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Physiological Response of Blackgram Varieties to Foliar Nutrition and Growth Regulators under Partial Shade in Coconut Orchard in Kerala

A.P. Pooja1,*, M. Ameena1, P. Arunjith1
1Department of Agronomy, College of Agriculture, Kerala Agricultural University, Vellayani, Thiruvananthapuram-695 522, Kerala, India.

ABSTRACT

Background: Coconut is one of the major plantation crops in Kerala which are planted at a wider spacing of 7.6 m × 7.6 m giving an ample opportunity for growing intercrops. To enhance soil fertility and productivity of crops inclusion of legumes is a viable option. Blackgram, though a shade sensitive crop performs well in partially shaded coconut gardens due to fast growing nature. The unique ability of biological nitrogen fixation, soil amelioration, carbon sequestration, low water requirement and capacity to withstand extreme drought, pulses have remained an integral component of coconut based cropping system. Since blackgram is a shade sensitive crop, to increase the productivity of blackgram in coconut garden, foliar spray of nutrients and growth regulators is a viable option. Under, shaded or stressed situations, nutrients and plant growth regulators provide optimum vegetative growth by regulating plant growth and architecture. The primary objective of the present study was to investigate the impact of foliar nutrition and growth regulator application on the physiology and yield of different blackgram varieties.

Methods: A field trial was conducted at College of Agriculture, Vellayani, Kerala, India during rabi 2020 and summer 2020-21. To find the performance of five blackgram varieties. The treatments consisted of five performing blackgram varieties under partial shade viz; Sumanjana, DBGV 5, VBN 5, VBN 6, CO 6 and foliar sprays of nutrients (19:19:19 NPK, sulphate of potash) and plant growth regulators [naphthalene acetic acid (NAA) and salicylic acid (SA)] alone and in combinations in split plot design at 45 and 60 days after sowing (DAS). 

Result: The study identified Sumanjana and DBGV 5 as shade adaptive varieties with suitable physiological traits. In both seasons, a higher stomatal conductance and lower stomatal index were observed, indicating a more efficient allocation of resources towards the photosynthetic machinery. Foliar spray of 19:19:19 (1%) and NAA at 40 mg/L and SA at 100 mg/L at 45 and 60 DAS resulted in higher seed yield, dry matter production and also higher net income and benefit cost ratio for these two varieties.  

INTRODUCTION

Among pulses, black gram is a much-preferred short duration crop as it survives better in all seasons either as sole crop, intercrop or catch crop accounting for 13% of total pulse area and 10% of total pulse production in the country. It is also adapted to be used as a catch crop as well as a contingency crop. To encourage and extend black gram cultivation in the prevailing situation of land fragmentation and low availability of cultivable lands in the state of Kerala, India, inclusion of blackgram in coconut gardens as an intercrop is a practical solution. Availability of variety with appreciable grain yield and shade tolerance is common limitation in the popularization of blackgram cultivation in coconut garden. 
 
There are varietal constraints leading to low productivity of blackgram along with other climatic and agronomic constraints. Among fertilizer application practices, foliar application of nutrients is a low-budget, yet effective method. Selection of suitable varieties to the situation coupled with foliar application of nutrients and/or growth regulators can improve the productivity of blackgram. Research works on the effect of foliar application of nutrients and growth regulators to enhance the productivity of blackgram under partial shade is meagre. They also enhance the source-sink relationship and stimulate photo-assimilate translocation thereby helping in effective flower formation, fruit and seed development, ultimately enhancing the productivity of crops. In this backdrop, the current study aimed to assess how foliar nutrition and plant growth regulators affect the physiology and yield of shade-tolerant blackgram varieties that were identified for this purpose.

