Dry-Storage and Light Exposure Reduce Dormancy of Arabian Desert Legumes More Than Temperature
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Bhatt, Bhat, Phartyal and Gallacher (2020). Seed Science and Technology, 48, 2, 247-255. https://doi.org/10.15258/sst.2020.48.2.12 Research Note Dry-storage and light exposure reduce dormancy of Arabian desert legumes more than temperature Arvind Bhatt1,2, N.R. Bhat1, Shyam S. Phartyal3* and David J. Gallacher4 1 Kuwait Institute for Scientific Research, P.O. Box 24885 Safat, 13109, Kuwait 2 Lushan Botanical Garden, Chinese Academy of Sciences, Lushan, Jiujiang City, China 3 Nalanda University, School of Ecology and Environment Studies, Rajgir, 803116, India 4 School of Life and Environmental Sciences, The University of Sydney, Narrabri NSW 2390, Australia * Author for correspondence (E-mail: [email protected]) (Submitted January 2020; Accepted May 2020; Published online May 2020) Abstract Propagation and conservation of desert plants are assisted by improved understanding of seed germination ecology. The effects of dry-storage on dormancy and germination were studied in seven desert legumes. Mature seeds were collected in summer 2017 and germinated within one week of collection (fresh) and after six months (dry-storage) under two temperature and two light regimes. Seed weight of two species increased 22-55% within 24 hours of water imbibition but others increased ≤ 7%. Germination ranged from 0-32% in fresh and 2-92% in dry-stored seeds, indicating a mix of non- and physically-dormant seeds at maturity. Dry-storage at ambient room temperature was effective at relieving dormancy, though the extent was species-dependent. Germination percentage increased in response to light exposure during incubation, while the effect of temperature was species-dependent. This variable response to dormancy alleviation may assist to spread the population risk of seedling survival in the harsh and variable environment of the Arabian desert. Keywords: arid, dry-storage, legumes, seed dormancy, seed germination, water-gap Experimental and discussion Desert plants are subjected to high temperature and high salinity, low soil water availability and acute nutrient deficiencies (Abu Sukar et al., 2007; Rewald et al., 2011). Each species exhibits strategies for population survival in these severe environmental conditions (Gutterman, 1993). Annual species allocate a higher proportion of resources to sexual reproduction than perennial species (Karlsson and Méndez, 2005). Populations of © 2020 Bhatt et al. This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: https://creativecommons.org/licenses/by-nc/4.0 247 A. BHATT, N.R. BHAT, S.S. PHARTYAL AND D.J. GALLACHER annual species are dependent on seed bank persistence, frequency of successful seedling establishment and rapid transition from germination to seed production (Gutterman, 2000). In contrast, populations of perennial species are dependent on producing multiple seed batches, thus reducing the reliance on seed banks and successful germination events (Escós et al., 2000). Populations of annual desert species are more sensitive to environmental variation since they are more dependent on successful seedling establishment in unfavourable seasons (Li, 2010). Interspecific variation in germination timing enables different species to occupy specific niches (Rathcke and Lacey, 1985). Germination timing affects population abundance by influencing seedling survival and is thus a crucial component of a species’ life-history strategy (Handley and Davy, 2005; Shahba and Qian, 2008). Dormancy at maturation is common in desert seeds, preventing germination during the unfavourably hot and dry summer (Iglesias-Fernández et al., 2011; Baskin and Baskin, 2014). Most species that produce mature seeds during summer thus do not germinate until the following winter, when soil moisture is more likely to persist after a rain event and seeds are able to germinate and seedlings to establish. The Arabian Peninsula is home to 50 Fabaceae species from 22 genera, with diverse growth habits, from annual herbs to woody perennials (Mandaville, 1990). Legumes are adapted to nitrogen-poor soils through their symbiotic association with rhizobia (Crews, 1999) which sometimes also benefits other plant species. Nitrogen is typically low in desert soils (Wolde-Meskel et al., 2004). Legumes are a preferred feed source for many desert ungulates (Graham and Vence, 2003). Physical dormancy (PY) is prevalent in legume seeds, due to a water-impenetrable palisade cell layer in the seed and/or fruit coat (Baskin and Baskin, 2014). PY is alleviated when the ‘water-gap’ in the palisade layer is breached and water enters the seed (Baskin and Baskin, 2014). This usually occurs when conditions are suitable for seedling establishment (Jayasuriya et al., 2007). The strength of the water-gap gradually declines over time, largely due to the seasonal and diurnal changes in temperature and moisture. These factors