Salinity and Variety Effects on Yield, Yield Attributes and Soil of Tomato under Greenhouse Conditions of Central Fiji

Soil salinity is one of the most significant limitations on plant growth, development and production all over the world and the harm produced by excessive salinity manifests itself as either a loss of plant productivity or plant mortality (Farooq, 2019). The buildup of various soluble salts in the root zone causes soil salinization (Farooq et al., 2021).
       
In 2021 tomatoes were planted on 163 ha with a production of 1633.05 tons. However, Fiji consumed about 3000 tons of tomatoes in the year 2020 (FAOSTAT, 2021).  The crop yield is low in comparison to that of some developed nations (Chand et al., 2022). Due to insufficient local production, Fiji imports 300 tons of tomatoes worth $2.5 million annually.
       
Tomato is a salinity-sensitive crop. It is noticed that soil salinity is increasing in new regions in Fiji. Increasing salinity is caused by a) intrusion of seawater due to river draining during winter, b) cyclones in the coastal region and c) influx of salt from the ground to the surface during the dry season via capillary movement (Rahman et al., 2018). As a consequence of sea level rise, climate change exacerbates coastal flooding, salinization and erosion (Morgan and Lese, 2011; Stocker, 2014; Sen, 2017). Due to the Large number of Oceanians along the coasts, saline intrusion threatens Pacific cash crops. Seawater intrusion is detrimental to fertile soils because it increases the saline content of the soil, preventing optimal growth and crop yields (Freeman, 2010). In Fiji, not enough research has been conducted on the effect of salinity on growth and yield characteristics of different crops including tomatoes. Therefore, the effects of various salt concentrations on the development and yield of three tomato varieties, namely Melrose, Alton and Alafua Large, were studied and an attempt was made to identify the tomato variety with the highest salt tolerance.

Experiment details
 
In a greenhouse at the Instructional Agriculture Farm Complex, College of Agriculture, Fisheries and Forestry, Koronivia Campus, Fiji National University, Fiji, a tomato field experiment was conducted from January to December 2022. The geographic coordinates of the studied area are 18°2' 30"-18°3' 36" South, 178°31' 17"-178°33' 10" East and the elevation ranges from 6 to 23 meters above mean sea level. The climate is tropical and the temperature was moderate (21°C-26°C) (Sachan et al., 2020). The experiment included two factors: three tomato varieties (V₁= Melrose; V₂= Alton; V₃= Alafua Large) and four salinity levels (T₁= Control with no application of saline water; T₂= 2 dSm^-1; T₃= 4 dSm^-1 and T₄= 8 dSm^-1) which were arranged in a randomized complete block design (RCBD) comprising three replications. The entire experiment was divided into three replications and each replication comprised of twelve plastic containers. Each of the 36 plastic containers in the experiment contained a single plant. Data on fruit length, fruit breadth, individual fruit weight, fruit numbers and fruit yield were collected. Pre- and post-harvest soil samples were collected and analyzed at the Fiji Agricultural Chemistry Laboratory, Koronivia Research Station, Fiji. Various soil attributes such as pH, electrical conductivity (EC), organic carbon (OC) and primary and secondary plant nutrients (total nitrogen, available phosphorus, available potassium, sodium, calcium and magnesium) were measured using standard analytical techniques (Sachan and Krishna, 2018).
 
Preparation and application of salt solution
 
NaCl concentrations of 0 dSm^-1, 2 dSm^-1, 4 dSm^-1 and 8 dSm^-1 were used as treatment levels. Sodium chloride (MERCK chemicals, AR grade) was measured in 0, 1.28, 2.56 and 5.12 g/L of water using an electric balance.  A one-liter-capacity watering cane was used to irrigate the soil after the measured salt had been correctly dissolved in water to irrigate each pot. In the experiment, irrigation with saltwater was permitted every seven days. In addition, freshwater was irrigated during the initial stage at a one-day interval.
 
Crop management
 
Selected tomato varieties seeds which are commonly grown around different parts of Fiji namely Alton, Alafua Large and Melrose were obtained from the Sigatoka Research Station, Ministry of Agriculture, Fiji. Tomato Seedlings were transferred to the pots once they were 2 weeks old, reaching 2 and 3 leaf stages. Fertilizers and manures were mixed as required in each pot. 1.0 g NPK (13:13:21), 0.5 g Urea and 270 g poultry manure (NPK at 200 kg/ha, Urea at 100 kg/ha and poultry manure at 12 t/ha) were applied basally to each pot (MoA, 2014). Urea was applied as the top dressing at the branching and flowering stages. 28-day-old healthy seedlings were uprooted and one seedling was transplanted into each pot. After planting, the seedlings were immediately watered. Plant protection measures were taken to prevent infestation of insects, weeds and diseases.
 
