T h e o r . Ap p l . G e n e t . 49 (1977) 253-258

~9~

~! ~ : :

~

R

~

:

9 by Springer-Verlag 1977

Ultraviolet Irradiation of Maize (Zea Mays L.) Pollen Grains I. Pollen Genotype Effects on Kernel Development P . L . P f a h l e r and H . F . L i n s k e n s U n i v e r s i t y of F l o r i d a , G a i n e s v i l l e , F l o r i d a (USA) and U n i v e r s i t y of N i j m e g e n , N i j m e g e n (the N e t h e r l a n d s ) S u m m a r y - M a t u r e p o l l e n g r a i n s f r o m two s i n g l e c r o s s h y b r i d s , Wf9 x H55 and K64 x K55, w e r e e x p o s e d t o e l e v en l e v e l s (0 to 6 . 8 0 e r g / c m ~ x 10 e at 0 . 6 8 i n t e r v a l s ) of u l t r a v i o l e t i r r a d i a t i o n and then w e r e u s e d to p o l l i n a t e t h e i r g e n e t i c s o u r c e . The n u m b e r and weight of t h e n o r m a l and s h r u n k e n ( p a r t i a l l y a b o r t e d ) k e r n e l s on e a c h e a r w e r e t a b u l a t e d . In g e n e r a l , t h e n u m b e r of n o r m a l k e r n e l s d e c r e a s e d and t h e n u m b e r and p e r c e n t a g e of s h r u n k e n k e r n e l s i n c r e a s e d with i n c r e a s i n g e x p o s u r e . H o w e v e r , s i g n i f i c a n t e x p o s u r e x h y b r i d i n t e r a c t i o n s w e r e p r e s e n t i n d i c a t i n g that the a m o u n t of c h a n g e d e p e n d e d on t h e h y b r i d . No c o n s i s t e n t r e l a t i o n s h i p b e t w e e n e x p o s u r e and e i t h e r n o r m a l o r s h r u n k e n k e r n e l weight w a s a p p a r e n t , but p o l l e n s o u r c e h y b r i d w as a c o n t r i b u t i n g f a c t o r . The e m b r y o w e i g h t and c o l e o p t i l e l e n g t h a f t e r g e r m i n a t i o n w e r e a l s o d e t e r m i n e d f o r t h e n o r m a l k e r n e l s . C h a n g e s in t h e s e c h a r a c t e r s by i r r a d i a t i o n w e r e a l s o s t r o n g l y i n f l u e n c e d by t h e h y b r i d . T h e s e r e s u l t s i n d i c a t e that t he d i r e c t i o n and m a g n i t u d e of the c h a n g e s in k e r n e l d e v e l o p m e n t p r o d u c e d by u l t r a v i o l e t a r e m o d i f i e d c o n s i d e r a b l y b y the g e n e t i c s o u r c e of th e p o l l e n g r a i n s . P r e s u m a b l y , g e n e t i c v a r i a t i o n f o r u l t r a v i o l e t r e s p o n s e i s p r e s e n t and s e l e c t i o n would be s u c c e s s f u l .

