Genotypes with high stem reserve mobilization can overcome the impact of rust on chickpea yield

Chickpea is one of the major pulse crops cultivated worldwide. It is affected by many fungal diseases including rust caused by the fungus Uromyces ciceris arietini. Its severity is mainly observed during the flowering and pod development stage and this causes considerable yield losses. The present investigation was carried out at ICAR-NIASM, Baramati during 2021–2022 to assess genotypic variation in stem reserve mobilization (SRM) and its influence on loss of grain yield due to rust infection on the basis of severity of rust, seed yield and test weight. The SRM was strongly correlated (R2 = 0.96) with yield irrespective of the severity of rust infection. Phule Vikram, a local chickpea genotype, recorded the highest grain yield (1506.6 kg/ha) and SRM (23.65 mg/g stem dry weight) while the lowest yield was recorded in Pusa Green 112 (476.7 kg/ha), a susceptible, genotype that had less SRM (3.44 mg/g stem dry weight) and partially filled grain. The results indicated that SRM has a major role in ensuring pod filling even under rust infection. Hence, it is suggested that the emphasis should be on SRM as a trait for selecting early generation breeding lines in the chickpea improvement program with precaution to reduce a penalty if any in terms of attracting pests by increased access to soluble sugars in high SRM genotypes.


Introduction
The Chickpea is one of the most important pulse crops after beans and pea, which belongs to the family Fabaceae (Gaur et al. 2010) and it is widely grown for its nutritious seeds. In India, chickpea contributes over 40% of the country's total pulse production and 75% of world chickpea production (Maurya and Kumar 2018). Earlier, foliar fungal diseases were least important without causing much yield loss, but chickpea rust has recently become a major threat to chickpea production (Patil 2015). Chickpea rust (Rutua) is a major foliar fungal disease caused by Uromyces ciceris arietini (Stuteville et al. 2010). It was reported for the first time in Mexico in 1961 and it is reported in more than 15 countries worldwide (Patil and Bhat 2013). In India, it is widespread in Maharashtra, Tamil Nadu, Bihar, Punjab, West Bengal, and Uttara Pradesh. Chickpea rust appears late in the season when the crop is at the maturity stage and moderately warm weather favours rust development. This disease is more serious and destructive in late sown crops and causes about 50-60% yield losses (Patil 2015).
Stem reserve mobilization is an active process of translocation of stored reserve (photoassimilates) from the stem to the grain. The leaf rust that affects the crop after the flowering stage can reduce the photosynthesis rate (Yahya et al. 2020). Due to impaired leaf photosynthesis during this period, stem reserve mobilization play a major role in grain filling in cereal crops (Blum 1998;Ehdaie et al. 2008). It may also contribute to grain weight (Nazir et al. 2021). Thus, the present study was to evaluate stem reserve mobilization in chickpea genotypes during rust infection.

Materials and methods
The present experiment was carried out at ICAR-NIASM, Baramati, Pune, India during 2021-22. The field experiment was laid out in randomized block design in three replications by using eight chickpea genotypes namely, BG-276, Pusa 1003, Pusa Green 112, ICCV-92944, IPC 06-11, JG-16, Vijay and Phule Vikram. Each plot had six lines of chickpea sown in 36 m length with 45 cm spacing. Observations such as stem reserve mobilization (SRM), seed yield and test weight were recorded under both infected and non infected chickpea plots.

SRM
Three plants were randomly selected from each plot at the grain filling stage and followed by the physiological maturity stage. The length of each plant's top five apical internodes was selected from the main stem and recorded their dry weight, firstly at the grain filling stage and second at the physiological maturity stage. SRM was calculated by using the formula suggested by Ehdaie and Waines (1996).

Stem reserve mobilization (mg∕g stem dry weight)
= Stem dry weight at the grain filling stage − Stem dry weight at the physiological maturity stage Grain yield (kg/ha) Grain yield was recorded after harvesting and threshing the plants per plot (kg/plot), and then converted into kg/ha by using the conversion formula suggested by Norman et al. (1995).
Test weight (g/100 seeds) The weight of 100 grains was recorded from each plot and expressed in grams (g).

Statistical analysis
Statistical analysis was performed using one-way analysis of variance (ANOVA) with LSD at p ≤ 0.05 and the Duncan multiple range tests was performed (Gomez and Gomez 1984).

