ONTOGENIC CHANGES IN THE CONSTRUCTION COST OF LEAVES, STEMS, FRUITS, AND ROOTS OF TOMATO PLANTS

The construction cost of a plant tissue, i.e. the amount of photoassim ilates used in the synthesis of a unit weight, varies with its biochem ical composition. Crop modellers use standard values published for a f ew groups of cultivated species. Yet, there are also intraspecific var iations in the construction cost in relation with the development of t he plant or organ. This research aimed at analysing the ontogenic chan ges in the construction cost of leaves, stems, roots, and fruits of to mato plants and the specific contribution of the mineral content to th ese changes, For that purpose, samples were harvested from the vegetat ive phase to the beginning of fruit production, The estimation of the construction cost was based on the contents of carbon, nitrogen and as h, In leaves, the construction cost decreased with the physiological a ge whereas, in stem internodes, it varied with the sympod number. Thes e ontogenic changes could partly be explained by different accumulatio ns of minerals, In contrast, the construction cost and the mineral con tent of fruits and roots remained fairly stable. On a whole plant basi s, the construction cost of the bulk of each category of organs varied much less, Most of the increase in the mean construction cost of the whole plant during the experiment was due to changes in the allocation ratio between the vegetative parts and the fruits. Attention of crop modellers is drawn to the importance of a precise estimation of the co nstruction cost and to the existence of ontogenic changes at the whole plant and organ levels.