Effect of Zinc Nutrition on Lathyrus (Lathyrus sativus L.) under Varying Sowing Dates in the Gangetic Plains

Prolonged use of input-intensive technologies in rice-wheat/rice-rice cropping patterns has exhausted the natural resource base of the Indo-Gangetic plains, resulting in declining total factor productivity of the system. Thus, the incorporation of pulses in cereal-based cropping sequences under such a scenario not only improves soil health and system productivity but also ensures food and nutritional security by establishing a sustainable agri-food system (Maji et al., 2019). The relatively higher stress tolerance of lathyrus (Lathyrus sativus L.) and its ability to withstand unfavourable weather conditions, along with minimal external input demand, make it an ideal pulse crop for resource-poor farmers. Lathyrus is rich in protein and different minerals, making it a preferred choice among the economically weaker sections of society due to its nutritive value and cheaper market price than other pulses. Additionally, due to its nutrient-dense stover, it is steadily gaining popularity as a forage crop and is a possible alternative to the nutritionally deficient paddy straw for animal feed. Despite the multiple agro-ecological and nutritional benefits of lathyrus, limited research interventions to improve lathyrus production have been taken up globally till date. The primary reason behind this paucity of research is the presence of the neurotoxin b-N-oxalyl-l-a,b-di amino-propionic acid (b-ODAP) in lathyrus stover and grains. Prolonged consumption of lathyrus causes lathyrism, a neurological condition that results in lower limb paralysis.
       
The timely sowing of winter crops in the lower Gangetic plains is often hindered by the shifting monsoon cycle, frequent occurrence of adverse weather events, late transplanting and the long duration of preceding monsoon rice. Therefore, to successfully include lathyrus into the region’s commonly practised rice-based systems, it is crucial to identify a temporally flexible sowing window that complies with the prevailing weather. Moreover, the zinc-deficient lowland rice soil severely limits the productivity of winter pulses grown after monsoon rice. The wide-ranging role of zinc in enzymatic activity, reproductive biology and stress tolerance makes it an important determinant of the final yield (Liu et al., 2019). Zinc fertilization is also an effective agronomic biofortification strategy to yield Zn-dense crops (Chaudhary et al., 2022). Hence, the current study was taken up to assess the effect of sowing dates and zinc nutrition on lathyrus growth, productivity, nutrient enrichment and economics.

A field study was set up during the winter seasons of 2018-19 and 2019-20 at ‘AB’ block farm, Bidhan Chandra Krishi Viswavidyalaya, to evaluate the effects of date of sowing and Zn fertilization on lathyrus. The study site’s geographical location was 22°58¢45²N and 88°25¢15²E, with an altitude of 9.75 m above mean sea level. The texture of the experimental soil was sandy clay loam with a neutral pH (7.3). The nutrient status of the soil was 0.50%, 285.11 kg ha^-1, 28.43 kg ha^-1, 153.60 kg ha^-1 and 0.54 ppm organic carbon, available nitrogen, phosphorus, potassium and DTPA extractable Zn, respectively. Year-wise temperature and rainfall data during the crop-growing period are provided in Fig 1.
 

       
The experimental design of the field study was a split plot with three replications. The main plot comprised two dates of sowing, viz., D₁: Early sowing (14 November) and D₂: Late sowing (29 November), while various Zn nutrition protocols were allotted to sub-plots, viz., Zn1: Control (No zinc application), Zn₂: 15 kg ZnSO₄ ha^-1 as basal, Zn3: 20 kg ZnSO₄ ha^-1 as basal, Zn₄: 25 kg ZnSO₄ ha^-1 as basal, Zn₅: 0.5% ZnSO₄ foliar spray, Zn₆: 1.0% ZnSO₄ foliar spray, Zn₇: Seed priming with 0.05% zinc solution, Zn⁸: Seed priming with 0.1% zinc solution. The foliar spraying for treatments Zn5 and Zn₆ was done at the pre-flowering and pod initiation stages and the priming duration for Zn₇ and Zn₈ was 6 hours.
       
The crop variety Prateek was sown with a spacing of 30 cm × 10 cm in a sub-plot size of 5 m × 4 m. A blanket basal dose of 20 kg N, 40 kg P₂O⁵ and 20 kg K₂O per ha in the forms of urea, single superphosphate and muriate of potash, respectively, was applied. Observations on plant height, root and shoot biomass and leaf area index were recorded for each treatment combination at 30, 50, 70 and 90 days after sowing (DAS). At harvest, the number of pods plant^-1, 100 seed weight (g), seed yield (kg ha^-1) and harvest index were recorded. For treatment-wise economic analysis, the prevalent market costs of the inputs, labour charges and the farm gate price of the output in West Bengal were considered.
       
