Keeping in view the aforementioned facts, we hypothesized that maize cultivars respond differently to different doses of foliage-applied K under varying levels of drought stress. The present study was carried out to assess the potential of K in alleviating the adverse effects of DS and improving water relations, photosynthetic characteristics and the growth of maize genotypes grown under well-watered and DS conditions.
Adequate exogenous supplies of K have shown beneficial effects in maintaining dry matter production, water retention, and membrane stability, compared to low K nutrition under drought stress conditions [ 25 ]. Similarly, K is essential under drought stress for maintaining cell turgor through osmotic adjustment [ 28 ] and transpiration through stomatal regulation [ 29 ]. K is generally a soil-applied mineral; however, its uptake by crop plants remains a challenge under moisture deficit conditions. Therefore, its deficiency in plants can be corrected through foliar application, which results in the rapid absorption and transportation in leaf tissues, leading to significantly higher crop yield [ 30 ]. Moreover, at the early plant growth stages, the root system may not have developed well enough to uptake nutrients from the soil. In such cases, foliage-applied K could be a viable option for supplying K to maize plants [ 31 ]. Thus, applying the optimum concentration of K through foliar application under moisture deficit conditions may be a realistic technique to enhance crop production [ 32 34 ]. Potassium application improved antioxidant activity, which imparted drought resistance in plants [ 33 ]. In addition, the metabolisms of plants depend on K concentration [ 25 27 ], while balanced K application assisted plants in boosting water use efficiency and grain yield. Moreover, foliar K enabled the plant to maintain cell turgor pressure under drought stress. The reduction in K application decreased the osmotic pressure and relative water content (RWC) in the plants, leading to a significant decline in the growth and yield of maize [ 35 ]. However, serious research gaps exist pertaining to the optimal doses of K for ameliorating the adverse effects of drought in maize under rainfed conditions. Moreover, very scant information is available regarding K dosage optimization for varying genotypes of maize under drought stress.
Under water stress, plant growth also gets suppressed, owing to the disruption of mineral nutrient transportation from the soil solution to the roots. Low soil moistures restrict root growth and therefore lower the uptake of nutrients through the roots [ 17 ]. Among all the macronutrients, potassium (K) is used as a stress alleviant plant nutrient that diminishes the adverse effects of abiotic stresses by improving the physiochemical as well as biological activities in plants [ 23 26 ]. Potassium application enables plants to survive under water deficit conditions by regulating rooting density, the turgidity of cells and the osmotic potential of cell walls [ 24 ]. The balanced application of K fertilizers improves plant yield and water use efficiency under moisture deficit circumstances [ 25 27 ].
Globally, maize, (L.) occupies a pivotal position among cereals in ensuring food security by virtue of its huge area under cultivation (197 m. ha) and production (1148 m.t.) [ 1 2 ]. It has the potential to adapt to a wide range of physiographic, soil, and climatic conditions [ 3 4 ]. However, recent climate changes, especially abiotic stresses including drought, are imposing a pronounced fluctuation in maize production, increasing the risk of food insecurity [ 5 7 ]. Moisture deficiency has emerged as one of the most serious threats in arid and semi-arid climates by causing up to 40% yield reduction in maize [ 8 10 ]. Drought stress (DS) adversely affects root proliferation [ 11 ], crop morphology and physiology, hence reducing crop yield [ 12 ]. DS also decreases the transportation of water and nutrients from the soil to the plant body, which reduces carbon dioxide (CO) assimilation and nutrient uptake [ 13 ]. This decline in nutrient uptake causes stunted growth and pronouncedly decreases crop productivity under the limited water conditions [ 14 ]. In addition, adverse effects imparted by DS depend on the intensity and duration of stress [ 15 16 ]. DS at critical growth stages, such as the seedling, vegetative or reproductive stages [ 17 18 ], leads to a severe reduction in relative water content and crop yield. Moreover, water deficit conditions at earlier growth stages when roots have not been fully developed results in stunted plant growth and a reduction in the binding of CO 20 ]. This leads to a significant reduction in the assimilation of photosynthates during the grain filling period, which produces shriveled grains and lower yield [ 19 22 ].
