عنوان مقاله [English]
Introduction: Most deciduous trees need low temperature to break flower bud dormancy. One of the most important abiotic stresses is low temperature which limits production of temperate fruits. Pear production has been considerably reduced in recent years. Important pear cultivars show different levels of resistance to cold. Cold compatibility followed by resistance increase is controlled genetically and contains several mechanisms which lead to production of different metabolites such as: polypeptides, amino acids and sugars. The object of this research was to evaluate the frost resistance of different ‘Dare Gazi’ genotypes and other pear cultivars in Mashhad climate condition.
Materials and Methods: This study was conducted to investigate the frost resistance of 23 ‘Dare Gazi’ pear genotypes and nine other cultivars include: ‘William’s’, ‘Bell de june’, ‘Spadona’, ‘Koshia’, ‘Domkaj’, ‘Torsh’, ‘Sebri’ and ‘Tabrizi’. Plant material contained vegetative and reproductive buds of one-year-old shoot samples which were collected from 25-year old trees on March 2014, four days after winter cold (-6.6 °C) in three directions of trees and sent to the laboratory. Frost damages of vegetative and reproductive buds were investigated based on visual observations (%), electrolyte leakage (EC) and proline content. EC was measured with a Metrohm 644 digital conductivity meter and proline content was measured based on Bates et al. (1973) method, using acid ninhydrin. The experiment was performed on completely randomized experimental design with three replications. Statistical analysis was carried out using MSTAT-C and Excel software. Mean values were compared using the least significance difference test (LSD) at 1% levels. Cluster analysis was conducted by SPSS 16 program.
Results and Discussion: Highest EC of reproductive buds was observed in ‘Dare Gazi’ 10, 19, ‘Tabrizi’ and ‘Torsh’ whereas ‘Dare Gazi’ 8, 18 and ‘Bell de June’ had the lowest EC. Based on visual observations, the least percentage of damaged reproductive buds was observed in ‘Dare Gazi’ 22 while ‘William’s’ suffered from frost at the highest damage level (96%). Göndör and Tóth (1998) studied 13 pear cultivars by microscopic observations of flower buds and found that ‘Packham's Triumph’ was relatively resistant under Hungarian ecological conditions. Honty et al. (2008) reported that Kaiser was the most sensitive pear cultivar to low temperatures during endodormancy and ecodormancy. Khorshidi et al. (2014) described that pear reproductive buds of ‘Ghodumi’ were the most tolerant. Considering the vegetative buds, ‘Dare Gazi’ 19 had the highest EC (74.47 %) which was not significantly different from ‘Dare Gazi’ 10, 20 and 3 whereas the lowest one was found in ‘William’s’(24.75%). The highest percentage of healthy vegetative buds was found in ‘Dare Gazi’ 1 (50%) which did not show a significant difference with ‘Dare Gazi’ 12, 5 and 7. ‘Tabrizi’ was the most sensitive and had most damaged vegetative buds (95.99%). Khorshidi et al. (2014) observed the least damaged vegetative buds in ‘Dare Gazi’ and the most damaged buds in ‘Boheme’ and ‘Ghodumi’. Palonen and Buszard (1997) mentioned that hardiness of woody tissue of apples did not seem related to flower bud hardiness. The highest proline content of reproductive buds was found in ‘Dare Gazi’ 20 (21.28 µmol g-1FW) and the lowest content observed in ‘Dare Gazi’ 2 (0.1 µmol g-1 FW). Young (1977) described that increase in proline was not correlated well with relative cold hardiness of citrus rootstocks. Data did not show any significant correlation between EC and proline content. Barka and Audran (1997) studied grape buds and shoots and reached a high negative correlation between proline content and frosttolerance. Yelonsky (1979) indicated that accumulation of proline was not correlated to cold hardiness. The results of present experiment were in agreement with Duncan and Jack (1987) findings which reported that increase in proline was not always correspondent to cold resistance. Based on the cluster analysis, the genotypes laid in two distinct groups. First group included ‘Dare Gazi’1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 17, 18, 19, 20, 23 and ‘Sebri’, ‘Domkaj’, ‘Bell de june’ and ‘Koshia’ and second group include ‘Dare Gazi’ 11,16, 21 and ‘William’s’, ‘Spadona’ and ‘Torsh’. Frost damage, EC and proline content were higher in the first group compared to second group.
Conclusion: Cosidering‘DareGazi’ genotypes, no correlation was found between proline content and frost damage rate. This shows that morphological differences among ‘Dare Gazi’ genotypes could be due to the existing of genetic variation of these genotypes or they are different clones of one cultivar.