Interactive effects of plant spacing and nutritional management on physicochemical quality attributes of water chestnut (Trapa natans var. bispinosa roxb.)

Introduction

Water chestnut (Trapa natans var. bispinosa Roxb.) is an edible aquatic angiosperm locally called as "Singhara". This is a significant aquatic warm season crop. This submerged rooted plant is commonly found on the water surface of lakes, tanks, and pools in India, particularly in Punjab, Bihar, Uttar Pradesh, Madhya Pradesh, Tamil Nadu, Maharashtra, and some parts of Uttarakhand (Puste, 2004; Takano and Kadono, 2005) [12,15] . In India, the water chestnut species Trapa bispinosa and Trapa quadrispinosa are frequently farmed. For commercial cultivation soil should be heavy, muddy and rich in organic matter. Organic manures in form of oil cakes, poultry manure and compost/FYM @ 8 t/ha are recommended development of for water growth chestnut and crop (Choudhary et al., 2003). Significant amount of phosphorus and potassium is needed for optimum growth of the crop. In West Bengal, application of 30-40 kg urea/ha during transplantation and another 20 kg/ha is recommended after 25-30 days of transplanting (Jana, 2016a) [7] . Soil rich in Zn and B result in increased number of fruits and fruit weight. For newly cultivated field, the fertilizer dose can be modified from NPK @ 60:20:20 kg/ha to NPK @ 60:40:30 kg/ha and planting distance should be maintained at 1m x 2m to give an optimum plant population of 4500 5000 plants/ha (Jana et al., 2019) [7] . Chemical fertilizers like Urea and Diammonium phosphate are applied at three times the recommended doses. To overcome this the farmers, treat the fruit with a bleaching agent to improve its appearance so that the market acceptability is high. It is hypothesized that varying doses of chemical fertilizers, Nano urea, and Jeevamrut could improve the performance of the crop without affecting its overall yield (Meena et al., 2023) [8] . The floating leaves are arranged in a rosette and are broadly rhomboid, triangular, deltoid or ovate in shape (Hummel and Kiviat, 2004) [4] . The leaves are sharply serrated with distinct venation and short hairs. Submerged leaves are alternate, finely divided and may reach up to 15 cm in length (Bitonti et al., 1996; Mehrhoff et al., 2003) [1,9] . The petioles possess a spongy structure that aids flotation. The plant bears white flowers with four petals and four sepals. Fruits are single-seeded horned nut-like structures that develop underwater and attain a width of approximately 3 cm (Mehrhoff et al., 2003) [9] . The stem is flexible, measuring 1-5 m in length, and remains anchored in the sediment through roots arising from the propagating seed hull Traditionally, water chestnut is consumed after boiling. It is used for the preparation of sweets and chapatti, while sun drying and oven drying are common preservation methods. Being low in calories and rich in nutrients, it has gained importance as a healthy food. The nut is gluten-free, cholesterol-free and rich in potassium, magnesium and dietary fibre. (Hummel and Kiviat, 2004) [4] . Water chestnut is a valued commodity in the culinary business due to its distinct flavour (Parker and Waldron, 1995) [11] .

Materials and Methods

Experimental site

Experiment was conducted during the Kharif seasons of the year 2023 and 2024 at Horticulture Farm No. 1, Department of Horticulture, School of Agricultural Sciences and Technology, Babasaheb Bhimrao Ambedkar University, Lucknow, Uttar Pradesh, India. The experimental farm is geographically located at 26.769559° N latitude and 80.927135° E longitude, with an elevation of approximately 121 m above mean sea level.

Experimental design and treatments

The experiment was conducted in a Factorial Randomized Block Design comprising two experimental factors and three replications. Factor A represented plant spacing, consisting of two levels: S₁ = 1.0 m × 1.0 m and S₂ = 2.0 m × 1.0 m. Factor B comprised thirteen nutritional regimes (T₁-T₁₃) involving conventional fertilizers, reduced fertilizer doses, recommended fertilizer application, nano-fertilizers and organic nutrient sources. The factorial combination of two spacing levels with thirteen nutritional regimes resulted in 26 treatment combinations (2 × 13). Each treatment combination was replicated three times, giving a total of 78 experimental units. The treatments were randomly allocated within each replication to minimize experimental variation and provide unbiased estimates of treatment effects. The factorial arrangement allowed assessment of the individual effects of spacing and nutritional regimes and their interaction (S × T) on the performance of water chestnut.

