3.1. Seed Quality
The traits related to seed quality including germination percentage, germination speed, seedling lengths of stem and root, and seedling dry weight were evaluated in 11 corn genotypes, including seven inbred lines and four F1
hybrids. Significant differences (p
≤ 0.01) among corn genotypes were observed for all traits related to seed quality in both BP test and AA test (Table 2
BP test had higher germination percentages than did AA test. For BP test, germination percentages ranged from 39.0% in C4 to 98.0% in H3, whereas, for AA-test, germination percentages ranged from 3.0% in C4 to 89.3% in H3. In general, hybrids performed better than did inbred lines for germination percentage except for B53, TSC/Su75 and TSC/H3, which were rather high similar to those of hybrids. These inbred lines are associated with sh2sh2 in TSC/Su75 and TSC/H3 or even sh2sh2 btbt in B53. The inbred lines that are associated with wxwx (D22, C4 and 101L) generally had low germination percentage, ranging from 12.7% in 101L to 40.0% in D22 in the BP test irrespective of the combined gene (either sh2sh2 or btbt). However, it is interesting to note here that E8 (btbt sh2sh2 wxwx) had a rather high germination percentage (79.0%) although it has the combination of three recessive genes. After accelerated aging, most genotypes reduced germination percentage sharply except for H3 and KGW#1, which could maintain high germination percentage (89.3% for H3 and 87.3% for KGW#1).
Germination speed followed the similar patterns of those for germination percentage in both BP test and AA test. In BP test, germination speed ranged from 6.0 seedlings per day in C4 to 13.7 seedlings per day in Su75, whereas, in AA test, germination speed ranged from 0.4 seedlings per day in C4 to 13.3 seedlings per day in H3. Again, hybrids were still better than inbred lines for germination speed, and the genotypes that could maintain high germination speed after accelerated aging were H3 (13.3 seedlings per day) and KGW#1 (10.6 seedlings per day).
In the BP test, the single recessive genotypes TSC/Su75 (sh2sh2) had the longest seedlings (10.7 cm) and it was not significantly different from check varieties Su75 (sh2sh2) (12.7 cm), H3 (sh2sh2) (12.5 cm), and KGW#1 (Btbt Wxwx) (10.9 cm). E8 (btbt sh2sh2 wxwx) (8.0 cm) and B53 (btbt sh2sh2) (8.5 cm) were shorter than Su75 (sh2sh2), H3 (sh2sh2), and KGW#1 (Btbt Wxwx), but they were still longer than the check varieties 101L (btbt wxwx) (8.3 cm) and D22 (sh2sh2 wxwx) (6.3 cm), whereas C4 (btbt wxwx) (5.7 cm) was lowest.
In AA test, TSC/Su75 (sh2sh2) (10.8 cm) and TSC/H3 (sh2sh2) (11.5 cm) had long seedlings but their seedlings were still shorter than those of check variety H3 (13.1 cm). Double recessive genotype D22 (sh2sh2 wxwx) (9.6 cm) has long seedlings compared to other triple recessive genotype E8 (btbt sh2sh2 wxwx) (4.8 cm) and double recessive genotypes, B53 (btb tsh2sh2) (4.3 cm), and C4 (btbt wxwx) (3.4 cm), and it was not significantly different from check varieties, Su75 (sh2sh2) (10.8 cm), KGW#1 (Btbt Wxwx) (9.6 cm) and TSC (sh2sh2) (8.4 cm).
In BP test, B53 (btbt sh2sh2) (0.068 g), and D22 (sh2sh2 wxwx) (0.066 g) had the highest seedling dry weight and they were not significantly different from check varieties, H3 (sh2sh2) (0.067 g) and KGW#1 (Btbt Wxwx) (0.064 g). In AA test, D22 (sh2sh2 wxwx) (0.068 g), TSC/Su75 (sh2sh2) (0.065 g), and E8 (btbt sh2sh2 wxwx) (0.059 g) had the highest seedling dry weight and they were not significantly different from check varieties, Su75 (sh2sh2), H3 (sh2sh2), and KGW#1 (Btbt Wxwx) (0.058 g, 0.061 g, and 0.058 g, respectively).
