Effects of decreased O2partial pressure P(O2) and increased CO2partial pressure p(CO2) on senescence of 7-day-old oat plants or excised leaves (Avena sativaL. cv. Victory) and broccoli miniflorets (Brassica oleraceaL. var.italica) were studied in terms of chlorophyll and protein loss, proteolytic activity, and stomatal behaviour. Senescence in darkness was delayed by decreasing p(O2), an effect which in oat leaves was unaffected by the CO2partial pressure, p(O2). However, increased P(CO2) at low O2levels gave the best colour and flavour retension in broccoli miniflorets. Chlorophyll level increased c. 25% during the first 2–3 days after plants or excised leaves were placed in darkness at a P(O2) of 0.5 kPa. Protein and chlorophyll breakdown were inhibited when p(O2) was 2 kPa or less. Excision of leaves accelerated breakdown when compared to whole plants regardless of p(O2). Modification of ubiquitin conjugates was observed in broccoli miniflorets stored at reduced p(O2). The data indicates ubiquit-independent proteolysis contributed to senescence of broccoli miniflorets.