4.2. Light
The seed of B. rapa subsp. chinensis, C. lanatus and S. retroflexum demonstrated positive photosensitivity, suggesting that seed germination of these species is a photochrome-mediated response, which is an attribute commonly observed among weeds ( Kettenring et al., 2006). Environmentally induced photosensitivity of seed is usually interpreted as an adaptation that ensures that seed germinates in places where there is a high probability of seedling establishment (Mpati, 2006). The seed of these three species is likely to prefer germinating at or near the soil surface. In the case of the ‘dabadaba’ landrace of B. rapa subsp. chinensis, this behaviour has been captured in indigenous knowledge. In Tshivenda, the word ‘dabadaba’ describes someone (or something) who is doing his or her own thing and does not follow the rules. According toTshikalange (2006) people in Venda named this land race of B. rapa subsp. chinensis‘dabadaba’, because it germinates like a weed, i.e. following soil disturbance. Soil disturbance practices, such as cultivation, expose buried seeds to light inducing germination of species that are light-sensitive ( Kettenring et al., 2006). Löwe and Dillenburg (2011) argued that light reliant species usually have small seeds that have faster photosynthetic cotyledons and growth, as appears to be the case for B. rapasubsp. chinensis and S. retroflexum, and shorter use of seed reserves than non-light reliant species. Aud and Ferraz (2012) agreed by pointing out that the limited resources contained in small seeds were compensated for by light dependency of the germination of such seeds. Conversely, in large-seeded species the seed reserves are greater and this tends to reduce their dependence on photosynthesis during the early stages of establishment (Khan, 2004).