4. Discussion4.1. Freezing InjuryCell membrane stability was widely used to express stress tolerance; and higher membrane stability could be correlated with abiotic stress tolerance [33]. In order to determine the response of sweet cherry cultivars grafted different rootstocks selleckbio to low temperatures (4��C, ?5��C, ?15��C, ?25��C), we measured membrane thermostability by electrolyte leakage method. Regarding the results of injury from bark tissues, it was the highest in NA stage than CA stage in all grafting combinations (Figure 1). Beside that, in the NA stage, higher than 50% injury was determined in bark tissues exposed to ?15��C and ?25��C treatments in all graft combinations. The injury was significantly greater in sweet cherry cultivars grafted on Gisela 5 rootstock compared with that on Mazzard at ?25��C.

The greatest injury by ?25��C treatment was observed in cv. Lambert grafted on Gisela 5 with ~77%. These results are in a good agreement with the result of other studies which shows that low temperature had usually been considered as the major cause of increased cell membrane permeability, relative conductivity, and injury index of plant tissue [26, 27, 34, 35]. To our knowledge, this is the first detailed study of membrane stability in sweet cherry cultivars/rootstock carried out under laboratory freezing tests.4.2. Soluble SugarsIt is well known that sugar metabolism is affected by temperature stress, and sugars accumulate in response to low-temperature stress [36]. Our results indicate that during NA period, bark tissues had significantly lower TSS contents than during the CA period (Figure 2(a)), which paralleled their freezing tolerance.

Increases in TSS ameliorate the impact of dehydration associated with freezing [9]. Consequently, seasonal changes in TSS content were related to changes in cold hardiness and air temperatures in sweet cherry. Accumulation of TSS during cold acclimation occurs in several species, such as peach [37], raspberry [36], olive [27, 38], cabbage [19, 39], arabidopsis [40], and wheat [41].In this study, reducing sugar and sucrose content were significantly greater in CA stage than in NA stage in all graft combinations (Figures 2(b) and 2(c)). However, reducing sugar content was higher in either sweet cherry cultivars grafted on Gisela 5 rootstock compared with Mazzard rootstock in CA stages (Figure 2(b)). On the other hand, sucrose content was higher in either sweet cherry cultivars grafted on Mazzard Dacomitinib rootstock compared with Gisela 5 rootstock in CA stages (Figure 2(c)). The most commonly accumulated soluble sugar in response to low temperature is sucrose [16]. However, sugar accumulation at low temperature is not limited to only sucrose.