Biophysical limitation of leaf cell elongation in source-reduced barley

Abstract.

The biophysical basis of reduced leaf elongation rate in source-reduced barley (Hordeum vulgare L. cv Golf) was studied. Reduction in source strength was achieved by removing the blade of leaves 1 and 2 at the time leaf 3 had emerged 3.0–6.7 cm from the encircling sheath. Third leaves of source-reduced plants elongated at 10–36% lower velocities than those of control plants. Removal of source leaves had no significant effect on maximum relative elemental growth rates (REGRs) and the length of the elongation zone (42–46 mm) but caused a shift of high REGR towards the basal portion of the elongation zone. Cell turgor was similar between treatments in the zone of maximal REGR (16–24 mm from base), but was significantly lower in source-reduced plants in the distal part of the elongation zone, where REGR was also lower. Throughout the elongation zone, osmolality and growth-associated water potential gradients were significantly smaller in source-reduced plants; bulk concentrations of sugars (hexoses, sucrose) were also lower. However, even in control plants, sugars contributed little to bulk osmotic pressure (6–11%). The most likely biophysical limitation to leaf (cell) elongation in source-reduced barley was a reduction in turgor in the distal half of the elongation zone. It is proposed that in the proximal half, increase in average tissue hydraulic conductance enabled source-reduced plants to maintain turgor and REGR at control level, while spending less energy on solute transport.