Dynamic aspects and enhancement of leaf elongation in rice

Some dynamic aspects of leaf elongation in rice were studied. Under both well watered and water-deficient conditions, leaf elongation rates were 15 to 30% greater during the day than during the night. Night temperatures below 27 C limited the rate of elongation at night but when night temperatures exceeded 27 C, night elongation rates exceeded rates during the day. The diurnal pattern of elongation was opposite to the pattern of bulk leaf turgor which was lower during the day than at night.Superimposed on the general diurnal pattern of leaf elongation were perturbations associated with the light/dark transitions. The rate of leaf elongation declined within minutes after illumination and remained low for 15 to 60 minutes, after which rapid rates ensued. The rate of leaf elongation was transiently accelerated within minutes after transition to dark and then declined to steady night rates after 30 to 60 minutes. Removal or covering of all subtending leaves eliminated these perturbations. Irrigation during the light-induced inhibition period did not influence leaf elongation rates of well watered plants but in stressed plants, high rates of elongation resumed immediately after irrigation.The rate of elongation was accelerated by hydrostatic pressure applied to roots of intact plants. The rate of leaf elongation increased with increasing pressure to about 5 bars and then showed no further increase with increasing pressure. This suggests that the rate of water uptake normally limits the rate of leaf elongation. The response to pressure could be altered by addition of an osmoticum to the root medium and elongation occurred only when the gradient of total water potential between the substrate and elongating leaf allowed water absorption. A model of leaf expansion based on water potential gradients is proposed to explain these observations.