The role of the apoplastic transport barriers for radial water and ion uptake in rice (Oryza sativa L.) and corn (Zea mays L.) roots
For rice, the overall radial hydraulic conductivity (Lpr) was lower than those of other cereal roots. The stele/endodermis, aerenchyma, and the outer part of roots (OPR) arrange in series and their resistances to the overall radial water flow are additive. However, the hydraulic resistance of the OPR was smaller by a factor of 30 than the overall values of root Lpr. Hence, the endodermis rather than the OPR limits water uptake. It appears that the OPR is constructed to provide a substantial barrier for oxygen rather than for water. The latter is transported down to root tips limiting root extension, which resulted to develop small rice root systems. Both cultivars used here (IR64 and Azucena) developed strong barriers to radial oxygen loss (ROL). The rates of ROL dramatically decreased along the root and reached values close to zero in basal parts. The data from this thesis show for the first time that radial uptake of water by rice roots is not limited by the OPR. Theoretical estimations suggested that the endodermis limits the rate of radial water flow and the resistance of the aerenchyma is in between that of the endodermis and the OPR. High values of the LpOPR could be either brought about by a large apoplastic component of water transport or by a high permeability of membranes of the living cells in the OPR or by both together. In order to quantify the relative contribution of the apoplastic vs cell-to-cell paths to the overall LpOPR, apoplastic pores of the OPR were either partially blocked by China ink particles or clogged with copper ferrocyanide precipitates. In another experiment, water channels (aquaporins) of the OPR were blocked with water channel blocker HgCl2. Resulted LpOPR values after the treatments suggested that proportionately greater apoplastic water flow across the OPR compared to cell-to-cell water flow. On average, 66-75% of water used extraprotoplastic path. This finding was further supported by substantial increases of the reflection coefficient of the OPR (ssOPR) after treatments with apoplastic blockers. Strongest evidence in favour of a predominant apoplastic water transport came from the comparison between diffusional (PdOPR, measured with heavy water, HDO) and osmotic water permeability (PfOPR) or hydraulic conductivity (LpOPR). The PfOPR was larger by a factor of 600-1400 than PdOPR. To obtain such huge values of Pf/Pd ratios are expected if the pathway involved a rather long porous path, i.e. a passage along the apoplast; this would offer a high diffusional resistance for HDO, but should be highly permeable in case of a bulk (hydraulic) water flow. Blockage of apoplastic pores with copper ferrocyanide precipitates significantly affected the bulk rather than the diffusive water flow and caused a 3-5-fold reduction of the PfOPR/PdOPR ratios. These findings suggested a prominent apoplastic bypass flow across the OPR of rice. Copper ferrocyanide precipitation technique with roots of rice and corn showed that CBs of the exo- and endodermis were not completely impermeable to Cu2+ ions. When offering Cu2+ and Fe(CN)64- on different sides, brown copper ferrocyanide crystals developed on the side where ferrocyanide was applied. This indicated that positively charged copper ions was moving through the barrier and cell walls, much faster than ferrocyanide with its four negative charges. There was a patchiness in the formation of precipitates, which correlated with the maturation of the exodermis in rice roots. Dense brown precipitates were observed around lateral root emergence points. These places may act as “open doors” for water and apoplastic tracer dyes. To some extent and depending on conditions and developmental state of roots, also ions may move through the apoplast into the xylem and may lead to increase the apoplastic bypass flow in roots. Hydrophobic aliphatic suberin is one of the major chemical compounds in plant roots that may act as an apoplastic barrier to water. To confirm this idea, total amounts of suberin were determined in corn and rice, and compared with their radial hydraulic conductivities. On average, exodermal cell walls of rice contained 6-fold greater aliphatic suberin than in corn hypodermis. In endodermal cell walls, amounts were 34-fold greater in rice than that of corn. Substantially higher amounts of suberin detected in apoplastic barriers of rice corresponded with substantially lower hydrostatic Lpr compared to corn. As the OPR of rice is highly porous and fairly permeable to water, it may argue that this holds true only for the endodermis. The results imply that some caution is required when discussing the role of suberin in terms of an efficient transport barrier for water. The simple view that just the amounts of suberin play the important role may not hold. A more detailed consideration of both the chemical nature of suberins and of the microstructure of deposits is required, i.e. how suberin impregnate wall pores.
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