Tải bản đầy đủ - 0 (trang)
VII. Physiological and Genetic Analysis of Heterosis

VII. Physiological and Genetic Analysis of Heterosis

Tải bản đầy đủ - 0trang

131



HETEROSIS



TABLE 111

Component Analysis of Heterosis in Plant Height in

Pliaseolus vulgaris (Coyne, 1965)



Genotype



Number

of

nodes



lnternodal

length

(cm)



Calculated plant

height

(cm)



Recorded plant

height

(cm)



PI

P,

PI X P z ( F I )



24.2

17.3

23.8



4.9

6.6

6.8



24.2 X 4.9 = 118.6

17.3 X 6.6 = 114.2

23.8 X 6 . 8 = 161.8



116.0

114.4

161.9



“overdominance” in plant height and which are analyzed at the level of

its constituents.

Two points are clear: ( 1 ) the inheritance of the number of nodes and

the average internodal length are independent characters, ( 2 ) the hybrid

followed the better parent in both the characters. Since the height is the

multiplicative product and is not additive, the final plant height of F, hybrid

appeared to be “overdominant.” It is then clear that the F, inherited both

the dominant characters and that the multiplicative gene action led to

heterosis in plant height.

In a second cross involving another set of parents, it was found that

heterosis in plant height was due to increased number of nodes (Table

IV). The internodal length remained the same, and therefore the degree

of heterosis was reduced. Apparently, in this case the longer growth period

was responsible for the greater number of nodes.

Plant height in sorghum hybrids CSH-1, CSH-2, and CSH-3 also showed

“overdominance” as compared to their inbred parents (Rao, 1970a,b,

1972; Khanna, 1974). Analysis of the number of nodes and the average

internodal length revealed that the higher node number and longer internodal length were dominant in hybrid CSH-2 (Table V) . Besides, the topmost internode in both the hybrids followed one of the parents. These

TABLE IV

Component Analysis of Plant Height in Field Beans

Phaseolus vulgaris (Coyne, 1965)



Genotype



Number

of

internodes



Internodal

length

(cm)



Calculated plant

height

(cm)



PI

PZ

PI X Pz (FI)



6.5

17.8

21 .o



5.9

6.1

6.4



6 . 5 X 5.9 = 38.3

17.8 X 6.1 = 108.6

21.0 X 6 . 4 = 134.4



Recorded plant

height

(cm)

38.6

105.5

135.1



132



SURESH K. SINHA AND RENU KHANNA



TABLE V

Component Analysis of Plant Height in Sorghurna

~~



Genotype



Nodes



Internodal

length

(cm)



msCK 60

IS 3691

msCK 60 X IS 3691



13

17

17



7.40

4.51

1.45



Plant height

calculated

(cm)



Plant height.

recorded

(cm)



13.0 X 7.40 = 96.2

17.0 X4.51 = 76.7

17.0 X 7.45 = 126.7



96.7

76.8

126.7



0 Plants were grown in the field at a population of 18 plants per square meter (Khanna

and Sinha, 1975b).



results show that “overdominance” in the height of sorghum hybrids, as

in Phuseolus vulgaris, can be explained on the basis of simple Mendelian

dominance of the component characters.

These examples can be further multiplied. In maize also the ‘Loverdominant” height of hybrids could be traced to the number of nodes and the

average internodal length (Nosberger, 1970). However, in most instances

authors in the past have recorded only the height of plants, and it is difficult

to analyze these results at the component level (Ashby, 1937; Powers,

1952; Shull, 1952; Doggett, 1969; Allison, 197 1 ; Patanothai and Atkins,

197 1 ;Rao and Venkateshwarlu, 197 1 ) .

Plant height is an important character in some crop plants since it is

either of direct economic significance o r can have a bearing on the yield

of the plant. For example, in jute and sugarcane, positive heterosis in plant

height would be desirable; and in coconut and arecanut, reduction in plant

height of plants would be desirable. If heterosis in plant height is the product of the components discussed above, as it appears to be, it should be

possible to choose the parents on the basis of these criteria. Obviously,

it will be difficult to obtain heterosis in jute by using both the tall parents

having an equal number of nodes and similar internodal length. Apparently

one parent should have a larger number of nodes and the other parent

should have longer internodes. Similarly, it may be possible to obtain

plants of desired height with a greater number of nodes, which are important for spathe-bearing capacity and hence productivity in coconut and

arecanut palms. However, these multiplicative effects would be regulated

by complementary photosynthetic potential, as will be discussed later.



