Growth and yield in taro (Colocasia esculenta (L.) Schott) grown using different planting materials and exposed to different morphological alteration treatments

The common taro used in this study had pale green petiole skin colour and broken-white inner corm colour. The planting materials (PMs) used consisted of cormel and two sucker sizes, i.e., with two leaves and four leaves. Cormels and suckers were selected for maximising uniformity within each category of PM. The PMs were sown onto pre-mixed growing substrate with a ratio of 2:1 (v:v) between topsoil and chicken manure. The substrate was rested into 30-cm height and 30-cm upper diameter pots and then watered to slowly settle the mix. Additional substrate was added to level the surface of substrate in all pots at a height of 25 cm. Substrate surface was at the same level, with four side holes positioned on each pot.

This experiment was arranged based on the randomised block design with two factors, i.e., planting materials (PMs) and morphological alteration (MA) treatments. Three kinds of PMs were used: cormel (PMC), suckers with two leaves (PMS2) and suckers with four leaves (PMS4). Two MA treatments were applied, consisting of removal of all suckers (MAS) and excision of the above-ground part of the mother plants (MAM). Non-treated plants (NMA) were used as control. Each of the 3 × 3 treatment combinations was replicated three times and each replication consisted of three taro plants. All plants were placed outdoors and directly exposed to full sunlight. The substrate was watered daily except on rainy days. Compound NPK (15:15:15, v:v:v) fertilizer was applied during the vegetative stage at 3 weeks after planting (WAP) (5.0 g), 7 WAP (7.5 g) and 13 WAP (10.0 g) per pot. The NPK fertilizer was not applied during corm enlargement or at maturity. The response of the taro plants to NPK application during the vegetative growth stage was continuously monitored for 28 days, starting after the day of application from 4 WAP to 8 WAP. Since only one plant species was used, i.e. common taro, and measurements were made under similar agroclimatic conditions, the use of soil plant analysis development (SPAD) value as a proxy for leaf nitrogen content was well founded. The concern of Xiong et al. (2015) on this matter was fully considered.

Measurement of the SPAD value – as a proxy for leaf nitrogen content – was conducted using a chlorophyll meter (SPAD-502Plus, Konica Minolta, Tokyo, Japan). The SPAD values were the average of the values at three points of measurement on each selected leaf. The directly measured traits were clustered into quantity-based, dimension-based and weight-based traits. Quantity-based traits included the number of suckers and leaves. The dimensional and weight-based traits included length, width, diameter, as well as the fresh and dry weights of leaf blade, midrib, petiole, sucker, corm, cormel and roots. The diameters of petiole, corm and cormel were measured using digital callipers (SH20, Taffware, Jakarta, Indonesia). Root length was obtained by measuring from the base of the plant to the tip of the longest root. The fresh weight of each part of the plant was taken directly by using digital scales. The dry weight of each part of the plant was measured after being dried in the oven at a temperature of 100 °C for 48 h. The leaf moisture content was calculated based on the difference between the fresh and dry weights of the leaf. The cormel/corm ratio was calculated based on the total dry weight of all cormels and the dry weight of the single main corm.

Routine data collections were conducted weekly for the quantity-based and dimension-based parameters of the above-ground organs. Destructive measurements were performed at 12 WAP, 20 WAP and 28 WAP for the weight-based parameters of the above- and below-ground organs. The analysis of variance (ANOVA) was carried out using the Statistical Analysis System (SAS), version 9.0, for Windows, based on the randomised block design with two factors. Differences among the mean values of significant treatments on any traits were tested using the least significant differences test at p ≤ 0.05.

High diversity has been recognised in the taro plant. Maretta et al. (2022) divided taro into three groups based on the plant stature, colour of the petiole and corm cross-section. The taro used in this study exhibited pale-green petiole skin and broken-white inner corm. Taro plants can be grown using different biomaterials, i.e. small cormel, sucker with two leaves or sucker with four leaves. In our previous study, large cormels (up to 70 g) exhibited fastest growth and yielded the largest plant during the vegetative phase (Lakitan et al., 2021). However, earlier development of corm in the mother plant was observed at 12 WAP in plants grown using cormel as the PM. At 12 WAP, there was no indication of corm development in plants grown using suckers with two and four leaves.

Taro plant is responsive to compound NPK fertilizer application. The SPAD value has been practically used as a reliable estimate for leaf nitrogen content. Yue et al. (2020) found that there was a strong correlation (R² = 0.94) between the SPAD value and leaf nitrogen content, irrespective of the growth stage. The leaf SPAD value significantly increased within 1 week after NPK application, and it took >3 weeks before the SPAD value dropped back to the pre-treated value. Raju and Byju (2019), based on their finding on NPK uptake by the taro plant, suggested an application of NPK at a ratio of about 4.7:1.0:6.4 for N, P and K, respectively. However, Lee et al. (2016) found that the ratio of tuber to total biomass decreased with increasing N fertilization rate. The decrease was related to an increase in the amount of assimilate allocated for shoot growth. Fertilizer use efficiency was decreased by increase of N and K fertilization. The highest taro corm yield was achieved at 150 kg · ha^–1 NPK fertilization.

Regardless of the planting materials (PMs) used, the number of leaves in the taro plant, on average, only varied between 4–6 leaves per individual plant. Legesse and Bekele (2021) testified that the taro plant maintained a maximum of eight leaves per plant throughout its life cycle. The plants actually continuously produced new leaves, but it was equalised by the death of the oldest leaf.

