Water Flow Across the Sieve Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. IV. Root Tips and Seed Coats

In the present paper, the theory developed in Part I of thisseries is applied to seed coats of Phaseolus vulgaris and somecombined data on root tips of Hordeum distichum and Hordeumvulgare. Because of the large back-pressures implied, it isconcluded that phloem transport into these primary sinks wouldbe physiologically impossible in the absence of a symplasticpathway for the unloading of water from sieve elements. In thiscase, unloading of water and sucrose will occur predominantlyas a pressure-driven flow of solution through plasmodesmata,although diffusion can contribute significantly to the plasmodesmatalsucrose flux. At least 20% of the plasmodesmata connecting sieveelements and adjacent cells must be unobstructed if large changesin turgor and osmotic pressure are to be avoided. Dependingon the membrane area available for water fluxes, it is possiblethat the difference in water potential across the sieve-tubeplasmalemma can lead to significant errors when axial turgorgradients are estimated from gradients of osmotic pressure andexternal water potential. The magnitude and even the sign ofthese errors is uncertain, but it is possible that sieve-tubeturgor pressures will be significantly underestimated in primarysinks Phloem, turgor, osmotic pressure, plasmodesmata, Munch hypothesis, Phloem unloading