Drought impacts on phloem transport have attracted attention only recently, despite the well-established, and empirically verified theories on drought impacts on water transport in plants in general. This is because studying phloem transport is challenging. Phloem tissue is relatively small and delicate, and it has often been assumed not to be impacted by drought, or having insignificant impact on plant function or survival compared to the xylem. New evidence, however, suggests that drought responses of the phloem might hold the key for predicting plant survival time during drought or revival capacity after drought. To maintain hydraulic equilibrium with the xylem, phloem water potential needs to osmotically adjust to match that of the xylem. During drought, this adjustment may increase the carbohydrate concentration, and viscosity of the phloem sap enough to significantly reduce phloem transport capacity, and potentially promote plant mortality. In this talk we combine results from theoretical considerations, scalable experiments with semipermeable tubing, and phase contrast synchrotron radiation microtomographic microscopy and light microscope imaging of xylem and phloem tissues of two coniferous species: Juniperus monosperma and Pinus edulis to discuss the impacts of drought on the phloem, and how phloem anatomy could impact maintenance of phloem transport during drought.
Results/Conclusions
Even if direct evidence of the need of phloem transport at low tissue water potentials is currently lacking, and we have only indirect evidence for phloem transport affecting plant survival time, our anatomical results support the view that phloem transport at low tissue water potentials matters. The high transport capacity at low water potentials, however, can be gained through various anatomical adaptations in addition to changing conduit or tissue size. These adaptations include high interconduit connectivity via high sieve pore density. Short and wide phloem conduits can facilitate more effective phloem transport at low tissue water potentials than long and narrow conduits. Conduits with walls that have a high permeability to water would require more frequent sugar loading zones to maintain a constant phloem flow rate than conduits with lower wall permeability. High wall permeability would make the conduits immune to viscosity effects on flow at low tissue water potentials, which might be a benefit even if it would require a constant supply of carbohydrates to maintain flow and turgor.