International Symposium on Asian Pears@(2001.8.25-29) 

Effect of Intermittent Warm Periods on Bud Break and Carbohydrate Contents of 

Japanese Pear (Pyrus pyrifolia Nakai) in Different Endodormancy Stages

  H. Honjo*, Y. Kobayashi, M. Watanabe and R. Fukui

Faculty of Agriculture, Utsunomiya University, Utsunomiya 321-8505, Japan

     Japanese pear cultivars are adapted to cold, at times fluctuating, winter temperatures. The chilling required to break endodormancy varies from a little less than 800 hours in cv. eKosuif to more than 1600 hours in some cultivars. More bud chilling may be required when the chilling during winter is interrupted by occasional warm periods than when the chilling is uninterrupted. Thus intermittent warm periods tend to reverse the effect of chilling. However, it should not be overlooked that endodormancy of flower bud is broken by exposure to transient high temperatures.

     The aim of this study is to evaluate the effect of intermittent warm periods on bud break and carbohydrate contents of Japanese pear in different endodormancy stages.

Materials and Methods

     Current shoots of two cultivars (eKosuif & eChojurof) were obtained from Utsunomiya University Farm. The eChojurof shoots with flower buds were collected on November 15 (100C hrs, chilling hours below 7.2C), December 6 (400C hrs), December 24, 1999 (700C hrs), and January 19, 2000 (1000C hrs). eKosuif shoots were collected on December 24 (700C hrs), and January 19 (1000C hrs). High temperature treatments were performed outdoors using a small chamber made of transparent polyolefin film. The shoots were bagged in the chamber, and the internal temperature was monitored using a data-logger at 30-min intervals. The heating degree-hours (heat unit, C hr) was calculated as an hourly summation of temperatures above 25C. After different periods of high temperature treatment, the shoots were cut off at one end in water, and forced in a growth chamber at 20C. Buds were observed every 2 days. Some shoots were sampled for carbohydrate analysis soon after and one week after high temperature treatment.

Results and Discussion

     Despite accumulation of chilling hours before high temperature treatment, intermittent warm periods increased bud break with increasing heat units in both cultivars. The onset of bud break was also forced with increasing heat units. For the December 6 sampling of eChojurof, the bud break with heat unit 800C hrs was observed in about 50 percent of the total buds (no significant increase in heat unit 0C hr) within 15 days after sampling, and the broken buds flowered on January 7 and thereafter. High temperature accumulation after 400 chilling hours stimulated flowering with increasing heat units.

     Glucose, fructose, sucrose and starch contents in the eChojurof shoot were measured by the enzymatic method (Boehringer Mannheim, F-kit). For the treatment on 1000 chilling hours (sampled January 19), glucose, fructose and starch contents increased during high temperature treatment. After 7 days of forcing at 20C, glucose and fructose contents decreased, but starch content continued to increase. For the treatment on 100 chilling hours (sampled November 15), starch content decreased during high temperature treatment and also after forcing. For the treatment on 400 chilling hours (sampled December 6), changes in carbohydrate contents resembled those for the treatment on 1000 chilling hours.