Tomato is highly consumed in Ghana. Ghana national tomato transporters and traders’ association [1
] reported that the country is the highest consumer of the product in Africa. It was further indicated that 90% of tomato produced in Burkina Faso (a neighbouring country) is consumed in Ghana. However, the production volume in Ghana remains low with an annual yield of 8.6 t ha−1
]. Producing enough to meet both local and international demand sustainably is not feasible without the state-of-the-art technology.
In 2014, some greenhouse facilities (tropically customized) were introduced into the country to boost tomato production sustainably. In spite of this, tomato production output has not yet improved significantly in comparison to Japan and the Netherlands. This is an indication that the appropriate cultivation systems at cost-effective levels have not so far been adopted.
Cultivation of tomato under hydroponics conditions for increased yield and quality has been reported. Work on the low-node order pinching at high plant density (LN&HD) cultivation system has been reported to be effective in enhancing productivity and quality of tomato [3
]. This system has been defined [4
] as, a short cultivation period of 70–120 days, where harvesting is made from 2–4 trusses at high plant-density. This cultivation system has been recommended due to its efficiency in resource utilisation. In contrast to soil cultivation, which has been affected by lots of challenges, this system is efficient in mitigating against such challenges. Efficient use of limited resources to achieve optimum output remains the desire of every rational producer (farmer). The use of water, nutrients, space, and labour is highly efficient with this system [5
]. A study was conducted earlier [6
] on three tropical tomato cultivars for yield under hydroponic conditions using a sub-irrigated pot cultivation. The study adopted the LN&HD system in recirculating nutrient film techniques (NFT). A yield of 84.9 t ha−1
per year was recorded in Jaguar with just about 50% yield loss to blossom end rot (BER).
Apart from the issue of BER, recirculating sub-irrigation NFT for low substrate culture results in reduced fruit size; hence, yields are consequently compromised. Because of this, it was reported [7
] that hydroponics by drip irrigation enhances good yield, higher fertilizer use efficiency, decrease production cost and reduce risk of environmental pollution. The drip method in recirculating hydroponics could also offer a better insight for determining water and nutrient uptake characteristics for a given cultivar under prevailing climatic conditions. The adequate amount with accurate timing of irrigation could influence the yield and quality of tomato [8
The LN&HD using recirculating drip hydroponics could be used to improve the yield of tomato. The system will also enhance water and nutrient use efficiencies, as reported [9
]. Plant roots that are involved in water and nutrient uptake are usually restricted in this system due to low substrate volume. These restricted roots are known to produce finer young roots [10
], which are efficient in uptake of water and nitrogen [11
]. A study on the attributes of water and nutrient uptake in tomato plant is of great importance. This is because the cost of fertilizers is high while water becomes somewhat scarce particularly in the dry seasons. This system will help in the proper use and management of resources at economic levels.
However, studies on most tropical tomato cultivars for growth, yield, water, and nutrient use efficiencies with the LN&HD are not available. In addition, this hydroponic cultivation system has never been reported in Ghana so far as tomato production is concerned. This system was ultimately intended to be established for tomato cultivation in Ghana. The current study sought to evaluate the growth, yield, water, and nutrient uptake attributes of tropical tomato under the LN&HD system using drip recirculating hydroponics in the greenhouse. The system was expected to become the most efficient and economical way of boosting tomato production in the tropics like Ghana.
2. Materials and Methods
The study was conducted in the greenhouse at Kashiwanoha campus of Chiba University, Japan between 29 August 2018, and 14 January 2019.
Cultivars used in the study were Jaguar and Momotaro York. These cultivars were obtained from Techisem, Savanna Seed Company limited, (Longué-Jumelle, France) and Takii Seeds Company Ltd (Kyoto, Japan) respectively.
Seeds were sown in cell trays using cocopeat as the sowing medium. The germinated seeds were kept in an artificial growth chamber. The chamber was equipped with a light intensity of 280 μmol m−2 s−1 for 16 h, 1000 µmol mol−1 CO2 and the day/night temperatures maintained at 23/18 °C. The constitution of the nutrient solution was 0.7 mM NH4-N, 8 mM NO3-N, 1.3 mM PO4-P, 2 mM K, 2 mM Ca, 1 mM Mg, 2 mM SO4-S, 3 ppm Fe, 0.5 ppm B, 0.5 ppm Mn, 0.02 ppm Cu, 0.05 ppm Zn, and 0.01 ppm Mo. An ebb and flow hydroponic technique was adopted in supplying the nutrient solution to the seedlings, once a day.