MATERIALS AND METHODS

The field trial was conducted at Instructional Farm, College of Agriculture, Vellayani, Thiruvananthapuram, Kerala during rabi 2020 and summer 2020-21 in partially shaded coconut garden. The field was situated at 8°25¢46²N latitude, 76°59¢ 24²E longitude and altitude of 29 m above the mean sea level. Soil of the experimental site was sandy clay loam in texture, belonging to the order Oxisols under Vellayani series. The soil was acidic in reaction (pH-6.1), with high organic carbon content (0.92%), low in N (188.16 kg ha-1), high in P (31.25 kg ha-1) and high in K (340.14 kg ha-1). The total rainfall received during rabi 2020 and summer 2020-21 was 288.6 mm and 237.1 mm respectively. The experiment was laid out in split plot design, replicated four times with five varieties (v1- Sumanjana, v2- DBGV 5, v3- VBN 5, v4- VBN 6, v5- CO 6). These varieties were selected after a preliminary screening under the partial shade in coconut garden in terms of yield per unit area (Pooja et al., 2021) for the main plot treatments. The duration of blackgram varieties in the shaded condition ranged between 90 to 110 days, which were more than that of open condition in general. Six foliar sprays of nutrients with and without plant growth regulators were the subplot treatments (f1: 19:19:19 (1%) at 45 and 60 DAS, f2: SOP (0.5%) at 45 and 60 DAS, f3: NAA 40 mg L-1 and salicylic acid 100 mg L-1 at pre-flowering (45 DAS) and 15 days later, f4: f3 + f1, f5: f3 + f2 and f6: Control - KAU POP).
 
Coconut gardens above 40 years of age having a light intensity between 40-46.5 klux, planted at spacing of 7.6 m × 7.6 m was selected and a distance of two-meter radius was left from the base of each coconut palm and beds of size 6 m length and 3 m width were taken as main plots in the interspace between coconut palms. The main plot was later divided into sub plots of size 3 m × 1 m and the seeds of blackgram and were sown at a spacing of 25 cm × 15 cm. The recommended nutrients (20:30:30 kg N: P2O5: K2O ha-1) was given through urea, rajphos and muriate of potash (KAU, 2016). Foliar spray of nutrients and plant growth regulators were done as per the treatments fixed. The foliar spray at 45 DAS coincided with the flower initiation stage and 60 DAS coincided with the completion of flowering and initiation of pod formation stage.
 
Plant protection measures were taken as and when required. Tobacco caterpillar (Spodoptera litura) incidence noticed at 20 DAS was controlled using quinolphos 25 EC (KRUSH) @ 2 mL L-1 and the incidence of spotted pod borer (Maruca vitrata) at pod maturity stage was controlled using chlorantraniliprole 18.5 SC (CORRAGEN®) @ 3 mL 10 L-1 and thiomethoxam (ACTARA 25% WG) @ 0.25 mL L-1. Collar rot disease caused by Rhizoctonia solani was managed by spraying carbendazim (BAVISTIN 50 WP) @ 1 g L-1. Data on physiological characters viz., leaf area index, chlorophyll content, stomatal index, stomatal conductance, soluble protein at 50% flowering were recorded from the observational plants and averages worked out. Harvest of pods was completed with three pickings. One day prior to harvest, the observational plants were pulled out and the yield attributing characters and root characters were recorded. The dry pods from individual net plot area were picked, sun dried and threshed separately to determine seed yield. The data was analysed statistically by analysis of variance (ANOVA) for split plot design and the significance was tested by F test.

RESULTS AND DISCUSSION

Physiological characters
 
Leaf area index was higher during summer compared to Rabi season (Table 1). Plants adapt to shading stress by increasing leaf area expansion for maximizing light interception. The varieties Sumanjana and CO 6 recorded higher values of LAI during both the seasons, due to the higher number of leaves recorded in these varieties. Higher leaf area is an adaptation to capture irradiance more efficiently under shaded situation for tolerant varieties, which could contribute to better photosynthesis. Sumanjana and CO 6 were found to be shade adaptive with higher LAI to compensate shade induced reduction in photosynthesis. The foliar application had a significant impact on leaf production and plants treated with major nutrients and plant growth regulators (PGR) showed superior performance in terms of leaf area index (LAI) during both seasons. This improvement could be attributed to the increased availability of nutrients from readily soluble fertilizer used in the foliar spray. Growth promoting substances such as NAA at 20 ppm had a positive effect on cell division and cell elongation leading to enhanced leaf expansion. 

