Jorge M. Santamaria

El papel de los estomas de las hojas de la papaya cv. Maradol Carica papaya bajo cultivo in vitro

Persian lime is one of the most important citrus fruits in Mexico. This species suffers from a physiopathy called Wood Pocket (WP), which is characterized by chlorosis in leaves and fruits, necrosis in the trunk, and the eventual death of the tree. The actual scientific knowledge about WP is very little; however, the producers, researchers, and technicians associate it with abiotic stress. In the present study, we observed that Persian limes exposed to temperatures over 45 °C for five months developed WP symptoms, while those kept at 25 °C during the same time did not. Both groups were then physiologically characterized. Our results show that WP strongly affects most fluorescence and gas exchange parameters. Specially, we observe that stomatal fluxes were negative in WP leaves. Our results suggest that WP in Persian lime plants might be a physiological affectation caused by the prolonged exposure to high temperatures. It might be possible that the physiological affectation (gas exchange and water use efficiency) could be associated with a high-temperature-induced abscisic acid accumulation, which, in turn, might have caused stomatal closure in WP-affected plants. However, this hypothesis must be corroborated experimentally.

Background: Papaya (C. papaya L.) is one of the most economically significant fruit crops in Mexico, as it is the leading global exporter. Although Yucatán is not among the country's top producing states, it holds great importance as the center of origin and distribution for this crop. This explains the current presence of wild papaya populations in unperturbed remote sites within the Yucatan State, which can serve as a genetic source for genetic improvement of current cultivated varieties, as they exhibit a higher level of tolerance to stress factors such as waterlogging caused by climate change. Therefore, it is essential to characterize the physiological response of these wild genotypes to abiotic stress factors. Objective: To characterize the physiological response to prolonged waterlogging stress of wild C. papaya genotypes. Methodology: A system was designed to induce waterlogging in three-months-old C. papaya plants. Plantlets were subjected to waterlogging (100% of roots submerged) for seven days. During the waterlogging stress, the following variables were measured every 24 hours: PSII efficiency (Fv/Fm), photosynthesis, transpiration, stomatal conductance, leaf loss, and petiole binding angle. Data collected from each parameter were analyzed using student t test analysis. Results: Seedlings from wild papaya populations exhibited a rapid response to waterlogging stress of some photosynthetic parameters, and with prolonged exposure, the plants eventually lose several leaves but papaya wild seedlings survive after 7 days of waterlogging (pw). Transpiration and stomatal conductance variables dropped after 24 hours pw and remained low throughout the experiment. Leaf loss began to be noticeable at around 72 hours pw. Additionally, leaf inclination angle exhibited variations starting from 24 hours pw, probably related to epinasty caused by ethylene accumulation. However, Fv/Fm remained high even after 7 d of being waterlogged. Implications: The results suggest that waterlogging stress has an impact on the physiology of wild papaya plants, however, this rapid response of closing stomata and maintaining photosynthesis could be a mechanism to tolerate this stress. The fact that wild papaya plants were able to maintain high values of Fv/Fm even after 7d of waterlogging, might be associated to waterlogging tolerance, as there are reports that in commercial papaya cultivars, this parameter rapidly decline during the first 24h to values much lower than those found here for wild papaya plants. Conclusions: It is necessary to perform further comparative studies between wild papaya genotypes vs commercial papaya cultivars and assess the performance of wild papaya plantlets during recovery periods once the stress is released.