Introduction P o l l i n a t i o n with p o l l e n g r a i n s e x p o s e d to u l t r a v i o l e t i r r a d i a t i o n is known to r e s u l t in a d e c r e a s e in t h e n u m b e r of n o r m a l l y d e v e l o p e d v i a b l e s e e d s and an i n c r e a s e in t h e n u m b e r of s h r u n k e n i n v i a b l e s e e d s ( B r e w b a k e r and E m e r y 1962). A p p a r e n t l y , u l t r a v i o l e t i n d u c e s n u c l e a r c h a n g e s in t h e p o l l e n g r a i n which d i s r u p t e m b r y o and e n d o s p e r m d e v e l o p m e n t . No d e t a i l e d i n f o r m a t i o n r e g a r d i n g t h e c o m p l e x r e l a tionships between exposure level, kernel developm e n t and p o l l e n s o u r c e i s a v a i l a b l e . T h e r e f o r e , t h e p u r p o s e of t h i s s tu d y w a s to d e t e r m i n e t h e n u m b e r and weight of n o r m a l and s h r u n k e n ( p a r t i a l l y a b o r t e d ) k e r n e l s p r o d u c e d a f t e r p o l l i n a t i o n with u l t r a v i o l e t i r r a d i a t e d p o l l e n g r a i n s f r o m two g e n e t i c s o u r c e s of maize. M a t e r i a l s and M e t h o d s L a r g e q u a n t i t i e s of m a t u r e p o l l e n g r a i n s f r o m two s i n g l e c r o s s h y b r i d s , Wf9 • and K64 x K 5 5 ( K ) , w e r e c o l l e c t e d on e a c h of two d a t e s , J u n e 1 ( D a t e 1) and J u n e 9 ( D a t e 2) in 1972 by t h e m e t h o d of P f a h l e r ( 1 9 6 5 ) . On e a c h d a t e t h e p o l l e n g r a i n s f r o m e a c h h y b r i d w e r e e x p o s e d to e l e v e n l e v e l s (0 to 6 . 8 0 e r g / c m 2 • 10 s at 0 . 6 8 i n t e r v a l s ) of u l t r a v i o l e t u s i n g an a p p a r a t u s which e x p o s e s g r a i n s to i r r a d i a t i o n f r o m all d i rections (Pfahler 1973). After irradiation, the pollen grains from each level w e r e used t o pollinate six e a r s of t h e i r s o u r c e . A l l p o l l i n a t i o n s w e r e m a d e with e q u a l

q u a n t i t i e s of p o l l e n g r a i n s in a m o u n t s a d e q u e t e to f u l ly p o l l i n a t e t h e e a r s . Thus, f o r e a c h l e v e l and hybrid, 12 e a r s ( s i x f r o m e a c h d a t e ) w e r e o b t a i n e d . On e a c h p o l l i n a t e d e a r , t h e n u m b e r s of t h e n o r m a l and s h r u n k e n ( p a r t i a l l y a b o r t e d ) k e r n e l s w e r e d e t e r m i n e d . The n o r m a l c l a s s i f i c a t i o n i n c l u d e d t h o s e k e r n e l s w h o s e e m b r y o and e n d o s p e r m h ad a n o r m a l s i z e and a p p e a r a n c e and w e r e v i a b l e . The s h r u n k e n classificatiun contained kernels whose embryo and/or e n d o s p e r m w e r e r e d u c e d in s i z e and d e f e c t i v e in a p p e a r a n c e and a l s o w e r e g e n e r a l l y i n v i a b l e . The d i f f e r e n c e s b e t w e e n t h e two k e r n e l t y p e s w e r e w e l l - d e fined and, in m o s t c a s e s , o b v i o u s . A p e r c e n t s h r u n k e n c l a s s i f i c a t i o n was c o m p u t e d b a s e d on the t o t a l n u m ber. An equal n u m b e r of n o r m a l k e r n e l s f r o m e a c h o f t h e 12 e a r s f r o m e a c h l e v e l and h y b r i d w a s m i x e d , e l i m i n a t i n g t h e e a r and d a t e v a r i a b l e . The s a m e p r o c e d u r e w as done with t h e s h r u n k e n k e r n e l s . F r o m e a c h of t h e r e s u l t i n g 22 g r o u p s of both n o r m a l and s h r u n k e n k e r n e l s , 100 k e r n e l s w e r e w e i g h e d , with s o m e exceptions noted in the tables. The e m b r y o s from the normal kernels in each group w e r e r e m o v e d after soaking the kernels in water for 24 hours at 60~ After removal, the e m b r y o s w e r e dried at 60~ for 72 hours. The weight of I00 e m b r y o s in each group was determined after drying. Coleoptile m e a s u r e ments w e r e taken on I00 normal kernels in eachgroup after germination for 7 days at 23~ Appropriate analyses of variance were performed on each character measured. Square root transformations w e r e applied to the shrunken kernel n u m b e r data before analysis to reduce variance heterogeneity. Inverse sine transformations w e r e applied to all percentage data before analysis. The m i n i m u m differences for significance in the tables w e r e obtained by m e a n s of the revised Duncan's ranges using for p only the m a x i m u m n u m b e r of m e a n s to be c o m p a r e d (Harter 1960).

