Radiation interception, photosynthetic performance and dry matter accumulation of onion (Allium cepa L.) under an onion-okra sequence cropping system

Introduction

Onion (Allium cepa L.) is the third most important vegetable crop in the world after potato and tomato. It is cultivated in about 58.42 lakh ha across nearly 140 countries, with an annual production of 111.27 million tonnes (Gupta et al. 2026) [7] . India has the largest area under onion cultivation (17.40 lakh ha) and is the leading producer (30.21 million tonnes), followed by China (11.29 lakh ha and 24.92 million tonnes). Together, India and China contribute nearly half of the world's onion production. The global average productivity of onion is 27.06 t ha -1 , while the Republic of Korea records the highest productivity (67.87 t ha -1 ). Despite being the largest producer, India has a relatively low productivity of 17.36 t ha -1 , ranking 93 rd globally. In India, onion is mainly cultivated in Maharashtra, Karnataka, Gujarat, Bihar, Madhya Pradesh, Rajasthan, Andhra Pradesh and Tamil Nadu, with Maharashtra and Karnataka contributing about 57% of the country's total production (Gupta et al. 2026) [7] . Onion contains carbohydrates (11.0 g), proteins (1.2 g), fiber (0.6 g), moisture (86.8 g) and several vitamins like vitamin A (0.012 mg), vitamin C (11 mg), thiamin (0.08 mg), riboflavin (0.01 mg) and niacin (0.2 mg) and also some minerals like phosphorus (39 mg), calcium (27 mg), sodium (1.0 mg), iron (0.7 mg) and potassium (157 mg) (Suresh, 2007) [22] . The outstanding characteristics of onion are the pungency, which is due to a volatile compound known as "Allylpropyl disulphide". It has got the effects of lowering the blood sugar fat and also having good coagulation effect (Vashi et al. 2011) [24] . Absorbed photosynthetically active radiation (APAR) and photosynthetic rate are key physiological determinants of crop productivity, as they regulate the conversion of intercepted solar radiation into biomass through photosynthesis. Efficient absorption and utilization of photosynthetically active radiation are essential for improving dry matter accumulation, growth and yield in onion (Gitelson et al. 2015) [4] . The availability of soil moisture and nutrients strongly influences canopy development, radiation absorption and photosynthetic efficiency, thereby affecting overall crop performance (Gregory et al. 1997) [6] . Although several studies have evaluated the individual effects of irrigation or nutrient management on onion productivity, information on their combined influence on APAR and photosynthetic rate under an onion-okra sequence cropping system remains limited. Therefore, understanding the physiological responses of onion under different irrigation and fertigation regimes is essential for developing resource-efficient production strategies and enhancing crop productivity. It was hypothesised that the optimisation of irrigation and fertigation under an onion-okra sequence cropping system would improve the absorption of photosynthetically active radiation, enhance photosynthetic efficiency and promote dry matter accumulation in onion. The combined application of appropriate irrigation scheduling and fertigation was expected to improve crop physiological performance through better water and nutrient availability. Therefore, the present study was undertaken to evaluate the effects of different irrigation regimes and fertigation levels on dry matter accumulation, absorbed photosynthetically active radiation (APAR) and photosynthetic rate of onion (Allium cepa L.) under an onion-okra sequence cropping system.

Materials and Methods

Experimental site

The present investigation was conducted rabi and summer seasons of 2023-24 and 2024-25 at Instructional Farm, All India Coordinated Research Project (AICRP) on Irrigation Water Management, MPKV, Rahuri is situated between 19° 36' 58" and 19° 36' 65" North latitude and between 74° 64' 40" and 74° 36' 44" East longitude. The altitude is 521 meter above mean sea level.

Experimental design and treatments

The experiment was conducted using a split-plot design with three replications, comprising 18 treatment combinations. The main plot treatments comprised three levels of irrigation based on crop evapotranspiration (ETc): 1.0 Etc (I 1 ), 0.8 Etc (I 2 ) and 0.6 Etc (I 3 ). While the sub-plot treatments included six fertigation levels: 100% N: P 2 O 5: K 2 O kg ha -1 through fertigation with no basal (F 1 ), 75% N: P 2 O 5: K 2 O kg ha -1 through fertigation with 25% N: P 2 O 5: K 2 O as basal (F 2 ), 75% N: P 2 O 5: K 2 O kg ha -1 through fertigation with no basal (F 3 ), 56% N: P 2 O 5: K 2 O through fertigation with 19% N: P 2 O 5: K 2 O kg ha -1 as basal (F 4 ), 100% N: P 2 O 5: K 2 O kg ha -1 through conventional fertilizers (F 5 ) and Absolute control (F 6 ). Irrigation treatments will be scheduled at an alternate day and depending on crop evapo-transpiration rate. The drip irrigation system consisted of 16 mm laterals with in-line drippers of 4.0 lph discharge spaced at 50 cm distance. Nutrients were applied through fertigation in 10 weekly splits according to crop growth stages of rabi onion. Nitrogen was applied uniformly throughout the crop period with 20, 30, 30 and 20% during 1-15, 16-45, 46-75 and 76-90 DAT, respectively. Phosphorus was applied in higher proportion during early growth stages to support root establishment and vegetative growth, while potassium application was increased during bulb development stage for better bulb enlargement and quality. Water soluble fertilizers viz., Urea, 19:19:19 and 12:61:00 were used as nutrient sources. Observations for dry matter, APAR and were recorded at 30, 60 and 90 DAT and at harvest.

