In vitro nodal cuttings of potato (Solanum tuberosum L.) `Atlantic' and `Russet Burbank' from bioreactor culture were hydroponically cultured for 28 days using a deep flow technique (DFT) system. The response of plant growth and photosynthesis to different levels of solution electrical conductivity (EC; 0.08, 0.15, 0.22 and 0.36 S·cm-1) and pH (3, 4, 5, 6 and 7) were studied. The best growth, characters of shoot length, total shoot and root fresh and dry weight, were obtained in nutrient solution of pH 6.0 and EC 0.15 S·cm-1 for `Atlantic', while pH 7.0 and EC 0.15 S·cm-1 were found to be best for `Russet Burbank'. Plantlet growth was reduced by low solution pH (3.0) and high EC level (0.36 S·cm-1). Photosynthetic rate, stomatal conductance, and transpiration rate were also found to be affected by EC levels. Down regulation of photosynthesis, as indicated by chlorophyll fluorescence results, were observed when potato plantlets were cultured under nutrient solution of higher EC level. Plantlet growth and photosynthetic rate increased as photosynthetic photon flux (PPF) levels increased from 50 to 250 μmol·m-2·s-1. Particularly, increasing PPF level had a more distinctive effect on plantlet growth than CO2 enrichment condition. It was apparent from this study that nutrient solution of pH 6.0 and 0.15 S·cm-1 EC in combination with high PPF level (250 μmol·m-2·s-1) were suitable for hydroponic culture of potato plantlets as it would maximize net photosynthetic rate, and achieve the highest growth rates.