Extract from article :
Masahumi Johkan, Masayuki Oda, Genjiro Mori
Graduate School of Life and Environmental Sciences, Osaka Prefecture University, Sakai 5998531, Japan
17 November 2007 :
4. Discussion
During acclimatization after grafting, no defoliation was observed in the grafted eggplant and tomato plants used in the present study; however, the opposite was true for the sweet pepper plants. This result was most apparent after grafting at an old stage. Sweet pepper plants defoliate as a result of water stress (Nitzsche et al., 1991) and the defoliation was previously observed in sweet pepper plants grafted at an old stage (Shirai and Hagimori, 2004). Although the survival rate of the eggplant and tomato plants was 100% after grafting at both the young and old stages, the survival rate in sweet pepper plants was low, 44%, at the old stage. These results confirmed the low survival rate of grafted sweet pepper plants compared with eggplant and tomato plants.
Callus formation, and the differentiation and connection of vascular bundles in the callus are essential for successful grafting. The differentiation of vascular bundles from the callus influences successful grafting (Moore and Walker, 1981; Moore, 1984), with active callus formation at the cut surface leading to the formation of a callus bridge between the scion and rootstock as well as vascular bundle differentiation(Andrews and Serrano, 1993). Moreover, insufficient connection of vascular bundles between the scion and rootstock decreases the quantity of water flow (Torii et al., 1992). Therefore, efficient callus formation is important for the formation of the graft union and survival of the scion, and it has been suggested that an ability to form callus is positively correlated with grafting success (Ogata, 1995). In our results, the callus was thinner in sweet pepper plants compared with eggplant and tomato plants, and it is therefore thought that the small amount of callus formation at the old stage is the cause of the low survival rate of grafted sweet pepper plants. The number of adventitious buds generated from cotyledons cultured on liquid MS medium was previously shown to be less in sweet pepper plants than tomato plants (Javier et al., 2001). This further supports the hypothesis that low survival rate of sweet pepper plants is attributed to slow callus formation and cell differentiation.
Generally, plant tissue at a younger age has a higher cell division and proliferation ability. In our experiments, the callus was thicker and the survival rate was better in young stage sweet pepper plants compared with old plants. Callus obtained from old tissue of pepper has a low rate of cell differentiation and division (De Donato et al., 1989). In sweet pepper plants grafted at the old stage, the low rates of callus formation might delay the connection of vascular bundles between the scions and rootstocks, cause water stress, and lower the survival rate of grafted plants.
AA acts an antioxidant through a catalytic reaction with APX (Shigeoka et al., 2002). Plants are injured by the cut in the grafting process, and the active oxygen concentration increases as a result of this injury stress (Garcia et al., 2004). The concentration of AA also increases after grafting in tomato (Wadano et al., 1999). Hyper-production of AA is thought to counteract injury stress, because AA has been shown to protect plants from oxidation (Tabata et al., 2001). Therefore, AA treatment after grafting was considered an effective way to protect plants from injury stress in grafted plants.
Although in the present study callus formation was promoted with AA treatments in the scions, there was no effect in the rootstocks. Since the rootstocks had a smaller leaf area—only the cotyledon in this experiment—it seems that little AA would have been absorbed by the plant. Therefore, improvement in the AA treatment of rootstocks may promote the survival rate of grafted sweet pepper plants.
Many physiological functions of AA have been reported besides protection of plants from active oxygen. AA is known to control cell differentiation (Arrigoni, 1994) and to promote callus division and growth (Tabata et al., 2001). Moreover, ascorbate oxidase, the substrate of which is AA, is also thought to have roles in cell division and hypertrophy (Kato and Esaka, 2000). These results indicate that AA treatment may result in callus formation at the cut surface of the scion via AA’s promotion of cell division. Since AA also promoted shoot formation from callus in tobacco tissue culture (Joy et al., 1988), it is thought that AA promotes not only callus formation but also differentiation of vascular bundles.
This study showed that sweet pepper plants form smaller amounts of callus at the cut surface of the stem than eggplant and tomato plants, and that the low survival rate of grafted sweet pepper plants was caused by the low rate of callus formation. It was also found that AA promotes callus formation at the cut surface of the scion and increases the survival rate of grafted sweet pepper plants.
End of extract.
A big thing to remember on grafting peppers :
the importance of grafting at young stage !
"
The survival rate of pepper plants grafted at the young stage (34 days after sowing) was at 89%, at the old stage (58 days after sowing), the survival rate was very low in the control : 44%"