254

Theor.

T a b l e 1. M e a n s q u a r e s f r o m t h e v a r i a n c e a n a l y s e s tionships S o u r c e of variation

Degrees freedom

Hybrid (H) Exposure (E) E • H Date (D) D • H D • E D • E x H Error

1 10 10 1 1 10 10 220

of

Normal number

kernel

80152** 441835** 15019** 1700 15781 10477* 34292** 4452

Appl. Genet.

involving kernel number andtheir

49 ( 1 9 7 7 )

rela-

Shrunken kernel number ~

Percent shrunken kernels b

21.03"* 204.35** 5.28** 1.63 12.87"* 6.17"* 9.46"* 1.76

121.58" 4045.46** 77.31"* 107.75* 53.21 24.10 29.43 20.37

Square root transformation b Inverse sine transformation * , ** F v a l u e s s i g n i f i c a n t a t t h e 8 a n d 1 ~ l e v e l r e s p e c t i v e l y

f l u e n c e d by u l t r a v i o l e t

Results

exposure

and that the magni-

t u d e of t h e c h a n g e s d e p e n d e d o n t h e h y b r i d i n v o l v e d . The influence of the main effect, date, and its inter-

Number

actions was quite inconsistent Mean squares

from the analyses

of v a r i a n c e

s e n t e d i n T a b l e 1. F o r a l l c h a r a c t e r s main effects,

hybrid and exposure,

tion, exposure

• hybrid,

The means associated

are pre-

measured,

the

and the interac-

the 5 % level and, in the majority of cases,

ance are presented

i n T a b l e 2. In b o t h h y b r i d s , decreased

exposures

at the 1

x hybrid interaction

ber and percent

and intermediate

kernels

Table2. Means associatedwith

were greatly in-

kernel number

Exposure Character

Hybrid

Normal kernel number a

W K

Shrunken kernel n u m b e r b

W K

Percent shrunken kernels c W K

Date

in-

the normal kernel num-

at the highest exposure

level

w a s a b o u t 20 S of t h e 0 l e v e l . T h e s i g n i f i c a n t e x p o s u r e

l e v e l . It i s a p p a r e n t t h a t i n b o t h h y b r i d s k e r n e l n u m shrunken

of v a r i -

creasing

ber so that the number

were significant at least at

and erratic.

with the analyses

and their relationship.

was associated

exposure

levels,

Each value represents

with the lower with W showing

the mean of 6 ears

( e r g / c m 2 • 105)

