Identification of maize genotypes tolerance to acid soil stress using multiple criteria

This study aims to select maize hybrids that can tolerate acidic soil stress using several tolerance indexes. The research took place at two sites. Muneng Experimental Farm has average soil (pH=6.8), and Moncongloe Experimental Farm has acid soil (pH=5.8) from January to April 2023. A Randomized Complete Block Design (RCBD) was employed to organize ten maize hybrids, namely TH 1, TH 2, TH 3, TH 4, TH 5, TH 6, TH 7, TH 8, NK 6172, and P 32, each replicated three times. The variable observation was yield, standardized to 15% moisture content. To determine the tolerance of maize, six tolerance indexes were applied: The Stress Tolerant Index (STI), the Stress Susceptibility Index, the Mean Productivity (MP), the Harmonic Mean, the Stress Relative index (SI), and the Stress Susceptibility Percentage Index (SSPI). The result showed that hybrids that tolerate acidic soil are TH 1 and TH 2, as indicated by their average rating and standard deviation. In contrast, TH 5 and TH 7 hybrids are sensitive to acidic soil conditions. The STI and MP are suitable tolerant indexes that can select acid-tolerant hybrids with high yields under optimum and acidic conditions.


Introduction
The availability of fertile land is limited, especially outside Java, which is a challenge for maize cultivation in Indonesia [1,2] Therefore, maize development is directed at marginal lands, such as those with high acidity.Acidic soils cover about 146.46 million ha in Indonesia [3].The fertility of these soils is low, and they have a high degree of Al saturation (ranging from 22.63% to 75.64%).The levels of nutrients such as macro, micro, and C-organic are also low (between 0.11% and 1.12%) [4][5][6].These circumstances can potentially negatively impact the maize plants' growth and productivity.
Most farmers prefer to grow hybrid maize, a commercial variety with a uniform look and a higher yield than composite maize [7,8].When cultivated in optimal conditions, hybrid maize has a high yield.However, it may yield less in stressful conditions [9].For example, hybrid maize yields between 6.52 tha -1 to 7.38 tha -1 on acidic soils.If the acidity is high, it can only yield between 2.4 tha -1 to 3.4 tha -1 [10].
One crucial abiotic stress affecting crop yields, particularly in maize, is acidic soil.Maize breeders try to screen genotypes with a high potential yield in normal and acidic soil.To assess how genotypes perform under stress, several tolerance indices have been recommended, such as the stress susceptibility index (SSI), which assess the relative genotype yield drop under optimal and stress conditions [11], the mean productivity index (MP), which indicates the mean yield across both environments [12], the geometric mean productivity (GMP) and the stress tolerance index (STI), which reflect the relative ranking of genotypes under stress and optimal conditions [13].However, no single index can adequately capture the complex nature of stress tolerance [14].A good index should have a strong relationship with yield in each environment and have the capability to differentiate between genotypes under both conditions [15].Therefore, combining indices may improve the selection efficiency for stress tolerance.This study aimed to select maize hybrids that can tolerate acidic soil stress using several tolerance indexes and determine the most suitable tolerance index for selecting maize hybrids with acidic soil tolerance.

Material and methods
The research took place at two sites.x EHW x SP ÷1.000 [16] (1) HA = Harvested Area (m 2 ) GM = Grain Moisture (%) EHW = Ear Harvested Weight (kg) SP = Shelling Percentage (%) Six tolerance indexes were used to determine the hybrid tolerance: [13] (2) Harmonic Mean (HM) = Stress Susceptibility Percentage Index (SSPI) = ( Ys represents the yield of a hybrid in acidic soil conditions, while Yp denotes the yield of the hybrid in normal conditions.Y ̅ s is the average yield of all hybrids under acidic soil conditions, and Y ̅ p is the average yield of all hybrids under normal condition.
The tolerance index value of each genotype is calculated and then ranked based on the index used.Then, the mean rating and standard deviation were calculated to determine genotypic tolerance.Lower SSI index values indicate tolerant hybrids.Higher STI, MP, HM, SI, and SSPI index values indicate tolerant hybrids.The average rank and standard deviation were counted to identify the hybrid's tolerance.The most tolerant hybrids were those with the lowest total mean and standard deviation [17]

