PARTITION OF PHOTOSYNTHATES BETWEEN SHOOT AND ROOT IN SPRING WHEAT (TRITICUM-AESTIVUM L) AS A FUNCTION OF SOIL-WATER POTENTIAL AND ROOT TEMPERATURE

Low soil water potential and low or high root temperatures are importa nt stresses affecting carbon allocation in plants. This study examines the effects of these stresses on carbon allocation from the perspecti ve of whole plant mass balance. Sixteen-day old spring wheat seedlings were placed in a growth room under precisely controlled root temperat ures and soil water potentials. Five soil water potential treatments, from -0.03 MPa to -0.25 MPa, and six root temperature treatments, from 12 to 32 degrees C were used. A mathematical model based on mass bala nce considerations was used, in combination with experimental measurem ents of rate of net photosynthesis, leaf area, and shoot/root dry mass es to determine photosynthate allocation between shoot and root. Parti tioning of photosynthates to roots was the lowest at 22-27 degrees C r oot temperature regardless soil water potential, and increased at both lower and higher root temperatures. Partitioning of photosynthates to the roots increased with decreasing soil water potential. Under the m ost favourable conditions, i.e. at -0.03 MPa soil water potential and 27 degrees C root temperature, the largest fraction, 57%, of photosynt hates was allocated to the shoots. Under the most stressed conditions, i.e. at -0.25 MPa soil water potential and 32 degrees C root temperat ure, the largest fraction, more than 80%, of photosynthates was alloca ted to roots.