In addition to the primary metabolism (growth) present in all living beings, plants have a secondary metabolism (protection) that allows production and accumulation of compounds of a diverse chemical nature called SMs, which are distributed differentially among taxonomic groups. According to their biosynthetic pathway, SMs are classified as terpenes, PCs, and nitrogenous compounds (Fang et al., 2011). Generally, SMs are bioactive agents of plants that can interfere with molecular targets in animals and microorganisms; others serve to attract pollinators and animals that disperse their seeds and can also function as antioxidants and ultraviolet radiation protection systems (Verma and Shukla, 2015).
SMs play an important role in the adaptation of plants to the environment and in counteracting stress conditions (Ahmed et al., 2014). The defense mechanisms of plants can be structural, such as cell wall thickening or lignification, or biochemical such as the production of PCs, phytoalexins, or enzymes (Anderson et al., 2005). Carotenoids play important roles in plant–environment interaction, including providing essential intermediaries for phytohormone biosynthesis (Esteban et al., 2015).
PC production is affected by the presence of pathogens and other stresses. In avocado (Persea americana Mill.) rootstocks inoculated with Phytophthora cinnamomi, the PC content was higher in asymptomatic plants, indicating that the defense mechanisms were activated (Andrade-Hoyos et al., 2015). In ‘Hass’ avocado fruit, the highest PC concentration was present in the exocarp (skin), probably because of the exposure of this tissue to stress conditions (Tesfay et al., 2010). In smooth-bark Mexican pine (Pinus pseudostrobus Lindl.) and sacred firs (Abies religiosa L.), the foliar concentration of photosynthetic pigments (chlorophyll A and B, carotenoids) generally decreases in response to stress or disease; on the other hand, environmental factors increase leaf structural chemistry (cellulose and lignin) in these species (Cambrón-Sandoval et al., 2011; España-Boquera et al., 2010).
Factors such as nutrient supply, temperature, light conditions, or carbon dioxide concentration in the atmosphere may influence the concentration of defense-related secondary compounds in plant tissues and, consequently, the distribution of energy expenditure between the primary metabolism and secondary metabolism (Gayler et al., 2008). The contribution of each factor varies between sites and each plant species responds with considerable physiological and genetic plasticity to maintain its dominant status in each habitat.
During plant phenology, the concentration of SMs varies between species, although their quantity and diversity is not universally related to the development of the plant. Early production of these compounds has ecological implications involving defense mechanisms, relationships with microorganisms and role of these compounds as a nitrogen reserve (De-la-Cruz Chacón et al., 2013). The harvest date and the maturity stage of fruit can affect the concentration of TPCs. Golukcu and Ozdemir (2010) found a decrease in PCs in the mesocarp of various avocado cultivars harvested at different dates; a similar phenomenon occurred in Mexican serviceberry [Malacomeles denticulata (Kunth) Jones] fruit where PCs decreased during maturation (Herrera-Hernández et al., 2013). In persimmon (Diopyros kaki Thunb.) fruit, the carotenoid concentration decreased during the first stages of development, then increased and reached its maximum value at harvest (Candir et al., 2009).
In Mexico, ‘Hass’ avocado is cultivated in different climates; therefore, the fruit is exposed to extreme climatic factors, especially temperature and solar radiation. In some markets, the price of ‘Hass’ avocado is lower when it presents pronounced skin roughness (Campos et al., 2011). A recent study showed that the thickness and roughness of ‘Hass’ skin was greater in producing areas with the highest ambient temperatures during fruit development (Salazar-García et al., 2016). PCs are involved in the adaptation of plants to the environment, in particular, their protective role against excessive solar radiation (Alonso-Amelot et al., 2004). It is unknown if environmental conditions affect the presence of SMs and lignin in the skin of ‘Hass’ avocado and if this is related to skin roughness. The aim of this research was to determine the concentration of TPCs, chlorophylls, TCARs, and lignin in the skin of ‘Hass’ avocado fruit during its development in three producing regions of Mexico.
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