Soil type

The soil of the experimental field (0-30 cm depth) was characterised as clayey in texture, alkaline in reaction (pH 8.13), moderate in organic carbon (0.49%), low in available nitrogen (254.3 kg ha -1 ), moderate in available phosphorus (18.7 kg ha -1 ) and very high in potassium (461.3 kg ha -1 ), respectively. The soil moisture constants viz., field capacity (33.33%), permanent wilting point (16.43%), bulk density (1.31 g cm -3 ) and porosity (49.34%) indicate that soil was good in water retention capacity. The available nutrient status of the soil was 254.3 kg ha -1 nitrogen, 18.7 kg ha -1 phosphorus and 451.3 kg ha -1 potassium.

Weather data

Weather data was collected daily from Meteorological Observatory located at AICRP on Irrigation Water Management Project, MPKV, Rahuri.

Five plants plot -1 were sampled randomly for determination of total dry matter including component parts i.e. leaves and bulb. The leaves and bulb were separated and chopped into small pieces, packed into brown paper bags and labeled properly. The samples were first air dried and then dried in hot air oven at about 65 ± 2°C temperature for 24 hours, until the final constant weight was observed.

Measurement of absorbed photosynthetically active radiation

Absorbed photosynthetically active radiation (APAR) was measured following the method described by Gallo and Daughtry (1986) [3] . Measurements were taken on a clear sunny day between 11:00 am and 1:00 pm to ensure uniform solar radiation. A LI-191SA Line Quantum Sensor (Li-COR, USA) was used for recording PAR values. The observations were recorded across the rows. Incident (PARi) was recorded by placing the line quantum sensor 1 ft above the crop canopy, facing upward towards the sky (upward direction). Transmitted PAR (PARt) was measured at ground level beneath the canopy with the sensor facing upward. Reflected PAR by soil (PARs) was measured by holding the sensor 1 ft above the soil surface and facing downward towards the soil, while reflected PAR by canopy plus soil (PARr) was measured by positioning the sensor 1 ft above the canopy and facing downward towards the crop canopy. Absorbed photosynthetically active radiation was calculated using the formula:

Where, APAR = absorbed photosynthetically active radiation PARi = Incident radiation PARc = reflected photosynthetically active radiation by canopy PARt = transmitted photosynthetically active radiation PARs = reflected photosynthetically active radiation by soil

Measurement of photosynthetic rate

The net photosynthetic rate (Pn) of onion leaves was measured using a LI-6400 Portable Photosynthesis System (LI-COR Biosciences, Lincoln, Nebraska, USA). Observations were recorded on the fully expanded, healthy third leaf from the top of five randomly selected plants in each treatment during the morning hours (09:00-11:00 h) under clear sky conditions to minimize diurnal variation and to ensure stable light and atmospheric conditions. The selected leaf was enclosed in the leaf chamber of the portable photosynthesis system and the net photosynthetic rate was determined based on infrared gas analysis (IRGA) following the manufacturer's operating procedure. For each plant, observations were taken on upper, middle and lower fully expanded leaves and the average value was used for statistical analysis. The photosynthetic rate was expressed as µmol CO₂ m -2 s -1 . Principle: The Infrared Gas Analyzer (IRGA) operates on the principle that carbon dioxide (CO 2 ) and water vapour (H 2 O) selectively absorb infrared radiation at specific wavelengths. During measurement, the leaf is enclosed in a chamber through which air of known CO 2 concentration is circulated. The reduction in CO 2 concentration due to photosynthesis and the increase in water vapour concentration due to transpiration are measured by infrared detectors. Based on these gas exchange measurements, the instrument automatically computes the photosynthetic rate.

Statistical analysis and interpretation of the data

The experimental data recorded during the two years of investigation were subjected to analysis of variance (ANOVA) appropriate for a split-plot design as described by Gomez and Gomez (1984) [5] . The irrigation regimes were considered as the main-plot treatments, whereas the fertigation levels were assigned to the sub-plots. The significance of treatment effects was tested at the 5% probability level (P ≤ 0.05). Whenever the treatment effects were found to be significant, treatment means were compared using the Critical Difference (CD) test at the 5% level of significance. Standard error of the mean [SE(m) ±] and critical difference (CD) values were computed for all the parameters. Pooled analysis over the two years was carried out after testing the homogeneity of error variances following the procedure described by Gomez and Gomez (1984) [5] .

