Knowledge of factors controlling leaf appearance is important for understanding climatic adaptation of a plant species. For quinoa ( Chenopodium quinoa Willd.) we show that both temperature and photoperiod control the rate of leaf appearance. Minimum phyllochron (thermal time between the appearance of two successive leaves as observed under short days) and photoperiod sensitivity of the phyllochron decreased as latitude of origin of a cultivar increased from 1˚13¢ N to 38˚46¢ S. Minimum phyllochron ranged from 21.8 ˚Cd in the Colombian cv. Nariño, to 15.9 ˚Cd in the Chilean cv. Baer (estimated for a common base temperature of 2˚C). Photoperiod sensitivity ranged from 1.2 ˚Cd h –1 in Nariño to insensitivity (0 ˚Cd h –1 ) in the Bolivian and Peruvian altiplano cvv. Kanckolla, Blanca de Juli and Sajama. The phyllochron sensitivities to photoperiod and temperature were linearly and positively associated with photoperiod and temperature sensitivities of time to visible flower buds (R 2 = 0.70 and 0.55, respectively, P < 0.05), so that shorter phyllochrons were associated with early flowering cultivars. Temperature sensitivity was highest in cvv. originating in cold or dry climates, and lowest for cvv. from more humid and warm climates. We suggest, therefore, that in its domestication as a crop plant, photoperiod sensitivity of quinoa has been selected for as a homeostatic mechanism to counteract the potentially reduced leaf area associated with early flowering under short days and high temperatures in the tropics.