Will future maize improvement programs leverage the canopy light-interception, photosynthetic, and biomass capacities of traditional accessions?

Experimental location

Plant screening trials were carried out at the Agro Tech Park, Malwatta (7°20′N and 81°44′E, altitude 16.0 m above sea level), in the Ampara region of Sri Lanka. This experimental test location is categorized as a dry zone, with sandy loam soils. The average monthly rainfall and temperature readings are 127.1 mm and 30.3 °C, respectively (Mubarak et al., 2022).

Plant material, growth conditions and experimental design

The experimental site was ploughed at the depth of 40–45 cm and levelled to facilitate soil physical properties that encourage maize plant root penetration (du Plessis, 2003). The field layout was designed with a randomized complete block design (RCBD), comprised of 10 treatments (10 maize germplasms) with four replicate (plots)/treatment (1.5 × 3.0 m² ). Ten plants were grown in each treatment plot. The tested maize germplasm included Pacific-999, an imported F₁ hybrid variety from Thailand, and eight high-performing local maize accession, which were designated as SEU2, SEU6, SEU9, SEU10, SEU14, SEU15, SEU16, and SEU17, and cv.Bhadra (control plant), a popular open pollinated recommended variety is released by the Department of Agriculture in Sri Lanka and was sowed at a spacing of 30 × 60 cm. Subsequently, the following standard agronomic practices were implemented as recommended by the Department of Agriculture, Sri Lanka, as follows. Seeds were directly sown into the field after being soaked for 24 h to encourage germination. A total of 1 week prior to planting, the basal fertilizer dressing was applied at the rates of 75 kg/ha of urea, 100 kg/ha of P₂O₅, and 50 kg/ha of K₂O. Another 250 kg/ha of urea was used as a top dressing after the first 4 weeks of crop establishment, and then earthing up was done between crop rows to reduce weed development. The flooding irrigation method was used to provide the required amount of water throughout the cropping season.

Plant growth and development measurements

Maize seed germination count was carried out 3 days after sowing and continued monitoring on daily basis until the first week of field planting to estimate germination percentage. The leaf count from randomly selected five plants per plot was recorded every week until the flag leaf of the plant emerged to calculate the average leaf development rate per week. Followed by the flag leaf development, the mean height of the crop stand was measured from five randomly selected plants from each plot and the measurement was taken from the soil surface to the flag leaf using 1 mm least count measuring tape. The mean days taken to 50% tassel emergence were recorded for each accession and compared. At the harvesting stage, the number of well-developed cobs per plant was counted. The plant physiological measurements were recorded at three distinct growth and developmental stages of maize accessions. At the early vegetative stage, (three weeks after field planting, three WAP) tasseling stage (six WAP) and grain filling stage (10 WAP).

Photosynthetically active radiation and leaf area index measurements

A ceptometer (AccuPAR, LP-80; Meter Group Inc., Pullman, WA, USA) was used to measure photosynthetically active radiation (PAR). When the exterior PAR sensor measurement was greater than 600 μmols m⁻¹ s⁻¹, the device was calibrated in accordance with the manufacturer’s instructions. Thereafter, the PAR at the above crop canopy was measured with an external light sensor (Apogee SQ110; Apogee, Santa Monica, CA, USA), while the below PAR was measured by placing 5 cm above the ground with the 80 cm probe which contains 80 independent sensors, spaced 1 cm apart (LP- 80, AccuPAR; Meter Group Inc., Pullman, WA, USA). The photosensors measure photosynthetically active radiation in the 400 to 700 nm waveband by placing it 5 cm above the ground. To reduce the portion of diffuse radiation into the plant canopy, at least 8–10 measurements were taken per plot between 10.30 a.m. and 1.30 p.m. Subsequently, light interception percentage (LI %) per individual canopy was calculated at the early vegetative stage (three WAP), tasseling (six WAP) and grain filling (10 WAP) stages respectively. Thereafter, the LAI was estimated in the above stages using the leaf distribution parameter (χ) and the ratio of below and above PAR level (Francone et al., 2014). Regression lines were fitted with LAI verse LI % to find out the optimum amount of solar radiation that the crop canopy be achieved. Subsequently, the light extinction coefficients (k) for radiation of each maize accession were calculated by applying Eq. (1) as stated by Monsi & Saeki (1953) and Lambert-Beer law. Then the graphs were plotted against ln (I/Io) vs LAI, which specifies k and the crop canopy architecture (Azam-Ali & Squire, 2002).

(1) $$I/I_o=e^{-kLAI}$$where I_o and I are above and below canopy PAR values. Subsequently, the fraction of radiation intercepted by the crop canopy (f) was computed (Eq. (2)).