Nutritional treatments

Thirteen nutritional regimes were evaluated in the experiment. The treatments consisted of a control, farmer's fertilizer practice, reduced fertilizer doses, recommended dose of fertilizer, nanofertilizers, Jeevamrut and nano-fertilizer-enriched organic nutrient sources. Details of fertilizers doses and their application are presented in Table 2.1.

Results and Discussion

The results pertaining to the physicochemical quality attributes of water chestnut revealed that both plant spacing and nutritional management had a significant influence on fruit quality during the two years of experimentation. The quality attributes, namely total soluble solids, fruit pH, reducing sugars, non-reducing sugars and total sugars, exhibited considerable variation in response to different plant spacings and nutritional regimes. The observed variations indicated that the spatial arrangement of plants and nutrient supply played an important role in modulating the physicochemical composition and quality characteristics of water chestnut fruits. The individual and interactive effects of plant spacing and nutritional management on the respective quality attributes are presented and discussed in the following sections.

Total soluble solids

The data presented in Table 3.1 revealed that total soluble solids (TSS) of water chestnut fruits were significantly influenced by plant spacing and nutritional regimes during the year 2023 and 2024. The interaction between plant spacing and nutritional regimes also exerted a significant effect on TSS during both years. The variation in TSS observed among the treatments indicated that both spatial availability and nutrient supply played an important role in determining the soluble solids content of water chestnut fruits. Plant spacing significantly influenced the TSS of water chestnut fruits during both years. During the year 2023, significantly higher TSS was recorded under wider spacing S₂ (2

), with a value of 6.16 °Brix, compared with 5.57 °Brix under closer spacing S₁ (1 m × 1 m). Similarly, during the year 2024, S₂ recorded higher TSS (6.32 °Brix) than S₁ (6.00 °Brix).

The higher TSS under wider spacing may be attributed to better availability of growth resources and reduced competition among plants, which could have favoured assimilate accumulation in developing fruits. The present findings are in agreement with the observations of Dwivedi et al., (2011) [3] , who reported considerable variation in TSS among water chestnut genotypes, ranging from 4.33 to 10.55 °Brix, indicating substantial variation in this important fruit-quality trait in Trapa natans var. bispinosa. Nutritional regimes significantly affected TSS during both years.

During the year 2023, the highest TSS was recorded with T₁₂ (Nano zinc fortified FYM) at 7.61 °Brix, followed by T₁₀ (Nano zinc fortified Jeevamrut) at 7.26 °Brix, whereas the lowest value was recorded under T₁ (Control) at 4.62 °Brix. During the year 2024, the highest TSS was again recorded under T₁₂, with 7.87 °Brix, followed by T₁₀ with 7.11 °Brix, while the lowest TSS was observed under T₁ (5.09 °Brix). The superior performance of T₁₂ may be associated with the combined contribution of FYM and nano zinc. Organic manure can provide nutrients progressively and improve the nutrient-supplying environment, whereas zinc is involved in several physiological and enzymatic processes associated with plant growth and metabolism. Consequently, improved nutrient availability may have enhanced photosynthetic productivity and the subsequent accumulation of soluble constituents in the fruit. The beneficial influence of balanced nutrient management on production and quality of water chestnut has also been reported by Puste et al., (2005) [12] , who demonstrated that integrated organic and inorganic nutrient management significantly influenced production and nutritional quality of aquatic food crops, including water chestnut. The interaction between plant spacing and nutritional regimes was significant during both years. During the year 2023, the maximum TSS was recorded with the combination S₂ × T₁₂ (2 m × 1 m + Nano zinc fortified FYM), registering 8.31 °Brix, followed by S₂ × T₁₀, which recorded 7.72 °Brix. The minimum value was recorded under S₁ × T₁, with 4.30 °Brix. A similar trend was observed during the year 2024. The highest TSS was again recorded under S₂ × T₁₂, with 8.02 °Brix, followed by S₂ × T₁₀ with 7.27 °Brix, whereas the lowest value was observed under S₁ × T₁, with 4.94 °Brix. The consistently superior performance of the S₂ × T₁₂ combination suggests that wider spacing combined with nano zinc fortified FYM provided a favourable environment for nutrient utilization and accumulation of soluble solids in the fruits. The occurrence of considerable variation in TSS among water chestnut genotypes has previously been documented by Dwivedi et al., (2011) [3] , while the direct assessment of °Brix as an important physicochemical quality parameter of Trapa natans fruits has also been reported by Rehman et al., (2024) [13] . Overall, the results indicated that wider spacing (S₂) and T₁₂ (Nano zinc fortified FYM) were most effective in improving TSS of water chestnut fruits. Among the interaction combinations, S₂ × T₁₂ consistently recorded the highest TSS during both years, indicating the potential of this combination for improving fruit quality. [13] specifically investigated pH and °Brix among water chestnut fruit types, confirming their relevance as physicochemical quality attributes. Plant spacing significantly influenced fruit pH during both years. During the year 2023, the wider spacing S₂ (2 m × 1 m) recorded a higher fruit pH of 5.54, compared with 5.43 under S₁ (1 m × 1 m). Similarly, during the year 2024, S₂ recorded a higher mean fruit pH of 5.51, compared with 5.42 under S₁. Nutritional regimes significantly influenced fruit pH during both years. During the year 2023, the highest fruit pH was recorded under T₁₂ (Nano zinc fortified FYM) at 5.86, followed by T₁₀ (Nano zinc fortified Jeevamrut) at 5.80. The lowest fruit pH was observed under T₁ (Control) at 5.14. During the year 2024, the same trend was observed. The highest fruit pH was recorded under T₁₂, with a value of 5.82, followed by T₁₀, with 5.76, whereas the lowest value was recorded under T₁ (5.13).