For zinc estimation, 0.5 g of ground seed and stover samples from each plot were placed separately in crucibles, followed by dry ashing in a muffled furnace at 550°C for 4 h. On the following day, 10 ml of 6.0 N HCl was added to the cool dry ash of each sample and thoroughly mixed. An aliquot of each sample was then collected after filtering through Whatman No. 1 filter paper. The volume makeup of the aliquots to 50 ml with double distilled water was done and finally the Zn concentration was estimated using an atomic absorption spectrophotometer (Perkin Elmer PinAAcle-900F, United States) and expressed in ppm. 
       
The treatment means were compared using Tukey’s HSD (honest significant difference) at p£0.05 after the relevant data were statistically examined by the analysis of variance (ANOVA) technique for split-plot design. The mean data of two years are presented here because ANOVA showed no significant variation (p > 0.05) between the years, among treatments or year x treatment interactions. The interaction effect between sowing dates and Zn nutrition was also found to be non-significant (p > 0.05, data not shown).

Plant height
 
The late-sown crop had significantly taller plants than the early-sown crop throughout the crop-growing period (Table 1). The early sown crop was exposed to a substantially higher average maximum temperature (28.7°C-28.8°C) during the first 30 DAS in both years of experimentation, which might have restricted the plant height initially. A mean daily maximum temperature of 25°C is considered the ceiling for heat stress in cool-season crops (Wahid et al., 2007). The late sown crop in the present experiment experienced favourable average maximum temperatures of 25.3°C and 25.2°C during the initial period in 2018-19 and 2019-20, respectively, which possibly improved its vegetative growth as evident from the increased crop height.
 

       
Zn nutrition significantly improved crop height compared to control because of its contribution to chlorophyll, cell division, meristematic physiology, cell volume, the formation of the cell wall and photosynthesis (Liu et al., 2019). Among the Zn fertilizations, basal application of ZnSO₄ at a rate of 25kg ha^-1 recorded significantly higher crop height (36.67 cm-110.63 cm) throughout the crop growth period.
 
Leaf area index
 
A sharp increase in LAI was observed after 50 DAS. In the present experiment, LAI continued to increase even after 50% flowering, with the maximum LAI being recorded at 90 DAS (Table 1). In determinate crops, the highest LAI is attained before the reproductive stage, but in indeterminate crops like pulses, the maximum LAI may be obtained after flowering. Congenial weather conditions might have facilitated better LAI development in D₂ sowing in the early stages and this early head start helped it to consistently maintain a higher LAI throughout the growing period. However, with the dip in mean temperature (17.3-18.2oC), the D₁ sown crop showed a sharp increase in LAI development during 50-90 DAS, indicating canopy adjustment in response to favourable weather conditions (Tesfaye et al., 2006). 
       
In the current study, Zn fertilization significantly improved LAI. Despite recording low LAI values on 30 and 50 DAS, lathyrus showed a rapid response to foliar nutrition, as evident from significantly higher LAI values (1.46-1.74) in Zn₅ and Zn₆ on 70 and 90 DAS. This indicates that in the case of foliar feeding, a quicker response to the applied nutrient can induce favourable canopy adjustment in lathyrus in Zn-deficient soils.
 
Root and shoot dry weight
 
D2 recorded significantly higher root weight than D₁ on 30 and 50 DAS, while no significant difference was observed between them during later stages of crop growth (Table 2). However, in the case of shoot biomass, D₂ recorded significantly higher values throughout the crop life. Exposure to a congenial thermal regime, especially during the first 30 DAS (25.3°C in 2018-19 and 25.2°C in 2019-20) possibly improved root and foliar growth in D₂. The initial boost in vegetative growth might have helped the D^­2 sown crop to sustain a favourable biomass accumulation even when exposed to high maximum temperatures (>27.0°C) in the reproductive stage. Sehgal et al., (2018) reported that, due to their efficient thermo-tolerance capacity, leaves exhibit high resilience at reproductive ceiling temperatures.
 

       
Zn₁ (control) recorded the highest root dry weight but the lowest shoot weight. The soil at the current experimental site had low (0.54 ppm) DTPA-extractable Zn. It is likely that in the Zn₁ treatment, where no Zn was applied, the crop developed a vigorous root system at the cost of shoot biomass to explore a larger volume of soil to acquire Zn. Yang et al., (2011) also reported that Zn deficiency augmented root growth in wheat. On the contrary, soil fertilization with 25 kg ZnSO₄ ha^-1 recorded significantly lower root weight than other levels of soil fertilization on 70 and 90 DAS but significantly higher shoot weight during the growing period. The addition of Zn fertilizer to the soil increases DTPA-Zn in the upper soil layer (Liu et al., 2019). This supports sufficient Zn absorption to meet the crop’s physiological needs from the upper layer itself, with no need for a robust root system to capture Zn from the deeper soil layer. Foliar nutrition recorded significantly lower root weight during crop life but showed significant improvement in shoot growth after Zn application, as observed on 70 and 90 DAS. Zn supplementation through foliar fertilization might have met the Zn demand of the crop by maintaining a favourable plant Zn status, ultimately resulting in enhanced shoot growth (Ahmed et al., 2023). This possibly also acted as a signal to offset the need to capture Zn from the soil, which, when perceived by the root, limited its growth.
 
