NITROGEN AND CARBON-ISOTOPE COMPOSITION OF WHEAT-GRAIN - ALTERATION DUE TO SINK-SOURCE MODIFICATIONS AT FLOWERING

An experiment of sink-source modifications was performed in field-grow n wheat at flowering on five varieties (four Triticum aestivum and one T. durum); among these, two varieties were non-awned. The sink-source modifications were blade removal (treatment B) and blade deletion and stalk and sheaths darkening (treatment C). Intact plants (treatment A ) acted as a control, In the three awned varieties, the effect of awn removal was also analysed. Two lots of tests were initially performed, with awns (treatments A(+), B-+, C-+) or with awns removed (treatment s A(-), B--, C--), Yields and carbon and nitrogen isotope compositions were then determined on seeds harvested at maturity. Blade deletion r educed the yield by one third whereas additional stalk and sheaths dar kening led to a two thirds decrease. Nitrogen yield was similarly affe cted but the deprivation due to treatment C was less marked for nitrog en than for carbon. Thus, treatment C produced grains which were notab ly richer in nitrogen than plants which had received treatment A and B , For each of the five varieties, the decrease in yield was highly cor related with a C-13 enrichment of grains (roughly from -28 to -25 part s per thousand). In contrast with the C-13 variations, there was no ge neral trend for N-15 evolution among treated plants. The final yield f or treated T. durum plants was strongly modified when the awns were re moved at flowering whereas no significant variations were noted in the other T. aestivum awned varieties. T. durum seeds obtained from plant s deprived of awns were notably depleted in C-13 and in N-15 compared to control plants. Isotope compositions of C and N of the different po ols supplying the grain filling were estimated and discussed in the co ntext of the physiology of the mother plants between flowering and mat urity.