Data recording and data analysis
 
Data on yield and yield attributes of tomatoes i.e., fruit length (cm), fruit breadth (cm), individual fruit weight (g), number of fruits plant^-1 and fruit yield plant^-1 (g) were recorded. Soil pH, organic matter (%), total nitrogen (%), available phosphorus (mg/kg), potassium (meq/100 g), calcium (meq/100 g) and sodium (meq/100 g) content in soil was also recorded after post-harvest soil sample analysis. The collected data were statistically analyzed to ascertain the statistical significance of the experimental results. After determining the means for each treatment, the F test was used to conduct analyses of variance for each character. Using the LSD test with a probability level of 5%.

Yield attributes
 
All the tomato yield attributes varied significantly with increased salinity except fruit length and width.
 
Fruit length and breadth
 
A significant effect of variety on the fruit length was recorded. A maximum of 4.22 cm of fruit length was recorded in Alafua Large followed by Melrose, however, the minimum 3.31 cm fruit length was recorded in Alton (Table 2). The minimum fruit length in Alton may be due to their genetic characteristics. The effect of salinity on the fruit length was non-significant. A maximum of 4.25 cm fruit length was observed from the application of the concentration of saltwater at EC 4 dSm^-1 however, a minimum of 3.55 cm was recorded from the application of the concentration of saltwater at EC 8 dSm^-1.  Salinity stress reduced marketable yield by reducing fruit size (Zhang et al., 2022). Ahmed et al., (2017) also supported the results and reported that plants irrigated with fresh water gave the tallest fruit followed by treatment (Freshwater) whereas the shortest fruit followed by treatment (10 dSm^-1). The combined effect of variety and salinity on the fruit length was statistically significant. A maximum of 4.41 cm fruit length was observed from the combination of Alafua Large and the application of the concentration of saltwater at EC 4 dSm^-1 and a minimum of 2.49 cm was observed from the combination of Alton and the application of the concentration of saltwater at EC 8 dSm^-1 (Table 2).
       
A significant effect of variety on the fruit breadth of tomatoes at harvest was recorded. The maximum 4.50 cm fruit breadth was recorded in Alafua Large whereas the minimum 2.89 cm was produced in Alton. Whereas the salinity effect on the fruit breadth of tomatoes at harvest was recorded as non-significant (Table 2). The minimum 3.39 cm fruit breadth was recorded from the application of the concentration of saltwater at EC 8 dSm^-1 whereas the maximum 4.12 cm fruit breadth was recorded from the application of the concentration of saltwater at EC 4 dSm^-1. The maximum 4.84 cm fruit breadth was recorded in combination with Alafua Large and the application of the concentration of saltwater at EC 4 dSm^-1 while the minimum 2.17 cm fruit breadth was recorded from the combination of Alton and the application of the concentration of saltwater at EC 8 dSm^-1 (Table 2).
 
Individual fruit weight
 
The effect of variety, salinity and their combinations was significant on individual fruit weight. The minimum 21.27 individual fruit weight was recorded in Alton whereas the maximum 45.60 g individual fruit weight was recorded in the Alafua Large (Table 2). The highest 40.24 g fruit weight was recorded from the application of the concentration of saltwater at EC 4 dSm^-1 and the lowest 30.14 g from the application of the concentration of saltwater at EC 8 dSm^-1. This might have been due to higher levels of salinity providing the plants with insufficient foods with decreased cell division, which contributed to the minimum fruit weight Ahmed et al., (2017). The maximum 56.21 g individual fruit weight was recorded from the Alafua Large and application of the concentration of saltwater at EC 4 dSm^-1and the lowest 14.29 g was observed from the combination of Alton and the application of concentration of saltwater at EC 8 dSm^-1 (Table 2).
 
Number of fruits
 
A significant variation between variety, salinity and their interaction in terms of the number of fruits per plant was recorded. The highest 7.83 fruits were produced by Alafua Large, whereas the lowest 5.41 fruits were by Alton (Table 2). The highest number of 9.11 fruits was obtained from the no saltwater application (Control), whereas the lowest 4.22 fruits were recorded from the application of concentration of saltwater at EC 8 dSm^-1. A maximum of 10.33 fruits per plant was recorded from Alafua Large and the application of saltwater concentration at EC 8 dSm^-1) while the minimum of 3.00 fruits from the combination of Alafua Large and application of saltwater concentration at EC 2 dSm^-1 (Table 2). A reduction in fruit number of 2.0% was reported with an increase of 1 dSm^-1 beyond the threshold value of 4.4 dSm^-1 (Magan et al., 2008). Ahmed et al., (2017) also supported this finding and reported the highest number of mature fruits per plant was produced by the T₁ treatment (Irrigation with fresh water) and then decreased gradually.
 
