Field emergence and growth parameters
Different seed biopriming treatments had a significant effect on the field emergence and plant growth parameters like, plant height at 30 DAS, plant height at final harvest, days to pod harvest and duration of harvest as compared to the fungicide treatment and untreated control (Table 1 and Fig 1). Amongst various treatments, seed biopriming with PGPR-1 +
Rhizobium strain B
1 (T
4) resulted in the highest percentage of field emergence (95.18%) followed by (93.70%) emergence in seed biopriming with
Rhizobium strain B
1 (T
2). These two treatments were, however, at par with each other. Similar trend was observed w.r.t. the plant growth parameters and seed biopriming with PGPR-1 +
Rhizobium strain B
1 (T
4) resulted in a significantly more plant height at 30 DAS (34.09 cm), plant height at final harvest (56.99 cm) and duration of harvest (18.67 days) than carbendazim seed treatment as well as untreated (control). The plant growth parameters observed after seed treatment with carbendazim
viz., plant height at 30 DAS (27.47 cm), plant height at final harvest (48.37cm) and duration of harvest (13.83 days) were significantly less than the biopriming treatments.
Under the present investigation, seed biopriming of French bean with a combination of PGPR-1 and
Rhizobium strain B
1 provided higher emergence and plant growth in field as compared to the rest of the treatments including fungicide treatment. These results are in accordance with the findings of
Junges et al., (2016), Entesari et al., (2013), Yadav et al., (2013) and
Ahmed et al., (2012) who have also reported enhancement in seedling emergence in bean, soybean and mungbean after biopriming the seeds with different bioagents. The effect of seed biopriming with the combination of PGPR and
Rhizobium strain B
1 on growth parameters was observed higher than that of biopriming with the individual bioagent under present study. Which might have happened because of the fact that
Rhizobium and PGPR share a common microhabitat, the root soil inter surface, where interactions between different microbial groups were reported during root colonization. Co-inoculation of
Rhizobium with PGPR enhanced nodulation and nitrogen fixations through the production of plant hormones like auxins, flavonoids, nod factor or enzymes in legumes (Bansal and Srivastava 2012).
Junges et al., (2016) who have also observed that the combined inoculation of bioagents was more effective than their individual inoculations. Seed biopriming increased germination rate and uniformity in emergence through metabolic repair during imbibition and build-up of germination related physiological processes. According to Taylor and Harman (1990) initiation of all these germination related physiological processes helped in the establishment and proliferation of bioagents on the spermosphere. The conditions on the seeds during priming increased the potential for microbial proliferation and several studies have demonstrated increased ability of applied bioagents to establish on seeds during biopriming or pre-germination processes.
Ahmed et al., (2012) have observed that increase in root and shoot growth of mungbean upon co-inoculation with
Rhizobium and
Pseudomonas was due to lowering of ethylene levels in germinating seeds by these strains. The increase in growth parameters could also be due to the fact that seed biopriming with bacterial antagonists increased the population of antagonists on the seeds and protected them from the attack of plant pathogens
(Callan et al., 1990). Biopriming ensures the entire seed batch is at the same point in the germination process, so that once planted, bioprimed seed showed more rapid and uniform emergence than unprimed seed (Rowse, 1996).
Yield attributes
The data in Table 2 and Fig 2 revealed that seed biopriming with PGPR-1 +
Rhizobium strain B
1 (T
4) also resulted in a significant improvement in yield attributes of French bean crop like, number of pods per plant, dry pod weight, pod yield per plant, number of seeds per pod, seed yield and 100 seed weight as compared to that of hydropriming, carbendazim seed treatment and untreated control. The number of pods per plant (20.17), dry pod weight (2.72 g), pod yield per plant (38.64 g), number of seeds per pod (7.17), seed yield (23.34 q/ha) and 100 seed weight (34.19 g) upon seed biopriming with PGPR-1 +
Rhizobium strain B
1 (T
4) were at par with that of seed biopriming with
Rhizobium strain B
1 (T
2) (Fig 3 and 4). The carbendazim seed treatment was found less effective than these biopriming treatments and resulted in significantly lesser number of pods per plant (11.50), dry pod weight (2.08 g), pod yield per plant (27.77g), number of seeds per pod (5.17), seed yield (17.98q/ ha) and 100 seed weight (30.44g), and it was at par w.r.t. some parameters with that of hydropriming and untreated (control).
