Plant in focus
The Allium Cepa is a vegetable, commonly known as the Onion. It is the most widely cultivated species of the genus Allium as it is valued for its flavours, antioxidant and therapeutic properties (Joshi & Sawant 2012). It belongs to the Amaryllidaceae family, consisting of mainly perennial and bulbous flowering plants (Taxonomy and etymology of Onions 2019). Onions are classified as being salt sensitive and electrically conductive (Joshi & Sawant 2012).
Why is Salt content in the soil important?
Salinity is one of the most serious abiotic sources of physiological stress in plants (Joshi & Sawant 2012). It can largely affect the growth of a plant (Shannon & Grieve 1998). When the salinity of a soil increases, it changes the soil's osmotic pressure, thereby forcing water to move from a lower to a higher salt concentration area. The osmotic effects of salinity contribute to reduced growth rate, changes in leaf colour, and developmental characteristics such as root/shoot ratio and maturity rate (Shannon & Grieve 1998). Specifically, in case of germination, seeds tend to lose moisture to soils of high salinity, thereby diminishing germination rates (Eoghan McCloskey n.d.).
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Data
From the Journal article on the Response of Onion Seed Germination to Salt levels (Joshi & Sawant 2012), data was collected on 6 varieties of Indian Onions and the salt tolerance was studied. The study showed that high salinity levels affect the plant growth at every stage thereby causing morphological, biochemical and molecular changes to adapt to the osmotic and oxidative stress. All the varieties were sterilized, soaked in distilled water and placed in culture dishes with solutions of four NaCl concentrations (0.25%, 0.5%, 0.75%, and 1.0%). Notes were made on the number of days taken for the seed to start germinate, changes in the length of the root and shoot. Seeds were considered to have germinated when the emerging radical elongated to 1mm (Joshi & Sawant 2012). Since the experiment was randomized, Biochemical methods such as Protein Extraction were used to explain factors subjected to variance.
Key findings and application
A delay in seed germination (4–8 days) and a decrease in germination percentage occurred with increasing salt concentration compared to controls. The maximum and minimum decline in germination percentage, compared to controls, was in variety 6 (82.0%) and variety 2 (36.81%), respectively, at a 1.0% NaCl level. This suggested that variety 2 was the most salt tolerant and variety 6 was the most salt sensitive (Table 2). These data identify a salt-tolerant variety that could be used for improvement of onion. Inhibition of germination due to salinity is attributed to osmotic stress, resulting in decreased water absorption by seeds (Joshi & Sawant 2012).
Growth reduction is generally observed in plants exposed to salinity stress. Declines in seedling shoot and root lengths with increasing NaCl level occurred in all varieties. This is likely due to NaCl limiting water absorption by seedlings. The decrease in shoot length was most significant in varieties 3, 4, and 6 at a 1.0% NaCl level (Table 3). The decline in root length was more than 50% at 1.0% NaCl level in all varieties (Table 3). The reduction in growth may be due to NaCl toxicity and unbalanced nutrient uptake by seedlings
Soluble protein content in germinating seeds is an important indicator of physiological status. An increase in total protein content of seeds in response to NaCl was recorded in all varieties (Table 3). Increases and declines in total protein content in response to salinity have been reported in several crops However, no association could be established between salt tolerance and protein content because variety 2, which appeared to be tolerant on the basis of seed germination and growth, did not show a rise in protein content due to NaCl level.
Accumulation of sugars and proline has been associated with salinity tolerance because they protect the cell by balancing the osmotic strength of cytosol with that of the vacuole and the external environment
Data sets for all varieties pooled individually for every treatment (Table 4) explained that seed germination and seedling growth were adversely affected, whereas total protein, carbohydrate, and proline content and catalase activity were enhanced under NaCl stress in comparison to control. Data sets for control and all NaCl levels pooled individually for each variety (Table 2) explained the varietal differences for various traits studied
Onion varieties could be characterized for salt tolerance at the level of germination and early seedling growth. When subjected to similar NaCl levels, variety 2 exhibited a minimum decline in seed germination percentage and shoot and root length, suggesting that this variety is the most salt tolerant, followed by varieties 5, 1, 3, 4, and 6. Only proline content could be associated with salt tolerance in variety 2. Percentage germination, seedling growth, and proline content may serve as potential indicators of salt tolerance in onion at the establishment stage. This study helped in identification of salt-tolerant varieties at the establishment stage. This information may be useful for further research and breeding of new onion varieties for salt tolerance. The results obtained needs to be examined under field conditions.
