Graft compatibility has been studied in vitro using callus tissues of apricot (Prunus armeniaca) and different Prunus rootstocks to form scion/rootstock combinations with different degrees of graft compatibility. In these species, incompatibility is manifested by a breakdown of the trees at the union area that can occur some years after grafting. Here, the possibility of obtaining an early detection method to determine graft incompatibility is explored by callus fusion in vitro. The adhesion of the two callus partners, the development of the cells at the contact surface (cell arrangement, intensity of cell-wall staining), and the presence of lipid and phenolic compounds have been studied during the first 3 weeks after grafting in both compatible and incompatible combinations. Differences were observed at the second and the third week of callus co-culture in most of the characters determined, although these differences were present as early as the first week in the case of phenolic compounds. The behaviour of the grafts grown in vitro was correlated to that of the same combinations in the field, suggesting that callus fusion in vitro could be a possible and reliable method for an early detection of graft incompatibility in different Prunus combinations.

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Pistachio (Pistacia vera L.) cultivars need to be grafted on suitable rootstocks for their propagation, but grafting success is uncertain, preventing producers to meet plant demand. Thus, there is a need for a better grafting method that in addition to increase success rate, allows us an early detection of success. Here, we have developed an alternative grafting technique that achieves these goals using young potted seedlings of terebinth rootstock and reduced size micropropagated scions from commercial cultivars. During micropropagation of pistachio cultivars we studied the effects of the application of antioxidants together with the growth regulators meta-Topolin and paclobutrazol on in vitro cultures to overcome the issues identified in the different phases of micropropagation: culture initiation, multiplication and rooting. Grafting succeeded using scions derived from in vitro cultured micro-shoots both from acclimated and non-acclimated plants. With this protocol, up to 75.7 % successful grafts were obtained, and grafted plants resumed growth from day 3, being ready for transplant a few months later. Sustained on our results, we present a simple and reliable system to reduce production time by ex-vitro grafting of shoottips from in vitro cultures onto 4 weeks old terebinth (Pistacia terebinthus L.) rootstock seedlings grown in pots.

Micropropagated plants are usually cultured in in vitro environments, with a high relative humidity and low light intensity, taking nutrients and energy from the culture medium. During acclimatization, physiological and structural changes allow micropropagated plants to adapt to the new environmental conditions, mainly to low relative humidity and high light intensity. As a result, plants become autotrophic and develop as normal plants. However, acclimatization is not easy in many species obtaining low survival percentages. In this work we present a method of acclimatization that yields high survival rates, based on the controlled exposure to low relative humidity. Micropropagated plants, exposed daily to low relative humidity, showed high survival rates during acclimatization under a plastic tunnel in the greenhouse. In contrast, only few plants survived when humidity was kept high continuously, indicating that the exposures to low relative humidity may stimulate the changes to become a functional plant. Care should be taken to keep the substrate well watered, in order to provide enough water to restore water loss from transpiration. In this way, plants soon started to adapt and new leaves were visible during the first week of acclimatization. In addition, water transport through the roots was stimulated since plants were able to recover from a moderate wilt after the exposures to low relative humidity. INTRODUCTIONMicropropagated plants need to adapt to the new environmental conditions, when transplanted to soil, through a process of acclimatization. Adaptation includes a number of hardening changes related with the control of water loss, and autotrophy. Micropropagated plants are usually cultured in in vitro conditions, with a high relative humidity and low light intensity, taking nutrients and energy from the culture medium. To increase plant survival after transfer to soil, different strategies were utilized. The use of antitranspirants to reduce water loss when exposed to a lower relative humidity, although can be a helpful treatment, did not fully solved the problem of plant growth and survival. The use of the latex polymer Anti-Stress 550 as an antitranspirant for acclimatizing micropropagated plantlets was investigated with in-vitro-rooted microshoots of walnut clone TRS (Voyiatzis and McGranahan, 1994). The survival rate of plants treated with latex and kept uncovered in the greenhouse was higher than that of the controls (66.5 and 55.0%, respectively). Earlier, several film-type antitranspirants were tested on tissue-cultured chrysanthemums and carnations (Sutter and Hutzell, 1984) at the time of transfer to a greenhouse. Although a silicone formulation had the greatest effect in reducing transpiration and water stress in transferred plants, it also stunted plant

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