- Stomatal Function
In most plant species, Kis the dominant cation responsible for turgor changes in guard cells during
ion-induced stomatal movement [4]. An increase in the K concentration of guard cells increases s and
results in uptake of water from the adjacent cells. The corresponding increase in turgor of the guard
cells results in stomatal opening. Closure of stomata in the dark is correlated with K efflux and a cor-
responding decrease in the s of the guard cells. Selectivity of transport systems for K over Na pro-
vides a fundamental limitation on the degree to which Na can substitute for this stomatal function in
plants [6]. Thus, although Na can substitute for K in vacuolar osmotic adjustment for a number of plant
species, it does not appear to be able to carry out this role for stomatal turgor [100,101]. However, for
Commelina communis, Na was able to replace K and was even more effective than K in stomatal func-
tion [102]. Thus, Na can have a role in stomatal physiology of such plants even if it is not an obligate
role [70].
However, from the differences betwen plant species with respect to the membrane permeability for
K and Na, one can suppose that in plant species with high permeability to Na (e.g., Beta vulgaris), K is at
least partially replaceable for this stomatal function [9]. Thus, Na may act as an alternative cation to K for
stomatal opening [103–109]. Our results with red beet indicate that stomatal conductance is nearly nor-
mal even when nearly 95% of the plant’s K was replaced by Na and Na levels in the leaf sap approached
200 mM (G.V. Subbarao et al., unpublished).
- Photosynthesis
Potassium is the dominant counterion for the light-induced Hinflux across the thylakoid membranes
[110] and for the establishment of the transmembrane pH gradient necessary for the synthesis of ATP
[111–113]. Also, in the formation of chloroplast structure, the translocation and storage of assimilates (su-
crose) in the sink tissue seem to depend on adequate K concentrations in the tissue [6].
In sugar beet, chloroplasts often contain high concentrations of Na [9], and Na and K are incorpo-
rated in the chloroplasts to a similar extent [114]. Because much of the leaf Na in sugar beet is concen-
trated in the chloroplasts, it is hypothesized that Na may have been involved in photosynthesis [115]. In
chloroplasts of Limonium vulgare, the Na content is even higher than K. Considering the beneficial effect
of Na in Beta vulgaris, it is possible that Na participates in photophosphorylation [9]. A prerequisite for
this function is high membrane permeability for Na. The high mobility of Na in crops such as sugar beet
suggests that this prerequisite is fulfilled. However, Na is unable to replace K in chlorophyll synthesis in
spinach, lettuce, and sugar beet [116,117]. For sugar beet, Na was able to replace K for chloroplast mul-
tiplication [9]. Nevertheless, photosynthetic rates of sugar beet declined substantially during K deficiency
even when Na was present [108,109]. In red beet, however, leaf photosynthetic rates were nearly normal
despite high levels of Na in leaf lamina (up to 100 g kg^1 dwtdry weight) [3]. Chlorophyll fluores-
cence and leaf Na levels are presented for red beet, spinach, and lettuce (Table 4). For red beet, leaf
chlorophyll levels and chlorophyll fluorescence (Fv/Fm) are not affected at tissue Na concentrations of 76
g kg^1 dwt (Table 4). In contrast, chlorophyll fluorescence is decreased substantially at leaf Na levels of
39.8 g kg^1 dwt in lettuce (Table 4).
- Counterion in Long-Distance Transport
Potassium is often the dominant counterion in long-distance transport as well as being the counterion dur-
ing storage of NO 3 in vacuoles [4]. As a consequence of NO 3 reduction in leaves, the remaining counte-
rion, K, requires the stoichiometric synthesis of organic acids (e.g., malate) for charge balance and pH
370 SUBBARAO ET AL.TABLE 4 Leaf Na Levels and Chlorophyll Fluorescence in Red Beet, Spinach, and Lettucea
Leaf Na concentration Chlorophyll fluorescence
Plant species (g kg^1 dwt) (Fv/Fm) ratio
Red beet (Beta vulgaris) 76.0 0.74
Spinach (Spinacea oleracea) 28.9 0.75
Lettuce (Lactusa sativa) 39.8 0.69
aRed beet is grown for 42 days after planting (DAP); spinach and lettuce are grown for 30 DAP using nutrient-film
technique where K and Na levels in the nutrient solutions were 0.25 and 4.75 mM, respectively.
Source: Subbarao and Wheeler unpublished data.