Rucinska-Sobkowiak, R; Nowaczyk, G; Krzeslowska, M; Rabeda, I; Jurga, S
Water status and diffusion transport were studied in the
roots of yellow lupine (Lupinus luteus L, cv. Juno) treated for 48h with two selected
concentrations of Pb(NO3)(2): 150 mg l(-1), which inhibited root growth by about 50% (medium
stress intensity), as well as 350 mg l(-1), which almost entirely suppressed root elongation
(severe stress intensity). Relative water content (RWC), which characterizes the degree of root
water saturation, slightly increased at the lower lead concentration and remained unchanged at
the higher lead dose. Ultrastructure analyses under a transmission electron microscope revealed
that plasmolysis was not evoked by lead in the apical part of the meristem. Moreover, direct
observation of meristem cells using Nomarsky optics indicated enhanced vacuolization in the
presence of both lead concentrations. These data suggest that the water status of the roots was
not affected by the metal. Due to the fact that proline is involved in the maintenance of turgor
in the cells, the metabolism of this amino acid was investigated. In the roots, the activity of
enzymes involved in proline synthesis, such as pyrroline-5-carboxylate synthetase (P5CS) and
ornithine aminotransferase (OAT), increased at 150 mg l(-1) Pb2+; nevertheless, proline content
was diminished at the lower lead concentration. This effect is likely the result of proline
degradation by proline dehydrogenase (PDH), since the activity of this enzyme increased at the
lower lead dose. On the other hand, in the presence of 350 mg l(-1) Pb2+, a low level of proline
was correlated with a decrease in the activity of P5CS and OAT, as well as unchanged PDH activity
in lupine roots. These data may imply that enzymatic synthesis of proline was strongly damaged by
the metal ions. The low level of proline in both experimental variants suggests that proline
accumulation is inessential to maintaining the osmotic uptake of water into root cells. NMR
spectroscopy showed that exposition of lupine seedlings to lead caused a deceleration in water
transport in the roots due to a reduction in the water transfer rate across the membranes
(transmembrane transfer) and vacuoles continuum, as well as water diffusion along the root
apoplast. Fluorescence staining and immunogold labeling showed the presence of callose strands in
cell walls and/or in the vicinity of them. In lead-treated lupine roots, callose was mainly
localized in the parenchyma cortex placed lengthwise to the vascular cylinder. Callose deposits
in the cell walls may reduce vacuolar transport, as well as increase cell wall resistance to
water flow. Deceleration of diffusional water movement to the vascular system, may in turn,
influence the rate of long-distance water transport to aerial parts of the plant. (C) 2012
Elsevier B.V. All rights reserved.