B.



HETEROSIS

I N LEAFAREA



Manifestation of greater leaf area is one of the important traits of heterotic hybrids. The importance of this factor in dry matter accumulation



133



HETEROSIS



and grain yield has been emphasized by several studies (Watson, 1952;

Yoshida, 1972). In Table VI, results of various experiments are given to

indicate the degree of heterosis in this character. From Table VI it is immediately clear that there appeared to be “overdominance” and heterosis for

leaf area in the F, hybrids of phaseolus beans (Phaseolus vulgaris), maize

(Zea mays), and sorghum (Sorghum vulgare) .

Leaf area is a product of leaf number and the size of leaves. The latter

can be further split into the length and width as its components. In maize,

sorghum, etc., the formula describing leaf area will be:

Leaf area



=



N(L X W X K )



where N = number of leaves; L = length of leaves; W = width of leaves;

K = factor to obtain leaf area of individual leaf.

In determinate plants, such as maize, sorghum, wheat, and others, variation in leaf number may be limited; therefore, the leaf size will become a

major component determining the leaf area.

Duarte and Adams (1963) made a genetic analysis of heterosis in leaf

area of Phaseolus vulgaris by splitting this character into its components,

the leaflet number and leaflet size (Tables VII and VIII) .

It is clear from this data that the F, hybrid followed one of the parents

in leaf number indicating dominance of larger leaf number over fewer leaf

number (Table VII). In leaflet size the F, was intermediate to the two

parents. The stability of both the characters was tested in field experiments

as well as in greenhouse (Table VIII). Plants had a much larger leaf area

when they were grown under field conditions, but the same relationship

was maintained between the hybrid and its parents under both field and

greenhouse conditions. The total leaf area of the hybrid plant exceeded

even the sum of the area of two parents in field-grown plants. Clearly the

TABLE VI

Heterosis in Leaf Area in Different Crop Plants (cm* Plant-’)



Crop



PI



Pt



P, X Pz

(FI)



Sorghum vulgaren

Pliaseolus vulgarisb

Zea maysC



1175



6964

2585



I590

1061 I

2323



2948

19249

3510



I,



Rao and Venkateshwarlu (1971).

Duarte and Adams ( I 963).

Ghildiyal and Sinha (1973).



Heterosis

over better

parent

66.08

81.40

35.78



134



SURESH K. SINHA AND RENU KHANNA



TABLE VII

Genetic Analysis of Leaf Area in Field Beans

(Phaseolus vulgaris): Field Data

(Duarte and Adams, 1963)

Per plant basis



Genotype



Leaflet

number



PI

P2

Pi X PP(FI)



83

320

334



Leaflet Total leaf

size

area

(cm2) (cm*/plant)

84

33

58



6964

I061 1

19249



TABLE VIll

Genetic Analysis of Leaf Area in Field Beans

Greenhouse Data (Duarte and Adams, 1963)

Per plant basis



Genotype

PI

P2



PI X PP(FI)



Leaflet

number



Leaflet

size

(crn2)



Total leaf

area

(cm*/plant)



26.7

63.7

68.2



147.1

63.9

105.5



3936

4109

8042



expression of hybrid vigor in leaf area was due to the multiplicative effect

of the dominant and partially dominant components in the hybrid. The

apparent “overdominance” was the result of simple Mendelian dominance.

Duarte and Adams (1963) concluded: “It should be clear that both the

heterosis and the alleged overdominant gene action exhibited for the compound trait in the hybrid can be attributed to multiplicative effects, in as

much as total leaf is compounded of size times number of leaflets and

these essentially independent components do not exhibit heterosis or overdominant gene action in themselves.”

In sorghum and maize there are several studies on the development of

leaf area (Ashby, 1930, 1937; Paddick, 1944; Nosberger, 1970; Quinby,

1970; Blum, 1970; Allison, 1971; Voldeng and Blackman, 1973; Donaldson and Blackman, 1973; Khanna, 1974). The data obtained by Nosberger

show that there was heterosis in leaf area of the plant, but in the number

of leaves the F, hybrid followed one of the parents. The width of leaves

was also intermediate.



