have modified the regular photosynthetic 

 pathway (C-3) so that they can be 

 effective at higher temperatures, higher 

 light levels, and lower C0 2 

 concentrations. Because of these 

 modifications, they are more productive in 

 tropical conditions than the C-3 plants. 

 They also do well in temperate summers. 

 The best known example of a C-4 plant is 

 corn (maize). 



3.1.1. Physiological Adaptations 



Water/salt balance . Terrestrial and 

 marsh plants must maintain contact with 

 the air, via stomata on their leaves, to 

 obtain C0 2 for photosynthesis. These 

 openings expose moist cell membranes to 

 the air so that plants lose water by 

 evaporation. This water must be replaced 

 from water surrounding the roots. Water 

 loss through the leaves and replacement 

 through the roots is termed 

 "evapotranspiration. " In the case of salt 

 marsh plants, the water surrounding the 

 roots is saline, which leads to a problem 

 of maintaining salt, as well as water, 

 balance. 



The outermost cells of Spartina 

 plants are waterproof so that water does 

 not enter the plant through leaves but 

 rather is supplied through the root/xylem 

 system. The saline water surrounding the 

 roots has an osmotic pressure of about -25 

 bars (about -25 atmospheres). Therefore, 

 a pull of about 25 atmospheres is required 

 to pull water through the root membranes 

 against the osmotic pressure of the soil 

 water. This pull is supplied by 

 evaporation at the leaf surface and is 

 transmitted along the columns of water in 

 the xylem system to the roots. Since the 

 water potential in air at even 98% 

 humidity is less than -25 bars, 

 evaporation can easily pull water out of 

 the pore solution, through the plant, and 

 out into the atmosphere. 



All the plant cells are in contact 

 with the internal water system and, 

 therefore, must have a lower osmotic 

 potential to maintain plant turgor. Most 

 higher plants simply wilt in seawater: 

 because the osmotic potential is lower in 

 the seawater than in the cells, water 

 moves out of the cells and into the 

 seawater producing a loss of turgor. 



Spartina solves this problem by 

 accumulating salts in the cell vacuoles. 

 As a result, the cells can maintain their 

 internal pressure (turgor) against the 25 

 atmosphere pull generated in the xylem 

 system. 



The root membrane preferentially 

 admits water, but small amount of salts 

 also pass into the plant. Although all 

 ions in saltwater are discriminated 

 against, some enter the plant more readily 

 than others. McGovern et al. (1979) found 

 that the ratio of sodium to potassium in 

 the xylem fluid (Na/K = 18.8) is similar 

 to that in seawater (Na/K = 27.7). 

 However, the ratios for sodium to sulfate, 

 calcium, and magnesium are greater in the 

 plant than in seawater (Na/S0 4 = 62 and 

 4.0; Na/Ca = 410 and 26.5; Na/Mg = 7300 

 and 8.3 for Spartina and seawater, 

 respectively) (McGovern et al. 1979). The 

 difference between these plant and 

 seawater ratios results from selective 

 uptake of ions by the plant. Similar 

 discrimination among ions has been found 

 in culture studies of Spartina (Smart and 

 Barko 1980). Measurements of the chloride 

 concentration in the xylem sap of Spartina 

 indicate it makes up about 5% of the 

 concentration in the pore water around the 

 roots (Teal, unpubl. data). The fact that 

 chloride is the principal anion in the sap 

 indicates a 20 to 1 discrimination against 

 the sum of the cations in seawater. 



Since salts in excess of the plant's 

 need enter the plant, a mechanism must 

 exist to eliminate the surplus. Spartina 

 has salt glands on its leaf surfaces that 

 can secrete a concentrated salt solution. 

 The secretion takes place against a very 

 high gradient. We have found the secreted 

 solution to be about 20 times as 

 concentrated as the solution in the xylem 

 (Teal et al., unpubl. data). In other 

 words, the plant can lose 19 times more 

 water through transpiration than through 

 secretion and still maintain its safe 

 balance. The secretion is 20 times more 

 concentrated than the sap which is 20 

 times less concentrated than the pore 

 water around the roots. Thus, the secre- 

 tion is approximately as saline as the 

 pore water. When this concentrated secre- 

 tion is exposed to the air, it typically 

 dries completely and forms salt crystals 

 that sparkle in the sun (Figure 6). 



12 



