Total dry matter production and physiological parameters
Moisture stress has a major impact on plant physiological traits, which in turn reduces crop yields and total dry matter output
(Hura et al., 2022). In the present study, moisture stress significantly inhibited the plant growth, in all groundnut genotypes significantly.
Under moisture stress condition, the SCMR was significantly higher in Kadiri-9 (47.00) and least by Kadiri-6 (30.22) (Fig 1A). The total dry matter production under moisture stress condition was significantly higher in TCGS-1694 (40.32 g/plant) and the genotype Kadiri-6 (28.26 g/plant) recorded lowest (Fig 1B). Moisture stress has been linked to a decrease in leaf chlorophyll due to the breakdown of pigments and there by reduction of photosynthates and dry matter production
(Trifunovi et al., 2021). Similar inhibitions of growth and development induced by drought stress have been reported for barley
(Istanbuli et al., 2021), tobacco
(Xu et al., 2022), Wheat
(Farid et al., 2021), millet
(Kalagare et al., 2021) etc.,
A crucial functional characteristic and indicators for calculating plant responses to environmental change is specific leaf area (SLA) and Relative water content (RWC)
(Chaimala et al., 2021). SLA and RWC reported a sharp decline in genotypes under moisture stress conditions over control conditions. Plants generally reduce SLA in response to moisture stress
(Andivia et al., 2021). Under moisture stress conditions, genotype Kadiri-9 (62.50%) recorded significantly higher RWC and least by Kadiri-6 (40.56%) (Fig 1C). Genotype, TCGS-1694 (253.00) showed the highest SLA value and lowest in Kadiri-6 (188.60), (Fig 1D). Recent findings were also similar to results of the present study, where a drought-tolerant genotype had higher RWC than the susceptible genotypes
(Khar et al., 2022).
Photosynthetic parameters
Photosynthesis and drought stress relationship is exceedingly complex. Photosynthetic attributes decreased significantly in moisture stress imposed plants. The photosynthetic attributes (Pn, gs, Ci and E) (Fig 2) were significantly higher in the genotype TCGS-1694 and the genotype Kadiri-6 recorded lowest. Under moisture stress, TCGS-1694 showed the highest Pn (24.56 µ mol CO
2 m
-2 sec
-1), gs (0.40 mole H
2‚ O m²/sec), Ci (198.54 µmole CO
2‚ mole
-1) and E (4.42 m. mole H
2‚ O m²/sec ) values, in contrast to Kadiri-6 (15.50 µ mol CO
2 m
-2 sec
-1, 0.09 mole H
2‚ O m²/sec, 125.63 µmole CO
2‚ mole
-1, 1.22 m. mole H
2‚ O m²/sec respectively), in which the minimum were recorded. This effect has also been reported for drought stressed cotton (
EL Sabagh et al., 2020), green gram (
Amarapalli, 2022) and sugarcane
(Misra et al., 2020).
Dry matter partitioning efficiency
Alongside of photosynthetic attributes, dry matter partitioning is the more important trait for the yield differences among the groundnut genotypes under control and moisture stress conditions. In the present study, mean data of 2 years regarding dry matter partitioning in the leaves, stem, roots and pods at 60, 80 days after sowing (DAS) and harvest for control and moistures stress conditions are depicted in Fig 3,4.
Under moisture stress conditions, at 60 DAS (Fig 4A) dry matter partitioning to leaves was significantly higher in the genotype, TCGS-1694 (60.69%). Groundnut genotype, TCGS-1694 with higher partitioning efficiency to leaf at 60 DAS representing more photosynthetic efficiency of the genotype. However, at 80 DAS (Fig 4B) and harvest (Fig 4C) dry matter partitioning to leaves was significantly higher in TCGS-2018 (39.81%). Genotype with higher partitioning from leaf to reproductive sinks coupled with better sink capacity is more important at 80 DAS under moisture stress conditions.
Both at 60 DAS and 80 DAS (Fig 4A,B) dry matter partitioning to stem was significantly higher in the Kadiri-6 (50% and 34.63%). Increased partitioning to stems result in lanky growth and lodging of plants. Both under control and stress conditions their no significant difference dry matter partitioing in roots at 60 DAS and 80 DAS respectively.
At 80 DAS and harvest (Fig 4 B and C) dry matter partitioning to pods was significantly higher in TCGS-1694 (54.97%). TCGS-1694 with higher Pn, total dry matter, dry partitioning to leaf resulted in higher dry matter partitioning to pods at grain filling phase. In plants, the reproductive sinks compete for dry matter with the vegetative organs thereby decreasing dry matter allocation to vegetative phase during seed development
(Zhang et al., 2022). However, this phenomenon may be supplanted by translocation of stored assimilates produced prior to pod/kernel production in groundnut. On the same line, the Kadiri-6 with higher partition efficiency was observed to stem rather than sink. Such differential behaviour in total dry matter partitioning and its distribution among groundnut genotypes could also ascribe to genotypes’ genetic characteristics (
Salazar Licea, 2022).
Yield and yield parameters
Dry matter partitioning to pods may be a major cause of yield differences for groundnut genotypes. Under moisture stress conditions, TCGS-1694 recorded significantly higher pod yield (1316.70 kg/ha), harvest index (34.06), shelling percentage (62.99%), kernel yield (2674.95 kg/ha) and 100 kernel weight (34.57 g) contrast to Kadiri-6 in which the lowest values (503.30 kg/ha, 20.66, 38.01%, 1324.10 kg/ha, 19.60 g respectively) were recorded (Table 1 and 2). Poorer dry matter partitioning observed in genotype Kadiri-6 may have contributed to its low yield despite the huge biomass partitioned to stem produced. Crop yield is increased by dry matter partitioned into sink (kernels). Genotype TCGS-1694 had significantly higher biomass coupled with higher partitioning efficiency to kernels in addition to high photosynthetic and harvest indices.
Variability of stress tolerance index (STI)
In the experiment, 7 genotypes of groundnut were subjected to evaluation at reproductive phase to moisture stress conditions (Table 2). Genotype TCGS-1694 recorded significantly higher stress tolerance index (63.71) and Kadiri-6 recorded lowest stress tolerance index (46.56). A high value of STI implies higher tolerance to stress. According to
Kamrani et al., (2018) selection based on STI helps to determine high yielding genotypes.
Principal component analysis (PCA) biplot
The distribution of genotypes and traits in the PCA biplot explained the high variability of traits for principal components. PCA biplot for photosynthetic and yield traits depicted the 86.4% (control conditions) (Fig 5) and 88.3% (stress conditions) (Fig 6) of variability in the raw data (Fig 1). In PCA of all 7 genotypes, TCGS-1694 and TCGS-1792 genotypes had a significantly higher response for photosynthetic traits, specific leaf area and yield traits, while TCGS-1784, TCGS-1862, Kadiri-9 genotypes had a higher response for relative water content, total dry matter, SCMR, harvest index and shelling percentage in both control and stress conditions. However, under control conditions TCGS-1862 had higher response for photosynthetic traits and pod yield. Genotypes TCGS-1694 and TCGS-1792 have been identified as drought-tolerant genotypes, while Kadiri-6 has been identified as susceptible genotype.