Table 1: Effect of varieties and foliar application on seed yield of black gram during summer 2020 and Rabi 2021.


 
Among the interaction effects of varieties and foliar sprays, Sumanjana supplied with 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering and 15 days later (v1f4) recorded higher LAI and was on par with v5f3 and v5f5 during summer season. In the Rabi season, v1f4, v2f4 and v3f4 were on par. Foliar application of fertilizers and PGRs improved the vegetative growth of the plant which contributed to higher LAI and improved photosynthetic efficiency. These results are in accordance with the findings of Dey et al., (2017) in mung bean. The higher leaf area index (LAI) observed in the plants could be attributed to the active role of auxins in promoting cell division and cell elongation. The positive influence of foliar spray of nutrients using mineral fertilizers could enhance nutrient availability, which in turn promotes cell division, cell elongation and overall crop growth and development.
 
Chlorophyll content
 
The results revealed that the chlorophyll content varied significantly among the varieties during both the seasons. During summer, the highest chlorophyll content was recorded by the variety DBGV 5 (1.96 mg g-1 fresh tissue) while during Rabi, Sumanjana recorded the same (2.36 mg g-1 fresh tissue). This observation could be attributed to the fact that leaves grown under shaded conditions tend to have a higher concentration of chlorophyll per unit of fresh mass. The increased chlorophyll content in shaded leaves led to enhanced photosynthetic efficiency. Kaleeswari et al., (2022) observed that with the increasing N levels in blackgram, leaf chlorophyll and nitrogen content increased linearly. The subplot factor f4 recorded higher chlorophyll content during both the seasons. The enhanced absorption and translocation of nutrients through foliar application could have contributed to increased chlorophyll synthesis and improved photosynthetic activity. This, in turn, can lead to higher efficiency in converting light energy into chemical energy, ultimately promoting better growth and productivity in plants. The increase in chlorophyll content reflects increased PS II photochemistry, photosynthates production and dry matter accumulation. Foliar application of salicylic acid influenced different physiological and biochemical aspects of green gram plant via increasing assimilation rate which revealed increase in chlorophyll content and hill reaction activity in the leaf. The treatment combination v1f4 recorded the highest chlorophyll that can augment the rate of photosynthesis for better accumulation of photoassimilates.
 
Stomatal index
 
It is the number of stomata compared to total number of epidermal cells expressed in percentage. Crops with lower stomatal index are preferred as they reduce transpiration rate and improve photosynthetic rate to the optimum. Sumanjana recorded the lowest stomatal index during both the seasons which could be inferred as a more efficient investment in photosynthetic machinery. Foliar spray significantly influenced stomatal index and f4 recorded lower stomatal index in summer. The observed effects could be attributed to the influence of foliar spray of salicylic acid, which can alter the balance of endogenous hormones in plants. This hormonal modulation may induce stomatal closure, reduce transpiration rates and ultimately improve the photosynthetic capacity of the plants. By reducing water loss through transpiration, the plants can maintain optimal leaf hydration and enhance their photosynthetic efficiency, leading to increased photosynthate production and subsequent dry matter accumulation. The lower stomatal index was recorded in v1f4 and was on par with v1f3 and v1f5. Similar results of lower stomatal index due to foliar spray of NAA was also reported by Jahan and Yasmin (2017). 
 
