Chlorophyll content in garlic leaves
As shown in Table 1 and Fig 2, Chl-
a and Chl-
b content in Garlic leaves were reduced under drought stress (T2) compared to the well-watered control (T
1). Here, reduced fresh weight, increased MDA and proline content in Garlic leaves (T
2) (Table 1) indicates that water scarcity has induced oxidative stress in plants.
Abid et al.,(2018) revealed that stomatal closure in drought stressed-wheat leaves, reduces carbon assimilation, cause an imbalance between excited electrons and their use in light reaction to trigger overgeneration of reactive oxygen species (ROS). Chlorophyll content in Garlic leaves might have reduced due to oxidative damage of chloroplast thylakoid membrane and inhibition of chlorophyll biosynthesis enzymes by ROS. Our suggestions were in favor of other findings that drought reduced chlorophyll content in maize hybrid varieties Xida-319 and Xida-889 (
Hussain et al., 2019) and Wheat
(Pozo et al., 2020). Routinely used fertilizers (BDF) (T3) insignificantly improved Chl-
a and Chl-
b content in Garlic leaves as compared with drought Garlic leaves (T
2). Supplementation of silica sprays to fertilizers (BDF) significantly increased leaves Chl-
a and Chl-
b content (T
4, T
5, T
6) in order 7.5 ml L
-1 <12.5 ml L
-1 <10.0 ml L
-1 over the Garlic grown under drought without any treatments (T
2). Previous studies of
Ma, (2012),
Sapre and Vakharia, (2016),
Greger et al., (2018),
Alamri et al., (2020) suggested that silicon deposits at the leaf bundle sheath and form the silica cuticle layer (epidermis) to reduce the transpirational water loss in plants. In our study, reduced MDA level and highly significantly increased proline (osmoprotectant) content in garlic treated with BDF+silica sprays (T
4, T
5, T
6) as compared to drought garlic (T
2) shows that silica has balanced water content in plants and might have prevented Chloroplast thylakoid membrane from oxidative damage. Our study is in favour with
Dehghanipoodeh et al., (2018) that Potassium silicate (K
2SiO
3) (10 mmol L
-1) maintained the water content of the Strawberry plants.
Sun et al., (2021) also showed that 15 g L
-1 of water-soluble silicon (Si-50G, Si-60G) has improved proline, sugar and protein (osmolytes) content to prevent membrane damage and increased chlorophyll content in maize seedlings.
Growth attributes of garlic plants
In our study, the Garlic plant’s growth was assessed based on height and the number of leaves on the plants. As shown in Table 2 and Fig 3, drought (T2) reduced garlic plant height significantly and the number of leaves plant
-1 insignificantly over the well-watered plant (T
1). It is evident that plants absorb growth nutrients from soil in a water-soluble form. Here, garlic plant’s height were reduced might be due to insufficient availability of water-soluble nutrients from soil. Secondly, reduced light-capturing pigments (Chl) possibly affected plants photosynthetic activity and diminished garlic plant height in drought stress. The basal dose of NPK fertilizers (T
3) has slightly improved plant growth against drought stress (T2). However, BDF+silica sprays (T
4,5,6) have significantly increased plant height over the drought plants(T
2) in order, 7.5 ml L
-1 <12.5 ml L
-1 <10 ml L
-1. Our study is in agreement with
Kowalska et al., (2020) that foliar sprays of orthosilicic acid (Si) (24%) increased plant height and number and weight of ear in spring wheat. Si increased plant growth by balancing water and nutrient content (
Rizwan et al., 2015). Similarly, 1000 kg ha
-1 treatment of potassium silicate (K
2SiO
3) increased the pH of the soil and P, Ca, Zn, S, Mo, Mn and Cu nutrients available to the plants (
Greger et al., 2018). 250 kg ha
-1 dose of Si (Diatomaceous earth, Calcium silicates, Bagasse ash) with a basal supply of fertilizer increased plant growth by enhancing nitrogen and potassium intake (
Gade et al., 2019),
(Minden et al., 2020). Various Osmolytes such as proline, antioxidants and secondary metabolites helps plants to combate drought stress (
Kusvuran and Dasgan, 2017). Silicon enhanced proline and antioxidants in wheat to reduce oxidative stress in term of H
2O
2 level (
Sapre and Vakharia, 2016). Similarly in our study, silica prevented plants lodging, increased proline content, reduced MDA level that might have increased photosynthetic food synthesis and uptake of essential nutrients from the soil for plants vegetative growth.
Garlic bulb quality
Garlic bulb quality is the main criteria for the farmers to earn a profitable income. A perusal of our data presented in Table 2 and Fig 4, revealed that drought (T
2) considerably reduced the fresh-dry weight and diameter of the Garlic bulb over the control (T
1). As per
Omar and Wabel (2010), the Garlic bulb is mainly constituted by water (65%), sugar, protein and fiber. Water deficiency and reduced leaves chlorophyll content might have declined photosynthetic food storage and affected bulb weight and diameter negatively. However, foliage applied silica sprays (7.5, 10, 12.5 ml L
-1) + NPK fertilizers (T
4,T
5,T
6) in the soil improved bulb diameter, fresh-dry weight in contrast to the bulbs grown under limited water supply (T
2). This is in favor with
Bhangare and Shinde (2020) that foliarly applied silicon has improved onion bulb diameter.
Dorairaj et al.,(2020) showed that silicon increased uprightness in rice plants and thus due to more photosynthetic activity weight of the panicle and 100 rice grains was increased. Foliage applied Si protected Soybean from lodging by lignin deposition and increased photosynthetic rate in soybean leaves.
Magd et al., (2013) reported that bulb weight depends on the quantity of organic food such as sugar and protein transferred from leaves to the bulb.
Hussaina et al., (2021) showed that foliage applied Si protected Soybean from lodging by lignin deposition and enhanced photosynthetic rate in Soybean leaves. Our studies show that silica mitigated water stress and increased chlorophyll content in Garlic plants (T
4, T
5, T
6) as compared to drought plants. Sufficient water availability and increased chlorophyll content in silica treated leaves might have increased photosynthetic food synthesis and translocation and accumulation of synthesised food to bulb increased bulb weight and diameter compared with the drought bulbs.