Effect of light and temperature on seed germination of selected African leafy vegetables

Abstract

Using laboratory incubation, the response of seed germination and emergence to variability in temperature and light was examined for spider flower (Cleome gynandra L.), amaranth (Amaranthus cruentus L.), non-heading Chinese cabbage (Brassica rapa L. subsp. chinensis), nightshade (Solanum retroflexum Dun.), pumpkin (Cucurbita maxima Duchesne), tsamma melon (Citrullus lanatus Thunb.), Jew's mallow (Corchorus olitorius L.) and cowpea (Vigna unguiculata (L.) Walp.). Effect of temperature on seed germination and emergence was monitored under continuous darkness and at constant temperatures that ranged from 4°C to 44°C with 4°C increments. The effect of light on seed germination was measured at 25°C. Temperature affected germination rate and final germination percentage of all species tested in ways that were characteristic for each of the species tested. Generally, optimum germination occurred at temperatures ranging between 29°C and 32°C but at higher temperatures for V. unguiculata (36°C) and C. olitorius (35°C). The minimum temperature for germination ranged between 8°C and 15°C, and the maximum between 36°C and 44°C. Optimum temperatures for seedling emergence ranged from 25°C to 31°C, the maximum between 32°C and 40°C and minimum between 2°C and 13°C. Light positively (p<0.01) affected onset of germination in A. cruentus, B. rapa subsp. chinensis and C. olitorius, and final germination percentage of B. rapa subsp. chinensis, C. lanatus and S. retroflexum. The results suggested that under South African conditions, seeds of the eight species will typically germinate optimally as temperatures rise during spring before the occurrence of very hot temperatures in summer. Due to their positive response to light, germination of B. rapa subsp. chinensis, C. lanatus and S. retroflexum seeds is expected to be optimal when sown at or close to the soil surface.