are therefore environmental signal detectors for dormancy break (Baskin and Baskin, 2000, 2014). PY can be broken in the laboratory through mechanical or acid scarification, percussion, or exposure to heat (moist or dry) or storage (moist or dry) (Phartyal et al., 2005; Bhatt et al., 2016; Koutouan- Kontchoi et al., 2020). Although most seeds in the studied genera are known to possess PY (Kondo, 1993; Patanè and Gresta, 2006; Kim et al., 2008; Long et al., 2012; Mondoni et al., 2013), the role of dry-storage in breaking PY is not well known. We compared the germination rate of freshly collected and dry-stored seeds of seven Arabian desert legumes. All species are widely distributed across the Arabian Peninsula and produce mature seeds in summer (April-June), with new seedlings emerging in early winter (Norton et al., 2009). We hypothesised that (1) fresh seeds would exhibit PY; (2) dry-storage would alleviate PY; and (3) that the tested temperature and light regimes for germination would reflect natural winter conditions. Mature seeds of all species were collected in late April 2017 from natural populations in Kuwait (table 1). Seeds of each species were collected from 30-35 spatially separated plants to represent the genetic diversity of the population. Seeds of each species were cleaned and divided into one batch for immediate germination (fresh) and another for 248 SEED DORMANCY IN ARABIAN DESERT LEGUMES dry-storage. Fresh seeds were tested for germination within one week of collection. Dry- stored seeds were kept indoors in brown paper bags at room storage (temperature 20 ± 2°C, relative humidity 15 ± 2%) and tested for germination in the first week of November. The dry-storage conditions are thought to be optimal for most orthodox seeds (Ellis et al., 1991), and November represents the month in Kuwait when soil moisture is likely to persist after rains. The mass of fresh seeds was determined by weighing three 50- seed replicates. Water permeability was assessed by recording the mass of three 25-seed replicates before and after placement on two sheets of Whatman No. 1 filter paper for 24 hours, moistened with 10 ml distilled water. Seed mass varied seven-fold from 0.52 mg for Lotus halophilus to 3.76 mg for Astragalus spinosus. It reveals perennial species (A. spinosus) produce heavier seeds than those of the annuals, reflecting previous findings of a positive correlation between seed mass and species’ lifespan (Baker, 1972; Leishman et al., 2000). In two of the seven species, water imbibition rate varied moderately (22.5 to 52.6%) while the remaining species showed only a slight (≤ 7%) gain during the first 24 hours of imbibition (table 1). Most legume seeds, if permeable, imbibe water close to 100% or more prior to seed germination (Baskin and Baskin, 2014). The moderate percentage of water imbibition indicates that the water-gap was open in only a moderate proportion of fresh seed and germination was therefore relatively low (6-32%). Fresh and dry-stored seeds were tested for germination at two night/day temperature regimes (15/20 and 20/30°C) and two light regimes (dark/light and dark/dark). Four 25- seed replicates per treatment were placed in 90 mm-diameter Petri-dishes containing two disks of Whatman No. 1 filter paper moistened with 10 ml of distilled water. Darkness was obtained by wrapping Petri-dishes in aluminum foil. The number of germinated seeds was counted daily for 28 days for seeds in the light and at the end of 28 days for seeds in the dark. Germination was defined as the emergence of the radicle to a length > 1 mm. All non-germinated seeds were cut with a scalpel to evaluate the status of the embryo. Seeds with a turgid whitish embryo were considered viable and those with a brownish (necrotised) embryo were considered non-viable. Germination percentage was calculated based on viable seeds. The relationship between mean germination percentage (dependent variable) and the value of three predictors (dry-storage, photoperiod and temperature) was compared for each species using a generalised linear model with a Poisson probability distribution and identity-link function. The Bonferroni correction was applied to assess significance, in which α = 0.0012. The effects of three predictors and their interaction in the model were tested by Wald chi-square values. All statistical analyses were carried out using SPSS 21. Germination of dry-stored seeds was significantly greater than fresh seeds for all species (figure 1) except A. corrugatus, which exhibited dormancy even after storage (table 1). Germination varied among species from 0 to 32% and 2 to 92% in fresh and dry- stored seeds, respectively. Dry-storage and photoperiod had a greater influence over seed germination than temperature regime in most species (table 2). Of the six species exhibiting germination, all exhibited a greater percentage with dry-storage, five with light exposure, and two were influenced by temperature regime but in opposite directions.