Water flow across the sieve tube boundary estimating turgor and some implications for phloem loading and unloading iv. root tips and seed coats
Annals of Botany 63(5): 571-580
In the present, paper the theory developed in Part I of this series is applied to seed coats of Phaseolus vulgaris and some combined data on root tips of Hordeum distichum and Hordeum vulgare. Because of the large back-pressure implied, it is concluded that phloem transport into these primary sinks would be physiologically impossible in the absence of a symplastic pathway for the unloading of water from sieve elements. In this case, unloading of water and sucrose will occur predominantly as a pressure-driven flow of solution through plasmodesmata, although diffusion can contribute significantly to the plasmodesmatal sucrose flux. At least 20% of the plasmodesmata connecting sieve elements and adjacent cells must be unobstructed if large changes in turgor and osmotic pressure are to be avoided. Depending on the membrane area available for water fluxes, it is possible that the difference in water potential across the sieve-tube plasmalemma can lead to significant errors when axial turgor gradients are estimated from gradients of osmotic pressure and external water potential. The magnitude and even the sign of these errors is uncertain, but it is possible that sieve-tube turgor pressures will be significantly underestimated in primary sinks.