Result and discussion
The effects of location, hybrids and the interactions on yield are displayed in Table 1.From Table 1, we can see a very significant effect of interaction on yield.The interaction between hybrids and location IOP Publishing doi:10.1088/1755-1315/1302/1/0120013 implies that different locations affect the yield of each hybrid differently [15].A hybrid may have high yields on normal land but low yields on acid soil and vice versa.Interaction between hybrid and location proves highly beneficial for plant breeding under stress conditions [18].Notably, the tolerance genotype performs more consistently than the susceptible one.These differences help breeders to choose the desired genotype.

Hybrid yield at normal and acid soil
The genotypes were clustered into four clusters based on their performance in optimal and stress conditions.Genotypes exhibiting high yield in both conditions comprised in Cluster A. Genotypes that display relatively high yield only when optimal conditions belong to Cluster B. Genotypes that primarily demonstrate high yields in stress conditions encompassed to Cluster C. Genotypes with low yields in both conditions are grouped in Cluster D [13].Cluster A hybrids are the best hybrids because they can produce high yields in both good and bad locations.They are very stable and can handle bad conditions well.Cluster B hybrids are only good for planting in good location Cluster B hybrids are best suited for cultivation in optimal conditions while Cluster C hybrids are specifically tailored for marginal lands, emphasizing their location-specific adaptability.Other hand Cluster D hybrids are the worst hybrids, characterized by their limited stress tolerance Table 3 presents the yields of various hybrids on two types of soils: normal and acidic.On normal soil, the mean yield of all hybrids was 9.39 tha-1.Among these hybrids, TH 1 is the highest performer on normal soil, boasting a yield of 10.92 tha-1.Conversely, TH 7 hybrids recorded the lowest (8.04 tha-1).The mean yield of all hybrids on acid soil was 6.18 t ha-1.P 36 led the way with the highest yield of 7.05 t ha-1, while TH 5 lagged with the lowest yield of 5.44 t ha-1.All hybrids showed a decrease in yield on acid soil compared to normal soil.The most significant reduction in seed yield was 4.56 t ha-1 for TH 5, and the most minor reduction was 1.31 t ha-1 for P 32.On average, the percentage decrease in yield across all hybrids was 33.52%, with TH 5 showing the highest reduction at 45.60% and P 32 exhibiting the lowest at 15.67%.

Hybrid tolerance to acid soil
Six different tolerance indices were applied to assess the hybrid tolerance levels under normal and acidic soil.Among these, Hybrid TH 1 demonstrated the highest tolerance based on the STI, MP, and HM indices, while Hybrid TH 7 was the most susceptible.Regarding the SSI and SI indices, Hybrid P 32 exhibited the highest tolerance, while TH 5 was the most susceptible.Finally, for the SSPI index, Hybrid TH 5 displayed the highest tolerance, while P 32 was the most susceptible (Table 2).There are different levels of hybrid tolerance on each index.A hybrid might be categorized as tolerant in one parameter while susceptible in another.For instance, there are conflicting indications when considering TH 5 and P 32.TH 5 is highly tolerant according to the SSPI index but susceptible when considering the SSI and SI indices.Conversely, P 32 is highly susceptible according to the SSPI index but displays tolerance when assessed using the SSI and SI indices.This can cause uncertainty in determining the tolerance level of hybrids to acid soils.An approach for ranking hybrids based on each tolerance index can address this uncertainty, which is then averaged to determine the tolerance level [19].This method has been applied to determine the tolerance rice to low Nitrogen [20], common bush bean to drought [21], maize to drought [22,23].
The data in Table 3 reveals distinctions among the various test hybrids concerning their responses to acid soil conditions.The total average ranking and standard deviation rank of TH 1 and TH 2 are 5.41 and 5.76, the lowest among all the test hybrids.Conversely, the test hybrids TH 5 and TH 7 showed the highest total average ranking and standard deviation rank of 10.61 and 10.81, respectively.These findings substantiate that TH 1 and TH 2 exhibit greater tolerance to acidic soil conditions, while TH 5 and TH 7 display heightened susceptibility compared to the other hybrid varieties.Additionally, it is noteworthy that the check variety P32 exhibits a higher sensitivity to acidic soils than the check variety NK 6172.