The relatively higher pH under T₁₂ and T₁₀ may be associated with improved nutrient availability and balanced metabolic activity during fruit development. The contribution of organic and inorganic nutrient sources towards improved production and quality characteristics in water chestnut has been documented by Puste et al., (2005) [12] . The present results are also relevant to the findings of Rehman et al., (2024) [13] , who demonstrated varietal differences in physicochemical properties, including pH and °Brix, of water chestnut fruits. Their study confirms that pH can vary among water chestnut materials and therefore represents a useful physicochemical quality parameter. The interaction between plant spacing and nutritional regimes significantly influenced fruit pH during both years. During the year 2023, the highest fruit pH was recorded under S₂ × T₁₂, with a value of 5.92, followed by S₂ × T₁₀, which recorded 5.88. The lowest value was recorded under S₁ × T₁, with 5.13. During the year 2024, S₂ × T₁₂ again recorded the highest fruit pH (5.90), followed by S₂ × T₁₀ (5.84), whereas the lowest value was observed under S₁ × T₁, with 5.12. Although S₂ generally produced higher fruit pH than S₁, slight reversals were observed in 2024 under T₂, T₃ and T₄. Therefore, the response should not be described as uniformly higher under S₂ for every nutritional treatment. The consistently superior performance of S₂ × T₁₂, however, indicates that the combination of wider spacing with nano zinc fortified FYM was particularly effective in enhancing fruit pH under the conditions of the present investigation. Overall, the results indicated that fruit pH was significantly affected by plant spacing and nutritional regimes. Wider spacing (S₂) and T₁₂ (Nano zinc fortified FYM) generally resulted in higher fruit pH, while the interaction S₂ × T₁₂ produced the maximum value during both years. These findings suggest that appropriate integration of plant spacing and nutrient management can modify important physicochemical quality characteristics of water chestnut fruits.

The data presented in Table 3.3 revealed that reducing sugar content of water chestnut fruits was significantly influenced by plant spacing, nutrient management and their interaction during both years of experimentation. Plant spacing significantly affected reducing sugar content in both years. During 2023, S₂ (2 m × 1 m) recorded higher reducing sugar content (1.73%) than S₁ (1 m × 1 m) (1.60%). Similarly, in 2024, S₂ recorded 1.74% compared with 1.61% under S₁. The higher value under wider spacing may be attributed to reduced plant competition and better availability of growth resources, favouring assimilate production and translocation towards developing fruits. Nutrient management significantly influenced reducing sugar content. In 2023, the highest value was recorded under T₁₂ (Nano zinc fortified FYM) (2.14%), followed by T₁₀ (Nano zinc fortified Jeevamrut) (2.00%), while T₁ (Control) recorded the lowest value (1.36%). A similar trend was observed in 2024, with T₁₂ recording 2.15%, followed by T₁₀ (2.01%), whereas T₁ recorded the minimum (1.37%). The superior response under T₁₂ may be associated with improved nutrient availability and physiological activity resulting from the combined effect of FYM and nano zinc. The present findings are supported by Babu and Dwivedi, (2012) [3] , who reported reducing sugar values ranging from 1.30 to 2.06% among water chestnut cultivars. Singh et al., reported about 1.27% reducing sugar in fresh water chestnut kernel, confirming its importance as a biochemical quality attribute.

