Results and Discussion

From the ongoing account, it is evident that

The physico-chemical characterisation of soil used during pot experiment along with variations due to the irrigation of different concentration of AsV is presented in Table 1. The pH of control soil was slightly alkaline (7.6) which decreased after repeated loading of 4, 8 and 12 mg l^-1 of AsV. The soil is rich in phosphorus (447.42 mg kg^-1) and availability of P fortified with addition of 4 mg l^-1 AsV (725.69 mg kg^-1) and further decrease at higher dose of AsV but this level is higher than control. The natural AsV concentration in control soil was low(2.3 mg kg^-1 dw) which increased upto 31 mg kg^-1 dry weight (dw) by irrigation of 12 mg l^-1 AsV. It was 24 and 26 mg kg^-1 dw at 4 and 8 mg l^-1 respectively. No changes were observed in case of other metals like Fe, Zn and S. Availability of nitrogen in natural soil was 0.49% and increased up to 1.18% at 4 mg l^-1 AsV and further decreased to 0.8% and 0.9% respectively at 8 and 12 mg l^-1 AsV. Various growth parameters were determined. Growth variables of selected rice cultivars havebeen presented in Figure 1A-D. The number of tillers was found to be a varietal characters which ranged from 17-22 in different rice cultivars, however, number of tillers increased in all the cultivars when 4 mg l^-1 AsV was added in the soil except Triguna where number of tillers increased upto 8 mg l^-1 AsV but decreased significantly at higher AsV supply (Figure 1A). There was no significant change observed in terms of plant height and panicle length up to 4 mg l^-1 As but at higher AsV doses (8 and 12 mg l^-1), significant decrease in plant height and panicle length were noticed (Figure 1 B, C) in all the varieties except in IET 4786 (Satabdi). In case of IET 4786 (Satabdi) plant height was decreased with increasing concentration of AsV supply in the soil and panicle length was decreased significantly only at 12 mg l^-1 of AsV supply. Similarly, grain weight (Figure 1A-D) was increased by AsVsupply only up to 4 mg l^-1 in all the cultivars in comparison to control but decreasing grain weight was observed insignificant at both the higher doses of AsV supply (8 & 12 mg l^-1) except IET 4786 (Satabdi) where decreasing in grain weight was significant with increasing supply of AsV. In general, AsV toxicity had led to early flowering in all selected rice cultivars consequently early seed maturity.

Supply of AsV at low concentrations promoted yield by enhancing nutrient availability such as P [9] and N2 as observed by increase in number of tillers and seed weight. Growth stimulation at low application of AsV has also been reported [10] while As hyperaccumulator Pteris vittata showed increased growth upto 100 mg kg^-1 soil AsV application [11]. In the present study, 12 mg l^-1 AsV application retarded the growth and yield of rice cultivars as reflected by reduced plant height, number of tillers and seed weight, possibly due to retardation of chlorophyll contents as observed in the present study and also reported in oat [12,13]. Arsenic may inhibit chlorophyll biosynthesis by interfering the activity of δ-ALAD in greening maize leaf segments under AsV stress [13]. Further, application of AsV at lower dose enhanced accumulation of other nutrients (Fe, P, Zn and S) however, higher dose limits nutrient uptake in rice [14]. Higher AsV concentrations, hampered uptake of essential nutrients such as Fe, Mn, Cu, Zn etc. may also be a reason for inhibition of photosynthetic pigments, reduced growth and number of tillers [15].

Therefore, higher dose of AsV limits rice plant from nutrients uptake which attributes negative effect on plant growth and thus reducing the yield of rice grains.

Table 1: Physico-chemical properties and metal composition of control pot soil and after supply of different AsV concentrations. All the values are mean of triplicates±S.D. ANOVA significant at p≤0.01. Different letters indicate significantly different values at a particular treatment (DMRT, p≤0.05).

Figure 1: Effect of AsV on number of tillers (A); plant height (B); panicle length (C); grain weight (D) in different rice cultivars. All values are mean of triplicates ±S.D. ANOVA significant at p≤0.01. Different capital letters indicate significantly different values among As treatments in a particular rice cultivar and small letters indicate significantly different values among rice cultivars at a particular treatment (DMRT, p≤0.05).