Gairola, Shabana, Mahmoud and Santo (2019). Seed Science and Technology, 47, 1, 53-58. https://doi.org/10.15258/sst.2019.47.1.06
Research Note
Seed germination of Kickxia acerbiana, a rare annual of the Arabian Desert
Sanjay Gairola1*, Hatem A. Shabana1, Tamer Mahmoud1 and Andrea Santo2
1 Sharjah Seed Bank and Herbarium, Sharjah Research Academy, P.O. Box 60999, Sharjah, United Arab Emirates 2 Independent Researcher, Selargius, Cagliari, Italy * Author for correspondence (E-mail: [email protected])
(Submitted June 2018; Accepted October 2018; Published online March 2019)
Abstract
Investigations on seeds of Kickxia acerbiana were undertaken to elucidate their germination characteristics. Germination was assessed in fluctuating temperatures, both in continuous darkness and in alternating light/dark (12-hour photoperiod), and in response to gibberellic acid pretreatment. Germination was favoured in light/ dark and fluctuating temperature regimes. Germination across all incubation temperatures in complete dark regimes was consistently low. Pretreatment with 2 mM gibberellic acid strongly promoted germination in the 12-/12-hour light/dark, 25/15 and 30/20°C regimes. In the light and without pretreatment with gibberellic acid, the optimum germination temperatures were 25/15 and 30/20°C, while at the highest temperature, 35/25°C, no more than approximately 20% of seeds germinated. The germination velocity of K. acerbiana seeds decreased with increase in GA concentration at 30/20°C temperature regime.
Keywords: germination, gibberellic acid, Kickxia acerbiana, temperature
Experimental and discussion
Kickxia acerbiana (Boiss.) Taeckh. & Boulos (Plantaginaceae) is a rare annual or short- lived perennial of the Arabian Desert, distributed in the mountainous habitats, mainly in rocky slopes and gravel of wadis. In the United Arab Emirates (UAE), seeds of this species ripen from May to late-June. Information on the germination requirements of this rare desert plant is lacking. Hence, it is imperative to investigate the seed germination response of K. acerbiana in relation to some environmental factors, mainly temperature and light. Knowledge of germination behaviour of rare and endangered species may contribute to improved understanding of their rarity, and be useful for their conservation (Schemske et al., 1994; Hilooğlu et al., 2018).
© 2019 Gairola 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 53 SANJAY GAIROLA, HATEM A. SHABANA, TAMER MAHMOUD AND ANDREA SANTO
Among several factors, light and fluctuating temperatures are two important factors triggering seed germination, however the responses can be species-specific (Kranner and Seal, 2013; Gul et al., 2013). It is known that pre-treatment with some germination- promoting chemical such as gibberellic acid (GA) is used in germination studies to break seed dormancy (Jones et al., 2016; El-Keblawy and Gairola, 2016; Peng et al., 2017). The response of seeds to the exogenous application of GAs mainly depends upon environmental factors (i.e., light and temperature) as well as on endogenous factors such as the level of physiological dormancy (Pereira et al., 2011). The aim of the present study was to evaluate the light dependence (photoblastism), the influence of alternating temperatures and the effect of giberellic acid pretreatment on K. acerbiana seed germination. Mature and fully dried seed capsules of K. acerbiana were randomly hand-picked from a very small population in mountainous slopes of Al Munaiy area in UAE during May 2016 (figure 1A). A small sample of seeds was collected, bearing in mind that the species is rare and its population balance in situ should not be disturbed. All seeds collected from the plants were combined into a bulk sample. Seeds were stored in dry, dark conditions at room temperature (approximately 23°C) for 3 weeks, until their use in the germination experiment. Seeds of this species are small (figure 1B). The 100-seed weight, measured by weighing five separate samples on an electronic balance (0.0001 g precision) according to ISTA (2018), was 4.1 mg.
(A)
(B)
Figure 1. Kickxia acerbiana plant in its habitat (A) and seeds (B).
