the critical leaf K levels were progressively reduced from 27 g kg^1 dwt in plants not fertilized with Na
to 5 g kg^1 dwt in plants receiving 400 mg Na per pot [165].
It has been suggested that as the tissue K concentration declines, the concentration in the cytoplasm
is preferentially maintained for operation of K-dependent processes in the cytoplasm [56,74]. Therefore,
initially all changes in tissue K concentration are likely to be at the expense of vacuolar K, with other so-
lutes being diverted to the vacuole to maintain s [56]. Leigh and Wyn Jones [89] argued that the cyto-
plasmic K concentration would be expected to decline to 15 g kg^1 dwt or less, which agrees with the val-
ues of critical K concentrations of 5 to 20 g kg^1 dwt reported for various tissues in a number of crops
[89]. This hypothesis may also explain the effects of other cations, such as Na and Mg, on tissue K con-
centrations. When these other cations are abundant in tissue, critical K concentrations range from 10 to
20 g kg^1 dwt, but when they are low, K values can increase up to 40 to 70 g kg^1 dwt depending on the
species [141,166]. For Italian ryegrass, the leaf K optimum decreased from 35 dwt to 8 g kg^1 dwt when
Na was provided as an alternative ion [141]. For fodder beet, sugar beet, red beet, oats, barley, ryegrass,
English ryegrass, turnips, lupins, red and white clover, potatoes, kale, and rapeseed, the optimal K levels
were lower when Na was supplied [3,9,24].
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 375
Figure 3 Leaf K levels of red beet, spinach, and lettuce in presence of adequate levels of Na in the nutrient
medium. (From G.V. Subbarao et al., unpublished data.)
Figure 4 Leaf Na levels and total dry matter production (expressed as percent of control) for red beet,
spinach, and lettuce for the same tissues as in Figure 3.