WHY DO LEAVES AND LEAF-CELLS OF N-LIMITED BARLEY ELONGATE AT REDUCED RATES

The objective of the present study was to assess whether, in barley, n itrogen supply limits the rate of leaf elongation through a reduction in (relative) cell elongation rate and whether this is attributable to a reduced turgor, a reduced availability of osmolytes or, by implicat ion, changed wall properties. Plants were grown on full-strength Hoagl and solution (''Hoagland''-plants), or on N-deficient Hoagland solutio n while receiving N at a relative addition rate of 16 or 8% N . plant- N-1. d(-1) (''16%-'' and ''8%-plants''). Hoagland-plants were demand-l imited, whereas 16%- and 8%-plants were supply-limited in N. Third lea ves were analysed for leaf elongation rate and final epidermal cell le ngth, and, within the basal growing region, for the spatial distributi on of relative segmental elongation rates (RSER, pin-pricking method), epidermal cell turgor (cell-pressure probe), osmotic pressure (OP, pi colitre osmometry and water potential (Psi). During the development of the third leaf, plants grew at relative growth rates (relative increa se in fresh weight) of 18.2, 15.6 and 8.1% . d(-1) (Hoagland-, 16%- an d 8%-plants, respectively). Final leaf length and leaf elongation rate were highest in Hoagland plants (ca. 34.1 cm and 2.33-2.60 mm . h(-1) , respectively), intermediate in 16%-plants (31.0 cm and 1.89-1.96 mm . h(-1)) and lowest in 8%-plants (29.4 cm and 1.41-1.58 mm . h(-1)). T hese differences were accompanied by only small differences in final c ell length, but large differences in cell-flux rates (146, 187 and 201 cells . cell-file(-1) . d(-1) in 8%-, 16%- and Hoagland-plants, respe ctively). The length of the growth zone (32-38 mm) was not much affect ed by N-levels (and nutrient technique). A decrease in RSER in the gro wth zone distal to 10 mm produced the significant effect of N-levels o n leaf elongation rate. In all treatments, cell turgor was almost cons tant throughout the growing region, as were cell OP and Psi in 16%- an d 8%-plants. In Hoagland-plants, however, cell OP increased by ca. 0.1 MPa within the zone of highest elongation rates and, as a consequence , cell Psi decreased simultaneously by 0.1 MPa. Cell Psi increased con siderably where elongation ceased. Within the zone where differences i n RSERs were highest between treatments (10-34 mm from base) average t urgor was lowest, OP highest and Psi most negative in Hoagland- compar ed to 8%- and 16%-plants (P < 0.001), but not significantly different between 8%- and 16%-plants.