The complex processes and several factors such as genotype, environment, soil born disease and storage time are involved in seed germination and seed vigor [18
]. In previous investigation, sweet corn with shrunken gene (sh2
) had high sugar content in endosperm and low starch, and sweet corm of this type had low germination percentage and poor seedling vigor because the storage food in the seed are not sufficient for normal seed germination [14
]. Low germination of sweet corn harvested at immature seed stage could be improved by seed priming [21
Sweet corn varieties with shrunken gene had more rapid seed imbibitions and higher respiration rate during four days after imbibitions [3
]. However, sweet corn varieties with more than one recessive gene controlling endosperm characters had lower germination than did the varieties with only one recessive gene. Although the new varieties with better seed germination than the parent (101L: btbt wxwx
), which was developed previously, have been developed so far, these varieties had little contribution to germination increase.
Seed germination and seedling vigor are related to yield [22
] and they are used as indicators for germination ability under field conditions. Harvest time [23
] and storage are also the factors affecting seed germination and seedling vigor. It is difficult to determine the most appropriate time for harvest because of the variations in genotype, environment, and maturity index. Number of days after pollination and seed moisture content [24
] were used as maturity indices for corn hybrids with shrunken gene (sh2
). Using seed moisture content as an indicator for maturity had better seed germination and seedling vigor than did using number of days after pollination.
3.2. Weights of Pericarp, Embryo, and Endosperm, Total Sugar Content, and Starch Content
Corn kernels have three important parts that might be related to seed quality and table quality of sweet corn. Pericarp is the outer cover of the embryo and endosperm. Embryo is the seed part that will develop to be a new plant, and endosperm is the part for food reserved. Total sugar content and starch content indicate the types of food reserved, which directly affect seed quality and table quality of vegetable corn. As the seeds of the inbred lines for evaluation were not sufficient, the analysis of kernel properties could be carried without replication. However, the data were average from three values.
Endosperm constituted the largest portion of the kernel, ranging from 0.46 g in C4 to 2.00 g in KGW#1 (Figure 2
). Embryo was somewhat greater than pericarp but it was not always true in some genotypes, ranging from 0.08 g in E8 to 0.33 g in Su75 and H3. Pericarp was the smallest portion of the kernel, ranging from 0.11 g in TSC/Su75 to 0.21 g in B52. For visualization of the kernel proportions for each genotype, the data were also presented in Figure 2
. Total sugar contents in general were higher than starch contents, ranging from 23.7 mg/g in KGW#1 to 230.0 mg/g in C4, whereas starch content ranged from 36.1 mg/g 101L in to 68.3 mg/g in C4.
For kernel proportions, types of corn genotypes (hybrid or inbred) did not determine these proportions, but the differences in proportions were mainly dependent on corn genotypes. However, types of corn genotypes clearly determined total sugar content and starch content as inbred lines were generally greater than hybrids for these traits. For table quality, H3, TSC, and TSC/Su75 seemed interesting as they had the lowest pericarp, and C4 and B53 are the best genotypes for high total sugar content and high starch content.
Carbohydrate compositions and physiological process during seed development are dependent on genes controlling endosperm characters [25
]. In this study, the inbred lines with shrunken gene (sh2
) had intermediate starch content similar to those of the inbred lines with brittle gene (bt
) and waxy (wx
) gene and shrunken gene and waxy gene. Our results were in agreement with those reported previously [12
In this study, starch content at seed maturity stage (30 days after pollination) slightly reduced compared to starch content at immature kernel stage (20 days after pollination). In the inbred line B73 of field corn, rapid increase in starch content occurred during pollination to 12 days after pollination and the starch content reached the highest value at 20–22 days after pollination, whereas the granule shape did not change. The number of starch granules did not increase during 23–30 days after pollination but starch granule shape did, forming more edges [26
]. In sweet corn Shuxuan1, starch content gradually reduced after 18 days of pollination until 30 days after pollination [20
]. Low starch content in sweet corn varieties with shrunken or brittle gene was due to low synthesis of starch during seed development and the high accumulation of sugar [27
In this study, small change in starch content was observed between starch contents at immature kernel stage and maturity stage. The low reduction in starch content in sweet corn varieties with shrunken gene was due to the reduction in amylase activity in aleurone layer, resulting in the inhibition of starch breakdown during germination [28
In the earlier work of our project, the inbred line101bt
) and the inbred line 216sh2
) had the highest sugar contents of 112.9 and 115 mL/gram fresh weight, respectively. However, the newly-developed sweet corn inbred lines, C4 (btbt wxwx
) and D22 (sh2sh2 wxwx
), had the highest sugar contents of 220.8 and 165.4 mL/gram fresh weight, which were much higher than those of the old inbred lines although they had the same genotypes [12
]. The earlier study showed that it is possible to develop the sweet corn inbred lines that had higher table quality by using the combinations of genes controlling endosperm characters.
