# GreenTray+
## 4 Discussion
With this simple movement, all plants are pulled out of the culture recipient and displayed in front of the observer, so they can easily be treated, inoculated, phenotyped, sampled, or pictured. By pulling the holding racks, plantlet roots can be observed, sampled, or pictured the same way as the aerial part of the plants. Since the container holding the culture medium is independent from the cultured plantlets, it can be aliquoted, supplemented, inoculated, and observed at any time during culture [39; 42]. These characteristics make the GreenTray(r) TIS superior to other commercial TIS bioreactors and containers to perform studies on abiotic [43; 44; 45; 46] or biotic [47; 48; 49; 50; 51; 52; 53; 54; 55] stress agents. The experimental conditions to assay the effects of pH and Ca\({}^{2+}\) availability in the rhizosphere were easily established and repeated with the GreenTray(r) design. The differential capacities of the _Prunus_ rootstocks Garmem(r) and Rootpac(r)20 to grow in alkaline conditions and Ca\({}^{2+}\) depletion, reported herein, were in accordance with what is known from agronomic trials in calcareous soils [56; 57]. Therefore, GreenTray(r) could be useful for many nutritional studies, including the characterization and selection of plant materials [36]. Forcing a reduction in the atmospheric humidity inside the GreenTray(r)'s culture vessel, although periodic and recovered rapidly through the evaporation of wet surfaces and transpiration through the plantlet leaf stomas, implies a lower water potential, which induces higher transpiration of the plantlet's leaves and a higher water uptake at the roots, reinforcing Ca\({}^{2+}\) transport to the shoots [41]. In these conditions, the rootstock Rootpac(r) 20 presented a significantly higher shoot Ca\({}^{2+}\) uptake than Garmem(r)[40]. All mechanisms of ventilation inside the bioreactor, for which the GreenTray(r) is especially suited for, increase the water potential in the atmospheric culture recipient, and this favors water and nutrient root uptake and translocation to the leaves. In vitro assays for tolerance to _Erwinia amylovora_ have always been approached in heterotrophic conditions [47; 48; 49; 50; 51; 52; 53; 54]. Although the results presented herein are for a reduced number of apple varieties, this is the first time that the test has been performed in the in vitro autotrophic conditions of the GreenTray(r), and the same plant material was tested against the same _Erwinia amylovora_ strain in a confined growth chamber with grafted potted plants. Since a very good correlation was found in terms of susceptibility to fire blight between the in vitro and in vivo growth, it is recommendable to use the GreenTray(r) bioreactor to select seedlings and obtain apple or pear varieties with higher tolerance to fire blight through cross-breeding. Further improvements will be needed in inoculating all plants with the same amount and concentration of the bacterial strain or obtaining a more uniform plant development before their inoculation. Consequently, the GreenTray(r), as it is developed, is a good tool to propagate plants in vitro at a lower cost and higher multiplication rates. Its design favors the in vitro growth of plantlets in autotrophic conditions, like hydroponic cultures, but in a highly controlled environment and with reduced space requirements. Bioassays performed in the GreenTray(r) to characterize the responses of plants to biotic and abiotic stress agents are more reliable than those performed in semisolid medium because of the autotrophic plant development conditions favored in this TIS bioreactor. Other plant in vitro culture methodologies applied to plant breeding, such as immature embryo rescue, somatic embryo maturation, adventitious regeneration, genetic transformation, physiological characterization, plant-pathogen interactions, and plant growth-promoting microorganism interactions with plants, can be studied under in vitro conditions with the GreenTray(r) as a tool [39; 42]. Conceptualization, R.D.-S., N.T.