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A rootstock trial has been established to develop dwarfing pear rootstock clones from Pyrus species. Seedlings that were raised from the open-pollinated seeds of P. betulaefolia Bunge (PB) and P. calleryana Decne (PC); and showed significant genetic dwarfing growth and good traits based on ease of clonal propagation were screened. Among those, four seedlings of PB named SPRB1, SPRB13, SPRB15, and SPRB22 (SPRB = Shinshu University Pear Rootstock P. betulaefolia clone) and six seedlings of PC named SPRC3, SPRC5, SPRC8, SPRC13, SPRC15, and SPRC20 (SPRC = Shinshu University Pear Rootstock P. calleryana clone) were selected. Clones that were generated from the selected seedlings had significantly higher rooting ability by softwood cuttings compared with the control during an 8-year test. Japanese pear 'Kosui' and European pears 'La France' and 'Conference' revealed a dwarfing growth habit when grafted onto the selected clones. All scion/stock combinations interacted significantly to affect shoot growth, trunk cross-sectional area (TCSA), and canopy spread. There was no noticeable difference in the fruit yield per tree of three cultivars grafted on PB and SPRB1, but the estimated yields per ha increased more than 20% in all cultivars on SPRB1. 'La France' trees on SPRB15 and SPRC20 began to bear early with significantly higher yields. Fruit quality characteristics of 'La France' and 'Conference' did not differ because of the influence of stock clones, however, 'Kosui'/SPRB1 had lower average fruit weight that corresponded to the lighter fruit skin color.

Response and tolerance to flooding of potted open-pollinated seedlings, four ungrafted dwarfing clonal selections of Pyrus betulaefolia Bunge (PB), six of P. calleryana Decne (PC), and the same topworked to the Japanese pear 'Kosui' were investigated. The 3-yr-old trees were flooded for 15 days or 12 weeks and then examined for tree survival, growth cessation, shoot growth, adventitious root formation, and defoliation. Although the tolerance to water stress was higher in PC than in PB, the responses varied within and between the species. Some SPRB clones with greater tolerance than the PC control and SPRC clones developed adventitious roots on the trunk above the water surface. This may be a morphological adaptation to the stress. Prolonged flooding separated the rootstocks into 4 overall rankings: most tolerant (SPRB22), very tolerant (SPRB13, SPRB15, PC1, PC2, SPRC15, and SPRC20), moderately tolerant (SPRC3 and SPRC8) and least tolerant (PB seedling, PB4, SPRB1, SPRC5, and SPRC13). PC and its selection flooded for 30 days had significantly higher shoot growth, new roots, and higher 2,3,5-triphenyltetrazolium chloride (TTC) reducing activity in the fine roots than the PB ones. Within the SPRB clones, SPRB15 and SPRB22 exhibited the most shoot and new root growth. From the analyses of long and short-term flooding data, we categorized the selections SPRB15, SPRB22, SPRC15, and SPRC20 that developed adventitious roots with extended root activity as having superior ability to withstand water stress.

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