CONTROL OF LEAF CELL ELONGATION IN BARLEY - GENERATION RATES OF OSMOTIC-PRESSURE AND TURGOR, AND GROWTH-ASSOCIATED WATER POTENTIAL GRADIENTS

In a previous study on the effects of N-supply on leaf cell elongation , the spatial distribution of relative cell elongation rates (RCER), e pidermal cell turgor, osmotic pressure (OP) and water potential (Psi) along the elongation zone of the third leaf of barley was determined ( W. Fricke et al. 1997, Planta 202: 522-530). The results suggested tha t in plants receiving N at fixed relative addition rates (N-supply lim itation of growth), cell elongation was rate-limited by the rate of so lute provision, whereas in plants growing on complete nutrient solutio n containing excessive amounts of N (N-demand limitation), cell elonga tion was rate-limited by the rate of water supply or wall yielding. In the present paper, these suggestions were tested further. The generat ion rates of cell OF, turgor and Psi along the elongation zone were ca lculated by applying the continuity equation of fluid dynamics to the previous data. To allow a more conclusive interpretation of results, a natomical data were collected and bulk solute concentrations determine d. The rate of OF generation generally exceeded the rate of turgor gen eration. As a result, negative values of cell Psi were created, partic ularly in demand-limited plants. These plants showed highest RCER alon g the elongation zone and a Psi gradient of at least -0.15 MPa between water source (xylem) and expanding epidermal cells. The latter was si milar to a theoretically predicted value (-0.18 MPa). Highest rates of OP generation were observed in demand-limited plants, with a maximum rate of 0.112 MPa.h(-1) at 16-20 mm from the leaf base. This was almos t twice the rate in N-supply-limited plants and implied that the cells in the leaf elongation zone were capable of importing (or synthesisin g) every minute almost 1 mM of osmolytes. Potassium, Cl- and NO3- were the main inorganic osmolytes (only determined for demand-limited plan ts). Their concentrations suggest that, unlike the situation in fully expanded epidermal cells, sugars are used to generate OP and turgor. A natomical data revealed that the zone of lateral cell expansion extend ed distally beyond the zone of cell elongation. It is concluded that l eaf cell expansion in barley relies on high rates of water and solute supply, rates that may not be sustainable during periods of sufficient N-supply (limitation by water supply: Psi gradients) or limiting N-su pply (limitation by solute provision: reduced OF-generation rates). To minimise the possibility of growth limitation by water and osmolyte p rovision, longitudinal and lateral cell expansion peak at different lo cations along the growth zone.