Yali is one of the best pear cultivars cultivated extensively in China and other countries. However, mortality rates of explants during the initial phase of tissue culture were found to be very high during the summer particularly from the shoot tip explants. A thorough investigation on the browning problem of Yali Aikansui and Abbe Fetel pears was done and their control measures are suggested in this paper. Emphasis has been given to the Yali pear. Shoot explants were collected from the field as well as from the plantlets grown in vitro during different months and observed for browning. The explants were excised and cultured in Murashige and Skoog (MS) solid and liquid medium as needed. The cultures were maintained in the dark as well as in a 16/8 hours light/dark photoperiod regime as needed and were kept in a growth room at (25 ¡ 2)uC temperature. According to the experimental results, Yali was found more severely infected by browning than the other two cultivars. Similarly, it was also found that, the intensity of browning was less during spring and increased with time and reached the maximum during summer. Shoot tips of Yali were found more severely infected by browning than the second node and other nodes. This accelerated the mortality rate up to 81% of shoot explants during the summer months of July and August. Browning was greatly influenced by water-soluble polyphenols, more of which were found in the shoot tip of explants collected from the field (significantly higher at P ( 0.05 and P ( 0.01). Total polyphenol contents were found less in the explants from plantlets grown in vitro, and less browning of medium appeared resulting in a lower mortality rate of explants. Collections of explants during early spring (P ( 0.05 and P ( 0.01) and use of other nodes rather shoot tip because explants (P ( 0.05 and P ( 0.01) were found to be better in preventing the browning problem. Finally, for curative measures to control browning, use of ascorbic acid at the rate of 100 mg? L 21 (P ( 0.05 and P ( 0.01), 0.02% polyvinyl pyrrolidone (PVP) in the culture medium (P ( 0.05), 96 hours dark treatment of other nodes (P ( 0.05), and 12 hours cold treatments of explants at 4uC (P ( 0.05 and P ( 0.01) prior to sterilization of explants, were found to be the best methods to control browning and therefore to increase the survival rate of cultured explants of the Yali pear.

The effects of growth regulators on the respiration metabolism of pear buds during dormant period were studied in this experiment. The results showed that, during early dormant period, the respiration intensity of pear buds was infirm and increased slowly. As the weather became very cold, the respiration intensity rapidly declined, and after that it changed a little. In the later stage of dormant period, the respiration intensity rose rapidly. The maximum value appeared on January 4th, at 0.390 mo1 O 2 $g -1 FW$min -1 , but thereafter declined to its original level. Under natural conditions, three respiratory pathways showed different changes. Pentose phosphate pathway might be the main reason for dormancy release in pear buds. The exogenous gibberellins were more efficient than salicylic acid in increasing the respiration rate. The exogenous SA appeared to play a more important role than exogenous GA 3 in phosphopentose pathway. The effect of gibberellins would be more effective than SA in tricarboxylic acid cycle (TCA). The respiration rate of glycolysis was not affected by gibberellins and salicylic acid.

The pear (Pyrus spp.) is one of the most important temperate fruit crops. The technique of adventitious shoot regeneration from leaves is considered to be one of the shortcuts in the research on pear genetic modification and cellular engineering, which, however, has not been widely used. As the regeneration frequency of pear leaves is usually very low, the research on adventitious shoot regeneration from pear leaves is eagerly needed. In this experiment, the factors affecting shoot and bud regeneration from the leaves of 'Zhongli 1' pear were studied, and an efficient protocol for shoot regeneration was established. The results showed that different types of basic media, different combinations of plant growth regulators, leaf placement on medium, periods of dark culture and the use of silver nitrate (AgNO 3 ) on culture media all significantly affected the adventitious shoot regeneration frequency of 'Zhongli 1' pear. The details are as follows: (1) Among three kinds of basic media, NN69 was better for 'Zhongli 1' shoot regeneration, followed by half (½) MS, while full MS had no effect on shoot regeneration; (2) Thidiazuron (TDZ) was better than 6-benzylaminopurine (6-BA) for 'Zhongli 1' regeneration, with an optimal concentration of 1.5 mg$L -1 , and the regeneration rate under this concentration could reach 85%, with 2.72 buds per leaf. 0.5 mg$L -1 indole-3-butyric acid (IBA), which induced a higher regeneration frequency, was a better choice for pear regeneration compared with 0.3 mg$L -1 naphthaleneacetic acid (NAA). Among the different combinations of plant growth regulators, TDZ + IBA was better for inducing high regeneration frequency; (3) The abaxial surface of leaves touching the medium was beneficial for leaves to uptake nutrients from the medium, and because of that, the regeneration frequency of leaves was significantly higher than that of leaves touching the medium with their adaxial surfaces (obverse side of leaf); (4) Dark culture was necessary for bud regeneration, and the best duration for dark culture of 'Zhongli 1' pear was 21 days; (5) The addition of 1.0 mg$L -1 AgNO 3 into the culture medium could promote adventitious shoot regeneration significantly. A high adventitious shoot regeneration frequency was obtained in this research, which will be beneficial for further research on efficient and stable in vitro plant regeneration systems and genetic modification of pear.

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