ferentially. In rice, four isozymes of malic dehydrogenase have been reported [138] and several isozymes
of-amylase have been isolated from seeds [139]. Because these isozymes have different properties, they
may have different ionic requirements for full activity and they may differ in their ability to function un-
der high levels of Na instead of K [9].
VI. GROWTH STIMULATION BY SODIUM
Growth stimulation by Na has both practical and scientific interest as it raises the possibility of applying
inexpensive, low-grade Na fertilizers [4]. The presence of Na in the growing medium has been reported
to have beneficial effects on the growth of numerous plants [25,140–143]. This stimulation is particularly
large for members of Chenopodiaceae. In the case of sugar beet, red beet, and spinach, Na stimulated
growth even when there appeared to be adequate K in the nutrient medium [23,142,144–150]. Many field
experiments showed that Na fertilization has improved the growth and yield of sugar beet [4,6,149]. How-
ever, the positive effects varied with cultivar, soil type, and climatic conditions [143,151–154]. Our stud-
ies with red beet showed that maximum growth is observed when both Na and K are present rather than
either of them alone [3]. Also, varietal differences are observed for the optimal ratio of K and Na in the
nutrient medium, with some varieties appearing to prefer higher levels of Na (Figure 1). However, no such
growth stimulation is observed for spinach (Figure 2).
372 SUBBARAO ET AL.Figure 1 Growth response of two red beet varieties to substitution of Na for K in the nutrient medium. Plants
are harvested at 42 days after sowing. (From Ref. 3.)
Figure 2 Growth response of two spinach cultivars to substitution of Na for K in the nutrient medium. Plants
are harvested at 42 days after sowing. (From G.V. Subbarao et al., unpublished data.)