Rust infestation and its effect on chickpea yield
The incidence of rust symptoms on chickpea was observed in the late-season during the flowering to pod filling stage, it might be due to a decrease in the immunity of plants against a pathogen due to initiation of plant senescence and reduced photosynthesis. Based on the previous report of Mayee and Datar (1986) and as suggested by the chickpea fungal pathologist through visual observations, the severity of rust ranges between 60 and 80% ( Fig. 1) in all the chickpea genotypes studied and it could have an impact on the efficiency of radiation capture and usage. Effects on photosynthesis rate and related parameters may mediate a reduction in radiation use efficiency. It decreases the accumulation of photoassimilates and it leads to poor pod filling. As indicated in Fig. 1b, the grain filling was affected by rust infection as compared to non-infected plants because of uneven distribution of photo assimilates and the weight of pods was decreased towards upper pods because initial or below pods were filled by stem Grain yield (kg∕ha) =(plot yield (kg) × 10, 000) ∕plot size in square metres reserve photoassimilates. The stem reserves contribute to pod filling of the firstly initiated pod and later pods will not fill due to reduced photosynthesis rate and it finally reduces quantity and quality of grain. This leads to poor seed yield over non infected plants as presented in Table 1.

SRM as a trait for selection
Under humid and warm weather conditions, chickpea encounters rutua/rust disease, which coincides with the flowering and pod filling stage. As a result, the current photosynthesis process either gets inhibited/ down-regulated and the crop promotes its senescence mechanism. Ultimately, the development of grain mainly depends upon the photoassimilates accumulated before the pod filling stage, stored in the stem as water-soluble carbohydrates. During the pod filling stage, these stored carbohydrates hydrolyse to transportable sugars and remobilize towards the grain development to maintain grain weight during pathogen infection. These stored reserves together with the regular photosynthesis support the grain filling, providing a good source-sink relationship. The overall hypothesis drawn is proposed as a model for stem reserve mobilization (source) into pod (sink) (Fig. 2).
As shown in Table 1 and Fig. 3c, SRM was recorded more in Phule Vikram (23.65 mg/g stem dry weight) followed by JG 16 (21.95 mg/g stem dry weight) and BG 276 (14.67 mg/g stem dry weight) and less in Pusa Green 112 (3.44 mg/g stem dry weight) genotype under rust infestation and in noninfected condition, the SRM was less as compared to rust infected condition because the plants were  not covered by rust infection, they were still in green and able to carried out photosynthesis, then photo assimilates were translocated by photosynthetic area of plants and genotype Phule Vikram (18.29 mg/g stem dry weight) and JG 16 (16.52 mg/g stem dry weight) were recorded more SRM then less was recorded in genotype Pusa Green 112 (3.36 mg/g stem dry weight). As mentioned earlier, the chickpea yield was reduced in infected condition as compared to non infected condition.  . 3a) and Table 1). In the present result showed a strong correlation (R2 ꞊ 0.96) between yield and stem reserve mobilization under rust-infected conditions (Fig. 3b). Nazir et al. (2021) in wheat, to improve yield under chemically induced stress and Blum et al. (1994) in wheat, under heat stress reported similar results for the contribution of SRM. Islam et al. (2021) showed contributions of reserve in the culm including sheath to grain filling in wheat under drought. The assimilation of stem reserves by pre-anthesis is an essential source for grain filling during stress conditions and this approach can be used for screening tolerant genotypes under terminal drought conditions (Salem et al. 2021). Gurumurthy et al. (2019) reported that the wheat grain yield improves under stress conditions by stem reserve mobilization trait. The accumulation and remobilization of stem water-soluble carbohydrates (WSC) are determinant physiological traits profoundly impacting yield potential in wheat under drought stress and molecular markers involved in this process and helping in marker-assisted selection to improve WSC-related traits in stem (Li et al. 2020).
The chickpea genotypes such as Phule Vikram and JG 16 are suitable genotypes under rust infestation conditions becouse of more stem reserve mobilization. Stem storage during the vegetative stage is the only source for improvement of grain yield under the severity of the chickpea rust. Alternatively, it can also be hypothesized that the genotypes which are rich in water-soluble sugars are prone to rust as these sugars are easily available for consumption. However, by the virtue of significant accumulation of SRM the genotypes can outperform others that are poor accumulators of SRM. Hence chickpea improvement program has to take into consideration both the disease resistance as well as SRM accumulation in a way to achieve optimum yield under biotic and abiotic stress conditions.