Background: Metals can be found in nature, but they are also generated by anthropogenic activities associated with technological waste, such as batteries and electronic components. These metals, when released to the environment may contaminate and cause significant health damage. Phytoremediation is a process to de-contaminate water bodies from metals using plants. Obviously, those plants should be able not only to uptake high amounts of metals, but also, they should be able to tolerate those high concentrations in their tissues. Salvinia minima is a fast-growing, floating aquatic fern and its ability to take up metals from the environment has been evaluated. Objective: To characterize the physiological and molecular response of Salvinia minima plants exposed to different metals. Methodology: A system was designed to expose Salvinia minima plants to different metals such as Pb, Ni, Cu, Zn and Li, for seven days at different concentrations levels and take samples every 24 h. Various parameters were evaluated including metal uptake, metal concentration in the medium, leaf appearance, PSII efficiency (Fv/Fm), photosynthesis, transpiration, stomatal conductance. Results: S. minima plants were capable to accumulate Pb (28 mg /g DW at 40 uM), Zn (10 mg/g at 80uM) Li (10 mg/g at 20mM), Cu (8 mg/g at 40 uM) and Ni (4 mg/g at 40 uM), at their roots. At short times (0-24 h) and at low concentrations (of certain metals) no significant physiological damage was observed, however, at high concentrations or longer exposure times (48-144 h), physiological damage can be observed in terms of decreased photosynthesis and PSII efficiency, transpiration and stomatal conductance. Implications: Salvinia minima plant are capable to take Pb, Ni, Cu, Zn and Li from the medium, which indicates that it is a good candidate to be used in the phytoremediation of water bodies contaminated with these metals. Conclusions: Salvinia minima plants take up and accumulate high concentrations of Pb, Ni, Cu, Zn and Li in their tissues, thus reducing the metal content of aqueous media. Even though at high concentrations those metals may affect its physiological performance, this occurs at much higher concentrations than those normally found in natural water bodies, therefore S. minima is certainly a good candidate to be used for the phytoremediation of water bodies contaminated with metals.

Background: Arabidopsis thaliana is the most widely used plant organism as a study model, due to its short life cycle and small genome. Among the most used accessions, the Columbia (Col-0) ecotype is widely used for molecular and genetic research, however the physiological response of this model plant to abiotic factors is relatively unknown. Objective: Given its relevance in studying gene functionality, it is essential to understand its physiology and morphology under individual stress conditions: water deficit stress (WDS), thermal shock (50 °C for 2 hours), and under the combined effect of both stress types (WDS + 50 °C for 2 hours). Methodology: 75-day-old A. thaliana Col-0 plants were used for the 3 stress treatments: 14d of WDS, 50 °C/2 h, and 14d WDS + 50 °C/2 h. The survival percentage, water potential, electrolyte leakage, PSII status, and gas exchange were evaluated. Results: A. thaliana plants exhibited susceptibility to prolonged levels of stress, demonstrating different physiological mechanisms to cope with individual and combined stresses. The analysis of the photochemical state of PSII indicated that Arabidopsis is more vulnerable to the 50 °C/2 h stress and to the combined WDS + 50 °C/2 h stress, than to water deficit stress. The WDS + 50 °C/2 h treatment caused greater membrane damage, more negative water potential, and lower gas exchange compared to the individual stress. Implications: This system is proposed for future molecular analyses involving the overexpression of cloned transcription factor genes from tolerant species, with the aim of extrapolating these findings to commercially relevant crops. Conclusion: The differential response observed under different types of stress in this model plant, may facilitate the elucidation of underlying molecular mechanisms, which should be a central focus in future research aiming to increase resilience to climate change factors in commercially important agricultural crops.

This chapter presents an efficient protocol for regenerating Carica papaya plants via somatic embryogenesis from immature zygotic embryos from economically important papaya genotypes. To achieve regenerated plants from somatic embryos, in the present protocol, four induction cycles are required, followed by one multiplication cycle and one regeneration cycle. With this optimized protocol, 80% of somatic embryos can be obtained in only 3.5 months. At this stage, calli containing more than 50% globular structures can be used for transformation (via agrobacterium, biobalistics, or any other transformation method). Once transformed, calli can be transferred to the following steps (multiplication, elongation, maturation, rooting, and ex vitro acclimatization) to regenerate a transformed somatic embryo-derived full plant.