Seed Vigour of Quality Protein Maize Varieties Belonging To Different Maturity Groups
Laboratory tests were carried out to investigate inherent quality differences among stored seeds of early, intermediate and late-maturing quality protein maize using completely randomized design (CRD) with three replications at the Seed Testing Laboratory of the Institute of Agricultural Research and Training, Obafemi Awolowo University, Moor Plantation, Ibadan, Nigeria for two consecutive years (2014 and 2015). From the results, germination and accelerated ageing germination traits were affected by the storage period. Similarly, the seed performance on germination, accelerated ageing and the conductivity test were seen to be better in early maturing than in other maturing maize genotypes. The speed of germination measured as the germination index was low; ranging from 3.24-3.68 days after sowing (DAS) irrespective of the maturity group. Seedling traits measured after physiological quality tests were equally affected by the storage period and the maturity group. All the seedling traits measured were better in the early maturing genotypes. Seed quality measured by laboratory quality assessments showed that genotypes in the early- maturing group are of high quality. Seeds of early-maturing genotypes retained significantly higher physical and physiological quality parameters and appear to have inherent potential to withstand effects of deterioration due to their slower rate of quality decline.
Quality Protein, Seed quality, Germination index, Storage, Maturity group
Maize is a major cereal crop for human nutrition worldwide. Several million people, particularly in the developing countries, derive their protein and calories requirements from maize. In many developing countries of Latin America, Africa and Asia, maize is the major staple food and often the only source of protein because animal protein is scarce and expensive and consequently, unavailable to a vast sector of the population . One of the main nutritional limitations of normal maize grain is its poor nutritional profile because of a deficiency in essential amino acids such as lysine, tryptophan and methionine . Normal maize has 10% protein which is of poor nutritional quality due to limiting concentrations of essential amino acids (lysine and tryptophan) which the human body cannot synthesise and has to be supplemented [3-5]. Therefore, adoption and cultivation of Quality Protein Maize (QPM) with high concentration of lysine and tryptophan could drastically reduce malnutrition, diseases and death among low income maize consumers in the developing countries, including Nigeria [6,7].
Maize production in the southern Guinea savannah (SGS) of Nigeria is constrained by several factors including drought, low soil nutrient status, susceptibility to pests and diseases, poor adaptation to the agro- ecologies [8,9] and limited availability of high quality seeds of known and traceable origin. Seed is a fundamental input; the most important and comparatively the least expensive of all external inputs in crop production. The quality of seeds, (physiological seed properties) which determines the potential for rapid, uniform emergence and development of normal seedlings under a wide range of field conditions  is an essential determinant of crop productivity. The use of seeds with high physiological potential is indispensable for success in agricultural production, and makes a significant contribution to rapid stand establishment and the initial development of vigorous seedlings in the field . Therefore, quality assurance during production, harvesting and post-harvest handling of harvested seed mass is an important consideration both in research and commercial seed production. Maize breeders have evaluated exotic germplasm for desirable agronomic and quality traits . However, seed quality is usually not one of the selection criteria. Seed scientists are concerned that breeders do not evaluate seed quality characters in their breeding programmes .
It is essential for producers that the improvement of the nutritional value of maize is not done at the expense of seed quality. Seed physiological quality is one of the major important factors affecting early performance and productivity of most agricultural crops. Generally there is inadequate seed quality assurance infrastructure for seed testing in developing countries. Quality assurance is required to increase crop productivity and enhance food security in these countries . Thus there exist ample opportunities in Nigeria to enhance crop productivity and food security through appropriate seed technologies. One of the main problems often observed in the field is poor seedling establishment which is influenced by seed quality, climatic conditions and field management practices [15,16]. Seed vgour is the sum of those properties of the seed that determine the activity and performance of seedlots of acceptable germination in a wide range of environments . Low vigour seed lots contribute to the development of uneven seedlings in plant stands and therefore reduce final yield. It has been widely reported that the longer the period of seed storage, the lower the seed quality and consequently the yield . Seed quality is a collective term for the state of a seed lot including genetic homogeneity, physical appearance, viability, vigour, and uniformity . Other characteristics such as specific chemical composition or resistance to certain insect pests and diseases also contribute to the quality of seed. In general, the quality of seed is measured in many ways. These include genetic and physical purity, germination, vigour, uniformity in size, freedom from seed-borne diseases, and other factors like heat and or mechanical damage  as well as pre-harvest sprouting [21,22] affecting seed performance in the field. Therefore, the study aimed to investigate inherent physiological quality differences among seeds of early, intermediate and late maturing maize genotypes.
Materials and Methods
The experimental biological materials comprised 12 Quality Protein Maize (QPM) genotypes belonging to three different maturity groups namely early, intermediate and late-maturity group which were stored for a period of 15-months. Seeds of nine of the QPM genotypes were developed at the Maize Improvement Programme of the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. The other three genotypes were obtained from the Cereals Improvement Programme of the Institute of Agricultural Research and Training (IAR&T), Moor Plantation, ibadan, where they were developed. The seeds of the genotypes were multiplied during the late season of 2013. Harvesting and processing were done in December 2013. After the preliminary evaluation of seed quality, the envelopes containing the genotypes were put inside polyethene bags for storage in the cold room of the IAR& T during the period of the study.
