Tropical and subtropical agroecosystems

Background. Bouteloua curtipendula (Michx.) Torr. is a grass native to Mexico with a high capacity for adaptation in areas with limited moisture. It has physiological and morphological characteristics that are of interest in genetic improvement programmes for the restoration of degraded areas and the establishment of pastures for livestock grazing. In addition, flow cytometry can provide useful information in the study of the plant genome of this grass, such as nuclear DNA content (2C value) and its correlation with ploidy levels. Objective. To determine the nuclear DNA content of 57 ecotypes of Bouteloua curtipendula (Michx.) to infer their ploidy level. Methodology. Nuclear DNA content was estimated using flow cytometry. The internal standard was Zea mays L. The analysis considered average fluorescence intensity indices with a coefficient of variation <5%. Based on the fluorescence index and the 2C DNA value of the internal standard, the absolute DNA content (2C) in pg of each ecotype was obtained. The level of ploidy was inferred from the data obtained. Results. The range of nuclear DNA content of the ecotypes analysed ranged from 1.66 to 5.67 pg. Four different ploidy groups were found, with the tetraploid level predominating with 39 ecotypes. Implications and Conclusion. There is considerable variation in the nuclear DNA content of B. curtipendula (Michx.) Torr., a valuable forage grass for rainfed pastures, and flow cytometry was useful in determining this variation.

Background: Chrysanthemum (Chrysanthemum morifolium) is one of the most economically important ornamental species in Mexico, particularly in the State of Mexico. Optimizing in vitro propagation protocols is crucial to overcome the limitations of conventional methods and meet the demand for high-quality plants. Objective: To evaluate the effect of different concentrations of benzyladenine (BAP) and sucrose on the in vitro organogenesis of C. morifolium var. Indianapolis. Methodology: A completely randomized experimental design with a 3x3 factorial arrangement, plus a control, was established. Three concentrations of BAP (0.0, 0.5, and 1.0 mg L⁻¹) and three of sucrose (30, 45, and 60 g L⁻¹) were evaluated in an MS basal medium. The control consisted of a standardized medium with kinetin (1.0 mg L⁻¹) and sucrose (40 g L⁻¹). Variables of survival, callogenesis frequency, leaf number, main shoot length, and root length were recorded after 70 days of culture. Data were analyzed by ANOVA and Tukey's test (α = 0.05). Results: The combination of 0.5 mg L⁻¹ BAP and 30 g L⁻¹ sucrose (T4) significantly promoted survival (100%), shoot length (15.7 mm), and leaf number (6.2). The highest frequency of callogenesis (95.8%) was observed in the absence of BAP and with the maximum sucrose concentration (T3). Root development was limited across all treatments, observed in only 10% of explants, with no statistically significant differences among the treatments that induced roots. Implications: Optimizing the balance between hormonal and carbon source is fundamental to directing in vitro morphogenesis. The identified protocol (0.5 mg L⁻¹ BAP and 30 g L⁻¹ sucrose) allows for efficient direct organogenesis, which can lead to faster and more uniform clonal propagation, benefiting the productivity and competitiveness of ornamental growers. Conclusion: The concentration of 0.5 mg L⁻¹ BAP combined with 30 g L⁻¹ sucrose constitutes an optimal stimulus for direct organogenesis in C. morifolium, maximizing survival and shoot development, while high sucrose concentrations induce a stress response leading to callogenesis.

