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Genetic Gain and Heritability of Seedling Characters Selected at a Low Temperature in Pearl Millet (Pennisetum typhoideum Rich.)

Totok Agung Dwi Haryanto, Tee-Kwon Shon and Tomohiko Yoshida
(Faculty of Agriculture, Kyushu University, Fukuoka 812-8581. Japan)

Abstract : One cycle of selection was applied for studying genetic gain and heritability of seedling characters such as shoot length and root length at a low temperature in two genetic populations. The correlation of these characters in yield components was also examined. Selection had a significant effect on seedling characters other than shoot-root ratio. Selection for a long shoot or root increased the percentage of germination, shoot length, root length, panicle length and panicle weight in the field. Shoot length was significantly correlated to root length. There was a relationship between seedling characters and yield components. A genetic gain for seedling characters was obtained. Shoot and root lengths are heritable characters and may be useful as selection criteria at low temperature. Heritability differed with the population selected for shoot or root length. Heritability values were 0.28〜0.69 for shot length and 0.37〜0.76 for root length. Further selection gain for a longer shoot and root should be possible.
Key words : Genetic gain, Germination, Heritability, Low temperature, Pearl millet, Recurrent Selection.
Accepted 26, May l997

Pearl millet (Pennisetum typhoideum Rich.) is one of the most extensively cultivated cereals, after rice, wheat, and sorghum, in arid to semi-arid regions. It is a principal food cereal cultivated on 25 million ha of the drought prone semi-arid regions of Africa and Indian subcontinent and has been grown most extensively as a forage crop in the USA, Australia, Southern Africa, and South America (Anand Kumar and Andrews, 1993 ; Burton, 1980 ; Burton and Wilson, 1995).
  There are many advantages in early planting of crops (Totok, et al. l997). It allows safe harvesting before the onset of rain or frost that may decrease seed yield and quality. It enables the marketing of an early and competitive produce (Blum, 1988) , and increases the number of options available for subsequent crops (Kane and Grabau, 1992).
  To develop early planting genotypes of pearl millet, information concerning germination at a low temperature is important, because low temperature stress is the most common environmental stress for germination (Blum, l988). In addition to reduction in germination, subsequent seedling growth is also inhibited by low temperatures.
  Some findings have reported germination of pearl millet. M'Ragwa et al. (l995) estimated the heritability value and genetic gain for seedling root length and coleoptyle length in pearl millet. They concluded that the coleoptyle and root lengths of seedlings were heritable and further selection gain should be possible. Yoshida and Sumida (l996) applied mass selection for germination at a low temperature, heavy grain weight and early heading, in a pearl millet population, and estimated the heritability from the genetic gain to be 0.26, 0.04, and 0.77 for germination, grain weight, and heading, respectively. Totok et al.(l997) reported that the pearl millet germination rate was low at low temperatures and increased by raising the temperature ; they concluded that pearl millet was capable of recovering the growth and yield in the field after exposure to low temperature stress at the germination stage and that selection for early spring planting is possible.
  In this study, the effect of one selection cycle on seedling characters at low temperatures were examined, and heritability values and genetic gain of the characters were estimated. The relationship between seedling characters and yield components were also examined.

Materials and Methods
One selection cycle for shoot and root length 5 days after seed germination at 15C was practiced with two pearl millet populations ; short stature (So) and tall stature (To) populations. So originated from an open pollinating variety "ICVM83074" and To from several early maturing breeding materials. These seeds were increased by open pollination in several seasons including May and August sowing (Totok et al. 1997 ; Yoshida and Sumida, 1996).

1. Selection procedure
   Fifty randomly chosen seeds of So and To were placed on filter paper in a petri dish, 9 cm in diameter, 4 petri dishes per group. The petri dishes were kept at 15C for germination. One mL of distilled water was supplied to each petri dish daily. After five days of incubation, the shoot length of germinated seeds was measured. The seedling with the longest 10% and the shortest 10% shoots were selected from each group and they were replanted in individual paper cup, 5cm X 5cm X 5cm in size, filled with fertile soil, and placed in the green house until they could be transplanted to pots for seed production. The same procedure was applied for root length selection. A total of eight sub-populations were created from 2 parents X 2 characters X 2-way selections.

2. Seed production of sub-populations
   The seedlings were transplanted to plastic pots filled with about 4kg sifted soil, 0.8g N, 0.8g P2O5 and 0.8g K2O per pot in May l996 in a green house. Two plants per pot and 6 pots per sub-population were arranged. A total of 120 plants in 60 pots, including So and T0, were prepared. Panicles were bagged before heading, polycrossed among individuals manually within a sub-population, and harvested on August 1996.