Busari et al. (2019) informed that some irrigation water management treatments, i.e. alternate wetting and drying, continuous flooding irrigation and wetting without flooding, did not negatively affect the number of leaves and the leaf area index in the mother plant. Further, Hidayatullah et al. (2020) found that taro plants positively responded to saturated water conditions by increasing the number of leaves. Derebe et al. (2019) found that leaf number and leaf area index were higher at higher altitudes (2,200 m above sea level).

In addition to corm as the main yield, the young leaf blade and petiole can be consumed as leafy vegetable (Miyasaka et al., 2019). Suckers can be used as a source of leafy vegetable. Total sucker biomass was consistently higher than that of the individual mother plant, indicating that the taro variety used in this study is suitable for production of leafy vegetable. Ogbonna et al. (2015) reported that the planting space in taro cultivation affected the ratio of harvested corm yield and the number of suckers for use as vegetable. Narrower planting space produced higher tuber yield per hectare, while wider planting space increased the number of suckers. Salam et al. (2016) added that use of larger cormel as the PM produced more suckers. These findings are useful for cultivating the taro plant to produce both corm as the main yield and young leaves as the additional yield.

The number of suckers also declined as the plant became older. This decline was associated with the switching of the assimilate's role from supporting shoot growth to development of corm as a stronger sink. The weakest sucker eventually died as the competition continued. Bekele and Boru (2020) revealed that the number of suckers could be significantly different among varieties. Therefore, suitable varieties for the double yield cultivation system are those with balanced shoot and corm development. The variety producing more suckers should be suitable for leafy vegetable production, and varieties with fewer suckers should be chosen for maximising corm yield. Agronomically, to some extent, taro plants suitable for any specific purposes can be created via MA treatment.

Fa'amatuainu and Amosa (2016) divided the accumulation of dry matter in taro plant into five parts, i.e., leaf blade, petiole, sucker, roots and corm/cormel. They found that dry matter accumulation and its partitioning to different plant parts were varied over the growth stages in the taro plant. In this study, the impacts of morphological alteration (MA) treatments were more consistent and significant on the leaf blade, petiole, roots and corm of the mother plant. Meanwhile, the impacts on parts of sucker were rarely considered. Instead, the attention was more concentrated on the cumulative cormel weight. Kaushal et al. (2015) reported that the corm and cormel of the taro plant were good sources of starch since the corms contain 70%–80% starch, have anti-oxidative and anti-inflammatory properties and are highly digestible. Saxby et al. (2021) added that the corm can be utilised as a carbohydrate alternative for improving the nutritional value for human health.

Corm size (based on diameter and length) was not related to the planting materials (PMs) used but was significantly affected by morphological alteration (MA) treatments. MA significantly affected the dry weights of the leaf blade, petiole and roots of the individual mother plants. Dry weights of the MAS-treated plants were significantly higher than those of normal plants; however, the morphological characteristics of MAM-treated mother plants were not significantly different from those of control plants.

At present, it might need extra efforts to find scientific articles on the response of taro plant to morphological alteration (MA) treatments. However, physical treatments such as goose-necking or decapitation of the mother plant have been reported to enhance sucker production in the banana plant (Bhende and Kurien, 2016). Sucker is a main PM in banana cultivation. A similar principle was used in the MAM-treated mother plants in taro, which aims to increase the number of suckers for production of young leaves as leafy vegetable.

Dorel et al. (2016) found that sucker removal eliminated potential competition for photo-assimilates between suckers and yield organ. A similar result was found in this study: corm size, corm weight and some other traits in the mother plant were significantly better in the MAS-treated plant harvested at 28 WAP. Datta et al. (2020) also reported that wider spacing (2.5 m × 2.5 m) with a single sucker as the PM produced the highest yield and all other morphological traits in the optimal range.

Initiation of corm development in taro plant was observed at 12 WAP. A fast enlarging period occurred during 12–20 WAP, and slowing down was observed during the next 8 weeks. Corm water contents measured at 12WAP, 20 WAP and 28 WAP were relatively unchanged and also were not affected by the different PMs used or the MA treatments. The optimum age for corm harvesting was 7–9 months after planting (MAP) (Boampong et al., 2018). Kristl et al. (2021) added that at around 8 months of age, total oxalate content in the corm was lower, and the water-soluble oxalate content increased as the plant grew older. In this study, corm weight, diameter and length were no longer affected by the PMs used at the ages of 20 WAP and 28 WAP or about 7 MAP. Oxalate causes irritation and burning sensation (acridity) in the throat and mouth on ingestion (Kaushal et al., 2015).

Cormel was developed later than the main corm. It took >12 weeks before the cormel began to develop into taro plants. Cormels can grow directly from the main corm or randomly develop from swelling roots. PMs did not consistently trigger cormel initiation. MAS treatment significantly increased cormel weight, but the MAM-treated plants did not significantly differ from the non-treated (NMA) plants in terms of the cormel enlargement process. Ubalua et al. (2016) described that corm initiation commenced at about 3 MAP, while cormel initiation followed afterwards. There was no information on the time lapse between corm and cormel initiation. However, cormels had been found at 20 WAP; therefore, it is reasonable to expect that cormel initiation occurred at around 15 WAP. The period of 3–6 MAP was characterised by a rapid increase in shoot growth. By the end of 6 MAP, shoot growth rate gradually declined, while corm and cormel continued to grow. Muinat et al. (2017) added that the number of cormels was positively correlated with the corm weight.