Seed quality assessment
The quality of seeds of 12 QPM genotypes (4 early, 4 intermediate and 4 late-maturing genotypes) was assessed with the following tests in the Seed Testing Laboratory of the Institute. The list of the maize genotypes used is presented in (Table 1).
Standard germination: Standard germination test was carried out in three replications of 50 seeds per replicate. Plastic germination bowls were filled with moistened sharp sand after sterilization and seeds were evenly spaced on the sand and thereafter thinly covered with moistened sand and lightly pressed for a good seed-substratum contact (Figure 1). The bowls were covered with nylon sheets to conserve moisture and kept at room temperature in the seed testing laboratory of the Institute. Germination counts were made daily from the 4th to 7th day after planting. On the 7th day, seedling analysis was carried out and the numbers of normal and abnormal seedlings were recorded. Germination was calculated as the percentage of seeds producing normal seedlings . After seedling analysis, five seedlings were selected at random for the determination of seedlings traits such as number of roots, root length, shoot length ,root dry weight, shoot dry weight, seedling length and seedling vigour index.
Electrolyte leakage (EL): Fifty intact and clean seeds in three replicates were counted, weighed, and placed in a 250 ml Pyrex conical glass flask containing 100 ml of distilled water. The flasks were covered with aluminum foil to prevent contamination and the flasks were gently shaken intermittently at 5-hours interval. Electrical conductivity measurements of the solutions (as an indicator of electrolyte leakage) were taken after 24 hours at 25 ℃ reference temperature using Mettler Toledo MC126 conductivity meter. All measurements were expressed as μ S cm-1g-1 and the values were calculated as suggested by :
Accelerated ageing: Ageing of seeds was determined by weighing 50 seeds from each subsample in three replicates. Thereafter, the seeds were placed on a wire mesh suspended over water inside accelerated ageing boxes with 4 1/8" × 4 1/8" × 1 1/8" dimension (Hoffman manufacturing brand) and then placed in an ageing chamber for 72-hours at a temperature of 43 ℃. The seeds were re-weighed after ageing to determine the amount of moisture gained during the ageing process. After the ageing period, the seeds were subjected to standard germination test as described above and the germination count on the 5th and 7th days' after planting. The results were expressed in percentage as done for standard germination. Accelerated ageing index (AAI) was similarly calculated from the accelerated ageing germination data.
Data collected were subjected to analysis of variance ANOVA separately for each maturity group and combined across maturity groups and the two years using general linear model procedures in statistical analysis system (SAS) version 9.2. , to compute mean squares for each character. Mean separation was done using Duncan Multiple Range Test (DMRT). Regression analysis was run to generate b-value (slope).
Inter-maturity group responses to storage durations on physiological quality of maize seeds
Mean values for physiological quality of maize genotypes of different maturity groups presented in (Table 2). All the traits were significantly affected by duration of storage when averaged over maturity groups. Hundred seed weight reduced by 5.46% 15-months after storage. At the initial stage of the trial, average germination was 82.78% μ S cm-1g-1 this was significantly reduced by about 13% 15 months after storage. Germination index (GI) and accelerated aging germination index (AAGI) were delayed by I day as compared to initial and 9 months after storage. Accelerated aging also reduced significantly by 44.9% from 66.25% to 36.49% at initial and 15-months after storage, respectively. Electrolyte leakage recorded 77.5% increase from 14.03 μ S cm-1g-1 to 24.90 μ S cm-1g-1 at initial and 15-month in storage. Early maturing genotypes showed highest mean values for hundred seed weight (HSW), standard germination (SG) and accelerated aging germination (AAG) but with significant and lower mean values for standard germination index (SGI), accelerated ageing germination index (AAGI) and Electrolyte leakage (EL). Number of roots (RN) was not affected until after 15-months in storage (Table 3). Root length (RLT), Root dry weight (RDW) and Shoot dry weight (SDW) progressively reduced and the difference was significant by 15 months in storage. There was a general trend of decrease in both seedling length (SDL) and seedling vigour index (SVI) during storage period with values of 20.43 cm and 1.59 after 15-months in storage from 25.81 cm and 2.17 respectively at the beginning of storage. All the seedling traits measured were highest in early-maturing genotypes followed by the late-maturing genotypes and intermediate genotypes.