3. Evaluation of germination at low temperatures
  The seedling characters of sub-populations and So and To were examined at a low temperature (15C). Four petri dishes were used for each sub-population as replications of completely randomized design. The data were subjected for analysis of variance according to Steel and Torrie (1980). The mean value of generation for each character was compared to their original parental mean value to estimate heritability and genetic gain values.

4. Yield components evaluation
   Seeds from sub-populations, namely : a long-shoot population (S1) and a long-root population (S2) selected from an So and a long-shoot population (T1) and a long-root population (T2) selected from To, were germinated in petri dishes with filter paper at 23rd July, transferred to paper pots at 3lst July and transplanted to field at l7th August 1996. Sub-populations each with 20 seedlings were grown in two rows per plot in a randomized complete block design with 4 replications. Space planting of 50X l0 cm between and within rows per plot, and 0.8g, N, 0.8g P205 and 0.8g K20 per plant were applied. Plant height, number of productive panicles, panicle length and panicle weight were observed as yield components and their correlations to the seedling characters at low temperatures were calculated.

5. Estimation of heritability
   Heritability (h12 and h22) was estimated using two formulas (Falconer, 1981 ; Fehr, l987) :
h12=(X1high - X1low)/(Xohigh - Xolow)
where, Xohigh and Xolow, are mean values of the character of individuals selected from the original population for high and low. characters respectively, and X1hiqh and X1low are mean values of the character of population selected after mating the individuals for high and low characters, respectively.
h22 = (X1sel - X1pop)/(Xosel - Xopop)
where, Aosel, Aopop, X1sel and X1pop, are the mean values of the character of individuals selected from the original population, that of the original population, that of the population derived from mating the selected individuals and that of the original population grown with Xsel, respectively. The h22 was calculated from the results of upward and downward selection responses.
The realized genetic gain was calculated as :
G= X1high - X1low and G=X1sel - X1pop

Results and Discussion

   Table 1 shows the analysis of variance for seedling characters after one selection cycle. Selection had a significant effect on the seedling characters, except for the shoot-root ratio. Sub-populations differed in shoot length, root length, and percentage of germination, as well as populations pooled across sub-populations. The absence of significant effect of selection on shoot-root ratio in this study revealed that this character was least influenced by selection. Table 2 shows the mean values of sub-populations pooled across parents and selected sub-populations.

1. Seedling characters
(1) Percentage of germination
The percentage of germination of the short population was higher than that of the tall population. The percentage of germination of sub-populations selected either for a short shoot or root was not significantly different from that of the parent population. Sub-populations pooled across parents selected for a long- shoot or root showed a higher percentage of germination than the parent population. The same results were obtained in the tall and short populations. This indicates that selection either for long shoot or root increased the percentage of germination at low temperature.
(2) Shoot length
   The shoot length of the tall population was longer than that of the short population. The shoot length of the sub-population selected for a long shoot was longer than those of the sub-populations selected either for a short shoot or root and of the original population. This means that selection for a long shoot or root was effective for the selection of long shoot characters. Sub-populations selected for a short shoot and root were not significantly different from each other, or from the original population in shoot length.
   In the tall population, the shoot length did not differ significantly among the sub-populations selected for a long shoot and root and the original population. However, the shoot of these sub-populations was longer than that of the sub-population selected far either a short shoot or root.
   In the short population, the shoot length of the sub-populations selected for a long shoot or root was similar to each other and to the original population. Both differed from the sub-population selected for a short shoot.
(3) Root length
   Root length of the short population. was significantly longer than that of the tall population. In the sub-populations pooled across parents, the root length of the population selected for a long shoot was similar to that of the sub-population selected for a long root, However, both were significantly different from the original populations, and sub-populations selected for a short shoot and root. Thus, selection for a long shoot or root was effective for selection of root length. No sigilificant difference was observed for root length among the sub-populations selected for a short shoot, short root, and the original population.
  On the other hand, sub-populations selected for a short shoot or root from the tall population had sigilificantly shorter shoots and roots than the original population. Selection is effective either for short shoot or root in the tall population.
  In the short population, no significant differences were observed among sub-populations and the original population. Selection for a short shoot or root also had no effect on root length although their root lengths were significantly different from those selected for a long shoot or root.
  There was no significant difference in the shoot-root ratio between the tall and short populations, among sub-populations pooled across parents and among sub-populations within the same parent. Richer et al. (l996) showed that the shoot-root ratio was sensitive to sub-optimal temperature. However, the sup-populations selected for either long and short shoot or root in this study had similar shoot-root ratios.

