Effects of Vegetative and Seedling Pear Rootstocks on Growth Characteristics and Water Potential of Pear Cultivar

Pear producers around the world are looking for faster return of capital and saving in labor costs, achieved well by high density orchard planting. Choosing a good combination of scion and rootstock is critical for production of fruit trees because the relationship between the rootstock and scion has a decisive effect on plant water relations, leaf gas exchange, plant size, flowering, fruit production, fruit quality and production efficiency. Nowadays in some regions, growers are using quince rootstock for pear orchard establishment. Using the quince rootstock alleviates many problems in pears orchard, but graft incompatibility between pear scion and the quince rootstock, and resistance to frost and alkaline soils are some problems restricting the use of this rootstock. In most pear-producing regions in Iran, pear seedling is used for pear propagation. Due to the problems of pear including overgrowth and late precocity (after 4 -5 years), this research evaluated the effects of pear clonal rootstock (Pyrodwarf, OHF and Quince c) on some growth characteristics in comparison to pear seedling (Pyrus communis).
This research was conducted during 3 years from 2014-2016 in Chenaran (36.6, 59.1) in northeast of Mashhad. Maximum and minimum temperatures were 40 and -22 ˚C, respectively, elevation 1176m and the average annual rainfall 240mm. The same tissue culture rootstocks from two cultivar Pyrodwarf and OHF with Quince rootstock and annual seedling from Dragazi pear were selected in August 2013 and T-budded with three commercial cultivars Spadona, Ntanz and Sebri. In the spring of 2014, after relieving frost danger, the trees were planted in field with row space 4×2m. In order to investigate dwarfing effect of rootstock on scion cultivar, some important vegetative factors that represent dwarfing effect of rootstock including trunk cross sectional area, height of tree, amount of lateral branch growth, tree growth rate during growth season, mean of growing buds on each lateral branch, and stem and leaf water potential were measured. This test was conducted in factorial based on randomized complete block design with 4 replications. Each plot was included one hybrid combination. MSTATC and Excel software were used for data analysis, and differences among means were compared by using LSD test.
Different rootstocks did not show any significant difference in terms of leaf water potential. Leaf water potential during the seasons had a constant time course about scion cultivar on all rootstock so while temperatures rise throughout the season it reduced the amount of leaf water potential. The effect of cultivar was significant on leaf water potential so that the highest water potential was related to Natanz and the least water potential was related to Serbi cultivar. The effect of rootstock and scion both on stem water potential was significant. Therefore the highest and lowest stem water potential was recorded for cultivars grafted onto the seedling and quince rootstock, respectively. In this study, a significant relationship was observed between minimum stem water potential (mid-day) and branch growth rate during the growing season. With reducing stem water potential as a result of temperature increase, the amount of branch growth reduced. The effect of both scion and rootstock on chlorophyll index was significant. In this investigation, all 4 rootstocks had different effects on chlorophyll index. The highest chlorophyll index was related to cultivar Natanz and the least was related to Spadona. Sebrie had medium chlorophyll index. Rootstock effect on vegetative growth of the scion was not significant but the effect of scion was significant at 5% level. Pear seedlings with vigorous growth had more long branches than other rootstocks. In the present research, seedling rootstock also induced higher growth of lateral branches. Furthermore, quince rootstock induced the least growth of lateral branch during 3 years of the investigation. In addition, quince rootstock had the minimum tree height and pear seedling had the maximum tree height. Internode length in dwarfing rootstock was less than seedling. Both scion and rootstock had significant effect on trunk sectional area so that at the end of three years, cultivars grafted on seedling rootstock had the highest trunk cross sectional area, and cultivars on quince rootstock had the lowest TCSA. Two vegetative pear rootstocks (Pyrodwarf and OHF) did not show significant difference from each other. Pyrodwarf and OHF rootstock showed good compatibility with Natanz and Spadona scions like seedling rootstock. On the other hand, the survival percentage on quince rootstock was really low.
This investigation showed that pear rootstock had less vegetative growth than seedling rootstock and induced dwarfing effect on scion growth during 3 years but quince rootstock had more dwarfing effect.