The pattern of activity of 1-aminocyclopropane carboxylic acid synthase (ACS) was similar to photosynthetic and growth traits observed at 30, 45, and 60 d after sowing in mustard (Brassica juncea L.) cultivars Varuna and RH 30 differing in photosynthetic capacity. Higher activity of ACS and therefore ethylene release in Varuna than RH 30 increased stomatal conductance, intercellular CO2 concentration, carboxylation rate (carbonic anhydrase and intrinsic water use efficiency), and thus net photosynthetic rate (PN) and leaf and plant dry masses (DM) at all sampling times. Moreover, Varuna also had larger leaf area which contributed to higher PN and DM. A positive correlation between ACS activity and PN and leaf area was found in both the cultivars. Thus ACS activity may affect PN through ethylene-induced changes on foliar gas exchange and leaf growth.