 1987). Apparently, below a certain tolerated tem perature limit the lightactivation system of the carbon reduction cycle becomes impaired (Rumich-Bayer & Krause 1986).…”

“…After long-term cold acclimation under a controlled p f d (260-300 pmol quanta m-2 s-1) the spinach leaves exhibited a substantially increased frost tolerance (cf. Klosson & Krause 1981). These hardened leaves showed phenomena of a weak photoinhibition, characterized by a slightly decreased Fy/F u ratio and quantum yield of photosynthesis (table 1).…”