The Effects of Light Intensity and Sucrose on Root Formation, Photosynthetic Ability, and Senescence in Detached Cotyledons of Sinapis alba L. and Raphanus sativus L.
The ability of detached cotyledons cultured in the light to assimilate 14CO2, was reduced by the presence of sucrose in the culture medium. This was due, at least in part, to an increased rate of chlorophyll loss and yellowing of the blade. When cotyledon discs were used, the inhibition of 14Carbon fixation by sucrose was even more marked than in entire cotyledons. This could be due to a higher level of penetration of the sucrose into discs or to the absence of the petiole which normally accumulates photosynthetic products. Sucrose culture also inhibited root production in cotyledons cultured in the light but promoted root formation in dark-grown or DCMU-treated cotyledons. The DCMU-inhibition of 14Carbon fixation by the blades was alleviated to some extent by sucrose culture. The sucrose effect on rooting was not permanent in that transfer into water from sucrose led to root formation although this was delayed and present in a lower proportion of cotyledons than the controls. Thus, although a carbohydrate source either from photosynthesis or as applied sucrose, is essential for root production to take place, the combination of culture in the light with the presence of sucrose in the medium may lead to an accumulation of carbohydrate to a level which directly or indirectly increases blade yellowing and inhibits root production.