Evaluation of hybrid maize tolerance under low nitrogen condition based on stress tolerant index and stress susceptibility index

Nitrogen (N) deficiency in maize could cause a yield decrease than normal conditions. Yield reduction is one of the main concerns of plant breeders in stress conditions selection. This study aimed to identify the low N tolerant hybrid maize. This research was conducted at the IP2TP field station of the Indonesian Cereal Research Institute in Maros, South Sulawesi, from June to October 2021. The experiment was arranged in the study using a nested Randomized Complete Block Design (RCBD). Replication was nested in the level of N fertilizer. Seventy-five maize genotypes and two levels of N fertilizer (N1=100 kg N ha−1 and N2=200 kg N ha−1) were used in this study. Each genotype was replicated two times at each fertilizer level. The variable observed was maize yield. Two tolerance indexes, such as the Stress Tolerant Index (STI) and Stress Susceptibility Index (SSI), were used to determine the maize’s tolerance to low N conditions. The results showed that the test hybrids showed tolerance level variations according to the SSI and STI indices. Seven hybrids were tolerant, and ten were susceptible based on two indices. Six hybrids were tolerant according to SSI but susceptible according to STI. Five hybrids were tolerant based on STI but susceptible based on SSI. Hybrid CML 465 x AVLN 109-3, AVLN 109-3 x VL1016518, CAL 1471 E x B 11, AVLN 86-4 X 1026-12, CML 465 x CAL 1473, VL1016518 x P7 and CML 161X CI 32 hybrids were tolerant maize based on SSI and STI indexes. These hybrids can be used as low N tolerant maize breeding materials in the next breeding program.


Introduction
The development of maize cultivation in Indonesia is directed to marginal land due to limited fertile land, especially in other agricultural areas in Indonesia besides Java.One of the characteristics of marginal land is low in nitrogen content [1,2].Intensive cultivation not followed by the return of organic matter into the soil caused the problem of the increase nitrogen deficiency, so the soils become poor in macronutrient element resources.Poor soil will affect the maize growth and development phase.
Maize, when cultivated in low N conditions will have a yield reduction.Yield reduction in low N conditions can reach 10-50% [3,4].It is necessary to increase the dose of N fertilizer to keep maize production high, but this will increase production costs and negatively impact the environment [5].Therefore, low N tolerant maize varieties are needed to increase Nitrogen efficiency [6].Breeding low N tolerant maize in China increased yield by 14% under low N conditions [7].The development of 1230 (2023) 012120 IOP Publishing doi:10.1088/1755-1315/1230/1/012120 2 adaptive maize varieties with low N fertilizer helps farmers and reduces the negative impact of high N fertilization on the environment.
Selection is an essential step in the improvement of low N tolerant maize.Yield loss is a significant concern in stress-tolerant plant breeding.Therefore, breeders emphasize yield under stress conditions in selecting stress-tolerant plants [8].Several kinds of indexing methods can screen plants tolerant to stress.In general, the index can be grouped into a susceptible index and a tolerance index.The susceptibility index distinguishes between tolerant and sensitive genotypes based on a negative correlation with yield, while the tolerance index is based on a positive correlation toward yield [9] Stress susceptibility index (SSI) was used to determine the tolerance level of a genotype to stress [10].Researchers also use the stress tolerance index (STI) to evaluate the genotypes' tolerance to stress conditions [11].A good tolerance index can distinguish genotypes under optimum and stress conditions and highly correlates to yield in both conditions [12].There is no precise index to select genotypes that are tolerant to stress conditions so far.A combination of stress indexes (tolerance and susceptibility indexes) can provide better criteria for increasing the selection of tolerance to stress conditions [13,14].Many researchers have used the tolerance index simultaneously in evaluating the tolerance of a genotype to stress i.e. rice [15], wheat [16], maize [17] and barley [18].It's shown to increase accuracy in the genotype tolerance screening results.Based on this, this research was carried out to identify the low N tolerant hybrid maize using the STI and SSI indexes.

Material and methods
The research was conducted at the Maros Experimental Farm, the Indonesia Cereal Research Institute (ICERI), South Sulawesi, from June to November 2021.The study used a nested Randomized Complete Block Design (RCBD).Replication was nested in the level of N fertilizer.This study used two N fertilizer levels: N1 (100 kg N ha -1 ) and N2 (200 kg N ha -1 ).Seventy-five maize hybrids were used in the experiment consisting of seventy-two test hybrids and three check varieties, namely BISI 18, P 36, and NK 7328 replicated two times in each fertilizer level.Fertilization in the N1 treatment (100 kg N ha -1 ) was applied once, i.e., ten days after planting (DAP), while in the N2 treatment (200 kg N ha -1 ), fertilization was applied twice, i.e., 50% at 10 DAP and the rest at 35 DAPS.The observed variable was yield which was corrected to 15% moisture that converted to units per hectare using the formula: Yield (t /ha) = 10.000HA x 100-GM 85 x EHW x SP ÷1.000 [19] (1) HA = Harvested Area (m 2 ) GM = Grain Moisture (%) EHW = Ear Harvested Weight (kg) SP = Shelling Percentage (%) Two tolerance indexes were used to determine the tolerant hybrid to low N conditions: a. Stress Susceptibility Index (SSI) Tolerance criteria based on SSI are as follows: SSI<1: tolerant, SSI≥1: susceptible.

