Chilling injury in coleus as influenced by photosynthetically active radiation, temperature and abscisic acid pretreatment. I. Morphological and physiological responses

Coleus blumei Benth. (PI No. 354190), a green‐leafed cultivar, was exposed to 5°C for 48 or 72 h after pretreatment for 48 h at two levels of photosynthetically active radiation (PAR) (8 and 320 μmol s−1 m−2), two temperatures (13 and 20°C), and two abscisic acid (ABA) levels (0 and 200 g m−3 of the racemic mixture). Plants given low PAR for only 48 h prior to chilling treatment (48 or 72 h at 5°C) showed increased protection against chilling injury while those given high PAR were severely injured. The former plants were darker green, contained greater concentrations of chlorophyll‐a, chlorophyll‐b, total chlorophyll and anthocyanin and generally had a lower abscission rate than the latter plants. There were no differences, however, in chlorophyll‐a/b ratio among plants grown at the two PAR levels, two temperatures or two ABA concentrations. Temperature and ABA pretreatment and number of hours at 5°C had no significant effect on chilling injury as measured by leaf chlorosis, but generally had a significant effect on leaf abcission, especially at 3 and 7 d after returning the plants to the greenhouse. Enclosing intact plants or excised shoots in plastic bags to maintain 100% relative humidity during 72 h chilling treatment failed to provide protection against chilling injury. These findings indicate that the protective effects of low PAR applied prior to chilling treatment may be as important or more important than that applied during chilling. They also indicate the importance of making careful measurements of PAR levels when conducting studies on chilling injury.