Background. In seasonally dry tropical forests, water availability determines the growth of plants. Diameter growth rate has a direct relationship with the density and relative water content in the wood and this relationship can be strengthened according to environmental seasonality. Objective. To determine the effect of seasonality in diameter growth and its relationship with wood density. Methods. The study was conducted at site Forest Reserve El Colegio de la Frontera Sur in Chetumal, Quintana Roo March 2010- March 2011. Diameter growth (DG), wood density (WD) and relative water content in wood (RWC) were determined during one year at two-week intervals. 16 tree species were studied. Results. The seasonality in rainfall directly influences the DG, this is corroborated by the high growth rate in the rainy season, plus WD is a determining factor in the growth rate, as it is directly related to RWC and the DG. In the dry season four functional groups were identified. A) soft wood trees with low WD, high RWC and high DG; B) WD trees with high, intermediate RWC and moderate DG; C) trees with high WD, intermediate RWC and slow DG; D) trees with high WD, under RWC without DG. Implication. The water availability significantly influences the growth rate and wood density. Conclusions. The growth rate is directly related to the WD and the RWC, this in turn is influenced by water availability. This relationship identifies a seasonal response and allows distinguishing functional groups.

Background: Soursop (Annona muricata L.) is a tropical fruit with important nutritional and medicinal properties. Clonal multiplication of soursop by in vitro tissue culture is the best alternative for genetic improvement and conservation purposes. Objective: To reduce the level of persistent microbial contamination in soursop explants to facilitate their introduction to in vitro tissue culture. Methodology: Leaf explants (LE) and internodal segments (IS) from ex vitro plants were disinfected with different concentrations of active chlorine and ethanol, and different combinations of antibiotics and fungicides: Treatment 1: Streptomycin:tetracycline with carbendazim (Strept100:Prozycar); Treatment 2: Streptomycin:tetracycline with benomyl (Strept100:Benomyl); Treatments 3, 4 and 5: Same as Treatment 2, with no ethanol wash and addition of 100, 200 or 300 ppm silver nanoparticles (AgNPs), respectively. Disinfected explants were cultivated on MS medium in the presence of different combinations of plant growth regulators (2,4-dichlorophenoxyacetic acid (2,4-D), benzyladenine (BA) or kinetin (Kin). Results: Strept100:Prozycar and Strept100:Benomyl combinations left ~96/98 % and ~60/54 % of LE/IS contaminated, respectively. The addition of AgNPs improved decontamination (~6% of LE and ~9% of IS, respectively). Simultaneous addition of 1.5 mg·mL-1 BA and 2.0 mg·mL-1 2,4-D, with a subsequent step with 0.5 mg·mL-1 Kin, led to the higher in vitro regeneration rates (~95.6% of LE and ~93.9% of IS). Implications: The reduction of internal contamination facilitates in vitro micropropagation of species of the Annonaceae family. Conclusion: The use of AgNPs was decisive to reduce internal contamination, improving conditions for the in vitro propagation of soursop tissues.

Background. Biomass biosynthesis and partitioning in plants significantly impact growth and depend on the relationship established between the plant’s photosynthetic (source) tissues and heterotrophic (sink) tissues. Several factors influence the source-sink relationship, including the establishment of interactions with soil microorganisms, such as arbuscular mycorrhizal fungi (AMF). During the interaction with AMF, a bidirectional flow of nutrients is established, whereby AMF receive carbon (C) from the plant in exchange for mineral nutrients, thereby altering the plant’s source-sink relationship and influencing plant growth. Objective. To analyse the mechanism that regulate and influence C partitioning in plants colonized by HMA as well as to expose the current theories that explain how the change of C partitioning in plants influenced by arbuscular mycorrhizal symbiosis alters plant growth. Methodology. Through a literature review, we explored the physiology of growth and C partitioning and allocation in plants, as well as studies concerning the growth responses exhibited by plants in relation to mycorrhizal symbiosis. Results. Colonization by AMF causes profound modifications in carbohydrate anabolism and catabolism, altering the source-sink relationship and potentially eliciting positive growth responses in plants. However, the alteration of the source-sink relationship can also be negative or neutral, indicating that AMF interaction may limit plant growth under certain conditions. Implications. Understanding the conditions and molecular mechanisms that favour the manifestation of positive mycorrhizal growth responses in plants will facilitate the agricultural management of this important association. Conclusion. Mycorrhizal growth response is determined by the alterations that AMF root colonization causes in C partitioning, allocation, and metabolism, which play a pivotal role in regulating plant growth.