2. Estimation of heritability
Table 3 shows the genetic gain realized at a selection intensity of 10 % for the seedling characters. Genetic gain calculated from (high-low selection) was 5.74 and 2.30 for root length, and 1.67 and 0.70 for shoot length, in the tall and short populations, respectively.
  The genetic gain calculated from (selection-population mean) of the long root was 2.70 and 1.40 in the tall and short populations, respectively and that of the short root was -3.04 and -0.90, respectively. The genetic gain of the long shoot, 0.93 and 0.56, respectively, was similar to that of the short shoot length, -0.74 and -0.l4, in the tall and short populations. This difference between the genetic gain obtained from upward and downward selection indicates an asymmetric selection response for the character.
  Table 3 shows the heritability values for seedling characters. The values of h12 varied from 0.28 to 0.76 and the values of h22 from 0.09 to 0.99. Heritability values in the tall population were higher than those in the short population. This means that the tall population was more heterogeneous and had a higher variability than the short population. Fehr (1987) explained that the estimation of heritability was influenced by the amount of genotypic variance present for a trait in the used population. This may explain that the higher genetic gain for seedling characters in the tall population than in the short population.
  Heritability values, 0.76 and 0,37 for root length in the tall and short populations, respectively, were similar to those for shoot length, 0.69 and 0.28, respectively. This suggests that the genetic gain is increased by selection for both shoot and root length.
  Considering the direction of selection, a long shoot or root is a more important character than a short shoot or root. The long shoot showed higher heritability values, 0.99 and 0.66 in the tall and short populations, respectively than short shoot, 0.50 and 0.09, respectively. Further selection for a longer shoot might be possible.
  Higher heritabilities of a long root 0.95 and 0.52 in the long and short population, respectively, than short shoot (0.58 and 0.26) in both the tall and short populations suggest the possibility of further selection for a long root.

3. Yield components
  Table 4 shows the mean square values for yield components of sub-populations selected under a low temperature and grown in a field. Sub-populations significantly differed in plant height, panicle length and panicle weight, but not in the number of productive panicles per plant. The mean values for various characters of sub-populations are shown in Table 5. The values in sub-populations selected from the tall population (T1, T2) were significantly different from those of the sub-populations selected from the short population (Si, S2) in all characters except for number of productive panicles. Plant height was higher in the tall population group (To, T1, T2) than the short population group (So, S1, S2), but panicle length of the former was shorter than the latter. No significant difference for plant height was observed among S1, S2 and So or among Ti, T2 and To.
  In the short population, the panicle of S1 was significantly longer than that of S2 and So, but that of S2 was not significantly different from that of So. The panicle weight of S1 was significantly higher than that of both S2 and So, but that of S2 was not significantly different from that of So.
  In the tall population, no significant difference was observed in panicle length among Tl, T2 and To. The panicle weight of T2 was significantly higher than that of To, but not that of T1.
 Thus, selection for a longer shoot increased the panicle length and panicle weight of the short population and selection for a longer root increased the panicle weight of the tall population.

4. Correlation between seedling characters and yield components
  Table 6 shows the correlation coefficients between seedling characters at low temperatures and yield components in the field among sub-populations. Although not significant, the correlation of shoot length to panicle length (0.519) and to panicle weight (0.503), and correlation of root length to panicle weight (0.553) suggests a relationship between the seedling characters and yield components.
  In conclusion, selection had a significant effect on seedling characters at low temperatures. Selection for a longer shoot or root increased the percentage of germination at low temperatures, panicle length and panicle weight in a field in addition to shoot and root lengths. Shoot length was always correlated with root length. Shoot and root lengths had high heritability and may be useful as selection criteria at low temperatures. Genetic gain for seedling characters should be possible, especially for a longer shoot and root length.

Table 1. Mean squares by ANOVA for germination and seedling characters after one 
selection cycle for  germination at a low temperature in pearl millet.
--------------------------------------------------------------
source                 df    Percent of       Shoot            Root          Shoot-root
                               germination (%)  length (mm,)  length (mm)  ratio (arcsin) 
--------------------------------------------------------------
Replication             3    8.27              0.04         0.63         2.35
Treatment               9    1186.48**     1.55**     21.55**     12.68
  pop. across sub-pop.  1    1488.41**    3.01**      32.97**     0.93
  Sub-population         4     1954.85**    2.38*        32.66**     19.43
  Parents vs sub-pop.   4     342.65*        0.35         7.59          8.78
--------------------------------------------------------------
**,* Significant at 5% and l% level of probability, respectively.