0

0.68

1.36

2.04

2.72

3.40

4.08

4.76

5.44

6.12

6.80

1 2 1 2

474 534 428 465

471 525 420 441

486 493 261 405

389 334 351 329

418 419 371 336

307 318 292 286

275 253 254 249

231 135 200 199

189 136 182 133

136 100 139 131

118 82 104 83

1 2 1 2

1.46 0.93 1.36 1.94

2.11 2.01 1.28 1.75

3.55 4.08 2.19 4.25

4.61 4.73 4.38 6.63

7.13 7.26 5.97 7.92

7.58 9.62 7.75 9.04

9.93 8.62 8.35 9.00

9.75 7.27 8.37 8.95

10.19 8.59 7.99 7.47

9.54 10.03 7.89 7.14

9.14 8.71 8.38 6.40

1 2 1 2

4.64 2.31 4.10 5.06

5.59 4.98 3.54 4.79

9.09 10.63 8.08 11.95

13.30 14.70 13.11 20.02

19.40 19.70 17.41 23.40

23.67 28.4 ! 24.52 28.21

30.88 28.40 28.12 29.80

32.85 28.72 30.59 32.85

36.58 36.62 30.78 33.93

41.08 40.81 45.12 43.19 34.17 39.38 31.93 35.79

a M i n i m u m d i f f e r e n c e s b e t w e e n a n y t w o n o r m a l k e r n e l m e a n s f o r s i g n i f i c a n c e a r e 98 a n d 127 a t t h e 5 a n d 1 level respectively Transformed (square root) means. Minimum differences between any two transformed shrunken kernel means for significance are 1.94 and 2.53 at the 5 and 1 ~ level respectively c Transformed (inverse sine) means. Minimum differences between any two transformed percent shrunken kernel means for significance are 6.61 and 8.61 at the 5 and 1 ~ level respectively

P . L . P f a h l e r and H . F .

L i n s k e n s : U l t r a v i o l e t I r r a d i a t i o n of M a i z e

(Zea mays

L.) P o l l e n G r a i n s . I.

255

Table 3. M e a n squares from the variance analyses involving kernel weight, e m b r y o weight and coleoptile length Source of variation

Degrees of freedom

N o r m a l kernel weight

Shrunken kernel weight

Embryo weight

Colecptile length

H y b r i d (H) Exposure (E) E • H Error

1 10 10 2178 =

9871.2 23970.7** 65990.9** 2589.6

71252.65"* 26853.86** 17878.68** 2504.71

277.57** 505.10"* 312.80"* 2 7 .8 3

162 9450** 9784** 1516

a

E r r o r d e g r e e s of f r e e d o m f o r s h r u n k e n k e r n e l weight w e r e 1935 ** F v a l u e s s i g n i f i c a n t at t h e I S l e v e l

less decrease in this range than K. A somewhat dif-

rate of increase of W

ferent pattern w a s found with shrunken n u m b e r and

the lower and intermediate exposure levels. Above

and K was almost identical at

percentage. In both hybrids, increasing exposure

4.76 e r g / c m 2 • 105, the rate of increase in K was

sharply increased both n u m b e r and percentage. H o w -

considerably lower than that in W .

ever, the significant exposure x hybrid interaction was primarily associated with the higher exposure levels. This response is obvious in Fig. 1, which is plotted in actual rather than transformed means. The Weight and Length M e a n s q u a r e s f r o m t h e a n a l y s e s of v a r i a n c e a r e p r e 1 I IIC " S H R U N K E N NUMBE~ foe % 9(

l

I I KERNELS W9 K ~ ....

I

I

I

I

I

s e n t e d in Table 3. F o r all c h a r a c t e r s m e a s u r e d , t he

I

50

m a i n e f f e c t , e x p o s u r e , and t h e i n t e r a c t i o n , e x p o s u r e 9 "~

x h y b r i d , w e r e s i g n i f i c a n t at t h e 1 ~ l e v e l . T h e s e r e s u l t s i n d i c a t e d that in both h y b r i d s e x p o s u r e to u l t r a -

W O ~ O K ~ .....

9

/\. ~....t\

8C

9

4O

t h e d e g r e e of c h a n g e w as r e l a t e d to t h e h y b r i d i n -

//

O

v i o l e t a l t e r e d k e r n e l weight and c o l e o p t i l e l e n g t h and volved.