The interaction between plant spacing and nutrient management was significant in both years. The highest reducing sugar content was recorded under S₂ × T₁₂, with 2.22% in 2023 and 2.23% in 2024, followed by S₂ × T₁₀ with 2.08 and 2.09%, respectively. The lowest values were recorded under S₁ × T₁, with 1.30% in 2023 and 1.31% in 2024. Wider spacing produced higher reducing sugar content than S₁ across all nutrient treatments, indicating a consistent advantage of S₂ for sugar accumulation. The consistently superior performance of S₂ × T₁₂ may be attributed to the combined effect of wider spacing and improved nutrient availability, which favoured photosynthesis, assimilate translocation and sugar accumulation in developing fruits. The findings are also in accordance with Dwivedi et al., (2011) [3] , who reported considerable variation in biochemical and physicochemical fruit attributes among Trapa natans var. bispinosa genotypes. Overall, wider spacing S₂ and T₁₂ (Nano zinc fortified FYM) resulted in higher reducing sugar content, while S₂ × T₁₂ recorded the maximum values in both years. Thus, their combined application appeared favourable for improving reducing sugar content and biochemical quality of water chestnut fruits.

The data presented in Table 3.4 revealed that non-reducing sugar content in water chestnut fruits was significantly influenced by plant spacing, nutrient management and their interaction during both year 2023 and 2024. Plant spacing significantly affected non-reducing sugar, with S₂ (2.94%) recording higher values than S₁ (2.91%) in 2023, while the corresponding values in 2024 were 2.95 and 2.92%, respectively. The higher value under S₂ may be associated with comparatively better availability of growth resources and reduced plant competition. Nutrient management significantly influenced non-reducing sugar content. Among the treatments, T₁₂ (Nano zinc fortified FYM) recorded the maximum value of 3.70% in 2023 and 3.71% in 2024, followed by T₁₀ (Nano zinc fortified Jeevamrut) with 3.56 and 3.57%, respectively, whereas the minimum was recorded under T₁ (Control), with 2.24 and 2.25%. The higher non-reducing sugar under T₁₂ may be attributed to the combined effect of FYM and nano zinc in improving nutrient availability and carbohydrate metabolism. Similar variation in non-reducing sugar among water chestnut genotypes was reported by Babu and Dwivedi, (2012) [3] , who recorded values up to 2.73%. Singh et al., (2010) [14] also reported substantial non-reducing sugar content in fresh water chestnut kernel. The interaction between plant spacing and nutrient management was significant in both years. The highest nonreducing sugar was recorded with S₂ × T₁₂ (3.74% in 2023 and 3.75% in 2024), followed by S₂ × T₁₀ (3.59 and 3.60%), while the lowest value was observed under S₁ × T₁ (2.24% in both years).

Although S₂ generally recorded higher values, the response varied with nutrient treatment, as equal or slightly lower values under S₂ were observed in T₁, T₄, T₇ and T₉. Overall, wider spacing combined with nano zinc fortified FYM (S₂ × T₁₂) was most favourable for enhancing non-reducing sugar content in water chestnut fruits.

The data presented in Table 3.5 revealed that total sugars content of water chestnut fruits was significantly influenced by plant spacing and nutritional regimes during the year 2023 and 2024. The interaction between plant spacing and nutritional regimes also exerted a significant effect on total sugars content during both years. The variation in total sugars content observed among the treatments indicated that spatial arrangement and nutritional management influenced sugar accumulation in developing water chestnut fruits. Plant spacing significantly influenced the total sugars content of water chestnut fruits during both years. During the year 2023, significantly higher total sugars content was recorded under wider spacing S₂ (2 m × 1 m), with a value of 4.67%, compared with 4.51% under closer spacing S₁ (1 m × 1 m). Similarly, during the year 2024, S₂ recorded higher total sugar content (4.69%) than S₁ (4.52%). The higher total sugars content under wider spacing may be associated with reduced competition among plants for available resources, which could have favoured assimilate production and its subsequent accumulation in developing fruits. Variability in biochemical attributes of water chestnut, including sugar content, has previously been reported among different collections of Trapa natans var. bispinosa by Babu and Dwivedi, (2012) [3] . The considerable variability in biochemical characteristics of water chestnut has also been recognized in subsequent literature. Nutritional regimes significantly affected total sugar content during both years.

