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 experim ents. In both experiments plants grown on vermiculite and maintained i n a controlled environment were submitted to a 7 d drought period gene rated by withholding water. Drought was followed by a 3 d recovery per iod in experiment II. As leaf elongation rate (LER) decreased during d eveloping drought both the growth zone length (initially 40 mm) and th e maximum relative elemental growth rate (initially 0.09 mm mm(-1) h(- 1) during the dark period of diurnal cycles) within the growth zone de clined. But the growth zone still exhibited a length of approximately 15 mm when LER approached 0 under severe drought (-2.0 MPa predawn lea f water potential). The growth potential of the basal 15-mm-long porti on of the leaf was conserved during the period when drought effected t he complete arrest of leaf elongation, A (retrospective) analysis of t he position-time relationships of epidermal cells identified on leaf r eplicas (experiment II) indicated that the cell flux out of the growth zone responded very sensitively to drought, Before drought the flux w as maximum at approximately 3.2 cells (cell file h)(-1) during the dar k 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 th e dark period. Cell elongation was also sensitive to drought. Under we ll-watered conditions epidermal cell elongation stopped when cells att ained a length of approximately 480 mu m. During developing drought ce lls stopped elongating at progressively shorter lengths. When LER had decreased to almost nil, cells stopped elongating at a length of appro ximately 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 t o resume expansion. Following rewatering cell flux out of the growth z one increased rapidly to and above the pre-drought level, but there wa s only a slow increase over time in the length at which cell elongatio n stopped. About 2 d elapsed until the leaf growth zone produced cells of similar length as before drought (i.e. approximately 480 mu m).