Abstract Spring morphotypes of white lupin (Lupinus albus L.) are very responsive to cool temperatures during seedling development as expressed in a number of subsequent developmental characters. Thermosensitivity was examined in a number of studies where seedlings were incubated at various stages of development and at differing temperatures. Plants were scored for thermosensitivity, as expressed by the number of mainstem and first order lateral vegetative nodes, at flowering. The stages of radicle emergence and split seed coat were observed to be the developmental stages when mainstem and first-order lateral meristems were most sensitive to incubation temperature. These data show that the optimum window for cold treating spring lupin seed is very narrow and occurs prior to seedling emergence. Seedlings at radicle emergence stage were incubated at temperatures ranging from 1 to 27 °C for 0 to 14 d in 2 d increments. For temperatures less than 12 °C, mainstem node number decreased curvilinearly with duration of incubation. For temperatures between 12 and 17 °C, mainstem node number decreased linearly with duration of incubation. However, mainstem node number increased linearly with duration of incubation at temperatures greater than 17 °C. The data show that 'warm' temperatures during early seedling development have a profound influence on subsequent plant morphology. Response to 'warm' temperatures is greater than to 'cool' temperatures and it is suggested that the differential response to warm vs. cold incubation temperatures could account for morphological and phenological differences observed within genotypes in field studies among sites and years. The lack of temperature response in one genotype, 'Start' may indicate thermoneutrality in this genotype.