(2) $$f=1-e^{(-kL)}$$

Leaf chlorophyll fluorescence parameters

A portable fluorometer (FluorPen FP 100; Photon Systems Instruments, Drasov, Czech Republic) was used to measure maximum quantum yield photosystem II (PSII) at light (Qy_light: Fv′/Fm′) and dark (Qy_dark: Fv/Fm) adapted leaf from each treatment and control. The flash pulse of the blue light LED emitter (455 nm) with the maximum intensity was set to 3,000 µmol m⁻²s⁻¹. The Qy_light measurements were taken on the fully developed youngest leaves from randomly selected five plants. The same leaves were covered with aluminium foil for 30 min to take Qy_dark (Naidoo & Naidoo, 2018). All the indicated measurements were taken at the three, six and 10 WAP stages.

Chlorophyll content

The leaf chlorophyll content in the SPAD unit was measured by employing SPAD 502 plus (Konica Minoltkoptics Inc., Wayne, NJ, USA). Initially, the instrument was calibrated then it was checked by a reading checker to ensure the calibration. The measurements were taken at three, six and 10 WAP stages. The average chlorophyll content measurement was taken from the base, middle, and tip of the youngest fully expanded leaves from randomly selected five plants per field plot.

Leaf gas exchange parameters

Net CO₂ assimilation rates (A_N) (µmol CO₂ m⁻² s⁻¹), transpiration rates (E) (mmol H₂O m⁻² s⁻¹), stomatal conductance (gs) (mmol H₂O m⁻² s⁻¹) were measured with an Infrared Gas Analyzer (LI-6800; LI-COR, Lincoln, NE, USA) portable photosynthesis system. Initially, machine calibration was performed including IRGA zeroing when the machine was attached with fresh Drierite^® and soda-lime. This procedure was precisely followed to avoid any temperature-induced zero shifts after acclimating to the chamber’s conditions (Ramazan et al., 2021). Before the spot leaf measurements, the warmup test was performed to check errors and faults that might undermine measurements. Then the following environmental conditions were maintained inside the chamber: leaf area of 6 cm², leaf temperature at 30 °C, air flow rates at 600 µmol s⁻¹, the relative air humidity within leaf chamber 70 ± 1%, CO₂ concentration at reference IRGA line was set at 410 µmol mol⁻¹, the light intensity was kept at 1,700 μmol mol^-1 photons m⁻² s⁻¹ (10% blue and 90% red). On the 45^th days after field planting, the youngest fully expanded leaves from three plants per plot were measured. Then the leaf was clamped and the measurement was taken after 5 min to stabilize the photosynthetic parameters. All photosynthetic parameters were measured from 9.00 am to 12.00 noon on clear, cloudless days to ensure the plants are engaged active photosynthetic state while minimizing plant stress due to light acclimation.

Above-ground dry mass and yield components

Once the plants had achieved their stage of physiological maturity, three randomly chosen plants from each plot were chosen and cut at the ground level. To acquire the above-ground biomass, the plant parts were divided into stem and cobs and oven-dried at 80 °C until completely dry (AGDM). Additionally, the fresh cobs were taken from four randomly chosen plants within each field plot in order to quantify the yield components and allow them to air dry so that the number of grains was recorded. At a moisture level of 12 percent, individual cob weight, and 100-kernels weight were measured.

Statistical analysis

Univariate analysis of variance (ANOVA) was carried out to evaluate the significant (p < 0.05) variations in maize accessions in comparison to control (cv.Bhadra). Estimation of pairwise phenotypic correlation between physiological characteristics of maize germplasms were performed using SPSS (version 25.00) software.

Plant growth and development parameters

The features of maize growth and development are shown in Table 1. Out of 10 germplasms, SEU2 had the highest plant germination percentage (84%) followed by the control maize variety (83%). The leaf development rates were evaluated. Only one germplasm (SEU17) produced considerably higher growth rates (two leaves/week⁻¹) than that of the control, while the other assessed germplasm had roughly identical leaf development rates (1.73 week⁻¹). In comparison to the other examined germplasm, the SEU2 had displayed the earliest days required for producing 50% tassel (44 d). In contrast, SEU17 showed delayed flowering characteristic (54 d) than that of control plants (46 d). In terms of plant height, SEU17 exhibited a significantly taller plants (221 cm), closely followed by SEU2 (201 cm), SEU10, and SEU14 respectively than cv. Bhadra (165 cm).

Each maize accession’s leaf area index (LAI) was compared. The control plants typically produce a lower LAI at 3 WAP (1.44), pronounced higher at six WAP (2.3), and a lower LAI at 10 WAP (1.93), therefore the mean LAI remained at 1.90 (Table 2). However, there were appreciable differences across the treatments. Out of all screened germplasms, SEU2 produced mean LAI of 2.3 and significantly enhanced LAI at six WAP (3.2) and 10 WAP (2.3). The majority of the tested maize accessions showed a similar trend. The LAI produced by the SEU9, SEU15, and Pacific-999 at 6 WAP was considerably higher than that of cv.Bhadra.