During the year 2023, the highest total sugar content was recorded with T₁₂ (Nano zinc fortified FYM) at 5.84%, followed by T₁₀ (Nano zinc fortified Jeevamrut) at 5.55%, whereas the minimum value was recorded under T₁ (Control) at 3.59%. A similar trend was observed during the year 2024, where T₁₂ again recorded the highest total sugar content (5.86%), followed by T₁₀ (5.57%), while the minimum value (3.61%) was observed under T₁ (Control). The comparatively higher total sugar content under T₁₂ may be associated with the improved nutrient-supplying environment resulting from the combination of FYM and nano zinc. Organic manure can contribute to the gradual supply of nutrients and improvement of the growing environment, while zinc is an essential micronutrient involved in several physiological and enzymatic functions of plants. Such improved nutritional conditions may favour photosynthetic assimilate production and carbohydrate accumulation in developing fruits.

The observed values are within the range of total sugar concentrations reported for water chestnut in earlier studies. Singh et al., (2010) [14] reported 5.63 ± 0.04% total sugars in fresh water chestnut kernel, while Babu and Dwivedi, (2012) [3] reported variation in total sugar content among water chestnut collections from the Lucknow region. The interaction between plant spacing and nutritional regimes was significant during both years. During the year 2023, the highest total sugar content was recorded with S₂ × T₁₂ (2 m × 1 m + Nano zinc fortified FYM), registering 5.96%, followed by S₂ × T₁₀, which recorded 5.67%, whereas the minimum value was recorded under S₁ × T₁ (Control), with 3.54%. A similar trend was observed during the year 2024. The highest total sugar content was again recorded under S₂ × T₁₂, with 5.98%, followed by S₂ × T₁₀ with 5.69%, whereas the minimum value was observed under S₁ × T₁, with 3.55%. The consistently higher total sugar content under S₂ × T₁₂ may indicate that wider spacing, when combined with nano zinc fortified FYM, provided a favourable nutritional and spatial environment for assimilate accumulation and sugar formation in developing fruits. The present findings are comparable with previously reported sugar concentrations in water chestnut. Singh et al., (2010) [14] recorded 5.63% total sugars in fresh water chestnut kernel, while the recent International Biological Flora: Trapa natans review summarized reported wet-basis sugar concentrations of approximately 4.80-5.63%, highlighting the variability of this quality attribute in the crop. Overall, the results indicated that wider spacing S₂ (2 m × 1 m) was superior to S₁ for total sugar accumulation, while T₁₂ (Nano zinc fortified FYM) recorded the highest total sugar content among the nutritional regimes. Among the interaction combinations, S₂ × T₁₂ consistently recorded the highest total sugar content during both years, followed by S₂ × T₁₀, whereas S₁ × T₁ recorded the minimum value. Thus, the combination of wider spacing with nano zinc fortified FYM showed considerable potential for improving the biochemical quality of water chestnut fruits.

Conclusion

The results of the present investigation demonstrated that plant spacing and nutritional management played an important role in determining the quality attributes of water chestnut fruits. Wider plant spacing S₂ (2 m × 1 m) consistently resulted in better values of total soluble solids, fruit pH, reducing sugar, non-reducing sugar and total sugars than closer spacing S₁ (1 m × 1 m), indicating a favourable effect of adequate growing space on fruit quality. Among the nutritional regimes, nano zinc fortified FYM (T₁₂) emerged as the most effective treatment, followed by nano zinc fortified Jeevamrut (T₁₀), whereas the control treatment recorded comparatively lower values for these quality parameters.

The significant interaction between spacing and nutritional regimes further indicated that the response of water chestnut quality attributes depended on their combined application. The combination of wider spacing with nano zinc fortified FYM (S₂ × T₁₂) consistently produced superior quality attributes during both years of experimentation. Therefore, under the agro-climatic conditions of Lucknow, the integration of wider plant spacing with nano zinc fortified FYM may be considered a promising nutritional and spatial management strategy for improving the quality of water chestnut fruits.

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