Results and Discussion

The influence of irrigation regimes and fertigation levels on the physiological growth parameters of onion (Allium cepa L.), namely dry matter accumulation, absorbed photosynthetically active radiation (APAR) and photosynthetic rate, under an onionokra sequence cropping system was investigated during the study period. The experimental findings are presented and discussed in relation to the treatment effects and relevant published literature. Dry matter (g plant -1 ) The periodical data on dry matter accumulation of onion as influenced by different irrigation regimes and fertigation levels are presented in Table 1.a and illustrated in Fig. 1. Dry matter accumulation increased progressively with crop growth, reflecting continuous biomass production and assimilate partitioning into vegetative and bulb tissues. The mean dry matter accumulation across treatments increased from 1.51 g plant -1 at 30 DAT to 5.71 g plant -1 at 60 DAT, 9.19 g plant -1 at 90 DAT and reached 14.49 g plant -1 at harvest on a pooled mean basis.

Effect of irrigation regimes

Irrigation regimes had no significant effect on dry matter accumulation at 30 DAT during either year. However, significant differences were observed at 60 and 90 DAT and at harvest. Irrigation scheduled at 1.0 ETc (I 1 ) recorded the highest dry matter accumulation at all later growth stages and remained statistically at par with 0.8 ETc (I 2 ). On a pooled basis, 1.0 ETc produced 5.78, 9.42 and 15.10 g plant -1 at 60 and 90 DAT and harvest, respectively, whereas the lowest dry matter accumulation was recorded under 0.6 ETc (I 3 ). The superior dry matter accumulation under 1.0 ETc may be attributed to the maintenance of favourable soil moisture throughout the crop growth period, which enhanced photosynthetic activity, nutrient uptake and assimilate translocation, resulting in greater biomass production. In contrast, moisture deficit under 0.6 ETc likely restricted physiological and metabolic activities, thereby reducing dry matter accumulation. Similar observations have been reported by Ramadass and Ramanathan (2017) [18] , Rani et al. (2018) [19] , Nandle et al. (2018) [14] and Maske (2021) [12] .

Effect of fertigation levels

Dry matter accumulation was not significantly influenced by fertigation treatments at 30 DAT; however, significant differences were observed at 60 and 90 DAT and harvest during both years. The application of 100% recommended N: P 2 O 5: K 2 O through fertigation without basal application (F 1 ) recorded the highest dry matter accumulation throughout the crop growth period. On a pooled mean basis, F 1 produced 5.90, 9.75 and 15.82 g plant -1 at 60 and 90 DAT and harvest, respectively and remained statistically at par with F 2 and F 3 at most growth stages. The lowest dry matter accumulation was consistently observed under the absolute control (F 6 ). Higher dry matter accumulation under F 1 could be attributed to the continuous and efficient supply of nutrients through fertigation, which enhanced nutrient availability, photosynthetic efficiency and assimilate production, thereby promoting greater biomass accumulation. These findings are in close agreement with those reported by Kakade et al. (2015) [11] , Yadav (2016) [26] , Pol (2017) [17] , Sushma Prasad (2019) [23] , Pathare (2021) [15] and Heredia et al. (2023) [10] .

Interaction effect

The interaction between irrigation regimes and fertigation levels significantly influenced dry matter accumulation at 90 DAT during both years and in the pooled analysis. The treatment combination 1.0 ETc + 100% recommended N: P 2 O 5: K 2 O through fertigation without basal application (I 1 F 1 ) recorded the highest dry matter accumulation (10.21 g plant -1 on pooled mean basis) and remained statistically at par with I 1 F 2 and I 1 F 3 . In contrast, the minimum dry matter accumulation (8.22 g plant -1 on pooled mean basis) was recorded under 0.6 ETc + absolute control (I 3 F 6 ). The superior performance of I 1 F 1 may be attributed to the synergistic effect of optimum soil moisture and continuous nutrient availability through fertigation, which enhanced photosynthetic efficiency, assimilate production and dry matter partitioning. Conversely, moisture stress coupled with inadequate nutrient supply under I 3 F 6 adversely affected crop growth, resulting in reduced biomass accumulation.

Absorbed photosynthetically active radiation (APAR)

The periodical data on absorbed photosynthetically active radiation (APAR) of onion as influenced by different irrigation regimes and fertigation levels are presented in Table 1. b and illustrated in Fig. 2. APAR increased progressively from 30 to 90 DAT and declined slightly at harvest during both years as well as in the pooled analysis. This trend reflected rapid canopy development and enhanced light interception during the active vegetative and bulb development stages, followed by a decline at maturity owing to leaf senescence and reduction in the photosynthetically active leaf area. On a pooled basis, the mean APAR increased from 132 μmol photons m -2 s -1 at 30 DAT to 348 μmol photons m -2 s -1 at 60 DAT and 394 μmol photons m -2 s -1 at 90 DAT, before decreasing to 332 μmol photons m -2 s -1 at harvest. Levels (F): F1-100% N: P2O5: K2O kg ha -1 through fertigation with no basal, F2-75% N: P2O5: K2O kg ha -1 through fertigation with 25% N: P2O5: K2O as basal, F3-75% N: P2O5: K2O kg ha -1 through fertigation with no basal, F4-56% N: P2O5: K2O through fertigation with 19% N: P2O5: K2O kg ha -1 as basal, F5-100% N: P2O5: K2O kg ha -1 through conventional fertilizers F6-Absolute control

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