2.2. The Hydroponic System
The system was arranged in benches of 15 m long. Each bench had panels that suspended the planting pot siting on troughs which served as a channel for draining unabsorbed water back to the return tank. Planting pots of 500 mL volume capacity were 90% filled with cocopeat. Spacing adopted was 0.2 by 1.3 m. The electrical conductivity (EC) of the nutrient solution was 0.12 S m−1, while the pH was maintained between 5.5 and 6.5. The nutrient solution was pumped to the root zone of each plant through drips periodically.
2.3. Transplanting and Treatments
Transplanting of seedlings was carried out at 21 days after germination. Plants were pinched (topped) at 42 and 56 days after transplanting (DAT) respectively for Jaguar and Momotaro after three leaves above the fourth truss were fully developed. The topping was carried out in line with the findings of [12
Flowers upon full opening were sprayed with 1 mL L−1 4-Chlorophenoxyacetic acid to enhance fruit set.
The morphometric parameters such as height, girth, leaf number, and chlorophyll content (SPAD) were measured fortnightly. Additionally, days to 50% flowering and fruiting were determined. Plant height and the girth were measured using a ruler and vernier calipers respectively. Plant girth was initially measured at the first two true leaves and subsequently at the leaf below the succeeding trusses. Chlorophyll content was measured with SPAD 502 plus (Konica-Minolta Inc., Tokyo, Japan). The chlorophyll content was determined on the leaves below the immediate truss as well as the leaves below the succeeding trusses.
2.5. Water and Nutrient Use Efficiencies
Water uptake for the first harvest (70 DAT) was determined as the initial water supplied less the amount of water left (unabsorbed). This parameter was measured periodically when the nutrient solution volume had drastically reduced thereby requiring a renewal. Water use efficiency (WUE) was determined as a ratio of fruit weight to total water uptake per plant at first harvest (70 DAT).
Nutrient uptake was determined by collecting samples of the nutrient solution at the end of each period. The samples were analysed for nutrients using the Dionex ICS1100 ion chromatography (Thermo Fisher Scientific Inc., Waltham, MA, USA). Nutrient use efficiency (NUE) was expressed as a ratio of the fruit fresh weight to the total nitrogen uptake per plant at first harvest.
2.6. Physiological Characteristics
Physiological attributes such as photosynthetic rate, leaf conductance and transpiration were measured using the LI-6400 (LI-COR, Lincoln, NE, USA). Measurement was carried out on the immediate leaf below the second truss between 1:30 and 2:30 p.m.
2.7. Growth Parameters
Relative growth rate (RGR) expressed as the rate of dry mass increase per unit of plant mass over a given time. Net assimilation rate (NAR) was expressed as dry matter increment per unit leaf area per unit of time. All the leaves were detached, and photo scanned using a camera. The scanned photos were analysed for the leaf area, using the Lia32 (https://www.agr.nagoya-u.ac.jp/~shinkan/LIA32/author-e.html
2.8. Dry Matter Partitioning
Distribution of dry mass (DM) was determined by measuring the leaf area, plant dry mass, shoot dry mass, root dry mass, dry mass allocated to fruits, and root-shoot ratio. The DM of leaf, stem, root, and fruit were determined through oven drying at 72 °C for ten days when dry weights of samples became constant. The DM contents were determined fortnightly and used to calculate the growth parameters.
Root tissue density (RTD) is a measure of resource uptake strategies in plants. Plants with higher RTD have more fine roots for the uptake of water and nutrients. RTD influences water and nutrient uptake by the fine roots with a corresponding dry matter investment as reported [13
]. Root dry matter concentration (RDMC) as indicated by [14
] was used as a proxy to determine RTD in the study. This method has also been recommended by [15
], as being relatively cheap, easy, and quick for determining RTD. The RTD was determined as root dry mass per unit root fresh mass.
2.9. Yield Component and Crop Productivity
Yield component was expressed in terms of fruit number per plant, fruit fresh weight per plant, and yield per area. Days to 50% flowering and fruiting were determined. Crop productivity was determined as a ratio of the fruit biomass to the plant biomass.
2.10. Fruit Quality
Components of fruit quality measured were total soluble solids (TSS), titratable acidity (TA), and TSS/TA ratio. TSS and TA were measured using K-BA100R spectrophotometer (Kubota, Yao, Japan) to scan the fruits. TSS/TA was determined as a fraction of TSS to TA.
2.11. Data Analysis
Data obtained was analysed using the GenStat (Rothamsted Research, Harpenden, UK) and the least significant difference (LSD0.05)) was used to separate the means.