Light and irrigation effects on chlorophyll fluorescence depend on seedling provenance in Magnolia pugana endemic of Mexico
Introduction
Environmental and ecological physiology enables understanding of the distribution and abundance of organisms in different environments based on environmental tolerances (Wikelski and Cooke, 2006). Observations, experiments, and mechanistic models indicate that many plant populations have the capacity to adjust to environmental change via phenotypic plasticity (e.g., ecophysiological adjustments) (Dawson et al., 2011). In this context, the ability of organisms to adapt to different environmental conditions could help populations of vulnerable species to reduce the danger of extinction.
The family Magnoliaceae is constituted by approximately 350 species and more than 50% of these species are on the IUCN Red List with some category of threat of extinction (Vázquez-García et al., 2017; Rivers et al., 2016). So far, at least 31 Magnolia species have been recorded in Mexico (Vázquez-García et al., 2012, 2013, 2017); six of these are distributed in the state of Jalisco, in western Mexico (Vázquez-García, 1994; Vázquez-García et al., 2002, 2012, 2013).
Magnolia pugana (H.H. Iltis & A. Vázquez) A. Vázquez & Carvajal, is distributed as a dominant species in riparian gallery forests in Western Mexico (Vázquez-García et al., 2002), surrounded by drier ecosystems (pine-oak and tropical seasonally dry forests). Their populations are geographically isolated, comprise only a few individuals, and are therefore regarded as critically endangered of extinction (Cicuzza et al., 2007) or endangered (Rivers et al., 2016). They are further hampered by low seed germination resulting from poor viability and dormancy mechanisms (Jacobo-Pereira et al., 2016).
Conservation of wild plants has a major focus on protecting populations through in situ activities (Higgins and Lynch, 2001). The establishment of artificial populations (ex situ) might provide individuals for in situ recuperation actions (Guerrant and Kaye, 2007). Because M. pugana is part of the forest cover that contributes to biodiversity, stabilizes soils, reduces erosion, and is in risk of extinction, it is essential to know the physiology of this species, especially regarding photosynthesis, to determine the environmental conditions that favor establishment, survival and optimal growth (Wikelski and Cooke, 2006).
In riparian forests, the amount of sunlight reaching below the canopy varies highly and can affect plant regeneration (Leakey et al., 2003; da Costa et al., 2019); furthermore, water influences the growth of plants and the spatial distribution in their habitats (Bianchini et al., 2001; Lemoine et al., 2001). In cleared habitats, elevated light levels may be detrimental to photosynthesis, e.g. due to photoinhibition (Demmig-Adams and Adams, 1992; Cavender-Bares and Bazzaz, 2004). In addition, gallery forests are vulnerable habitats as affected by climate change, in which extreme events are projected such as droughts, which can have adverse effects on plant functioning (Garssen et al., 2014). For example, studies in riparian plants show physiological adjustment to cope with water deficiency such as rapid stomatal closure, which can affect photosynthetic capacity (Kozlowski and Pallardy, 2002).
Light and water deficit stress can affect photosynthetic variables such as chlorophyll fluorescence parameters (Cavender-Bares and Bazzaz, 2004). For example, full sunlight and severe water deficit stress decreased the effective quantum yield of PSII in Aloe vera (Hazrati et al., 2016). Thus, these abiotic conditions could also affect chlorophyll fluorescence of M. pugana seedlings. However, it is very important to differentiate between phenotypic plasticity and genotypic adaptation in the plant responses. Thirty years ago, a cultivated population of Magnolia pugana was established in the gardens of the “Centro Universitario de Ciencias Biológicas y Agropecuarias de la Universidad de Guadalajara”, the genotypic diversity of which is expected to be similar to the original wild population (Muñiz-Castro et al., 2020). Thus, we expected phenotypic plasticity, defined as the fraction of the total phenotypic variation found in a given variable that is explained by differences between environments (Valladares et al., 2005), in the M. pugana chlorophyll fluorescence responses.
Chlorophyll fluorescence measurements allow to determine the amount of light required for the saturation of photosystem II (PPFDsat) and the maximum electron transfer rate (ETRmax), based on a light response curve (Rascher et al., 2000; Arroyo-Pérez et al., 2017). Likewise, it determines the amount of light effectively used by the plant for photochemical work, i.e., the maximum primary leaf efficiency and the amount of energy that the plant will emit as heat if exposed to excess light or drought (Demmig-Adams and Adams, 1992; Singh and Thakur, 2018).
The purpose of our research was to evaluate the variables related to chlorophyll fluorescence: ΦPSII and ETR for seedlings of two populations (wild and cultivated) of M. pugana in response to different levels of radiation and watering. Because wild populations are subject to higher natural selection pressures by the environment compared to cultivated populations, they can be expected to possess greater capacities for tolerance to water deficit and excess light stress (Primack and Kang, 1989) and higher photochemical efficiency of PSII and electron transfer rate (Rotundo and Cipriotti, 2017; Padhan and Panda, 2018). We tested whether ΦPSII and ETR were higher for seedlings from the wild population than for the cultivated population, thus contributing to high tolerance to excess light and water deficit stress. This research leads to elucidate the ecophysiological responses of M. pugana seedlings in the face of current and future environmental change challenges for its conservation and ecological restoration.
Section snippets
Study area
The study area is located in central Jalisco, in the physiographic province of Eje Neovolcánico (Trans-Mexican Volcanic Belt), in western Mexico. The studied Magnolia pugana populations are found in the municipality of Zapopan, in the State of Jalisco. The first population is situated in the gardens of the University Center for Biological and Agricultural Sciences (CUCBA, 20° 44′ 53.6″ N, 103° 30′ 52.2″ W), at 1659 m a. s. l. It consists of 20 trees that were planted in the 1980s from seeds
Results
The maximum quantum efficiency of photosystem II (Fv/Fm) in seedlings of M. pugana was not affected by the seedling provenance (F1,36 = 1.01, P = 0.3) (CUCBA = 0.54 ± 0.01; SN = 0.50 ± 0.01), light levels (F1,36 = 2.2, P = 0.15) (high light: 0.51 ± 0.01; low light: 0.50 ± 0.01), or irrigation levels (F2,36 = 1.3, P = 0.28) (constant irrigation: 0.51 ± 0.01; 15 d without irrigation: 0.52 ± 0.01; 25 d without irrigation: 0.50 ± 0.01).
For the effective quantum yield of PSII (фPSII), there was a
Discussion
This is the first study to evaluate the response of chlorophyll fluorescence in seedlings of Magnolia pugana under different irrigation and light levels. The photochemical reactions of photosynthesis are sensitive to high irradiation, which affects the maximum quantum efficiency and effective quantum yield of photosystem II (фPSII) (Keren and Krieger-Liszkay, 2011). The reduction in the maximum quantum efficiency of photosystem II (Fv/Fm) in terms of radiation, known as photoinhibition, is the
CRediT authorship contribution statement
Olivia Hernández-González: Formal analysis, Methodology, Writing - original draft. Rosa L. Romo-Campos: Formal analysis, Methodology, Investigation, Writing - original draft, Writing - review & editing. Miguel Á. Muñiz-Castro: Formal analysis, Investigation, Writing - original draft. Joel Flores: Investigation, Methodology, Formal analysis, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to Alejandro Muñoz for his support with equipment, to Adriana Avendaño for her assistance in the greenhouse, Gerardo Hernández and Edilia de la Rosa for reviewing the manuscript and to Karla García for her help in collecting data.
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