b. Stress tolerant index
Tolerance criteria based on SSI are as follows STI >1,0: tolerant, 0,5 < STI ≤1,0: medium tolerant, STI ≤ 0,5: susceptible Information: Ys = hybrid's yield under Low N condition Yp = hybrid's yield under optimum N conditions Ys = mean of all hybrids' yield under Low N condition Yp = mean of all hybrids' yield under optimum N condition

Result and discussion
Analysis of variance in Table 1 showed significant differences in the effect of nitrogen levels, hybrids, and the interactions between nitrogen levels and hybrids on yield character.Interaction between nitrogen levels and hybrids indicated that the genotypes used in this experiment had variation responses to the N fertilizer level changes [20] .There was a difference in hybrid yields rank between 100 kg N ha -1 and 200 kg N ha -1 fertilization.It implicated the differences in hybrid maize's tolerance to N fertilization levels decreased.The average yield at 100 kg N N ha -1 fertilization was 4.85 ton ha -1 .The AVLN 86-4 X 1026-12 hybrid obtained the highest 100 kg N N ha -1 fertilization yield (7.64 ton ha -1 ).The B11 x CML 465 hybrid had the lowest yield of 1.01 ton ha -1 at 200 kg N ha -1 fertilization, the highest yield was found in hybrid AVLN 109-3 X AVLN 83-4 (9.71 ton ha -1 ), and the lowest yield was in hybrid B11 x CML 465 (3.29 ton ha -1 ) with an average yield of 6.93 ton ha -1 (Table 2).
The hybrids could divided into four groups based on the yield under optimal and stress conditions [21].Group A has a high yield both under optimal and stress conditions.Group B has a relatively high yield only under optimum condition.Group C hybrid, which has a relatively high yield under stress condition.Group D has a low yield under optimal and stress conditions (10).There were twenty-eight hybrids included in group A. Sixteen hybrids belonged to group Twelve hybrids belonged to group C. Nineteen hybrids included in group D (Table 2).A=Genotype with a high yield under optimal and stress conditions.B= Genotype with a high yield only under optimal conditions, C= Genotype with a high yield only under stress conditions, D= Genotype with a low yield under optimal and stress conditions.
Each hybrid suffers varied yield reductions at each level of N fertilization.The average yield reduction for all hybrids was 2.08 ton ha -1 .The AVLN 86-2 X CAL 1473 hybrid suffers the highest yield reduction, about 5.87 ton ha -1 , decreasing from 6.91 ton ha -1 to 1.03 ton ha -1 .Hybrid AVLN 109-3 x VL1016518 did not suffer a yield decrease at 100 kg N ha -1 fertilization (Table 2).
The difference of each hybrid yield reduction indicated hybrid tolerance differences at different N levels.Table 2 showed that 39 hybrids were tolerant and 36 were susceptible based on the SSI index.The AVLN 109-3 x VL1016518 hybrid was the most tolerant genotype, while the AVLN 86-2 X CAL 1473 hybrid was the most susceptible genotype according to the SSI index.It can be seen from the SSI value of hybrid AVLN 109-3 x VL1016518 is the smallest (0.00), while AVLN 86-2 X CAL 1473 is the largest (2.84).The SSI index provides information on the hybrid tolerance based on its ability to suppress yield loss in stressed conditions regardless of the yield level [22].A hybrid with a low yield could have the same relative SSI value as a hybrid with a high yield if the yield reduction of both hybrids were the same.It can be seen on CAL 1471 E x VL 1016492 and VL1016492 x 122-2 hybrids.Both crosses showed the same SSI value (0.33).Under both fertilization levels, CAL 1471 E x VL 1016492 yield was below all hybrids' average yield, while the VL1016492 x 122-2 was above the average.Therefore, the selection of tolerant hybrids based on the SSI index must also consider the yield so that the selection will be more accurate.
Based on the STI index, sixteen hybrids were susceptible, forty-seven hybrids were moderate tolerant, and thirteen hybrids were tolerant to low N level (Table 2).The B11 x CML 465 hybrid was the most susceptible, while the AVLN 86-4 X 1026-12 was the most tolerant.The STI value of B11 x CML 465 was 0.07, with 100 kg N ha -1 and 200 kg N ha -1 fertilization yields of 1.01 ha -1 and 3.29 ton ha -1 , respectively.The STI value of AVLN 86-4 X 1026-12 was 1.49, with yields of 7.64 ton ha -1 at 100 kg N ha -1 and 9.33 ton ha -1 at 200 kg N ha -1 fertilization.
The STI index can provide an overview of high-yielding maize genotypes under normal and stress conditions.The STI index could distinguish tolerant genotypes with high yield potential under normal and stress conditions [23,24].Table 2 reveals that the tolerant and medium tolerant hybrids have aboveaverage yields at both fertilization levels.
The SSI and STI indexes have different characteristics in the hybrid selection under stress conditions.A genotype could have the same or different tolerance based on those two indexes [25].A hybrid could include in the tolerant category according to the SSI index but susceptible according to the STI or vice versa.Therefore, it is necessary to make a selection involving both indexes so the selection can be carried out correctly [26].The simultaneous tolerance selection genotype based on the SSI and STI indexes will be very useful.Hybrids with tolerant STI and SSI had yields between 6.18-7.64 ton ha -1 at 100 kg N ha -1 fertilizer level and 7.05-9.33ton ha -1 at 200 kg N ha -1 fertilizer level.SSI values ranged from 0.00 to 0.92, and SSI and STI values for hybrids that are susceptible based on SSI and STI are 1.29-2.84and 0.07-0.46,respectively.The yield at 100 kg N ha -1 fertilization level was 1.01-3.40ton ha -1 , and at 200 kg N ha -1 fertilization level was 3.29-5.87ton ha -1  Hybrids that are tolerant according to SSI but susceptible to STI are AVLN 109-3 X AVLN 86-4, AVLN 83-4 X AVLN 122-2, B11 x VL144077, CAL 1471 E x VL 1016492, MAL 03 x CAL 1473 and VL1440077 x B11.SSI and STI values are between 0.12-0.75 and 0.30-0.44,respectively.The hybrid yields were 3.58-4.17N ton ha -1 at 100 kg/ha fertilizer level and 3.97-5.20ton ha -1 at 200 kg N ha -1 fertilizer level.Hybrids in this group belong to group D.
AVLN 86-4 X CAL 1473, CML161 x 122-2, CML 465 x 118-2, MGOLD x 62-8, and P 36 hybrids are tolerant based on STI but susceptible based on SSI.The hybrid has an STI value between 1.00-1.27and an SSI value ranging from 1.02-1.35.The yield of this hybrid was 5.41-6.39ton ha -1 at 100 kg/ha and 8.47-9.53ton ha -1 at 200 kg N ha -1 .All hybrids in this group belong to group A. hybrids in this group are hybrids that are suitable under optimal conditions.