Crop growth rate (CGR)
 
The crop exhibited early slow growth (30-50 DAS), followed by rapid growth during the late vegetative and early flowering stages (50-70 DAS) (Fig 2). CGR then showed a sharp decline during the late flowering and pod development stages (70 -90 DAS). The dip in overall CGR during 70-90 DAS might be because of the diversion of current assimilates more towards pod formation and filling than vegetative growth. The late sown crop recorded a significantly higher CGR during 30-70 DAS. Despite the D₂ sown crop being subjected to high temperatures during 70-90 DAS, no significant difference in CGR between the two dates of sowing was seen. An early head start in TDM production and LAI due to a favourable thermal regime in the early stages may have helped the D₂ sown crop sustain a higher CGR even while experiencing high maximum temperatures (27.3 and 27.7oC) during 70-90 DAS. Although the threshold heat stress temperature of the lathyrus has not been specifically reported, it is capable of withstanding extreme temperatures (Dixit et al., 2016). Likewise, significant improvements in TDM accumulation and LAI of lathyrus in response to Zn nutrition were reflected in increased CGR. For instance, favourable canopy adjustment as an immediate response to foliar Zn nutrition resulted in improved biomass accumulation and significantly increased CGR (5.43 g m^-2 day^-1) in Zn₆ during 50-70 DAS.
 

 
Yield attributes, yield, grain and straw Zn content and economics
 
The late sown crop (D2) recorded significantly higher pods plant^-1 (24), 100 seed weight (7.40 g), harvest index (0.40) and yield (1380 kg ha^-1) than the early sown crop due to overall improved crop growth favoured by congenial weather conditions (Table 3). Foliar nutrition with Zn, irrespective of doses, was observed to be more effective than soil fertilization in improving yield (1512-1647 kg ha^-1). Therefore, growing conditions (Tzudir et al., 2016) and Zn management (Kumar et al., 2016) favouring improved biomass accumulation, growth rates and partitioning of assimilates to sink in lathyrus are likely to fare well in terms of productivity.
       
Interestingly, the dates of sowing exerted no significant influence on grain and stover Zn content (Table 3). Domingos and Bilsborrow (2021) also observed no significant influence of sowing conditions on Fe and Zn content in buckwheat. Basal application of 25 kg ZnSO₄ ha^-1 and 1.0 % ZnSO₄ foliar spray at pre-flowering and pod initiation stages recorded significantly higher grain Zn (29.10- 30.36 ppm) and stover Zn (40.37- 40.89 ppm) content. The higher Zn content in these two treatments was likely due to improved assimilation and absorption of the applied nutrient, which enhanced crop growth and produced Zn-enriched seeds (Dhaliwal et al., 2021).


       
The D₂ sown crop recorded a 15.0% higher gross return (GR) and GR: COC ratio than D₁ due to the crop’s better yield performance (Table 4). Foliar nutrition with 1.0% ZnSO₄ fetched maximum gross return (Rs. 62638 ha^-1) and GR: COC ratio (1.67) closely followed by foliar nutrition with 0.5% ZnSO₄, thereby making them feasible agronomic interventions for resource-poor farmers.

Exposure to a lower thermal regime in the early stages improves vegetative growth in late-sown (29 November) lathyrus, which might help the crop sustain adequate biomass accumulation and yield even when subjected to high temperatures later in crop life. Improved absorption and assimilation of the applied Zn with foliar fertilization of 1.0% ZnSO₄ at pre-flowering and pod development stages resulted in lathyrus yield intensification and the production of Zn-dense seed and stover. Striking a balance between root and shoot growth under suboptimal conditions is likely a crucial determinant of final yield in lathyrus. Hence, lathyrus can be conveniently accommodated with adequate foliar Zn nutrition in Zn-deficient rice-based systems of the lower Gangetic plains, where delayed sowing and short winters adversely affect the growth of post-rice crops.

The authors duly acknowledge AICRP-MULLaRP, Bidhan Chandra Krishi Viswavidyalaya, Kalyani, West Bengal, for supporting this study.

None.