Stomatal conductance
 
It is the ability of stomata for gaseous exchange. During both the seasons, Sumanjana consistently exhibited the highest stomatal conductance during the flowering stage. This suggested that Sumanjana had a higher rate of gas exchange, allowing for efficient uptake of carbon dioxide and release of oxygen without significant loss of photosynthetic productivity. As a result, these plants were able to express their genetic potential and achieve optimal growth and development even under partial shade. Foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering and 15 days later (f4) resulted in higher stomatal conductance during both the seasons. Increased stomatal conductance with foliar spray of salicylic acid was earlier reported by (Khan et al., 2010). The main plot and subplot effect was reflected in interaction with v1f4 having the highest stomatal conductance during both the seasons. The content of soluble protein was unaffected due to main effects and interaction effects during both the seasons. However, the content was higher during the summer season compared to Rabi season. 
 
Yield
 
The cumulative effect of growth attributing characters was manifested in the final yield, which showed considerable variation across different varieties in both seasons (Table 2). Both Sumanjana and DBGV 5 exhibited higher seed yield per plant in both seasons. The foliar spray also significantly affected the seed yield per plant and foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg  L-1 and SA 100 mg L-1 at pre-flowering and 15 days later (f4) resulted in the highest seed yield per plant, during both the seasons. The control treatment as per KAU package was having the lowest seed yield per plant during both the seasons. 
 
Among the treatment combinations, higher seed yield was produced by Sumanjana and DBGV 5 along with foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering and 15 days later. The increased yield might be attributed to the balanced export of nutrients during critical stages, such as flowering and fruit setting, along with the use of growth regulators that promote greater flower retention. The availability of nitrogenous fertilizers promoted photosynthesis, leading to enhanced utilization of photosynthates and an increased allocation of these resources towards the economically valuable parts of the plant. This observation aligns with the current findings. Sarkar et al. (2021) documented that foliar fertilization is best suited for Rabi pulses rather than top dressing or placement of fertilizers. 

Table 2: Effect of varieties and foliar application on seed yield and dry matter production of blackgram during summer 2020 and Rabi 2021.


 
Sumanjana and DBGV 5 recorded higher seed yield/ha during both the seasons. The subplot treatment of foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering and 15 days later (f4) revealed the highest seed yield ha-1 during both the seasons. Reducing the abortion rate could have significantly increased the number pods per plant and number of seeds per pod in soybean as observed by (Khatun et al., 2016). Reduced flower drop and increased pod set percentage due to the foliar application of nutrients might have increased seed yield of pulses.
 
Main and subplot effects were reflected in the interaction and higher seed yield was recorded in Sumanjana along with foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1  at pre-flowering and 15 days later (v1f4) and was on par with DBGV 5 along with foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1  at pre-flowering and 15 days later (v2f4) in the summer season. During Rabi season, the highest seed was recorded in v1f4 followed by v2f4. Foliar nutrients usually penetrate the leaf cuticle or stomata and enter the cells facilitating the easy entry of nutrients. The increase in the number of pods per plant and seed yield per plant could be attributed to the effective fulfilment of crop demand through higher assimilation and translocation of photosynthates from source (leaves) to sink (pods). This process was facilitated by the foliar spray of 19:19:19, with a balanced ratio providing the required nutrients for optimal growth. 
 
Pooled analysis on seed yield revealed Sumanjana and DBGV 5 to be better performers under partial shade with seed yield of 1489 kg ha-1 and 1473 kg ha-1 respectively. The subplot factor f4 recorded the highest seed yield (1505 kg ha-1).  The combination of main and subplot factors as v1f4 recorded the highest seed yield (1725 kg ha-1) followed by v2f4 (1626 kg ha-1) which is deduced to be due to the positive interaction of individual effects (Fig 1). Better crop growth enhanced the absorption of nutrients through root and enhanced the synthesis of IAA, carbohydrate and N metabolism which ultimately led to higher economic yield. The production of higher seed yield due to nutrients and growth regulators could be attributed to the fact that plants treated with macro nutrients and growth regulators remained physiologically more active to build up sufficient food reserves for the developing flowers and seeds.

Fig 1: Interaction effect of varieties and foliar spray of nutrients and plant growth regulators on pooled seed yield.


