Important functions of water relations are considered to be related to genome size diversity in gymnosperms. We investigated relationships among genome size, dimensional characteristics of conductive cells, and water relations parameters by using young seedlings of sixPinusspecies. Xylem hydraulic conductivity was not correlated with genome size and dimensional characteristics of conductive cells, but the water potential at the turgor loss point was.Pinusspecies with large genome sizes had thick cell walls and small ratios of lumen radii to cell wall thickness in their conductive cells, and those species lost their turgor to tissue dehydration al low water potentials. The characteristics observed in the present study may contribute to pine drought tolerance.