Beta-amylase activity. No alpha-amylase activity. Involved in cold resistance. Mediates the accumulation of maltose upon freezing stress, thus contributing to the protection of the photosynthetic electron transport chain. Plays a role in the circadian-regulated starch degradation and maltose metabolism in chloroplasts, especially at night. More active on phosphorylated glucan. Interacts directly with starch or other alpha-1,4-glucan. Hydrolysis of (1->4)-alpha-D-glucosidic linkages in polysaccharides so as to remove successive maltose units from the non-reducing ends of the chains. Redox regulation; active in reducing conditions, inactive in oxidizing conditions. Optimum pH is 6-7. Expressed in vascular tissue of cotyledons, leaves, petioles, stems, petals, siliques and roots, particularly in phloem, as well as in photosynthetic tissues. By cold stress. Light-mediated circadian regulation; highest levels at dawn and at dusk in long days (LD) but only at dusk in short days (SD). Repressed by trehalose in a ABI4-dependent manner, this effect is reversed in the presence of sucrose. Slightly retarded growth rate and reduced starch breakdown in leaves during the night. Belongs to the glycosyl hydrolase 14 family. Truncated C-terminus. Truncated C-terminus. Truncated N-terminus. Truncated N-terminus.