This experiment was conducted at the greenhouse of the Department of Horticulture, PMAS-Arid Agriculture University, Rawalpindi, Pakistan, to study the effect of potassium sulfate (K2SO4) on two maize cultivars, Islamabad gold (drought tolerant) and Azam (drought susceptible), grown under DS. These cultivars were collected from the National Agricultural Research Council, Islamabad, Pakistan, and are high yielding and locally preferred for cultivation. The design of the experiment was a completely randomized design (CRD) with three replicates for each treatment. Earthen pots 20 cm in diameter and 45 cm in height were filled with an 8 kg mixture of soil and farmyard manure (FYM). Soil and FYM were mixed in a 2:1 ratio, and 6 seeds per pot were manually sown to avoid germination constraints. A soil sample from pot soil indicated that the soil was alkaline in nature with a 7.20 pH, an electrical conductivity of 0.75 dS m−1, 0.48% organic matter and 0.03% N, along with available phosphorous and K contents of 4.30 and 78 mg kg−1, respectively. Both maize cultivars were grown in pots under well-watered conditions. Thinning was done manually at the 3-leaf stage, and only 2 plants per pot were kept to maintain the optimum plant population and promote the establishment of seedlings. Drought was imposed after one week of thinning. For imposing drought, pots were divided into three sets, and drought was maintained as follows: (i) well-watered (WW) (80% water holding capacity-WHC); (ii) mild drought (MD) (60% WHC); and (iii) severe drought (SD) (40% WHC). To maintain WHC, pots were weighed after every 3–4 days of drought imposition, and a difference in weight was achieved by adding water to attain the required weight. After one week of imposing drought, a foliar spray of water and K2SO4 was applied twice in 3-day intervals. For this purpose, plants were treated as follows: (i) water spray (control) using distilled water; (ii) 1% K2SO4 (1 g K2SO4 was dissolved in 1L of distilled water); (iii) 2% K2SO4 (2 g K2SO4 was dissolved in 1L of distilled water), whereas Tween 20 at 0.2 mL L−1 was used as a surfactant. Before spraying, the soil surfaces of treated pots were covered with polythene sheets to avoid contamination. NPK, in the form of urea, DAP and SOP, was applied in quantities of 0.6, 0.27 and 0.06 g per pot, respectively. Seedlings were harvested 60 days after sowing for the measurement of morphological traits.
RWC = (FW − DW)/(TW − DW) × 100
For measuring leaf relative water content (RWC), samples were randomly collected from each treatment and packed into polythene bags. The leaves’ fresh weight (FW) was measured by using a digital weighing balance. Then, the leaves were soaked in distilled water for 24 h to make them turgid, and then the turgid weight (TW) was measured using a digital weighing balance. Thereafter, the leaves were oven-dried at 75 °C for 72 h, and subsequently, the dry weight (DW) was measured. The RWC was calculated by using the below formula:
FW = Fresh weight of leaves
DW = Dry weight of leaves
TW = Turgid weight of leaves.
Gas exchange parameters, viz. stomatal conductance (Gs), photosynthesis rate (Pn) and transpiration rate (Tr), were measured using an infrared gas analyzer (IRGA Leaf Chamber Analyzer, Type LCA-4, manufacturer, city and state abbreviation, USA) two weeks after the foliar spray of KSO. For this purpose, fully developed expanded leaves were selected from each experimental pot and inserted in an IRGA Leaf Chamber Analyzer (Type LCA-4, manufacturer, city and state abbreviation, USA) [ 36 ]. Measurements were taken from 9:00 a.m. to 11:00 a.m. on a clear, sunny day by keeping the COconcentration at 400-μmol mol 36 ].
Chlorophyll a (mg/g FW) = [12.7(OD 663) − 2.69(OD 645) × V/1000 × W]
Chlorophyll b (mg/g FW) = [22.9(OD 645) − 4.68(OD 663) × V/1000 × W]
Total Chlorophyll (mg/g FW) = [20.2(OD 645) − 8.02(OD 663) × V/1000 × W]
Chlorophyll (a, b and total) contents were measured by using the method proposed by Ref. [ 37 ]. About 1 g of fresh leaves were taken and divided into segments that were kept in 80% acetone solution for a whole night. Then, samples were centrifuged at 14,000 rpm for 5 min, and supernatant was observed at 645 nm and 663 nm wavelengths using a UV spectrophotometer (Unicam 8620). Chlorophyll a, b and total contents were measured using the following formulawhere V = volume of the leaf extract (mL), W = weight of fresh leaf tissue (g).
µmol proline g−1 fresh weight = (µg proline mL−1 × mL of toluene/115.5)/(g of sample).
The proline content was measured according to the procedure developed by [ 38 ]. Homogenization of 0.5 g of fresh leaves was done with 10 mL of 3% sulfosalicylic acid (C7H6O6S) solution. Filtration of the homogenized mixture was done with 2 ml of acid ninhydrin and 2 ml of glacial acid in a test tube. The mixture was incubated for 60 min at 100 °C. Then, the mixture was cooled in an ice bath and 4 mL of toluene was mixed into it while stirring simultaneously for 60 s. A spectrophotometer with 520-nanometer wavelength radiations was used to measure chromophore absorption. Proline contents were calculated on the basis of fresh weight from the optimized curve using the following formula:
Plants were uprooted two weeks after foliar application of K2SO4 from well-watered and drought-stressed treatments, and the shoot length (SL) of plants was measured using a meter scale. After measuring the SL, the roots were separated, and their root length (RL) was measured using the same scale and then averaged to determine the RL per plant. After the measurement of the RL, the shoot and root from each treatment were placed into envelopes and dried in a hot air oven at 75 °C for 72 h. After drying the samples, the shoot dry weight (SDW) and root dry weight (RDW) was measured using a digital weighing balance.
Data for different attributes were analyzed using Statistix 8.1 software to determine the significance among the treatments. Tukey’s test at a 5% probability level was executed to find out the significant differences among treatment means [ 39 ]. Bar graphs using standard error were prepared in Microsoft Excel 2007.