Fruit yield
 
Fruit yield differed significantly between variety and salinity. The highest 352.10 g fruit yield per plant was recorded in Alafua Large and the lowest 140.41 g was recorded in Alton (Table 2). The fruit yield decreased as the salinity level increased. The highest 323.52 g fruit yield was recorded from the no saltwater application (Control) followed by the application of the concentration of saltwater at EC 2 dSm^-1 and the lowest 145.59 g was recorded from the application of the concentration of saltwater at EC 8 dSm^-1 (Table 1). A reduction in fruit yield was noticed with the increasing salinity. This result agreed with the findings of Mou (2021); El-mogy et al., (2018), Islam et al., (2011) and Mazumder (2016 a and b). Zhang et al., (2022) discovered that the overall yield of tomatoes was remarkably decreased at the salinity of 5 dSm^-1 and above. The highest fruit yield of 432.17 g was observed from Alafua Large and no saltwater application (Control) and the lowest 64.69 g was recorded from Alton and the application of the concentration of saltwater at EC 8 dSm^-1 (Table 2). Salinity may cause a significant decrease in photosynthesis and an increase in transpiration rate, resulting in a deficiency of assimilation to the developing organs and consequently delaying or halting growth (El-Hendaway et al., 2005). In addition, the application of NaCl solution to planting soil could have detrimental effects on the soil’s physicochemical properties (Fontes and Ronchi, 2002). Salinity negatively affects the yield of crop plants by reducing the availability of soil moisture and due to the toxicity effects of sodium and chloride ions at high concentrations in the plant (Mallick et al., 2020).
 

 

 
Soil properties
 
The addition of NaCl solution to planting soil could have detrimental effects on the soil’s physicochemical properties (Fontes and Ronchi, 2002). Therefore, the post-harvest plant soil’s physicochemical properties, including pH, OM, Na, K, Ca, P and total N, were investigated. The results of potting soil after NaCl salinity (2, 4 and 8 dSm^-1) irrigation are shown in Table 3. The maximum 7.15 soil pH was recorded from the application of the concentration of saltwater at EC 8 dSm^-1, while the minimum 6.84 soil pH was recorded from no saltwater application (Control). The soil pH increased with the application of saline water to the pot plants. The initial pH of the potting soil was 6.2 and after increasing salinity, the maximum pH reached a value of 7.15 (8 dSm^-1). For tomato cultivation, the optimum soil pH range lies between 6.00 to 6.8, so the application of moderate levels of salinity maintains soil pH within the tolerable range. Similar results were reported by Rahman et al., (2018). The organic matter decreased with the addition of NaCl-solution to the soil, from 1.96% in the control to 1.82% in T₄ (8 dSm^-1). A maximum of 0.43% total nitrogen was recorded from the no saltwater application (Control), while a minimum of 0.39% was recorded from the application of the concentration of saltwater at EC 2 dSm^-1. A decrease in total nitrogen was recorded due to the application of NaCl to irrigation water (Fontes and Ronchi, 2002 and Rahman et al., 2018). The maximum 192.78 mg/kg available phosphorus was recorded from no saltwater application (Control), while the minimum 160.33 mg/kg was recorded from the application of the concentration of saltwater at EC 2 dSm^-1. The addition of NaCl reduced the amount of available phosphorus in the soil. A maximum of 2.06 meq/100 g potassium was recorded from the application of the concentration of saltwater at EC 2 dS^-1, while a minimum of 1.75 meq/100 g was recorded from the application of the concentration of saltwater at EC 4 dSm^-1. The addition of NaCl reduced the amount of potassium in the soil. A maximum of 33.65 meq/100 g of Calcium was recorded from the application of the concentration of saltwater at EC 2 dSm^-1, while a minimum of 30.38 meq/100 g was recorded from the application of the concentration of saltwater at EC 4 dSm^-1. The addition of NaCl solution reduced the concentration of Ca. The maximum of 2.47 meq/100 g sodium was recorded from the application of the concentration of saltwater at EC 8 dSm^-1, while the minimum of 0.65 meq/100 g was recorded from the control with no saltwater application (Control). The Na concentration eventually increased from 1.11 meq/100 g to 2.47 meq/100 g (8 dSm^-1) in treated soil. In response to the application of NaCl solution to the pot,  almost every studied parameter  (OM, Total N, K, Ca and Available P) decreased with salinity, except pH and Na (Table 3). The decrease in these could be the cause of the decrease in tomato plant growth and development. Thus, it is observed that as soil salinity increases, growth-influencing parameters change and reduce yields.
 

The tomato growth and yield varied with the varieties and salinity levels. Alafua Large performed better than all other varieties and recorded higher yields at higher salinity levels. In combined effect of variety and the salt concentration Alafua Large showed a better yield performance than other varieties when combined with the lower salinity levels. In response to the application of NaCl solution to the pot, studied parameters (pH, Total nitrogen and Na) increased with salinity, whereas OM, P, K and Ca recorded reduction. Hence, from the results, Alafua Large is comparatively more salt tolerant than the other varieties. However, further studies can be conducted in field or pot conditions with more tomato varieties using more salt concentrations to identify any suitable varieties to cultivate under the saline-prone areas of Fiji.

The authors sincerely acknowledge the College of Agriculture, Fisheries and Forestry, Fiji National University, Fiji for providing facilities and assistance in conducting this research.