Growth characteristics of Japanese pear cultivar 'Kosui' (Pyrus pyrifolia Nakai) on different trunk interstocks

The effects of trunk interstocks on tree growth, fruit growth, axillary flower bud development, fruit quality and crops in Japanese pear cultivar "Kosui" were studied. The PCA analysis of 11-year cultivation tests from 1984, when is an initial fruition year, to 1994 provided the following conclusion as the growth characteristics of "Kosui" on several trunk interstocks. (1) The influence of trunk interstocks appeared clearly at the girth of trunk. "Chojuro" and "Yamanashi" (Pyrus pyrifolia Nakai, which seedlings are used as rootstock) were separated for inferior type of trunk girth; oppositely "Nijisseiki", "Waseaka" and "Manshyumamenashi" (a hybrid clone selected from Pyrus betulaefolia Bunge * Pyrus pyrifolia Nakai, which seedlings are used as rootstock) were for superior type. (2) "Nijisseiki" as a trunk interstock was well at the growth of trunk and primary scaffold limb trunk, and also a tree crown expansion at the young age was better than other interstocks. Under the condition of same pruning method, shoots sprouted from lateral fruiting branches and a primary scaffold limb was satisfactorily. Growth of fruits and development of axillary flower bud on the waiting shoot was superbly; in addition, fruit cracking were lest and total crops of 3 year's were most among other interstocks without "Manshyumamenashi". From these result "Nijisseiki" was evaluated that it has the best productivity as a trunk interstock for "Kosui" among the combination of this investigation. However, it was considered that summer pruning of shoots on the base part (40cm at length) of fruit lateral fruiting branches at young fruit stage was important to improve soluble solids concentration (Brix, degree of refract meter) of fruit because of significantly low compared with "Chojuro". Lower ratio of wood-leaf (Cw/F) of shoots at bottom part of bearing branch in June was considered one of reasons why "Nijisseiki" has highest productivity. (3) "Chojuro" as a trunk interstock for "Kosui" showed the best of Brix and was well at the growth of fruit, although it was most inferior to others at the growth of trunk and the tree canopy expansion. Especially, this tendency was clear in 1993, when it was record-breaking low temperature summer years. However, few sprouting of shoots from the primary scaffold limb in "Chojuro" interstocks was considered to cause high possibility of fruit cracking compared with others. Adding to them, total crops were inferior to "Nijisseiki". Considering these characteristics, "Chojuro" was evaluated to be available in such some chilly-summer districts as "Yamase" zone. (4) "Manshyumamenashi" as a trunk interstock showed same characteristics as "Nijisseiki", but some individuals showed same characteristics as "Cyojuro", in addition, the development of axillary flower buds is inferior to "Nijisseiki" and "Cyojyuro" significantly in the low temperature summer. And there was diffusion of characteristics depending on trees. These 3 interstocks showed hopeful productivity more than "Kosui" trunk interstock for the control. (5) "Shinsui" and "Waseaka" as a trunk interstock was inferior significantly to "Nijisseiki" and "Chojuro" at the bearing of axillary flower buds on waiting shoots, fruit weight and total crops. However, there was no significant difference with "Kosui" for the control at every item estimating as productive factors. (6) Differences of other trunk interstock systems in comparison to "Kosui" for control wasn't clear. "Yamanashi" presented the resemblance of "Cyojuro" in the growth of trunk's girth. but as a trunk interstock, it was thought to be more sensitive to the over-crops than "Manshyumamenashi". (7) The improvement of productivity by "Nijisseiki", "Cyojuro", and "Manshyumamenashi" interstocks demonstrated the synthetic effect of a trunk interstock system. These facts show the important role of trunk in regard to the practical productivity of "Kosui" in fields. (8) And this research showed evidences of the fact that the pruning method affects on fruit weight and development of axillary flower buds. So as to evaluate the productivity, it is necessary to regard the pruning method for reconditions. The following indexes were effectual on evaluations of productivity: 1) bearing division percentage of primary scaffold limb: 2) waiting-shoot / lateral-branch ratio: 3) lateral branch density. (9) It was obvious that yield relied on numbers (or density) of lateral fruiting branch and the adjustment of fruit bearing level per a tree by thinning. Consequently, it was considered that evaluations of productivity by only yield quantitative were irrational. (10) As effectual items in the growth investigation to estimate the effect of reserves, the followings were thought to be applicable, i.e. the circumference of primary scaffold limb, it's enlargement per one year, fruit numbers after pre-thinning (by which one fruit per spur is selected in early May), shoot numbers at the bottom part of lateral fruiting branch and primary scaffold limb, length and the ratio of wood-leaf (Cw/F) of shoots pruned at summer. Especially, such the evaluation of shoots pruned after 45 days fromfull bloom at basal 40cm of lateral fruiting branch as numbers, length and the ratio of wood-leaf (Cw/F) was expected for the convenient tree assessment. (11) As effectual items in the growth investigation to evaluate productivity, yield, fruit weight, Brix (and hardness), axillary flower bud percentage on shoot were thought to be applicable. "Yield efficiency", which means fruit yield per 1 square cm of trunk cross-sectional area, cannot adaptable necessarily for index to evaluate total productivity by the reason why the yield efficiency is reflected heavily to trunk girth, which is characteristic of interstock species, and yield is reliable on thinning level managed by human. But it was estimated that the yield efficiency of primary scaffold limb primary scaffold limb has the relationship with Brix by PCA (and factor analysis after rotation of PCA by normal Varimax method) in the examination accomplished under the same pruning criteria. With respect to the yield efficiency of trunk, PCA analysis indicated that it is useful to survey the advantage of fruit weight and axillary flower bud development reciprocally.