Laserfiche WebLink
<br />l""T'J <br />i"- <br />r- <br />c~ that could be utilized for growth, If all salts had to be <br />,~ actively transported throughout the plant, growth would be much <br />C) less efficient. <br /> <br />A water and solute continuum links the plant to the water in the <br />soil and water vapor in the air, Water is lifted from the <br />roots, through specialized cells (xylem), to the top of the <br />plant by fo~ces of evaporation from the leaf coupled with the <br />properties of water, when guard cells are open, Thus, water <br />movement in tall trees can be physically explained in terms of <br />the evaporating power of their leaves, the absorbing power of <br />thei~ roots, and the physical property of cohesion in the water <br />in their conducting strands, This is transpiration, <br /> <br />Roots receive the mate~ials needed for growth and maintenance <br />through a downward flowing conduit (the phloem). It is through <br />this channel that the sugars and other organic compounds <br />produced in the leaves are transported throughout the plant, <br />Roots and non-photosynthetic tissues of the plant metabolize <br />these compounds, obtaining the energy and carbon needed to <br />create their structu~e and perfo~m thei~ functions. <br /> <br />Actively growing portions of the root are responsible for salt <br />uptake, a process necessary to p~ovide the plant with the basic <br />nutrients it requires, This provides an osmotic gradient, <br />leading to water uptake, Subsequent events depend upon the type <br />of plant under consideration, <br /> <br />B, Mechanis.s of Salt Tolerance <br /> <br />Figu~e 5 illustrates three different types of plant adaptations <br />to salt. The first is representative of freshwater plants, or <br />glycophytes, similar to those most commonly employed in modern <br />agriculture, The roots of these plants actively absorb ions <br />principally in response to the total osmotic potential of soil <br />water, without regard to ionic composition. Howev'er, following <br />absorption, ions are selected and actively excluded, dependent <br />upon concentration and type, According to Kramer (1984), plants <br />in this category possess various types of mechanisms, all energy <br />demanding, that prevent salts from reaching the stem, except in <br />small amounts, Maas and Hoffman (1977) identified and discussed <br />many of these species, finding that, in general, plant response <br />is directly ~elated to the duration of exposure to salinity, <br />Alfalfa, for example, was found to respond to a mean salinity, <br />weighted according to the amount of water absorbed in the root <br />zone (Bernstein and Francois, 1973). <br /> <br />The significance of this is that. as salinity increases. these <br />plants must expend increasingly larger amounts of energy on <br />salinity control, drawing from energy otherwise available fo~ <br />production, As a result, they become either noncompetitive or <br />unable to survive in saline environments. <br /> <br />14, <br />