In general, sugar content at immature kernel stage was higher than at mature kernel stage. However, the reductions in sugar content varied depending on genotype. The carbohydrate compositions in endosperm of sweet corn and other types of corn have been studied [2
]. Sugar content increased from pollination and peaked at 15–20 days after pollination. After the endosperm sugar reached the highest content, the sugar content gradually reduced until it was lowest at physiological maturity stage. In our earlier work in 6 inbred lines of waxy corn, sugar contents were highest during 15–19 days after pollination, and they gradually reduced during 20–40 days after pollination [30
]. The soluble sugar gradually conversed to starch and accumulated in endosperm during seed development (14–42 days after pollination) [20
3.3. Correlations among Traits Related to Seed Quality and Carbohydrate Characters
Most traits related to seed quality were inter-related except for seedling dry weight, which has low correlation with germination percentage determined by BP test, germination speed determined by BP test, germination percentage determined by AA test and germination speed determined by AA test (Table 3
). The correlations among these traits were better in AA test than BP test as all correlation coefficients were significant, ranging from 0.63 * between seedling dry weight and germination speed to 0.99 ** between germination speed and germination percentage. However, when both methods are considered, germination percentage, germination speed, stem length and root length were inter-related and correlation coefficients ranged from 0.74 ** between root length and germination speed using the BP test to 0.99 ** between germination speed and germination percentage using the AA test.
For the relationships between seed quality traits with kernel characters and carbohydrate characters, pericarp weight, endosperm weight and starch content were not significantly correlated with all traits related to seed quality (Table 4
), whereas embryo weight and total sugar content were significantly correlated with all traits related to seed quality especially for AA test. Embryo weight was positively and significantly correlated with germination percentage and germination speed in AA test, whereas total sugar content was negatively and significantly correlated with germination percentage and germination speed in AA test.
Therefore, selection of corn genotypes with larger embryo might improve seed quality, and H3 (0.33 g) and Su75 (0.33 g) are promising for this traits. As pericarp was not correlated with all traits related to seed quality, selection of genotypes with low pericarp is highly recommended for improved table quality, and TSC/Su75 (0.11 g), TSC (0.13 g), and H3 (0.14 g) are worth selecting.
In this study, seed quality parameters such as germination speed, germination percentage, stem length, root length, and seedling dry weight were related to each other and the correlations after accelerated aging were somewhat higher than those of newly-harvested seeds. In previous studies in corn, accelerated aging reduced shoot length, root length, germination percentage, and germination speed [21
]. The results in this study supported previous findings. Germination percentage under laboratory conditions could be used for production of germination percentage under field condition as this parameter evaluated under laboratory condition was positively and significantly correlated with that evaluated under field condition [18
]. Germination speed also had the similar pattern of germination percentage as they had positive and significant correlation both under BP test and AA test [32
]. These findings imply that the seed with high seedling vigor has high germination speed. In corn germination percentage of newly harvested seeds was closely related with germination percentage of accelerated aged seeds [33
Food in the endosperm was another factor affecting germination process of seeds [34
] as seeds require a source of energy for growth [5
]. In this study, sugar had a detrimental effect on seed germination. Germination percentage under field condition was negatively correlated with sugar content in dry seeds [35
However, total starch and total sugar alone are not sufficient to predict seed germination and seedling vigor as several factors such as genotype, soil born disease, environmental factor, seed storage [18
], seed weight, and seed coat [19
], all have an effect.