Intra-maturity group differences in physiological quality of maize seeds and the resultant seedlings
The genotypic mean values for all traits, evaluated in each maturity group are presented in (Table 4). Laboratory SG (%) ranged from 41.11% to 95.89%. The lowest SG (%) of less than 50 was recorded in two of the intermediate genotypes (41.11% and 44.67% for ILE-1-OB and POP66-SR/ACR 91, respectively). Irrespective of the maturity group, the SGI was low ranging from 3.24-3.68 days after sowing (DAS). Average AAG ranged from 36.11 in ART/98/SW5 (intermediate) to 83.11 in TZE-POPDT (early) respectively. Maximal level of performance in AAG was reached in 2 out of the 4 genotypes in early maturing group. The difference in AAGI among the 12 maize genotypes was small. Electrolyte leakage values ranged from 7.77 μS cm-1g-1 in EVDT-W99 (early) to 62.42 μScm-1g-1 in ILE-1-OB (intermediate). All the measured seedling traits after physiological quality tests were highest in the early-maturing genotypes (Table 5). Number of root ranged from 4.84 in EVDT-W99 to 3.16 in ILE-1-OB. SVI also ranged from 2.53 in TZE-YPOPDT to 1.50 in ART/98/SW 5-OB.
Inter-maturity group responses to storage durations on physiological quality of maize seeds
Successful crop production requires the use of high quality seeds to achieve good crop establishment count and high crop yield. High quality seeds are demanded by the end users to ensure maximum seedling emergence and stand establishment in the field . High quality seeds can perform well in the field, ensuring optimum stand establishment and satisfactory grain yield under a wide range of environmental conditions . The most obvious component of seed quality is germination capacity and germination tests are used worldwide to determine the maximum germination potential of a seed lot under optimum conditions. Therefore germination capacity should be considered a priority in seed production. The significant effect of storage period across maturity groups on germination indicates that seed lots deteriorated progressively as storage period increased. Generally, germination percentage decreased from 0 month to 15-month of duration at the rate of 0.88. Seed deterioration was still evident in the seed lots after 15-months in storage due to a day' delay in the speed of germination. This could be ascribed to loss of vigour due to ageing during storage. This implies that no matter how perfect the storage facilities and conditions are, the potential of seed lot deteriorates with storage period. This agreed with the works of Gupta and Aneja  who reported a marked reduction in vigour due to seed ageing and Arefi and Abdi  that reported reduced viability, rate and capacity of germination also due to seed ageing. Lower viability recorded in the intermediate-maturing genotype is in agreement with the findings of Mahesha, et al.,  who reported that varieties differed significantly in germination percentage.
Intra-maturity group differences in physiological quality of maize seeds and the resultant seedlings
The vigour of the early materials was better than the other maturing groups due to faster emergence as measured by the germination index. Seed weight as an index of seed quality was high in the early-maturing genotype and this contributed greatly to the quality of early maturing genotypes. Large seeds of the early-maturing genotypes enhanced the germination percentage, the production of more vigorous seedlings, higher, seedling length, dry weight and seedling vigour index. This agreed with the works of [31,32] they reported that seed weight and seed nutrient content affect plant growth at seedling stage. From this study, potential performances such as storage quality and physiological quality were assessed using accelerated ageing (AAG) and electrolyte leakage (EL). The AAG made it possible to determine if certain seed lots had a lesser or greater germination capacity, and to determine the storage potential of seed lots . Seed vigour as assessed by AAG test showed a declining trend in viability after ageing and all other traits were significantly affected by seed ageing as indicated by the variation in values over storage period. Reduced seed germination following seed ageing treatment might have resulted from the increased solute leakage following imbibition which is usually accompanied with inevitable exist of some necessary materials for germination and normal seedling growth. This result is in agreement with those of [34-36]. The authors worked with different crops and reported that electrical conductivity and germination rate tests are suitable for evaluation of seed vigor in both laboratory and field experiments. Low conductivity value at the beginning of the trial showed that all the seed lots used were of high vigour.
Based on the above discussion it is concluded that: Significant mean squares for maturity groups and storage durations for all the measured traits in the laboratory seed quality assessment indicated that the genotypes used are of different physiological status and storage potentials. Seeds of early, intermediate and late-maturing genotypes exhibited inherent differences in physical and physiological seed quality. Seeds of early genotypes had significantly higher physical and physiological quality parameters. They appeared to have inherent potential to withstand deterioration due to their slower rate of deterioration.
This work was supported by Research by Research Grant from Federal Ministry of Agriculture and Rural Development through the Institute of Agricultural Research and Training, Obafemi Awolowo University, Moor Plantation, Ibadan, Oyo State, Nigeria.
Declaration of Competing Interest
Credit Authorship Contribution Statement
This work was carried out by the first author under the supervision of the second author. It was a PhD work of the first author. Authors Olasoji Julius Oluseyi and Ajayi Sunday Adesola designed the study. Author Olasoji Julius Oluseyi collected data, performed the statistical analysis, wrote the protocol and wrote the first draft of the manuscript under the supervision of author Ajayi Sunday Adesola. Both authors read and approved the final manuscript.
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Olasoji Julius Oluseyi, Institute of Agricultural Research and Training, Obafemi, Awolowo University, PMB 5029, Moor Plantation, Ibadan, Oyo State, Nigeria.
© 2020 Olasoji JO, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.