The composite water and solute transport of barley (Hordeum vulgare) roots: effect of suberized barriers

Abstract

Background and Aims Roots have complex anatomical structures, and certain localized cell layers develop suberized apoplastic barriers. The size and tightness of these barriers depend on the growth conditions and on the age of the root. Such complex anatomical structures result in a composite water and solute transport in roots. Methods Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography–mass spectometry (GC-MS). The hydraulic conductivity of roots (Lpr) and of cortical cells (Lpc) was measured using root and cell pressure probes. Key Results When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones. However, they developed an endodermis. Endodermal Casparian bands first appeared as ‘dots’ as early as at 20 mm from the apex, whereas a patchy suberin lamellae appeared at 60 mm. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lpr. Comparison of Lpr and Lpc suggested that cell to cell pathways dominate for water transport in roots. However, the calculation of Lpr from Lpc showed that at least 26 % of water transport occurs through the apoplast. Roots had different solute permeabilities (Psr) and reflection coefficients (σsr) for the solutes used. The σsr was below unity for the solutes, which have virtually zero permeability for semi-permeable membranes. Conclusions Suberized endodermis significantly reduces Lpr of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series (cortex, endodermis) in addition to the currently included components arranged in parallel (apoplastic, cell to cell pathways).

Seminal roots of wild and cultivated barley differentially respond to osmotic stress in gene expression, suberization, and hydraulic conductivity

“…The qualitative suberin composition in terms of substance classes and single detected suberin monomers was identical between cultivated and wild barley, and it also fits to published data of other barley cultivars such as Golf (Ranathunge et al, ) and Scarlett (Kreszies et al, ). This suggests that the root suberin monomer composition is genetically well conserved in barley even under stress conditions.…”

“…So far it has been described that very different from other crops, for example, wheat, maize, and rice (Ranathunge, Lin, et al, ; Ranathunge, Schreiber, Bi, & Rothstein, ; Schreiber, Franke, Hartmann, Ranathunge, & Steudle, ), barley does not form an exodermis characterized by Casparian bands and suberin lamellae (Coffey et al, ; Gitto & Fricke, ; Jackson, ; Knipfer & Fricke, ; Kreszies et al, ; Ranathunge et al, ). Thus, it was an exciting observation that in response to osmotic stress, the wild barley accession Jordan formed an exodermis in the basal parts of about 20% of the investigated seminal roots (Figure ).…”

“…Thus, Pakistan having significantly longer roots compared with Morex (Figure ) and lacking a pronounced suberization in zone B (Figure ) can keep its overall radial hydraulic conductivity in the presence of osmotic stress. Radial water uptake in barley roots mostly occurs through the weakly suberized younger root zone including the root tip, whereas water uptake is significantly decreased in the strongly suberized basal part of the root (Ranathunge et al, ; Sanderson, ). Thus, in Morex with an enhanced suberization, a decreased hydrostatic Lp r can be expected.…”

Osmotic stress enhances suberization of apoplastic barriers in barley seminal roots: analysis of chemical, transcriptomic and physiological responses

“…This is very different from other crop plants, such as rice and maize, which develop a strong exodermis in response to stress (Schreiber et al ., ; Ranathunge et al ., 2011a, ). Our results on barley seminal root anatomy are consistent with previous studies (Knipfer & Fricke, ; Ranathunge et al ., ).…”

Root cortical senescence decreases root respiration, nutrient content and radial water and nutrient transport in barley

“…Individual suberin monomers were identified through a homemade mass spectrometry suberin library. Barley does not develop a suberized exodermis, and therefore, suberin detected in this assay is endodermal suberin (Ranathunge et al ., ). Results of suberin analyses were expressed on the basis of total DW of original isolates used for the depolymerization, the surface area of the endodermis or root cross section (data not shown) and root segment length (data not shown) with similar results.…”