Solute Transport via Xylem and Phloem in Two Wheat Genotypes Differing in Drought Susceptibility

More frequent and more severe extreme events (e.g. flooding, drought, heat waves) were predicted from climate models for the next decades. Impacts of such events are relevant for crop productivity. Effects on a whole-plant level are the focus of this study. The influence of an extended drought period on solute allocation via the phloem was compared in two Bulgarian wheat varieties differing in drought susceptibility: “Sadovo” (drought-sensitive) and “Katya” (drought-tolerant). Young plants grown hydroponically on standard nutrient solution were transferred at the beginning of the experiment to fresh medium (controls) or to medium with polyethylene glycol 6000 (artificial drought). Radiolabeled solution (57Co, 109Cd, 134Cs) was introduced via a flap into the lamina of the third or fourth leaf of young plants (youngest fully expanded leaf before stress treatment). Export and allocation of the radiolabel were monitored by gamma spectrometry. 57Co, 109Cd and 134Cs are suitable radionuclides, since they are not metabolized and not released in gaseous form. The exported radionuclides were mainly found in the roots (30-50%) and in the younger leaves (30-60%), while only traces reached the older leaves indicating that only small quantities were transported back from the roots to the shoot via the xylem. A comparable percentage was transported from leaf 4 to leaves 5 and 6 in both varieties under control conditions. A much higher percentage was found in leaf 5 than in leaf 6 of “Katya” subjected to drought, while “Sadovo” was affected in the opposite direction. The results suggest that adaptations in the source/sink network are important for the drought susceptibility of wheat varieties.