54 SEED GERMINATION OF KICKXIA ACERBIANA
Pre-sowing treatments with different concentrations of gibberellic acid (GA4+7, hereafter referred to as ‘GA’) were applied to evaluate the germination response of K. acerbiana seeds. The seeds were soaked in 1, 2 and 4 mM GA solutions for 24 hours (three volumes of GA solution for each volume of seeds) at room temperature. After the treatment, seeds were set to germinate at different temperatures with two photoperiods. Untreated seeds were sown in distilled water (0 mM GA) and the same light and temperature conditions, and were used as a control. Seeds were germinated in 90 mm-lidded Petri dishes containing three disks of Whatman No. 1 filter paper. The filter papers were moistened with distilled water. Four replicates of 25 seeds each were used per treatment. The Petri dishes were placed in three incubators (280A, LMS) at daily temperature regimes of 25/15, 30/20 and 35/25°C in 12-hour light/12-hour dark or in continuous dark conditions. The incubation temperatures were used to approximate the temperature fluctuations in the natural habitats of this species during the growing season. Dark conditions were achieved by wrapping the Petri dishes with two layers of aluminum foil. Compared with the light treatment, a little more water was added to each dish in dark treatment to maintain moisture conditions for a longer period. In addition, to prevent excessive water loss, the Petri dishes were wrapped with a strip of parafilm. Germinated seeds were counted and removed every alternate day for 30 days in the light treatments and at the end of the experiment (after 30 days) in the dark treatments. Final germination percentages were calculated, and the rate of germination was estimated by using a modified Timson’s index of germination velocity (TI): TI = ∑ G/t, where G is the percentage of seed germination at 2-day intervals and t is the total germination period. Using this index, a higher value indicates more rapid germination. For all the data, normality was assessed using the Shapiro-Wilk test. Arcsine-transformed germination percentages of light and temperature experiments and log10-transformed TI were analysed by ANOVA and Fisher’s Least Significant Differences (LSD) post hoc test. Statistical analyses were performed using the statistical software Statistica 7.0 for Windows (Software Statsoft Release 7). Germination of K. acerbiana was significantly affected by light, temperature and GA concentration (P < 0.01, table 1), and by their interactions. K. acerbiana seeds were able to germinate more over tested low and moderate temperatures under light/dark regime and relatively few in total darkness. Overall, germination decreased with the increase in temperatures (figure 2). The studied low and moderate temperatures usually prevail during the time of effective rainfalls (i.e. December - February) in UAE deserts, which might favour successful germination and seedling establishment of plant species in natural habitats. In addition, after rainfall and depending on the temperature, a certain portion of the total number of seeds in the population might germinate and show an opportunistic strategy for germination. Responses to environmental cues have evolved into specific germination strategies that reflects species adaptations to different habitats and trigger germination at favourable sites and times for seedling establishment (Willis et al., 2014). Both the incubation temperature (T) and the GA concentration (G) had a highly significant effect (P < 0.001) on germination velocity (table 2). The fastest germination was observed in seeds incubated at 25/15 and 30/20°C under the control conditions (untreated seeds) and all used GA concentrations. Faster germination of K. acerbiana seeds at low and moderate
55 SANJAY GAIROLA, HATEM A. SHABANA, TAMER MAHMOUD AND ANDREA SANTO temperatures will offer progressively more suitable conditions for the establishment of seedlings, as the favourable time for seedling establishment under harsh environmental conditions of the arid deserts is short and often unpredictable. The slowest germination for seeds at 35/25°C indicates the hedging strategy against the risk of moist conditions being temporary, perhaps as there will be lower chance for seedling establishment with scanty and uncertain rainfall late in the growing seasons. In general, an early or later germination comes at a cost, especially in arid desert habitats where there is an increasing risk of exposure to unfavorable conditions. Thus, successful establishment of seedling will strongly depend on the post-germination environmental conditions.
Table 1. Three-way ANOVA results for the effect of light (L), temperature (T), GA concentration (G) and their interactions on final germination of Kickxia acerbiana seeds. **P < 0.01 and ***P < 0.001.
