The water content of plants is very high (up to 90% by weight), but this water i
ID: 258132 • Letter: T
Question
The water content of plants is very high (up to 90% by weight), but this water is in a flux. Water is absorbed through the roots, rises as sap and evaporates from the leaves. The primary forces (pressures) that determine water transport include gravitational pressure (Pg) and osmotic pressure (?). Assume that the pressure generated due to all other factors can be lumped together (as Po). Now consider the following two plants
(a) The 3800 year old General Sherman giant sequoia located in the Sequoia National park, CA which stands 272 feet high (and has a diameter of 36.5 feet at its base)
(b) A red mangrove that can grow as tall as 80 feet. It grows in the delta/ estuary regions of the tropics, and its roots can often submerged in sea-water.
Concentrations of various ions (in m-mol/L) in the sap and extracellular solutions at the roots in both cases are provided in the table below.
For which of the two plants, does one need a greater contribution from the “other” factors to transport water to the highest located cells ?
Sequoia Sap Seawater Mangrove Sap 20 Ion K+ Nat2 Cl- NO3- 3 Ca + 7 Mg++ 8 10 450 530 0 10 50 20 0 2 20 30 0) 2 20Explanation / Answer
Ans. The numerous factors contributing to the transport of water to highest located cells would be greater in case of the latter one in the mangrooves. In case of mangrooves due to enhanced salinity the transport of water would be hindered due to osmotic distress. Due to enhanced salinity there would be stomatal closure and reduced supply of carbon dioxide serving as a major hindrance for the supply of water and minerals.
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