​Impact of Foliar Spray of Plant Growth Retardants with Potassium on Growth Traits, Gas Exchange Parameters and Grain Yield in Foxtail Millet (Panicum italicum L.)

Small millets hold great potential in contributing food and nutritional security of the country which are called as powerhouse of nutrients and climate resilient crops. They have high fiber, quality protein and minerals composition being called as “nutri-cereals” (Durgad et al., 2019). Among the millets, foxtail millet is mostly grown to meet the domestic needs of the rural people and used as an energy source for pregnant women, sick people and children. Recently, the importance of foxtail millet is recognized as diabetic food (Hariprasanna, 2016).
       
The biggest challenge for agriculture is to meet the food with nutritional security for increasing world population. Increasing the production of millets is first importance because of nutri-cereals. Improving crop yield under rainfed condition is important, to maintain food security and improve the livelihoods of the poor (Yadav et al., 2015). The potentiality of foxtail millet has not been exploited in India due to mainly growing in rainfed condition, not recommended potash fertilizers and not altered the source-sink relation by using PGRs. Greater proportion of photo-assimilates is diverted for production and maintenance of vegetative plant parts rather than translocation to reproductive parts and hence, the plant puts on more plant height and higher biomass (Secondo and Reddy, 2018). Therefore, any attempts to reduce plant height would be expected to improve the yield by diverting the photo-assimilates to reproductive structures. CCC has been most widely used to reduce plant height, stimulate tillering, distribution of assimilates to sink and limits risk of lodging (Emam, 2011). Anosheh et al., (2016) reported that foliar application of cycocel improved the grain yield of wheat and maize. Latifkar et al., (2014) also reported that the application of cycocel increased the thousand grain weight and grain yield significantly in wheat. Foliar application of mepiquat chloride significantly increased the photosynthetic rate and seed yield in greengram (Rajesh et al., 2014).
       
Potassium is an important primary nutrient and foliar spray of potassium increased the growth and yield of maize crop has been reported by Ali et al., (2016) under rainfed condition. Proper nutrient management especially potassium is also a key issue in achieving higher productivity of any crop. Keeping this in view, a field experiment was conducted to investigate the impact of plant growth retardants like CCC and MC with potassium on growth traits, gas exchange parameters and grain yield in foxtail millet.

The study was conducted at Regional Research Station, Tamil Nadu Agricultural University, Paiyur during the kharif season of 2019-2021. The seeds of ATL 1 variety were directly sown to field with the spacing of 30 × 10 cm. As per the recommendations of TNAU Crop Production Guide, 44 kg of nitrogen and 22 kg of phosphorus per hectare were applied as basal. Plant growth retardants viz. CCC and MC and nutrient potassium sulphate solutions alone and combinations includes CCC (250 ppm), MC (250 ppm), K₂SO₄ (1%), CCC (250 ppm) + K₂SO₄ (1%) and MC (250 ppm) + K₂SO₄ (1%) were prepared and applied as foliar spray at 35 days after sowing. The control was maintained with water spray and an absolute control maintained without any spray. The experiment was imposed with seven treatments replicated thrice and adopted statistics of randomized block design.
 
Measurement of growth parameters
 
Plant height was measured from the ground level to the tip of the growing point. The plant was uprooted and the root was taken with minimum damage and the length from the cotyledonary node to the root tip was measured and expressed as cm. Number of leaves was determined by counting the leaves from the base to tip of the plant in each replication and mean value expressed in numbers.
       
Leaf area duration (LAD) was determined by using the formula of Power et al., (1967) as follows and expressed in days.
 
Where,
L₁= Leaf area index at first stage.
L₂= Leaf area index at second stage.
t₂- t₁ = Time interval in days.
       
Crop growth rate (CGR) was calculated based on the formula designed by Watson (1958) and the value is expressed as g m^-2 day^-1.
       
Where,
W₂ and W₁ are whole plant dry weight at time t₂ and t₁ respectively. t₂ - t₁ is time interval.
P is spacing in m².
 