7(

I/

Q --~-,*....4 /

50

The m e a n s of e a c h h y b r i d at t h e v a r i o u s e x p o s u r e l e v e l s a r e shown i n T a b l e 4 a n d a r e p l o t t e d as a ~ of O

ms

exposure in Fig.2. For normal kernel weight, shrunz

50

ken kernel weight and e m b r y o weight, increasing exposure increased the m e a n s in W

40!

ii/~~//

20

30

or no change in K. The m e a n s in W

and produced little significantly ex-

ceeded the O exposure level at all exposure levels, i

/ e,

20

I0

=, , /

nificantly different from the O exposure level. A n

I0

~

whereas the m e a n s in K at most levels w e r e not sig-

entirely different relationship e m e r g e d with coleop-

I 0

l 0.68

I I 1.36 2 . 0 4 r

I

I

I

2.72

3.40

4.08

I 4.76 5)

-o

I

I

I

5.44

6.12

6.80

tile length. With this character, a reduction was found with increasing exposure. Significant differences from the O exposure level were found at the intermediate and higher exposures in W

Fig. I. The effect of various ultraviolet exposure levels on the n u m b e r and percent of shrunken kernels obtained on the two hybrids

while the m e a n of K was

significantly different from the O exposure level only at 6.80 e r g / c m 2 x 105 .

256

T h e o r . A p p l . G e n e t . 49 (1977)

Table 4. Mean a s s o c i a t e d w i t h k e r n e l w e i g h t , e m b r y o w e i g h t and c o l e o p t i l e l e n g t h . E a c h v a l u e r e p r e s e n t s the m e a n of 100 m e a s u r e m e n t s with e x c e p t i o n s n o t ed E x p o s u r e ( e r g / c m 2 x 105) Character Normal kernel weight ( m g ) a Shrunken kernel weight ( m g ) b Embryo weight ( m g ) r Coleoptile length ( m m ) d

Hybrid 0

W K

0.68

1.36

2.04

2.72

3.40

4.08

4.76

5.44

6.12

6.80

286.14 313.33 301.68 328.33 331.06 346.40 335.46 371.34 368.15 339.05 336.35 341.31 337.22 354.41 353.33 359.37 337.50 320.57 313.26 330.19 341.68 315.06

W K

38.88 e 67.07 t 8 3 . 5 2 60.696 7 4 . 9 5 6 1 0 4 . 7 8

94.29 7 5 .5 7

97.87 119.19 7 8 .0 0 68.68

71.62 5 8 .5 3

8 3 .4 5 79.12

86.73 5 9 .1 8

71.31 73.77

80. 33 59.75

W K

25.13 28.22

28.36 28.46

2 7 .1 2 29.66

30.23 29.34

31.01 32.22

32.01 30.81

3 0 .0 3 29.61

33.47 27.87

32.39 29.76

2 9 .7 1 2 6 .6 8

28.99 28.02

W K

118 111

125 104

111 111

111 102

117 102

94 104

105 113

98 119

100 112

99 111

99 83

M i n i m u m d i f f e r e n c e s b e t w e e n any 5 and 1 ~ l e v e l r e s p e c t i v e l y b M i n i m u m d i f f e r e n c e s b e t w e e n any the 5 and 1 ~ l e v e l r e s p e c t i v e l y r M i n i m u m d i f f e r e n c e s b e t w e e n any level respectively M i n i m u m d i f f e r e n c e s b e t w e e n any level respectively The m e a n of 35, 57, 37, and

two n o r m a l k e r n e l weight m e a n s f o r s i g n i f i c a n c e a r e 17.54 and 2 2 . 75 at t h e two s h r u n k e n k e r n e l weight m e a n s f o r s i g n i f i c a n c e a r e 17.25 and 22.37 at two e m b r y o weight m e a n s f o r s i g n i f i c a n c e a r e 1.82 and 2.36 at t h e 5 and 1 two c o l e o p t i l e leflgth m e a n s f o r s i g n i f i c a n c e a r e 13 and 17 at the 5 and 1 28 m e a s u r e m e n t s r e s p e c t i v e l y

u n d a m a g e d p o l l e n g r a i n s . It is o b v i o u s f r o m t h i s study

Discussion

that induced d a m a g e d o e s not c o m p l e t e l y inhibit the In m a i z e , k e r n e l a b o r t i o n r e s u l t i n g f r o m p o l l i n a t i o n

f e r t i l i z a t i o n a b i l i t y of the d a m a g e d p o l l e n g r a i n s .

