Assessment of Suitable Growth Condition of Salvinia spp. for Sustainability of Floating Agriculture to Mitigate Climate Change Challenges on Crop Cultivation

Floating agriculture is an indigenous cultivation practice in the southern waterlogged areas of Bangladesh. Water hyacinth and other macrophytes were utilized in this agricultural method to prepare the floating bed. Among the numerous macrophytes used as mulch in floating beds, Salvinia spp. is a common species. To meet the local farmer's demand, it is necessary to improve the growth rates of these macrophytes so that they can be available year-round for preparing floating beds. For this purpose, an experiment was conducted to assess the growth conditions of Salvinia spp.


Introduction
Bangladesh is among the countries on Earth where climate change is seen as a severe challenge due to its location.Aside from this situation, practically every year, two-thirds of Bangladesh's land wetland stays underwater for eight months (Islam et al., 2011), greatly endangering the country's agriculture industry.The available research suggests that climate change will severely hinder crop output, putting the nation's food security at risk (Karim et al., 1999;Palop et al., 2010;Warrick and Ahmad 2012).One creative, indigenous knowledge and technique-based, climate-smart agricultural method that can help with these issues is floating agriculture.This method of cultivating vegetables creates new chances for the local environment (Chowdhury 2004).Farmers in Bangladesh's southern floodplains, especially in the districts of Barishal, Gopalganj, and Pirojpur, have been engaged in this agricultural technique for over a century (Asaduzzaman 2004;Islam and Atkins 2007;Haq and Nawaz 2009;Irfanullah 2009;IUCN Bangladesh 2005).
Local farmers in Bangladesh's south prepare floating beds using a variety of macrophytes, including water hyacinth (Eichhornia crassipes), Asian water moss (Salvinia molesta), Kariba weed (Salvinia cucullata), Topapana (Pistia stratiotes), Khudipana (Lemna minor), Kutipana (Azolla pinnata), and Sonapana (Spirodelapolyrhiza). Local farmers frequently use Salvinia spp. as a mulching material for their cultivation procedure out of all these floating macrophytes.Local farmers assert that macrophytes (Salvinia spp.) are placed on the floating bed several times during the growing season.Because of this, a significant number of Salvinia plant samples are required to preserve the quality of the floating bed, including improved crop growth and nutritional status.Salvinia cucullata and S. molesta are two of the many species that live longest on floating beds and give crops more nutrients and support.Farmers mostly use these species in floating beds for crop cultivation.
Salvinia is a visually appealing, rapidly increasing, and free-floating aquatic fern that grows naturally in various settings, particularly in water rich in nutrients.Salvinia must be improved upon to sustain a floating bed and be readily available throughout the year.Farmers add urea to water bodies to promote Salvinia's vegetative growth, although the dosage quantity can occasionally have adverse effects.Additionally, it is still being determined how successful urea is at promoting Salvinia growth.In addition, plants may experience mineral deficiencies and NH 4 + toxicity when there is a significant nitrogen (N) input to water bodies.These factors highly influence the amount of sunshine, salvinia growth, and water quality.Earlier research had conducted several studies to determine Salvinia's ideal growing conditions and doubling period.Jampeetong et al. (2012) examined the growth and morphological responses, nitrogen uptake, and resource allocation in four aquatic macrophytes using three distinct inorganic nitrogen treatments, i.e.NH 4 + , NO 3 -, or/both NH 4 + and NO 3 -.An experiment was carried out in Bangladesh to learn more about these species' nitrogen intake and mineral allocation.This requires measuring the ideal nutrient combination for Salvinia spp.'s rapid growth and understanding how nutrients affect the species' growth rate, morphological responses, and nutrient allocation.
In this background the objective of the present investigation was to estimate the growth rate and doubling time, to determine the morphological responses and vegetative growth patterns under different nutrient conditions and to measure the allocation of nutrients taken up by different parts of Salvinia spp.from the water bodies.

Experimental site
The Department of Botany at the University of Barishal, which is situated at 22° 39′ 46.0′′ N latitude and 90° 21′ 53.5′′ E longitude, is where the current experiment was carried out in a net house.

Plant materials
Salvinia molesta and S. cucullata, two free-floating species of water fern were used at the present investigation which were taken from ponds at Regional Agriculture Research Station (RARS), Rahamatpur, Barishal.