Correlation analysis
The calculation of the correlation between the tolerance index and yields in both normal (Yp) and acidic soil conditions (Ys) was conducted (Figure 2) to identify the most suitable index to select the tolerance hybrid.Put differently, the most suitable cultivars and indexes can be assessed by correlation analysis between yield and tolerance indexes.[24].An appropriate tolerance index should significantly correlate with yield in the two land conditions [25]. Figure 2 shows that all tolerance indexes correlate significantly to Yp except HM.Index SSI, STI, MP, and SSPI display a positive correlation; meanwhile, SI is negatively correlated.Contrarily, all tolerance indices reveal a significant correlation with Ys.SSI and SSPI negatively correlated, whereas the others positively correlated.
Using a tolerance index, those exhibiting a positive association with Yp but a negative association with Ys (SSI and SSPI) tend to choose hybrids within cluster C characterized by low Yp and high Ys [26].Contrarily to the SSI and SPI indexes, selection using the SI index is prone to selecting a hybrid in cluster B with high Yp but low Ys [27].A proper tolerance index can select the tolerant hybrid with high Yp and Ys [28].It means that the index must positively correlate to Yp and Ys.The correlation value between STI and MP with Yp and Ys are significant and positive.This criterion confirms that the STI and MP can select acid-tolerant hybrids with high yields under both optimal and acidic conditions.The previous studies on millet [29], spring wheat [30], and rice [31] used STI and MP to determine the tolerant genotype with high Yp and Ys.

Conclusion
TH 1 and TH 2 hybrids exhibit tolerance to acidic soil, as indicated by their average rating and standard deviation, whereas TH 5 and TH 7 hybrids are sensitive to acidic soil conditions.The STI and MP are suitable tolerant indexes that can select acid-tolerant hybrids with high yields under optimum and acidic conditions.

Figure 1 .
Figure 1.Venn Diagram of Hybrids yields in normal and acidic soil.

Figure 1
Figure 1 illustrates the categorization of hybrids into different clusters.Cluster A comprises two hybrids, specifically TH 1 and TH 8. Cluster B consists of three hybrids: TH 2, TH 3, and TH 5. Cluster C includes hybrids TH 7, NK 6172, and P 32.Additionally, Cluster D is composed of hybrids TH 4 and TH 6.Cluster A hybrids are the best hybrids because they can produce high yields in both good and bad locations.They are very stable and can handle bad conditions well.Cluster B hybrids are only good for planting in good location Cluster B hybrids are best suited for cultivation in optimal conditions while Cluster C hybrids are specifically tailored for marginal lands, emphasizing their location-specific

Table 1 .
Effect location, hybrids, and interactions to yield

Table 2 .
Hybrids yields in normal and acid soil and the tolerance indices.
Note: Yp= Yield under normal soil, Ys= Yield under acid soil, STI = Stress Tolerant Index, SSI = Susceptibility Index, MP = Mean Productivity, HM = Harmonic Mean, SI = Stress Relative Index, SSPI = Stress Susceptibility Percentage Index

Table 3 .
Ranking, average ranking, standard deviation, total average ranking and standard deviation rank of hybrid tolerance indexes Note: Yp= Yield under normal soil, Ys= Yield under acid soil, STI = Stress Tolerant Index, SSI = Susceptibility Index, MP = Mean Productivity, HM = Harmonic Mean, SI = Stress Relative Index, SSPI = Stress Susceptibility Percentage Index, AR= Average Rank.SDR=Standard Deviation of Rank.