Toshihiro Umebayashi

During the course of pine wilt disease (PWD), xylem dysfunction is induced (in the absence of water stress) by invasion of pinewood nematodes (PWN) into resin canals, and the infected pine trees wilt. When PWNs are inoculated to Japanese black pine seedlings, massive embolisms develop at the inoculation site. The developmental process of massive embolization to form an embolized zone is not well understood. We studied the development of xylem dysfunction around the inoculation site of one-year-old stems of PWN-infected seedlings using a combination of compact MRI and cryo-SEM. We visualized pathological changes in tracheids and resin canals in the xylem, and found that embolisms occurred in PWN-inoculated seedlings at a negative hydraulic pressure of −0.5 MPa. Resin canals in healthy trees were filled with resin. In PWN-infected seedlings, abnormal embolisms and resin were observed around resin canal near the inoculation wound. Empty tracheids were observed more extensively than resin-filled tracheids. We suggest that abnormal embolisms and resin leakage occur from damages of resin canals. Resin may seep into the tracheids when cell wall of the axial and radial resin canals is crushed. These embolisms under high water potential are unavoidable under natural conditions.

The dye uptake and heat transfer methods are used to understand sap movement in mature tree stems, but correlation of results between both methods is weak. The circumferential variation of sap movement is detected generally in the heated transfer method. On the other hand, the tangential movement such as helical ascent is tracked in dye uptake method. It is still unclear whether the circumferential and the tangential movements in both methods mean same and/or related phenomenon. In this study, we monitored sap flux density ( F _d ) on the east and west sides in intact stems of Betula ermanii Cham. using the thermal-dissipation method to identify F _d response to the creation of artificial flows through severed vessels by drill treatments. We then tracked sap flow by injecting dye solution. The F _d values in east side tended to be higher than that in west side before drilling treatment. Similar F _d values in both sides were detected after drilling treatments, and dye heights were similar to F _d values after drilling in any sides. Simple comparison of water ascent using both methods can lead to errors, because water ascent from severed vessels may differ from water ascent via roots.

Deciduous ring-porous species in Japan shed all of their leaves under severe water stress before large vessels in earlywood are embolized, and embolization take place during winter. Water in deciduous ring-porous species is mainly conducted upward via large earlywood vessels of the current year. Water columns in large vessels are vulnerable to drought-induced and freeze stress-induced embolisms. Although a vulnerability curve can be created to estimate the hydraulic capacity of plants, it remains unclear why the loss of conductivity in potted plants of ring-porous species does not reach 100 % under severe drought stress. In this study, two deciduous ring-porous species in Japan (Kalopanax septemlobus and Toxicodendron trichocarpum) were used to explain the species-specific pattern in the water-conducting pathway of the stem. We monitored and visualized the spatial distribution of xylem embolisms in the stem of K. septemlobus saplings under drought stress and freeze stress using compact magnetic resonance imaging and cryo-scanning microscopy. In addition, we evaluated the water ascent in the stems of both species using a dye uptake method. Although embolisms of large vessels were observed under drought stress and in winter, all leaves were dropped to avoid fatal water loss after embolization of some large vessels. In contrast, all large vessels were embolized in winter. Larger-diameter vessels of latewood in T. trichocarpum tended to function in trees growing in the warm temperate zone. Thus, our results suggest that the unclear curve may be derived from a discrepancy between leaf water potential and actual water potential in the xylem under severe drought stress. The frequency of xylem embolisms in deciduous ring-porous species in Japan mainly depends on the number of freeze–thaw cycles.

Although previous studies have suggested that branch dieback and whole-plant death due to drought stress occur at 50–88% loss of stem hydraulic conductivity (P₅₀ and P₈₈, respectively), the dynamics of catastrophic failure in the water-conducting pathways in whole plants subjected to drought remain poorly understood. We examined the dynamics of drought stress tolerance in 3-year-old Japanese black pine (Pinus thunbergii Parl.). We nondestructively monitored (i) the spatial distribution of drought-induced embolisms in the stem at greater than P₅₀ and (ii) recovery from embolisms following rehydration. Stem water distributions were visualized by cryo-scanning electron microscopy. The percentages of both embolized area and loss of hydraulic conductivity showed similar patterns of increase, although the water loss in xylem increased markedly at –5.0 MPa or less. One seedling that had reached 72% loss of the water-conducting area survived and the xylem water potential recovered to –0.3 MPa. We concluded that Japanese black pines may need to maintain water-filled tracheids within earlywood of the current-year xylem under natural conditions to avoid disconnection of water movement between the stem and the tops of branches. It is necessary to determine the spatial distribution of embolisms around the point of the lethal threshold to gain an improved understanding of plant survival under conditions of drought.