Cultural Practices and Growth Study
The collected plants were washed thrice with distilled water.The homogeneously sized fronds were placed in a hydroponic system and were thought to be inoculum.Half-strength Hoagland solution (HS) was used in the 1.5 L plastic containers for the experiments, and the pH was adjusted to 7. For every treatment, three replications were conducted, using six inoculums in each dose, while the control (0 mM) was kept in a similar state.To make up for the water volume lost in the pots, distilled water was added.After 28 days, the plants were cleaned and harvested to collect morphological data.The equation RGRw = (ln W 2 ln W 1 )/(t 2 -t 1 ) was used to calculate the relative growth rate (d -1 ).W 1 and W 2 stand for the plant's beginning and final dry weight, and t 1 and t 2 stand for the starting and end time.Moretti et al. (1988) provided the following formula for doubling time (in days): t log 2 [log (wtw0−1)]1.

Contents of chlorophyll
A spectrophotometer (Model: T60 UV Spectrophotometer) was used to measure the total chlorophyll content, chlor-a, and chlor-b at 645 and 663 nm (Yoshida et al., 1976).

Statistical analysis
Analysis of variance (ANOVA) and significance of the difference among the means was evaluated according to the least significant difference test (LSD) at the 5% level of probability (Gomez and Gomez 1984) using STATISTIX 10.

Number of leaves and leaf area
Leaf number and total leaf area are the primary factors that affect a plant's photosynthesis and, subsequently, it′s biomass production (Reddy and Matcha, 2010).Our findings align with this conclusion as we also observed a direct correlation between plant leaf number and total leaf area with biomass production.They affected the total number of leaves and leaf area (Table 1).In the case of both Salvinia spp., the most significant number of leaves and leaf area were seen at 1 mM of ammonium, followed by 1 mM and 5 mM of phosphate.

Root number and root length
Different combinations and concentrations of nutrients highly affected the root number/plant and length of the root (Table 2).The highest number of roots were seen at 1 mM ammonium and 5 mM phosphate, respectively, suggesting fresh shoot development occurs more quickly under these nutritional conditions.However, the longest roots of S. molesta were found at 1mM of phosphate treatment, and root length was reduced with increasing concentration above 5mM for all nutrient-fed plants.

Total biomass
The findings showed that the nutrients and their concentrations significantly impacted the fresh and dry biomass of Salvinia spp.(Fig. 1).Plants grown with ammonium and phosphate produced the highest fresh and dry weight.For all nutrients, Salvinia spp.Exhibited the highest fresh and dry weight when exposed to 1 mM to 5 mM concentrations.In S. molesta, urea-fed plants showed the maximum biomass at 1 mM, then decreased and not survive above 5 mM, whereas S. cucullata could not survive above 1 mM.On the other hand, Nitrate-fed plants showed lower biomass than ammonium and phosphate.In a study of Salvinia species, the highest dry matter yields were obtained from plants that received NH4-N l-1 to compare with the same level of NO3-N or urea-N (Cary and Weerts 1983a).

Relative growth rate and doubling time
The nutrients and their concentrations impacted the growth of Salvinia spp.(Fig. 2).The plants' relative growth rates (RGRs) regarding nutrients were higher in phosphate and ammonium than in nitrate and urea.Similar results were found in S. molesta (Cary and Weerts 1984) and Glyceria maxima, where plants grown in NH 4 + had a higher growth rate than NO 3 - (Tylova-Munzarova et al., 2005).The maximum relative growth rate (0.085 mg g -1 day -1 ) was observed in S. molesta, which grew at 5 mM nutrient concentration of phosphate.
In S. cucullata, the highest (0.067 mg g -1 day -1 ) RGRs were observed at 1 mM of NH 4 + , but at higher concentrations of NH 4 + , the RGRs of the plants were significantly lower.Ammonium is more favorable over nitrate as an N-source for many species of aquatic macrophytes, possibly because of the lower energy needed for its uptake and assimilation (Miller and Cramer 2005;Fang et al., 2007;Jampeetong and Brix 2009a).The minimum doubling time (DT) of the frond number was recorded (4.17 days) in 1 mM of NH 4 + , and the maximum DT of the frond number was recorded (6.84 days) in 1 mM of urea for S. molesta.In the case of S. cucullata, the minimum DT of the frond number was recorded (5.41 days) in 5 mM of PO 4 3-and the maximum DT of the frond number was recorded (22.10 days) in 10 mM of NO 3 -(Fig.3).It was apparent that the sources of NH 4 + and PO 4 3-were superior in resulting in higher numbers of leaves, slightly higher relative growth rates, and shorter doubling times than plants fed with the same level of NO 3 -N or urea-N.A similar pattern of results was also observed by Cary and Weerts (1983a), Finlayson (1984), and Mitchell and Tur (1975) for Salvinia.

