Evidence that salt gland ion excretion is an active, metabolically driven process is varied, includ-
ing effects of temperature [100], light [101], oxygen pressure [88], and metabolic inhibitors [102] on
excretion rate as well as selectivity of ion excretion. Excretion is typically highly selective for Naand
Cl[102–104], although other ions may be excreted in minute amounts, such as K, Ca^2 , and Mg^2
[34,44,74,88]. Comparison of salt gland excretion rates among studies is difficult because of the vary-
ing influence of environmental factors, such as light and temperature, cumulative days of exposure to
salt stress, and plant factors such as leaf age [85]. Also, units of measurement differ, one fundamental
difference being whether excretion rates are based on leaf area or leaf weight. Finally, excretion rate is
not static but is influenced by saline ion concentrations in the growing media. Increasing media salin-
ity generally stimulates excretion up to an optimal level above which excretion rate may decline [88].
Maximum excretion rate was reported to occur at 150 to 200 mM media NaCl (8–13 dS m^1 ) in mod-
erately tolerant Chloridoid species such as bermudagrass, Rhodesgrass, goosegrass [Eleusine indica
(L.) Gaertn.], and Kallar grass [92,102,104]. However, excretion was maximal at 200 mM NaCl (17 dS
m^1 ) in desert saltgrass and Spartinaspp. [88] and 300 mM NaCl (23 dS m^1 ) in Sporobolus virgini-
cus[40].
Among seven grasses belonging to the subfamily Chloridoideae, shoot Naand Clconcentrations
were negatively correlated, whereas salt tolerance was positively correlated with salt gland Naand Cl
GROWTH AND PHYSIOLOGICAL ADAPTATIONS OF GRASSES TO SALINITY STRESS 629
Figure 5 Scanning electron micrographs of adaxial leaf surfaces. (A) Salt gland of desert saltgrass (Distich-
lis spicatavar.stricta). Only cap cell is visible—basal cell is embedded in epidermis. G, salt gland; P, papillae.
(B) Salt gland of bermudagrass (Cynodon dactylon). G, salt gland; P, papillae. (C) Salt gland of buffalograss
(Buchloë dactyloides).