Comparison of pattern reception of photosynthetically active radiation (PAR) and subsequent light attenuation properties in the canopies, at three WAP, six WAP and 10 WAP indicated that the SEU2 showed increasing percentage, while the control variety showed a lower percentage in all stages (47%, 64% and 63% respectively). The magnitude of the LAI and light extinction characteristics may account for variation in light attenuation (Table 3).

A strong positive regression (R² = 0.92) between LAI and LI percent (Fig. 1) was found in the tested germplasm, showing that maize canopies with lower LAI exhibited lowered light interception percent. According to regression model, the maximum of 80% light interception can be obtained when the maize canopy reaches LAI = 3.0 to 3.5, and thereafter, producing LAI beyond 3.5 did not increase the light interception. Similar to this, there was a negative association between In(I/I₀) and LAI (R² = 0.95). We estimated the light extinction coefficient (k) of each tested germplasm at various growth stages using the slope of the regression line (Table 4). The mean k-values were shown non-significantly different, ranged from 0.33 in SEU14 to 0.50 in SEU9, all maize germplasm showed an overall k of 0.5, indicating that the leaves produced were upright/eractophelic.

Additionally, the fractional light absorption (f) by the canopies of maize tends to vary depending on the stage of growth (Table 3). There were no significant differences in f between the local germplasms and the control plants, although values ranged from 0.4 in SEU 14 to 0.59 in SEU 10. The f was lower at the three WAP, reached its peak at the six WAP for canopies, and then declined at the 10 WAP. The accession SEU10 had the greatest value (f = 0.59), while SEU2 and SEU9 had similar values to cv.Bhadra (f = 0.57), the remaining treatments had values lower than 0.4, demonstrating that the maize accessions varied in their fractional interception of solar energy. The tested maize accessions did not differ significantly in terms of their chlorophyll contents (Table S1). In this case, nearly all of the accessions had chlorophyll levels in their leaves that were greater than >48.0 SPAD units.

Chlorophyll fluorescence parameters

There were no appreciable differences in the mean maximum quantum yield by photosystem II (PSII) at light (Q_{y light}: Fv/Fm) adapted leaf, which ranged between 0.64 to 0.68 (Table 5). A similar pattern was followed by Qy _dark, with the only exception being the greater value of SEU6 and Pacific-999 at six WAP.

Leaf gas exchange parameters

The photosynthetic leaf exchange parameters of maize plants were measured at six WAP (Table 6). The novel hybrid variety Pacific-999 had denoted with significantly increased rate of photosynthesis A_N (30.45 μmol CO₂ m⁻²s⁻¹) in comparison to the control plant cv.Bhadra (23.2 μmol CO₂ m⁻²s⁻¹). Importantly, the native germplasm SEU17, SEU9 and SEU2 displayed significantly increased photosynthesis rates of 29.7, 28.9 and 26.2 (μmol CO₂ m⁻²s⁻¹) respectively than that of cv. Bhadra. It was possible to observe comparable variations in E, Ci, and gs among the studied maize germplasm.

The control plants and variety Pacific-999 showed increased stomatal conductance (0.29 mmol m⁻²s⁻¹), while SEU6 had significantly lowest than that of control variety. However, there were non-significant difference among other tested germplasm. Transpiration rates ranged between 2.45–3.98 mmol m⁻²s⁻¹ among the tested germplasm and SEU6 showed significantly lower than the control, whereas there was no significant difference among other tested germplasm Fig. 2 and Table 7 show the analysis of the biomass and yield characteristics of maize accessions. With regard to the yield components, SEU2 produced significantly higher above-ground dry mass (AGDM) of 458.4 g Plant⁻¹ than that of the control (290 g Plant⁻¹) (Fig. 2A). The remaining germplasm typically had AGDM values that were comparable to the control plant. Similar trend was seen for cob yield per plant basis, SEU2 produced the heaviest cobs (231.5 ± 2.2 g ) among the germplasm examined (Fig. 2B); which was significantly higher than the control (158.17 ± 1.63 g). Moreover, there were significantly higher 100 grain weight observed in SEU2, 9, 15, 16, and Pacific-999 among them (Fig. 2D). The lengthiest cob was observed in SEU2 closely followed by SEU15, SEU6, and Pacific-999 were significantly longer than the the cv.Bhadra. The highest number of kernels were seen in a single row of cob in Pacific-999 followed by SEU2.

Interesting characteristics of the maize accessions were revealed by the correlation matrix among the plant attributes. Overall, a significantly positive correlation between the percentage of PAR light interception (LI %), and A_N (0.21), AGDM (0.29) and weight of 100 kernels (0.34). Although the number of kernels developed in a cob (0.37) was shown significantly negative, indicating that the light penetration throughout the canopy is advantageous for the maize plant’s ability to synthesize photosynthates and engage carbohydrate utilization mechanisms. Additionally, significant correlation was evident between photosynthetic traits and grain yield characteristics in maize accessions, notably with respect to AGDM (0.59), cob weight (0.61), and kernel weight (0.385), respectively (Table S2).