5. Conclusion
Seed of A. cruentus, B. rapa subsp. chinensis, C. gynandra, C. lanatus and C. maximagerminated optimally at 29–32 °C, but V. unguiculata (36 °C) and C. olitorius (35 °C) had higher optima. Accordingly, seed of these species is expected to germinate optimally when planted during spring before the occurrence of hot mid-summer temperatures. Delaying planting to summer appears most appropriate for C. olitorius and V. unguiculatabecause their seeds germinated optimally at 35 °C. However, seed of V. unguiculatatogether with that of B. rapa subsp. chinensis and S. retroflexum emerged best when the temperature ranged between 20 °C and 25 °C, indicating their preference for cool climates or cool periods for seedling emergence during the year in areas where the climate is hot. The ability to emerge at low temperatures, which applied to most crop species tested, indicated that seed could be planted when the temperatures are still relatively low, knowing that the process of emergence will accelerate as temperatures rise towards the optimum. The cardinal temperatures that were determined for the different African leafy vegetables could help to predict their suitability for new localities, but germination and emergence are function of multiple environmental factors. In this study, the experiments were conducted under controlled environmental conditions of constant temperatures, whereas under natural conditions temperature is subject to diurnal fluctuations. Consequently, the cardinal temperatures that were presented could differ from those applying in nature. Emergence has not been studied in sufficient detail to permit reliable predictions for the African leafy vegetable species covered by this study. Future studies should focus on the interactive effects of responsible factors to refine the results obtained in this study.