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, nitrogen 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 implication, changed wall properties. Plants were grown on full-strength Hoagland solution ("Hoagland"-plants), or on N-deficient Hoagland solution 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-limited, whereas 16%- and 8%-plants were supply-limited in N. Third leaves were analysed for leaf elongation rate and final epidermal cell length, and, within the basal growing region, for the spatial distribution of relative segmental elongation rates (RSER, pin-pricking method), epidermal cell turgor (cell-pressure probe), osmotic pressure (OP, picolitre osmometry) and water potential (Ψ). During the development of the third leaf, plants grew at relative growth rates (relative increase in fresh weight) of 18.2, 15.6 and 8.1% · d-1 (Hoagland-, 16%- and 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). These differences were accompanied by only small differences in final cell length, but large differences in cell-flux rates (146, 187 and 201 cells · cell-file-1 · d-1 in 8%-, 16%- and Hoagland-plants, respectively). The length of the growth zone (32—38 mm) was not much affected by N-levels (and nutrient technique). A decrease in RSER in the growth zone distal to 10 mm produced the significant effect of N-levels on leaf elongation rate. In all treatments, cell turgor was almost constant throughout the growing region, as were cell OP and Ψ in 16%- and 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 Ψ decreased simultaneously by 0.1 MPa. Cell Ψ increased considerably where elongation ceased. Within the zone where differences in RSERs were highest between treatments (10—34 mm from base) average turgor was lowest, OP highest and Ψ most negative in Hoagland- compared to 8%- and 16%-plants (P < 0.001), but not significantly different between 8%- and 16%-plants.