According to another study on the Salt Effects on Germination (Miyamoto 1989), five Onion cultivars grown in southwestern USA with the help of furrow irrigation were studied. These crops were subjected to leeching and salt accumulation on the top soil. Experiments were conducted. Soil water contents at saturation, 0.01 and 0.1 MPa, were 0.25, 0.14, and 0.02 kg kg-I, respectively. Four different experiments, all arranged in a split plot de- sign with five replicates, were conducted concurrently in a greenhouse at diurnal temperatures of 15 to 25 'C.
The first experiment was designed to measure seed ger- mination when incubated in saline solutions 1 to 5 of Table 1. Rectangular sponges (127 X 254 X 13 mm) were soaked in the solutions and drained to retain approximately 0.44 ml of the solution per cm3 of the sponges. Fifty seeds were sandwiched between two sponges, according to the proce- dure developed by Taylor and Lankford (1969). These sponges were placed in plastic bags, set in a shaded portion of the greenhouse, and the germinated seeds were counted every 2 to 3 d for 2 wk. Seeds were considered germinated when the length of the radicle exceeded that of the seed. Ungerminated seeds were then transferred to the sponge soaked in distilled water and their germination monitored for 1 wk.
When incubated in a 0.8 dS m-l solution all tested cultivars began germinating in 2 to 3 d, and achieved over 90% germination in 5 d. Ringmaster had the high- est germination when measured on Day 3. Seed ger- mination slowed with increasing salinity. At 20 dS m-I, for example, 7 to 9 d had elapsed to achieve 80% germination using the data from New Mexico BR-1
The ANOVA revealed a significant, but not great, difference in germination amoing the cultivars 5 d after seeding. Cultivar effects on germination be- came obscured after about l wk and diminished at the final count made 2 wk after seeding. A signif- icant reduction in germination, averaged over the cul- tivars, occurred at 13 dS m-' on d 5 and 27 dS m-* at the final count. The seeds incubated in a 38 dS m-l solution did not germinate. However, upon the transfer to distillled water, these seeds germinated over 80% within 24 h and an additional 5 to 10% during the next 24 h in[ all the tested cultivars.
Soluble salts in subirrigated pots (Exp. 2) accumu- lated mostly in the top 5 mm of the soil
Onion seed is ordinarily planted 7 to 15 mm deep in pre-irrigated beds. Soil salinity of reshaped beds at these depths are probably close to those of irrigation waters for about a week after irrigation. Sal- inity of irrigation waters used for onion usually does not exceed 2 to 3 dS m-I, which is well below the threshold salinity for seed germination. Soil water depletion near the seed can increase salinity of the soil solution substantially if irrigation is unduly delayed. When this occurs, the seed is likely to ger- minate following the next irrigation, as evidenced by the rapid germination following incubation in highly saline waters. It is, therefore, unlikely that ger- mination becomes a limiting process for establishment unless soil is saline or seed is sown less than about 5 mm deep.
Upon emergence seedlings are subject to at least three different forms of salt damage. Cultivar selection may prove effective in improving establishment if the selection is made on the basis of cotyledon salt tolerance and of seedling survival rates, but not on the ability of seed to germinate in highly saline solutions. Seed priming with osmotic solutions and fluid-drilling of pregerminated seed may also be effective as they can shorten the time required for emergence. In terms of cultural practices seeding into pre-irrigated and reshaped beds is preferred to seeding into dry beds with no prior irrigation. Pre-irrigation and reshaping of the beds help remove the salts accumulated prior to seeding. The use of shallow seeding, but not less than 5 mm deep, and the use of evaporation deterrents also deserve evaluation as a means of improving establishment in saline areas.
Summary/conclusions
It is important to study the salt tolerance of different plant varieties to improve production and yield of the crop. Determination of germination potential of seeds is important to gain information about the degree of success in early establishment of plants under saline conditions.
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