DROUGHT EFFECTS ON CELLULAR AND SPATIAL PARAMETERS OF LEAF GROWTH IN TALL FESCUE

The effect of drought and recovery on cellular and spatial parameters of the growth process in tall fescue leaves was studied in two experiments. In both experiments plants grown on vermiculite and maintained in a controlled environment were submitted to a 7 d drought period generated by withholding water. Drought was followed by a 3 d recovery period in experiment II. As leaf elongation rate (LER) decreased during developing drought both the growth zone length (initially 40 mm) and the maximum relative elemental growth rate (initially 0.09 mm mm(-1) h(-1) during the dark period of diurnal cycles) within the growth zone declined. But the growth zone still exhibited a length of approximately 15 mm when LER approached 0 under severe drought (-2.0 MPa predawn leaf water potential). The growth potential of the basal 15-mm-long portion of the leaf was conserved during the period when drought effected the complete arrest of leaf elongation, A (retrospective) analysis of the position-time relationships of epidermal cells identified on leaf replicas (experiment II) indicated that the cell flux out of the growth zone responded very sensitively to drought, Before drought the flux was maximum at approximately 3.2 cells (cell file h)(-1) during the dark period, Flux decreased to 0 when leaf elongation stopped. Flux also varied diurnally both under well-watered and droughted conditions. In well-watered conditions it was about 30% less during the light than the dark period. Cell elongation was also sensitive to drought. Under well-watered conditions epidermal cell elongation stopped when cells attained a length of approximately 480 mu m. During developing drought cells stopped elongating at progressively shorter lengths. When LER had decreased to almost nil, cells stopped elongating at a length of approximately 250 mu m. When drought was relieved following a 2 d complete arrest of leaf elongation then cells shorter than 250 mu m were able to resume expansion. Following rewatering cell flux out of the growth zone increased rapidly to and above the pre-drought level, but there was only a slow increase over time in the length at which cell elongation stopped. About 2 d elapsed until the leaf growth zone produced cells of similar length as before drought (i.e. approximately 480 mu m).