Recently, plant growth–promoting endophytic fungi with gibberellins-producing ability were isolated from the roots of coastal sand dune plants. Carex kobomugi Ohwi in coastal sand dunes was treated with the supernatant solution of Penicillium citrinum KACC43900 isolated from Ixeris repens (L.) A. Gray. After treatment, the length of leaf blades, photosynthesis, chlorophyll fluorescence, and soil respiration were measured in the field. The contents of chlorophyll and carotenoid were determined with leaf blades selected for measurement of chlorophyll fluorescence. Fungal culture filtrate (FCF) increased leaf blade length, the contents of chlorophyll a (Chl a) and chlorophyll b (Chl b), total chlorophyll (total Chl [a b]), and carotenoid in leaf blades of C. kobomugi. FCF also increased net photosynthetic rate (A), transpiration rate (E), carboxylation efficiency (A/Ci), and water-use efficiency (A/E). However, the ratio of variable fluorescence to maximal fluorescence (Fv/Fm) did not show significant difference between control and FCF-treated plants. Soil respiration rates were higher in the site treated with FCF than the control. Although growth response and physiological characteristics of C. kobomugi were different according to the treatment methods of FCF, injection or spray treatment, plant growth of C. kobomugi was increased with FCF treatment.