Measurement of gas exchange parameters and grain yield
 
Measurement of gas exchange parameters was performed by using Portable Photosynthesis System (PPS) (Model LCpro-SD., ADC BioScientific Ltd., Hoddesdon, UK) equipped with a halogen lamp (6400-02B LED) positioned on the cuvette. Third leaf from top was used for the measurements with replicated thrice. The photosynthetic rate, transpiration rate and leaf temperature were measured between 9 AM and 11.30 AM. The grain yield was recorded after harvest and expressed in terms of tonnes per hectare. The data on various parameters were analyzed statistically as per the procedure suggested by Gomez and Gomez (1984).

Impact of plant growth retardants and potassium on growth traits
 
Optimum plant height is desirable for higher productivity especially in millets. In the present study, control plants recorded highest plant height of 116.7 cm while foliar spray of CCC recorded lowest plant height (100.7 cm). However, moderate plant height of 110.4 cm was observed in K₂SO₄ spray followed by CCC + K₂SO₄ (Table 1). The reduction in the plant height by the application of CCC might be due to the growth retardant action by reducing cell elongation and cell division. Mansuroglu et al., (2009) stated that the plant growth retardants inhibit the formation of active gibberellins and reduce the plant height. However, moderate plant height by CCC with K₂SO₄ might be due to potassium which acts as growth nutrient and enzyme activator. Potassium increased the plant height, tiller numbers and yield in berseem (Akshit et al., 2020).
 

       
Highest root length was recorded by 1% K₂SO₄ which is on par with CCC + K₂SO₄ and MC + K₂SO₄ while lowest length in absolute control (Table 1). In the present study, root length increased up to 18.4% by potassium sulphate spray alone. Increment of root length by potassium might be due to the effective translocation of photo-assimilates to root growth and its action of compatible osmolyte keeps the leaf water potential lower than that of root water potential which ultimately induces the root growth. Root length increased significantly by the application of potassium in tobacco (Rad and Boland, 2017). Interestingly, growth retardants CCC and MC also increased the root length by foliar spray alone and also along with potassium. Anosheh et al., (2016) reported that the application of cycocel increased the root length by the enhancement of IAA content.
       
Foliar application of CCC reduced the number of leaves per plant and maintained optimum number along with potassium. Among the treatments, highest number of leaves (11.3) was recorded by absolute control and CCC alone registered lowest number (Table 1). Vishal et al., (2016) found that the plant height and number of leaves were significantly reduced by CCC. The optimum number of leaves (9.0) was recorded by CCC with potassium which is on par with potassium alone (9.7). The enhancement of leaf number by the addition of potassium with CCC might be due to the role of potassium on turgor maintenance leads to increased plant height. Potassium increased the plant height and number of leaves in Brassica rapa through increased protein synthesis (Truong, 2017).
       
The highest LAD value of 46.10 days was recorded by CCC with potassium which is on par values with K₂SO₄ alone (45.90 days). The increment of LAD by CCC might be due to maintaining leaf area and LAI through increased retention capacity. The present study revealed that the increment of leaf area by CCC and MC might be the reason for the improvement of LAD. Sivakumar and Surendar (2020) reported that the foliar application of brassinolide along with potassium showed superior LAD value might be due to maintaining leaf area in finger millet.
       
CGR explains that the dry matter accumulation per unit land area and unit time. Foliar spray of 1% K₂SO₄ recorded superior CGR value of 14.10 followed by CCC with potassium (13.92). Absolute control recorded least value (11.69) of CGR (Table 1). There was an increment of 20.6% and 19.1% CGR values by the application of K₂SO₄ alone and CCC with K₂SO₄ respectively. The positive role of potassium on CGR might be due to the effective transport of photo-assimilates to various parts of plants. Mumtaz et al., (2012) found that the increasing rate of potassium increased the LAI and CGR in canola cultivars. Foliar application of CCC increased the CGR value in mustard (Banerjee et al., 2012).
 
