AN ARCHITECTURAL ANALYSIS OF THE ELONGATION OF FIELD-GROWN SUNFLOWER ROOT SYSTEMS - ELEMENTS FOR MODELING THE EFFECTS OF TEMPERATURE AND INTERCEPTED RADIATION

The effects of photosynthetic photon flux density (PPFD) and soil temp erature on root system elongation rate have been analysed by using an architectural framework, Root elongation rate was analysed by consider ing three terms, (i) the branch appearance rate, (ii) the individual e longation rates of the taproot and branches and (iii) the proportion o f branches which stop elongating. Large ranges of PPFD and soil temper ature were obtained in a series of field and growth chamber experiment s, In the field, the growth of root systems experiencing day-to-day na tural fluctuation of PPFD and temperature was followed, and some of th e plants under study were shaded, In the growth chamber, plants experi enced contrasting and constant PPFDs and root temperatures, The direct effect of apex temperature on individual root elongation rate was sur prisingly low in the range 13-25 degrees C, except for the first days after germination. Root elongation rate was essentially related to int ercepted PPFD and to distance to the source, both in the field and in the growth chamber. Branch appearance rate substantially varied among days and environmental conditions. It was essentially linked to taproo t elongation rate, as the profile of branch density along the taproot was quite stable. The length of the taproot segment carrying newly app eared branches on a given day was equal to taproot elongation on this day, plus a 'buffering term' which transiently increased if taproot el ongation rate slowed down. The proportion of branches which stopped el ongating a short distance from the taproot ranged from 50-80% and was, therefore, a major architectural variable, although it is not taken i nto account in current architectural models, A set of equations accoun ting for the variabilities in elongation rate, branch appearance rate and proportion of branches which stop elongating, as a function of int ercepted PPFD and apex temperature is proposed. These equations apply for both field and growth chamber experiments.