The depresion of mitosis as a result of rubidium action illustrated by the lower frequency of mitosis was found in an investigation made on roots of wheat seedlings grown on a nutrient medium which the total amount of potassium or a half of it was replaced by rubidium. The lower mitotic activity was caused by limited number of nuclei beginning prophase and by a prolonged duration of metaphase.

Measurements were carried out of the assimilation area, NAR value, the crop growth rate (C) and of the yield of roots and leaves of sugar beet plants in a production field during two successive vegetation years. An interdependence was found between the formation of the assimilation area in the canopy and the final yield of biomass. The assimilation area depended mainly on the date of sowing. In 1975, the sowing was earlier by 15 days, amid this resulted in a much more favourable LAI and in a higher yield of biomass. During both vegetation years, a violent decrease of the crop growth rate was observed at the end of August and at the beginning of September. This may be due to an unfavourable change in the ratio of the area of younger, photosynthetically active leaves to older, less active leaves and also to the increased participation of the loss of the assimilates resulting from stronger respiration of the fast growing roots while the photosynthesis of the whole plants decreases.