Table 2. Mean values of germination and seedling characters after one selection cycle for 
germination at a low temperature in pearl millet.
--------------------------------------------------------------
Factor             percent of      Shoot          Root       Shoot-root ratio
                    germination (% )  length (mm)  length (mm)    (arcsin)
--------------------------------------------------------------
Pooled across sub-pop.
  Tall population               51.70a         2.06a        7.60a         17.79a  
  Short population              63.90b         2.61b       9.41b         l8.09a   
Pooled across parents                                                                   
  Original pop.                   47.00a         2.16a      8.40a         l6.19a   
  Long shoot sub-pop.          79.50b         2.90b       l0.66b        17.86a  
  Long root sub-pop.            69.25b        2.91b       l0.45b        17.99a  
  Short shoot sub-pop.          44.75a         1.72a       6.61a         17.24a  
  Short root sub-pop.           46.50a         1.98a       6.43a         20.42a  
Sub-population                                                                          
  Original tall pop.              47.50b          1.90b     7.78b         14.78a  
  Long shoot tall sub-pop.     7l.50c          2.83c       10.56c        17.41a  
  Long root tall sub-pop.        68.00c         2.84c      10.48c       17.48a  
  Short shoot tall sub-pop.     28.00a         1.16a       4.45a         17.41a  
  Short root tall sub-pop.        43.50b        1.59ab     4.74a         21.86a  
  Original short pop.            50.50b         2.42c       9.02bcd      17.60a  
  Long shoot short sub-pop     87.50d         2.98c     10.75d        18.32a   
  Long root short sub-pop.      70.50c         2.99c      10.42cd      18.50a   
  Short shoot short sub-pop     61.50c         2.28bc    8.77bcd      17.06a  
  Short root short sub-pop.      49.50b        2.38c      8.12bc        l8.98a   
c.v.(%)                            12.77           21.l2      20.59         16.52   
--------------------------------------------------------------
For each factor, means followed by the same letter within a column are not significantly 
different at 5% level of probability according to ANOVA protected LSD test.


Table 3. Genetic gain and heritability of characters selected
  for germination at a low ternperature in pearl millet.
--------------------------------------------------------------
 character               G                 h12
                       (X1hi-X1low)
--------------------------------------------------------------
 Shoot length tall pop.     1.67     0.69
 Root length tall pop.      5.74     0.76
 Shoot length short pop.     0.70     0.28
 Root length short pop.     2.30     0.37
--------------------------------------------------------------
                             G                 h22
                    (X1sel-X1pop)
--------------------------------------------------------------
 Tall population
 Long shoot         0.93    0.99
 Long root          2.70    0.95
  short shoot          -0.74     0.50
  short root           - 3.04     0.58
 Short population
 Long Shoot         0.56    0.66
 Long root          1.40    0.52
  short shoot          -0. 14      0.09
  short root           -0.90      0.26
--------------------------------------------------------------
 G is the genetic gain at a selection intensity of 10 %, and h12 and h22 are heritabilities 
calculated from the values of (high selection-low selection) and (selection - population 
mean) , respectively.


Table 4. Mean squares by ANOVA for yield components of sub-populations selected for 
germination at a low temperature.
--------------------------------------------------------------
Source       df    Plant       No. of prod.   Panicle    Panicle
                 height (cm)   panicle    length (cm)   weight (g)
--------------------------------------------------------------
Replication      3   383.21*    0.208     7.01     39.83*
Sub-population   5   7074.85**    0.028    2411*   47.67**
--------------------------------------------------------------
*,** Significant at 5% and l% level of probability, respectively.

 Table 5. Mean value of yield components of sub-populations selected for germination at a
   low temperature.
--------------------------------------------------------------
 Sub-pop.     Plant height   No. of prod,    Panicle    Panicle weight
             (cm)        Panicles      length (mm)     (g)
--------------------------------------------------------------
   So       94.5a      1.31a      20.6ab      17.4a
   S1       95.6a      1.45a      25.6c       26.1c
   S2       9l.3a      1.25a      22.4b       20.2ab
   To       l68.9b       1.31a      l8.9a       19.8ab
   Tl       l72.6b       1.31a      l9.9a       24.4bc
   T2       170.1b      1.2la      19.6a       24.9c
 c.v. (%)      6.8      19.8        7.6        14.7
--------------------------------------------------------------
 Means followed by the same letter within a column were not significantly different at 5% 
level of probability according to ANOVA protected LSD test. For So〜T2 , see text.

 Table 6. Correlations between germination and seedling characters and yield
 components among sub-populations selected for germination at a low temperature.
--------------------------------------------------------------
             Root   Plant   Panicle  Panicle
             length  height  length  weight
--------------------------------------------------------------
 Shoot length    0.684*  -0.193  0.519  0.503
 Root length         -0.217  0.392  0.553
 Plant height               0.534  0. 135
 Panicle length                   0.4 84
--------------------------------------------------------------
 * Significant at 5% level of probability.

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*In Japanese with English abstract.
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