 
Among the treatment combinations, Sumanjana (v1) with f4 produced higher LAI, chlorophyll content and stomatal conductance, at flowering during summer and Rabi. LAI, Higher measure of leafiness of the crop in relation to land area (LAI) resulted in increased photosynthetic efficiency coupled with increased capacity of utilization of photo accumulates by the growing tissues and their utilization reflected in the yield attributes. The enhanced chlorophyll content indicated an improved capacity for light absorption, while increased stomatal conductance suggested better CO2 uptake. These factors combined enhanced the efficiency of photosynthesis, resulting in an increased production of photosynthates. Higher source capacity along with better translocation and partitioning to the reproductive sinks might have contributed final yield of shade tolerant varieties in this experiment. 
 
A correlation analysis was done to examine the linear relationships among physiological parameters with yield (Table 3a and 3b). The analysis revealed that seed yield had a significant and positive correlation with chlorophyll content and stomatal conductance during the summer, while with chlorophyll content alone during the Rabi season. The significant association between chlorophyll content and seed yield at the phenotypic levels was reported by (Baroowa et al., 2015). On the other hand, the stomatal index showed a significant negative correlation with seed yield during the summer and had a weak correlation during the Rabi season. This suggests that the physiological characteristics, namely chlorophyll content and stomatal conductance, directly influenced the yield of blackgram under conditions of low light intensity.

Table 3a: Effect of correlation on physiology, seed yield, haulm yield and TDMP of blackgram during summer 2020.



Table 3b: Effect of correlation on physiology, seed yield, haulm yield and TDMP of blackgram during Rabi 2020-21.


 
Economic analysis
 
The economic analysis for the different combinations revealed that the varieties Sumanjana (v1) or DBGV 5 (v2) supplied with foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering and 15 days later (f4) found to be profitable recording higher gross returns, net returns and benefit cost ratio (BCR) during both the seasons (Table 4). The higher economic benefits could be attributed to the higher seed yields realized, the treatments being the cumulative effects of the varieties and application of foliar nutrients and plant growth regulators. 

Table 4: Effect of varieties and foliar application on economics.


 
Mean values of net return (₹ 70411 ha-1) and B: C ratio (2.04) were also the highest for the treatment v1f4, followed by the v2f4. The treatment combination v1f4 and v2f4 produced ₹ 21378 and ₹ 16050 as additional mean net income compared to its respective controls. There was 30.36 per cent and 26.09 per cent increase in net income in v1f4 and v2f4 compared to control (v1f6 and v2f6 respectively). Mamathasree (2014) and Kumar et al., (2018) obtained higher net returns and BCR in red gram and blackgram respectively due to foliar application of water-soluble fertilizer 19:19:19 at 2 per cent at flowering and pod development stages. Yalagar et al., (2021) also found that, application of recommended nutrients to pigeonpea coupled with foliar spray of 19:19:19 (1%) at flower initiation and peak flowering stage is optimum for higher net returns (₹ 65908 ha-1) and BCR (3.49). Shwetha et al., (2021) registered higher net return (₹ 161,159 ha-1) and B:C ratio (4.19) in pigeon pea due to the foliar application of pulse magic (1%) + 19:19:19 (1%) + 50 ppm NAA + Borax (0.2%).

CONCLUSION

The study demonstrated that foliar spray had a significant impact on leaf production in blackgram and shade adaptive varieties treated with a combination of major nutrients and plant growth regulators exhibited superior physiological characteristics and yield. The present study revealed that, the varieties DBGV 5 and Sumanjana, could perform better under partially shaded coconut garden. Higher yield could be realized in these varieties under partial shade with recommended dose of KAU POP supplemented with foliar spray of 19:19:19 (1%) at 45 and 60 DAS + foliar spray of NAA 40 mg L-1 and SA 100 mg L-1 at pre-flowering (35 DAS) and 15 days later. Sumanjana and DBGV 5 with f4 recorded higher mean net income and mean B:C ratio.

CONFLICT OF INTEREST

There is no conflict of interest among authors in publication.

REFERENCES


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