Figure 1 .
Figure 1.Venn diagram of hybrid tolerance based on SSI and STI indexes.

Fig 1
Fig 1 shows that seven hybrids were tolerant according to SSI and STI indexes.ten hybrids were susceptible based on SSI and STI.Six hybrids were tolerant according to SSI but susceptible according to STI.Five hybrids were tolerant based on STI but susceptible based on SSI.Hybrids that were medium tolerant based on STI and tolerant based on SSI twenty-six.Furthermore, twenty-one hybrids were medium tolerant based on STI and susceptible based on SSI.Hybrids with tolerant STI and SSI had yields between 6.18-7.64 ton ha -1 at 100 kg N ha -1 fertilizer level and 7.05-9.33ton ha -1 at 200 kg N ha -1 fertilizer level.SSI values ranged from 0.00 to 0.92, and SSI S: Susceptible based on SSI SSI T: Tolerant based on SSI STI S: Susceptible based on STI STI T: Tolerant based on STI STI MT: Medium Tolerant based on STI STI values from 1.02 to 1.49.The hybrids are CML 465 x AVLN 109-3, AVLN 109-3 x VL1016518, CAL 1471 E x B 11AVLN 86-4 X 1026-12, CML 465 x CAL 1473 VL1016518 x P7, CML 161X CI 32.These hybrids include in group A. These hybrids have the opportunity to be used as breeding materials for low N tolerant maize in the next breeding program.
test hybrids showed tolerance level variations according to the SSI and STI indices.Seven tolerant hybrids and ten susceptible hybrids based on both indices.Six hybrids were tolerant according to SSI but susceptible according to STI.Five hybrids were tolerant based on STI but susceptible based on SSI. b.Hybrid CML 465 x AVLN 109-3, AVLN 109-3 x VL1016518, CAL 1471 E x B 11, AVLN 86-4 X 1026-12, CML 465 x CAL 1473, VL1016518 x P7 and CML 161X CI 32 hybrids were tolerant maize based on SSI and STI indexes.These hybrids have the opportunity to be used as breeding materials for low N tolerant maize in the next breeding program

Table 1 .
Variance analysis of nitrogen level, hybrids and their interactions effect to hybrid maize yield.

Table 2 .
Yield at 100 kg N ha -1 and 200 kg N ha -1 fertilization, yield decreasing value, Stress Susceptibility Index (SSI) and Stress Tolerance Index (STI) values for tested hybrid maize.

Table 2 .
Yield at 100 kg N ha -1 and 200 kg N ha -1 fertilization, yield decreasing value, Stress Susceptibility Index (SSI) and Stress Tolerance Index (STI) values for tested hybrid maize (Cont.)