135



HETEROSIS



TABLE IX

Leaf Area Development in Sorghum (Quinby. 1970)

Leaf characters

No. of leaves

Leaf width: 7th leaf

11th leaf

14th leaf

Maximum leaf width



P1



Pt



Pi X P2 (FI)



17

24

66

81

81



18

23

73

88

90



18

27

70

90

90



Quinby (1970) made a detailed study of leaf area development in sorghum (Table IX).It is clear from his data that the hybrid followed the

poor parent in the number of leaves. In seedlings the leaf width of the

hybrids was more than that of either parent, but ultimately one of the

parents reached the same maximum width as the hybrid. Leaf size is a

character that is influenced by the environment (Humphries and Wheeler,

1963). Therefore R. Khanna and S. K. Sinha (unpublished, 1974) determined the 1ength:width ratio in the leaves of inbred parents and hybrids

of sorghum (Table X ) . These data show that the hybrids were either intermediate or followed one of the parents. When the leaf area of top four

leaves after anthesis, which are major contributors to grain yield, was computed by Quinby (1970) and Khanna and Sinha (1975b), it also showed

dominance (Table XI).

The results discussed above, therefore, once again indicate that the

alleged heterosis in leaf area can be analyzed physiologically to components

that have simple Mendelian dominance. This concept can be of far-reaching importance in crops where leaf material is the final economic product,

such as tobacco, tea, and forages. It is difficult to find instances where heterotic leaf area obtained in these crops was analyzed as described above.

TABLE X

Leaf Length: Width Ratio in Sorghum

Hybrids and Their Parents at Anthesis'

Genotype



Length: width ratio



msCK 608

msCK 60 X IS 3691

IS 3691

2219B X IS 3691

22 19B



8.66

8.02

9.36

9.12

9 . I7



From Khanna and Sinha (1975b).



136



SURESH K. SINHA AND RENU KHANNA



TABLE XI



Leaf Blade Area of Top Four Leaves (cm*/plant)



“3095

*I045



2408

1258



21 52

1607



3092

1557



3330

1131



Quinby (1 970).

Khanna and Sinha (1975b).



However, if this concept is valid, then it should be possible to obtain

“directed” heterosis in this economically important character. Furthermore,

as will be discussed later, in cereals some specific leaves are major contributors of photosynthates after anthesis (Thorne, 1965; Allison and Watson,

1966; Asana, 1968; Yoshida, 1972). There could be a possibility of obtaining the desired leaf area on the plant if the inheritance of component

characters is well understood.



C. HETEROSIS

I N PRODUCTION

OF DRYMATTER

The F, hybrids exhibiting heterosis, besides being larger in form and

appearance, are known to be heavier than their parents (Ashby, 1930,

1932; Sprague, 1936; Whaley, 1952; Rao and Venkateshwarlu, 1971 ;

Khanna and Sinha, 1975b). Since this fact was recognized very early in

the development of the concept of heterosis, there have been several attempts by plant physiologists to explain greater dry matter production in

F, hybrids (Ashby, 1932; Allison, 1971; Voldeng and Blackman, 1973;

Donaldson and Blackman, 1973; Khanna, 1974). In spite of the various

approaches to this problem, a satisfactory answer still eludes. However,

a significant fact that emerges from all studies is that the big differences

observed between inbreds and hybrids at maturity, are not there for the

first few weeks (Ashby, 1932; Sprague, 1936; Whaley, 1939; Nosberger,

1970; Allison, 197 1 ; Rao and Venkateshwarlu, 1971 ; Voldeng and Blackman, 1973). Initial differences between inbreds and hybrids are small,

but they keep on enlarging as the time passes. It is also not possible to

achieve the same dry matter production by delayed sowing of the hybrid.

The data obtained by Nosberger (1970), Allison (1971), and Voldeng

and Blackman (1973) on different inbred parents and hybrids are reproduced in Figs. 1-3. The curves obtained by all these workers clearly suggest that the enlarged differences could be due to the compound interest

law of growth (Blackman, 1919). Let us examine the components of dry

matter production (Figs. 1-3).



137



HETEROSIS



1500 -



?



c



c



a

0



1

1



;.



1000-



3

ZI

L



U



0



c



z



50 1

100



3



4



5



6



7



8



9



Weeks a f t e r sowing



FIG. 1 . Total dry matter produced in maize hybrid and its inbreds. ---,

L; -.