Factor SS DF MS F P
Light (L) 36750.65 1 36750.65 576.10 ***
Temperature (T) 3528.70 2 1764.35 27.65 ***
GA concentration (G) 30846.71 3 10282.24 161.18 ***
L × T 1861.31 2 930.65 14.58 ***
L × G 11389.38 3 3796.46 59.51 ***
T × G 3070.73 6 511.79 8.02 ***
L × T × G 1610.05 6 268.34 4.20 **
Error 4592.96 72 63.79
b a Light a A Dark 0 mM 100 a A AB AB a b 1 mM a a B B B B 2 mM 80 4 mM c A 60
40 b c B B a b A Germination (%) a 20 C a ab a A A a b b B B a A C A A 0 25/15 30/20 35/25 25/15 30/20 35/25 Temperature (°C)
Figure 2. Final germination of Kickxia acerbiana seeds in light (12/12) and dark (0/24) conditions. Bars with the same letters are not significantly different at P < 0.05 (three-way ANOVA followed by Fisher’s Least Significant Difference post hoc test). For each photoperiod, capital letters were used to compare final germination among the GA concentrations at the same temperature, while lower-case letters were used to compare final germination at the same GA concentration among different temperatures.
56 SEED GERMINATION OF KICKXIA ACERBIANA
Table 2. Timson’s index values (TI) for germinating seeds of Kickxia acerbiana at alternating temperatures and following pretreatment with GA at different concentrations. Capital letters in columns are related to the same temperature, while lower-case letters in rows to the same GA concentration. Values with different letters indicate significant differences at P < 0.05 (Fisher Least Significant Difference post hoc test). Temperature (°C) GA concentration (mM) 25/15 30/20 35/25
0 (Control) 3.72 ± 0.80aBC 4.50 ± 0.42aA 0.95 ± 0.25bB
1 4.03 ± 0.44aB 4.62 ± 0.27aA 0.27 ± 0.21bC
2 5.38 ± 0.33aA 4.54 ± 0aA 3.15 ± 1.36bA
4 4.31 ± 0.17aB 3.86 ± 0.60aA 1.60 ± 0.36bB
It is known that light requirements for germination are more likely in small-seeded than in large-seeded species (Milberg et al., 2000), which is interpreted as a way to prevent small seeds with limited resources from germinating far from the soil surface (Fenner and Thompson, 2005). The highest germination percentages at low and moderate temperatures under control conditions (figure 2) suggest that most K. acerbiana seeds might germinate if moisture availability is sufficient for seed germination and subsequent seedling establishment. Under the light regime, application of GA enhanced seed germination slightly at all temperatures where the highest germination was recorded for seeds treated with 2 mM GA (figure 2). However, GA plays a significant role when seeds were incubated at dark conditions, where seed germination was either nil or comparably low. The lack of germination in darkness in the presence of GA likely indicates that light might be needed for GA activation and this might be the case of K. acerbiana, on the basis of our results. It has been reported by Baskin and Baskin (2014) that many desert annuals can germinate indifferently under light or darkness, however, other species require necessarily light to germinate. The Royal Botanic Gardens Kew (2018), using an agar substrate, in combination with GA3, recorded up to 95% germination for seeds of
Kickxia spuria. Among others, the positive role of exogenously applied GA3 in increasing seed germination of some of the UAE’s desert annuals (El-Keblawy and Gairola, 2016) and Arabian desert shrub Helianthemum lippii (Zaman et al., 2009) has been reported. Seeds of K. acerbiana naturally mature in May-June, when rain events are very rare and the average temperature is 25-27°C (minimum) to 39-41°C (maximum). As reported for many arid and semiarid desert annuals, the dispersed seeds of K. acerbiana are likely to enter the soil seed bank during harsh dry summer period until conditions for germination become more favourable. The period between November and March, when temperature and moisture conditions are favourable in natural habitat, is most likely to trigger seed germination and result in successful seedling establishment of this species. Moreover, fast germination of non-dormant seeds of K. acerbiana can be considered an important trait that allows seeds to germinate during short periods of moisture availability. It can be inferred that the fresh mature seeds are non-dormant (germinate well under optimum conditions) but enter conditional dormancy when exposed to sub-optimal conditions.
57 SANJAY GAIROLA, HATEM A. SHABANA, TAMER MAHMOUD AND ANDREA SANTO
Application of GA overcomes the conditional dormancy to some extent. However, the obtained responses for temperatures warrant further studies to investigate sensitivity of seeds to tolerate abiotic stresses, which could be relevant in terms of future possible climate change. Collectively, results of the present study provide the basis for further seed testing of this rare annual of the Arabian deserts.
Acknowledgements
The authors thank Dr. Amr Abdul-Hamid, Director General of the Sharjah Research Academy for encouragement and support.
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