Alfalfa (Medicago sativa L.) growing on K-depleted, semiarid, calcareo
us soil displays three K-deficiency symptoms: (i) white spots on leafl
ets (W), (ii) marginal chlorosis (M), and (iii) normal leaflets on stu
nted plants (N). In a greenhouse study, we examined partitioning and h
eritability of K and Na nutritional traits in three alfalfa clones and
their crosses, and also how K and Na soil fertility levels affected b
iomass production, genetic dependence of K and Na uptake and partition
ing, and K use efficiency. Mother plants were propagated vegetatively
and by seed from phenotype crosses in a low-K, low-Na, calcareous soil
rooting medium. One trial used K- and Na-amended soil. Harvests were
at early bloom. Leaves contained more K than stems, with no difference
among phenotypes; Na was uniform in stems, but leaves of M contained
much more Na than leaves of N and W. With nonamended soil, K and Na up
take and partitioning among leaves, stems, and roots were significantl
y affected by cross. The Na:K ratio in leaves, stems, and roots was st
rongly influenced by phenotype and cross. Biomass production of leaves
and stems differed among crosses and was increased by K fertilization
; Na fertilization affected biomass only of leaves, and only in crosse
s MxM and NxN (Na may have partially replaced K). The shoot:root ratio
was affected only by cross. Potassium use efficiency was affected by
cross and by Na fertilization. Broad-sense heritability for K and Na c
oncentration was high, especially for Na in leaves and stems. Uptake a
nd partitioning of Na and K in phenotype crosses supported narrow-sens
e heritability as well. Genetics, rather than soil K-Na competition, g
overned Na uptake, translocation, and utilization by phenotypes. Accor
dingly, characterization of K nutrition in alfalfa must consider Na le
vels. As phenotypes of alfalfa cultivars range widely in K-Na nutritio
nal traits, it should be possible to select for high yield and K use e
fficiency.