L x R. (From Nosberger, 1970.)



R; -,



*



--,



Two major components of dry matter production are the leaf area and

the net photosynthesis rate per unit area or the net assimilation rate. The

dry matter production by a single leaf has the following relationship:

Dry matter production



=



NAR X Leaf area



The accumulation of dry matter by a photosynthetically active surface

eventually leads to the absolute growth rate of the plant. However, given

a certain amount of dry weight, how the plant adds more dry weight to

itself is expressed by the relative growth rate (RGR). According to the

law of compound interest, the dry matter added over the time is added

to itself; hence, the higher the dry matter production per unit, so much

greater than proportionate would be the further accumulation.

Let us presume, that there are two plants A and B that differed in leaf

area by 20% at the 1-week stage because of a lag in emergence in A.

The reasons for a lag in emergence will be described in a later section.

With an advantage of 20% in leaf area, even if the rate of photosynthesis



138



SURESH K. SINHA AND RENU KHANNA

600



-



500 -



400 c

C



-a

0



\



-



P



4-



c



300-



Dl

.-



;



0)



200-



100-



-



n7



9



11



13



15



17



19



21



23



Weeks after sowing



FIG.2. Changes with time in total dry weight of shoot of two maize inbreds

(N and S) and their hybrid (N x S). 0-0,

N; 0-0, S ; A-A,

N x S.

(From Allison, 1971.)



remains same, there will be greater and greater investment in the emergence

and development of new leaves until their maximum number is achieved.

This will then be associated with greater dry matter production. Thus in

the initial stages the leaf area ratio (LAR) would be an important determinant of dry matter production. Therefore, there would be no necessity to

presume the existence of some unexplicable physiological stimulus in the

hybrid.

Ashby (1930, 1932, 1937) studied RGR of inbred parents and hybrid,

but found no heterosis. He did find consistent increase in absolute growth

rate. Recently in a comprehensive study Nosberger ( 1970) found heterosis

in net assimilation rate (NAR) on the basis of unit leaf area. But the

heterosis was over the midparent, and the hybrid seemed to follow the

better parent. However, when data were expressed on the basis of unit

amount of chlorophyll, there was heterosis with respect even to the better

parent. These observations were valid only during the seedling stage. At



139



HETEROSIS



-+

pr)



8 -



7 -



H



0



c



0'



I



50



I



I



I



100



I



150



Days from emergence



FIG.3. Changes with time in dry weight of the shoot in maize inbreds and hybrids.

H; D-B,

J ; 0-0,

H x J. (From Voldeng and Blackman, 1973.)



A-A,



this time, when the leaves are expanding, the LAR may itself become an

important factor in determining the amount of chlorophyll per unit area.

Voldeng and Blackman (1973) and Donaldson and Blackman (1973)

examined a set of six triplets (two inbred parents and their hybrid) in

maize for various growth parameters including NAR, LAR, and RGR over

a period of 150 days. They reached the conclusion that there was heterosis

in RGR in early stages of growth, but the hybrids were either intermediate

or followed one of the parents in NAR. It was interesting that RGR in

their experiments became nonheterotic after about 50 days of growth,

apparently the time when all leaves had emerged. Besides, they raised the

inbreds and hybrids at the same plant population. It is not unlikely that

the mutual shading of leaves might have influenced light penetration more

in hybrids than inbreds resulting in reduced RGR of the hybrid at later

stages of growth. This would make the hybrid follow at some time one

parent, yet at another stage be intermediate in RGR (Table XII).

V. Balasubramanian, P. Shantha Kumari, and S. K. Sinha (unpublished) studied RGR, NAR, and LAR for the first 4 weeks after germina-



140



SURESH K. SINHA AND RENU KHANNA



TABLE XI1

Changes within and between Triplets in the Relative Growth Rates (RGR) of

Maize lnbreds and Their Hybrids“

~~



RGR (g g-1 week-1)

Weeks from sowing



Components of

triplets



Dent

W 182 EN

W 79 A

W 182 EN X W 79 A



1



2



3



4



5



6



-0.12

-0.11

-0.11



-0.12

-0.18

-0.27



0.87

1.02

0.96



0.51

0.61

0.72



0.63

0.72

0.92



0.46

0.58

0.58



0.86

0.90

1.11

0.18



0.56

0.67

0.63

0.33



0.65

0.82

0.91

0.32



0.62

0.42

0.49

0.29



Flint



Fi



EPI

F7 X EPI

LSD



-0.10

-0.13

-0.06

0.04



0.21

0.11

0.32

0.11



Donaldson and Blackman (1973).



tion in maize inbreds and hybrids. They also found slight heterosis in RGR

between week-2 and week-3 and week-3 and week-4 stages. The LAR

was low during this period.