In the present investigation, higher seed yield was recorded when the seeds of French bean were bioprimed with PGPR-1 +
Rhizobium strain B
1 (T
4). The present findings are in agreement with that of Yadegari and Rahmani (2010) who have also found that co-inoculation with PGPRs and
Rhizobium increased seed yield in common bean as compared to the uninoculated (control). The increased seed yield was attributed to the increased physiological activities like synthesis of chlorophyll, carbohydrates, amino acids and translocation of photosynthates into developing pods and seeds in bioprimed plants.
Abadeh et al., (2013) have also observed increased grain yield in red lentil after seed biopriming with PGPR as compared to the non-bioprimed seeds. Inoculation of seeds with PGPR and
Rhizobium resulted in early and better nodulation which was translated into higher shoot nitrogen accumulation and there by increased higher seed yield. This increased seed yield in bioprimed plants was due to the increased uptake and availability of major and minor nutrients together with growth regulators, enzymes and amino acids at all the crucial stages of growth and development. These physiological changes caused more number of pods and bolder seeds in them
(Bais et al., 2006). Rhizobacteria have also been found to induce phytoalexins in roots and these phytoalexins protect the roots against pathogens and help their development, mobilization of insoluble nutrients followed by enhancement of uptake by plants
(Valverde et al., 2006).
Incidence of diseases
The data presented in Table 3 and Fig 5 revealed that the seed biopriming with bioagents including PGPR,
Rhizobium and
Trichoderma alone and their combinations significantly affected the incidence of
Rhizoctonia root rot and angular leaf spot diseases in French bean. Amongst different treatments, minimum incidence of
Rhizoctonia root rot (2.60%) as well as Angular leaf spot (2.97 %) was observed in the seed biopriming with PGPR-1 +
Rhizobium strain B
1 (T
4) followed by 3.70 and 3.34 % in
Rhizobium strain B
1 (T
2), 3.71 and 4.07 % in PGPR-1 +
Rhizobium strain B
1 +
Trichoderma viride (T
7), respectively. These three biopriming treatments were statistically at par with each other. The incidence of root rot and angular leaf spot in French bean plots grown after seed treatment with carbendazim @ 0.2% was observed to be 9.26 and 9.63% which was significantly higher than all the biopriming treatments. The disease incidence in untreated plots was observed maximum which was significantly higher than all the other treatments.
In the present investigation, the seed biopriming with PGPR-1 +
Rhizobium strain B
1was found most effective treatment in reducing the diseases like
Rhizoctonia root rot and Angular leaf spot in French bean. There existed no report in literature on the effect of seed biopriming in reduction of the root rot disease in French bean. Hence, this seems to be the first report in this regard. However, many workers have reported that seed biopriming with biological control agents reduced the incidence of various diseases in different leguminous crops such as faba bean (El-Mougy and Kader, 2008), soybean
(Begum et al., 2010) and green gram
(El-Mohamedy et al., 2015). Besides, growth promotion activities, PGPRs and Rhizobia also act as biocontrol agents against some plant pathogens and inhibit the progress of the pathogens following different mechanisms like, competition, lysis, antibiosis, siderophore production and hyper-parasitism. More importantly, the seed biopriming with PGPR, rhizobia and
Trichoderma triggered the defence mechanisms during the initial germination process involving synthesis of salicylic acid and jasmonic acid and thus provided basal resistance against pathogens. These signals play important role in induction of induced systemic resistance (ISR) and systemic acquired resistance (SAR) and both are effective against a broad spectrum of pathogens
(Mahmood et al., 2016). Application of these organisms through seed biopriming might have activated and followed the same mechanism and resulted in decreased incidence of
Rhizoctonia root rot and angular leaf spot diseases. In addition to this these beneficial organisms also help in increased nutrient uptake and plant growth as discussed earlier and a healthy plant is less affected by the attack of pathogens.