Impact of plant growth retardants and potassium on gas exchange parameters
 
Application of K₂SO₄ alone and CCC with K₂SO₄ increased the photosynthetic rate and transpiration rate. Among the treatments, CCC + K₂SO₄ recorded higher photosynthetic rate of 26.84 which is on par with potassium alone (26.45) and the lower was recorded in absolute control (Fig 1). The beneficial effect of CCC and MC on photosynthetic rate is due to the increment of CO₂ conductance through stomatal movement in cotton (Kumar et al., 2005).
 

       
CCC with potassium increased the photosynthetic rate up to 14.2% and potassium alone 12.5% over control. Increased photosynthetic rate by potassium might be due to its action on stomatal opening. Rad and Boland (2017) noticed that the addition of potassium increased the photosynthetic rate under drought condition in tobacco.
       
Foliar spray of CCC with potassium increased the transpiration rate and reduced the leaf temperature compared to unsprayed one. The highest transpiration rate of 17.94 was registered by CCC + potassium which is on par with MC + potassium (17.56) and potassium alone (17.42). An increment of 15.6% transpiration rate was observed by CCC with potassium (Fig 2). Foliar spray of CCC and MC increased the transpiration rate was associated with increased stomatal conductance in soybean genotypes (Kumar et al., 2002). Potassium plays a major role on stomatal opening through its compatible osmolyte nature thus increased the transpiration rate. Zaina and Ismail (2016) reported that enhancement of transpiration rate was observed by the application of potassium in rice.
 

       
Absolute control registered higher leaf temperature of 30.6°C followed by water spray control (30.4°C). However, the lowest leaf temperature of 29.1°C was found in 1% K₂SO₄ which is on par with CCC + K₂SO₄ (29.2°C). Maintenance of transpiration rate under rainfed condition is an essential to maintain the photosynthetic rate and leaf temperature. In the present study, leaf temperature was reduced up to 1.5°C and 1.4°C by the application of potassium alone and CCC with potassium respectively compared to absolute control. Foliar application of cycocel reduced the leaf temperature in tuberose (Khondoker et al., 2019). The reduction of leaf temperature by the addition of potassium might be due its stomatal opening function. Athnere et al., (2020) found that the foliar spray of 1% potash recorded minimum leaf temperature and higher grain yield in summer greengram.
 
Impact of plant growth retardants and potassium on grain yield and BC ratio
 
The highest grain yield of 2.13 tonnes ha^-1 registered by CCC + K₂SO₄ which is on par with K₂SO₄ alone (2.11) and MC + potassium (2.10) while the lowest was recorded in absolute control (1.80). The grain yield increased up to 18.3% by CCC with potassium and 17.2% by K₂SO₄ alone (Fig 3) compared to absolute control. The yield increment by CCC + K₂SO₄ might be due to the increased root length, CGR, LAD and photosynthetic rate which are directly contribute to the photosynthesis and ultimately yield.
       

 
Foliar application of CCC increased the grain yield in wheat up to 13.5% compared to control under water deficit condition (Jiriaie and Sajedii, 2012). Qin et al., (2020) reported that the foliar application of CCC reduced the plant height and increased the grain yield in wheat. Split potassium fertilization increased the dry matter content and quality parameters in tomato (Vasileva and Dinev, 2021). The highest benefit cost (BC) ratio of 2.75 was registered by potassium alone followed by MC + potassium (2.33) and CCC + potassium (2.32). Even though, the highest grain yield was recorded by CCC with potassium, lower BC ratio was found due to higher cost of CCC than MC and potassium.

Foliar spray of CCC (250 ppm) with K₂SO₄ (1%) at 35 days after sowing recorded higher grain yield of 2.13 tonnes ha^-1 with the yield increment of 18.3% compared to absolute control. However foliar spray of 1% potassium sulphate alone recorded 2.11 tonnes grain yield per hectare with highest BC ratio of 2.75 in foxtail millet. Therefore, foliar spray of 1% K₂SO₄ may be recommended to increase the grain yield and also attain higher benefit cost ratio in foxtail millet under rainfed condition.