The data described above suggest no special mechanism for the accumulation of more dry matter in heterotic hybrids. In early stages,

when the leaves of hybrids are in the expansion phase in achieving

length and width, they tend to have a lower LAR. This possibly provides

an advantage as a component of RGR, which apparently becomes heterotic. However, once this phase is over and the majority of leaves are mature, any advantage in RGR disappears because of proportionate decrease

in the number and area of young expanding leaves. Thus, the component

analysis taken over a period clearly indicates that none of the components

of growth parameters show “heterosis.” The hybrid follows either one or

the other parent or is, usually, intermediate.



D. HETEROSIS

IN RATEOF



PHOTOSYNTHESIS



Do hybrids have a higher rate of photosynthesis per unit area as compared to their inbred parents? There are very few studies on this aspect.

Moss (1960) reported no heterosis in the rate of photosynthesis per unit

area of leaf in maize. Fousova and Avratovscukova (1967) and Heichel

and Musgrave (1969), however, observed heterosis in the rate of photosynthesis in F, hybrids obtained by crossing divergent parents. Heterosis

in the rate of photosynthesis was observed by Khanna and Sinha (1975c),



HETEROSIS



141



Nagy et al. ( 1972), and in sorghum and maize, respectively, in the seedling

stage.

In the first place it seems to be an established fact that variation in

the rate of photosynthesis and its components exists at the cultivar level

(Hageman et al., 1967; Reddy and Sinha, 1970; Khan and Tsunoda, 1970;

Sinha and Khanna, 1972; Wallace et al., 1972). What seems to influence

results on photosynthesis is the stage of plant growth. Moss (1960) determined photosynthesis when plants had reached maturity and found no heterosis. Heichel and Musgrave ( 1969) determined photosynthesis of the

fifth or sixth leaf from the top in maize, usually bearing an ear in its axil.

According to them the “sink” was fully developed. The hybrid usually has

a greater “sink” capacity than the inbred parents. Would the differences

observed by Heichel and Musgrave (1969) then be the result of “sink”

effect? It has been shown by Wareing et al. (1968) that partial defoliation

or sink removal influences the rate of photosynthesis in beans and maize.

Nonetheless it must be conceded that the results of Heichel and Musgrave

(1969) do exhibit the maximum potential of the rate of photosynthesis

that could be reached in hybrids but do not necessarily bring out the genetic

potential of inbreds. There is considerable evidence that the increased

“sink” potential leads to enhanced rate of photosynthesis (Stoy, 1969).

Therefore, the comparisons of photosynthetic rates in inbreds and hybrids

should be independent of “sink” capacity if one is to arrive at valid

conclusions.

Higher photosynthetic rates in hybrids have been observed in seedlings

(Sarkissian and Huffaker, 1962; Nagy et al., 1972; Khanna and Sinha,

197%). I n these studies the topmost leaf was usually employed for the

determination of photosynthesis. Alternatively, a specific leaf, say the third

leaf was assayed. Since there is a lag in the emergence and expansion of

leaves in the inbreds and hybrids, the leaves that are compared are not

at the same physiological maturity. In some experiments the leaves may

be expanding and younger if they are topmost. They could be more mature

if the same leaf is being assayed in inbreds and hybrids. Depending upon

these factors one could obtain positive heterosis, no heterosis, or negative

heterosis. Therefore, the photosynthetic rate of a specific leaf from its

emergence to maturation and senescence can provide a better understanding of the expression of this character in relation to heterosis. One

can obtain the maximum rates as well as average rates over a period.



I . Components of Photosynthesis

Although there is no clear evidence in favor of heterosis in photosynthesis, yet the possibility of obtaining heterosis in photosynthesis cannot be

entirely ruled out. It would then be of some value to examine the compo-



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

VII. Physiological and Genetic Analysis of Heterosis

Tải bản đầy đủ ngay(0 tr)

×