The accumulated ion content in plant tissues
The accumulated ion contents (NO 3 -, Na + , K + , PO 4 3 , NH 4 +) in both leaves and roots differed between S. molesta and S. cucullata.NO 3 -and NH 4 + content in leaves and roots were significantly affected in S. molesta.On the other hand, K + , PO 4 3-, and NH 4 + contents were affected considerably in the leaves and roots of S. cucullata.However, Na + and K + contents were reversely involved in the leaves and roots of both plants (Table 3, Fig. 4).Accumulation of ion contents in tissues varied with species and types of tissues, but differences between treatments were negligible.Konnerup and Brix (2010) observed that the NO 3 uptake by Cannabis indica L. was not affected by the type of nitrogen sources.In this study, NH 4 + content in plant tissue was higher in ammonium-fed plants than in other nutrient-fed plants because of the availability and less energy to uptake.Different studies have indicated that NH 4 + fed aquatic macrophytes grow well because less energy is needed for NH 4 + uptake than other nitrogen sources (Room and Thomas 1986;Fang et al., 2007;Jampeetong and Brix 2009a;Konnerup and Brix 2010).

Chlorophyll contents
The concentrations of nutrients impacted the amounts of chlorophyll (Fig. 5).At 1 mM and 5 mM concentrations of NH 4 + , respectively, the maximum amount of chlorophyll was found in S. molesta and S. cucullata.Still, the chlorophyll contents declined as the NH 4 + concentration in the media increased.According to Jampeetong et al. (2012a), Salvinia spp.'s chlorophyll contents decreased as NH 4 + supply increased, mainly when NH 4 + levels were above 10 mM.This reduction in chlorophyll content may impact the plants' rate of photosynthetic activity.Plants fed ammonium had higher levels of chlorophyll than those provided phosphate, nitrate, or urea.

Conclusions
The results indicated that ammonium and phosphate-fed plants had better RGR, leaf number, total biomass, leaf area per plant, chlorophyll content, and less doubling time than nitrate and urea.In terms of concentration, nutrient concentrations between 1 mM and 5 mM produced better results.The results of the study suggest that plants responded reasonably well to individual doses of 1 mM and 5 mM of phosphate and ammonium.Consequently, it could be helpful to carry out more research on the combination application of these nutrients to see if it can result in even greater biomass production and plant development.

Figure 1 .
Figure 1.Combine effect of different concentrations of nutrients on (a) total fresh biomass production, (b) total dry biomass production (mean ± SD) of Salvinia spp.

Figure 3 .
Figure 3.The doubling times (DT in days) of S. molestaand S. cucullataplants in different concentrations of nutrients.

Figure 5 .
Figure 5. Amount of chlorophyll (mean ± SD) in the fresh leaf tissue of (a) S. molesta and (b) S. cucullataunder different concentration of nutrients.

Table 1 .
Combine effect of different concentrations of nutrients on leaves number and leaf areaof Salvinia spp.
* (P ≤ 0.05), ** (P ≤ 0.01), NS (non-significant).Means (± SD) were calculated from three replicates for each treatment.Values with different letters are significantly different at P ≤ 0.05 applying the LSD test.Blank entries indicate plants were not survived)

Table 2 .
Combine effect of different concentrations of nutrients on root number and root length of Salvinia spp.
*(P ≤ 0.05) and ** (P ≤ 0.01).Means (± SD) were calculated from three replicates for each treatment.Values with different letters are significantly different at P ≤ 0.05 applying the LSD test.Blank entries indicate plants were not survived)

Table 3 .
Results of two-way ANOVA (F-value) of Chlorophyll contents and accumulated ion contents in S. molesta and S. cucullata grown in different concentrations of nutrient.