海岸砂丘では、一般的に、植物は汀線と平行に帯状分布をしており、また、汀線に近いほど土壌NaCl濃度が高い。これは単に環境要因と植生分布の相関を示しているにすぎない。そこで、ケカモノハシ，コウボウムギ，ハマヒルガオの３種の砂丘植物をNaCl濃度が0〜100mMの水耕液で栽培し、初期生長に対するNaCl濃度の影響を調べた。この結果、いずれのNaCl濃度でも、最も汀線近くまで分布するハマヒルガオのＲＧＲが最も大きく、最も内陸に分布するケカモノハシのＲＧＲが最も小さかった。またＲＧＲの大小は主にＬＡＲ（葉の量）の大小と対応していた。以上により、内陸に分布するケカモノハシは、より汀線近くに分布する２種よりも、NaCl耐性が低いことが示された。
The effects of soil-water salinity on growth and photosynthesis
of three coastal dune plants were examined by salt-treatment
in order to clarify the causal relationship between salinity and
plant distribution in a dune habitat. Plants were cultivated
hydroponically at three salinity levels: 0, 10 and 100 mM NaCl.
With the 100 mM salt treatment, Calystegia soldanella (C3 species) had the highest relative growth rate
(RGR) (0.085 g/g/d), followed by Carex kobomugi (C3) (0.066), and Ischaemum anthephoroides
(C4) (0.060). This order coincides with
the distribution pattern of the three species on coastal dunes;
Calystegia soldanella is generally distributed in more
seaward areas whereas I. anthephoroidesoccurs further
inland. The order of RGR was determined exclusively by leaf area
ratio (LAR) among the three species. Due to its C4
pathway, I. anthephoroides had higher net photosynthetic
rate (Pn) and net assimilation rate (NAR) than the two C3 plants at all NaCl concentrations, despite its
low RGR. This apparent discrepancy is explainable by differences
of LAR among the three species; LAR of I. anthephoroides
was lowest, and about half that of Calystegia soldanella.
These results suggest that LAR is one of the main determinants
of salt tolerance based on RGR, whereas Pn or NAR may not be significant.