CORESTA Meeting, Agronomy/Phytopathology, Oxford, 1995

Using changes in electrical conductivity and sucrose content of flue-cured tobacco leaves to assess ripeness more objectively

Tobacco and Cotton Research Institute, Rustenburg, South Africa
No objective method to measure ripeness of flue-cured tobacco exists at this stage. Leaf ripeness is still assessed in a subjective manner according to leaf colour. Several factors, such as water logging, under fertilization and cultivar differences can all contribute to a false ripeness. The loss of membrane permeability is seen as the first step in the ageing process, because it precedes the degradation of chlorophyll. When ageing commences, bonding of cations to cell walls and membranes starts to decline. If this actually happens, the electrical conductivity (µS) of those cells and membranes might decline as cations come loose and are translocated out of the leaves. Theoretically it may be possible to use the change in electrical conductivity of a leaf to assess it's ripenses more accurately. In field trials, the change in electric conductivity of the different leaf positions on the stem was measured over a period of time. Changes in leaf thickness, relative turgidity and chlorophyll content were also monitored. Whatever the influence of these parameters, electrical conductivity still declined as the chlorophyll content of the leaves declined, i.e. start to ripen or age. Results showed that the best quality tobacco grown on an Arcadia- and Marikana soil series was obtained as soon as the electrical conductivity in the leaf varied between 27.0 and 29.9 µS (micro Siemens). This was especially applicable to leaves 5-12. The best quality for leaves 5-12 grown on a Clansthal soil series were obtained when the conductivity varied between 24.0 µS. Whatever the soiltype used, leaf numbers 1-4 should probably be harvested when the conductivity varies between 27.0 and 29.9 µS at harvesting. Combining the change in sucrose content of the green leaf, measured with a hand held refractometer, with the conductivity may possibly lead to a more objective assessment of ripeness before harvest.