This study was conducted on Scurrula yadoriki (Sieb.) Danser and its host trees to examine leaf water potential and leaf water relations using the pressure-volume technique. Compared to the various hosts, S. yadoriki had more negative leaf water potentials, suggesting water flow toward the parasite from the host. There was no significant difference in photosynthetic rates between S. yadoriki and Machilus thunbergii or Acer amoenum. However, the parasite exhibited substantially higher daytime transpiration rate than the host trees. These results suggest that the parasite was able to obtain sufficient water and nutrients from host species because the lower leaf water potential at midday enabled it to maintain photosynthetic activity and growth. Leaves of the parasite also maintained a higher turgor pressure than host trees, suggesting that this parasite has a competitive advantage in the acquisition of water from the host species.

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To elucidate the influence of parasitism by sap-sucking insects on the growth of seedlings, the moisture condition and incidence of death in natural seedlings of Todo fir (Abies sachalinensis; tree height, about 50 cm; age, about 30 years) parasitized by Todo fir aphids (Cinara todocola) were measured using a pressure chamber. When the plot was set up for this study, the seedlings were divided into three groups according to the level and influence of parasitism by Cinara todocola: “minimal parasitism”, “considerable parasitism but normal growth”, and “considerable parasitism and abnormal growth”. The minimally parasitized seedlings showed higher midday xylem water potential compared to considerably parasitized normal growing seedlings; however, no significant difference was observed between the two groups. This may be because Cinara todocola suck sap from phloem and not xylem. The midday xylem water potential of abnormally growing seedlings that withered due to parasitism was ≤-2 MPa, and most of these seedlings died later. The dead seedlings in the study plot were observed each year, and the maximum mortality reached over 10%.

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