Evidence for the Consequences of a Barrier to Solute Diffusion Between the Apoplast and Vascular Bundles in Sugarcane Stalk Tissue

Abstract

In a previous study we found that the apoplast of mature sugarcane stalk tissue contains up to 700 mM sucrose. In the current study, we found that xylem sap, exuded under root pressure from decapitated stalks, was virtually free of sucrose. This suggested that the apoplast of sugarcane stalk tissue contains at least two separate compartments: one within the free space of the vascular bundles, which is nearly free of sucrose, and another in the free space of the surrounding storage tissue. Anatomical observations indicated that these putative compartments were separated by the sclerenchymatous bundle sheath cell walls that were suberised and lignified early in development, constituting a barrier to the movement of relatively large molecular weight solutes but not water. It was hypothesised that this semipermeability would enable sucrose and other solutes in the apoplast of the storage tissue to provide a gradient for osmotic water flow from the xylem, generating a hydrostatic pressure in the apoplast. Additional lines of evidence were obtained to support this hypothesis: (i) apoplastic dyes were restricted to the xylem and did not accumulate in the apoplast of storage tissue when water-stressed plants were rehydrated, (ii) water potential measured with in situ psychrometers decreased when sections of intact stalks were cut, (iii) mature internode tissue of well-watered plants often cracks after maximum fresh and dry weight accumulation, and (iv) internode sections typically shrink in diameter immediately upon excision. The existence of a semipermeable barrier separating the vascular bundles from the storage parenchyma apoplast would require that phloem unloading involve a symplastic step in order to traverse the barrier. The presence of plasmodesmatal connections between companion, sclerenchyrna, and storage parenchyma cells supported this conclusion.