with u l t r a v i o l e t - i r r a d i a t e d p o l l e n g r a i n s h a s b e e n

H o w e v e r , the r e l a t i o n s h i p b e t w e e n i r r a d i a t i o n d a -

r e c o g n i z e d ( B r e w b a k e r and E m e r y 1962, S t a d l e r and

m a g e and f e r t i l i z a t i o n a b i l i t y m a y not be c o m p l e t e l y

S p r a g u e 1936, S t a d l e r and S p r a g u e 1937, S t a d l e r a n d

independent s i n c e a p o l l e n s o u r c e e f f e c t w as e vi de nt

U b e r 1942). H o w e v e r , no e x p e r i m e n t a l r e s u l t s r e l a -

in t h i s study. This i n d e p e n d e n c e of d a m a g e and f e r t i -

ting a b o r t i o n to e x p o s u r e l e v e l h a v e b e e n r e p o r t e d ,

l i z a t i o n a b i l i t y w as o b s e r v e d in a study i n v o l v i n g

s i n c e t h e m a i n t h r u s t of m o s t s t u d i e s d e a lt with the

g a m m a i r r a d i a t i o n of m a i z e p o l l e n g r a i n s , but d i f -

m u t a g e n i c a c t i v i t i e s of t h i s agent on s p e c i f i c loci o r

f e r e n c e s b e t w e e n p o l l e n s o u r c e s w e r e not s i g n i f i c a n t

chromosomes. Also, ultraviolet irradiation studies

( P f a h l e r 1967). An i n v i t r o

w i t h m a i z e p o l l e n g r a i n s a r e d i f f i c u l t b e c a u s e of t h e

i n d i c a t e d that t h e g e r m i n a t i o n p e r c e n t and p o l l e n t u b e

l i m i t e d p e n e t r a t i o n of u l t r a v i o l e t and the l a r g e d i a -

g r o w t h of m a i z e p o l l e n g r a i n s f r o m one g e n e t i c s o u r c e

m e t e r and e c c e n t r i c n u c l e a r o r i e n t a t i o n within m a i z e

d e c r e a s e d only s l i g h t l y when t h e g r a i n s w e r e e x p o s e d

p o l l e n g r a i n s ( B r e w b a k e r and E m e r y

to t h e l e v e l s u s e d in t h i s s t u d y . O u r l i m i t e d k n o w l e d g e

1962, U b e r

st u d y ( P f a h l e r 1973)

1939). The a p p a r a t u s u s e d in t h i s s t u d y r e d u c e s t h e s e

in t h i s a r e a s u g g e s t s that t h e r e l a t i o n s h i p b e t w e e n i r -

p r o b l e m s to a m i n i m u m ( P f a h l e r 1973). O u r p r e s e n t

r a d i a t i o n d a m a g e and f e r t i l i z a t i o n a b i l i t y i s i n d e p e n -

k n o w l e d g e , gained l a r g e l y f r o m r e s e a r c h with o t h e r

dent. However, the d i f f e r e n c e s between pollen s o u r c e s

s p e c i e s , i n d i c a t e s that s e e d a b o r t i o n r e s u l t s f r o m

found in t h e p r e s e n t study i n d i c a t e that t h i s o b s e r v a -

c y t o g e n e t i c d a m a g e o r d o m i n a n t l e t h a l i t y i n d u c e d in

tion m a y h a v e to be m o d i f i e d , at l e a s t in r e g a r d to

t h e m a t u r e p o l l e n g r a i n at t h e t i m e o f e x p o s u r e . To

ult r a v i o l et .

f e r t i l i z e t h e o v u l e and p r o d u c e a b o r t i o n , th e p o l l e n

A n o t h e r i m p o r t a n t c o n s i d e r a t i o n that s h o u l d be i n -

grains containing i r r a d i a t i o n - i n d u c e d damage must

c l u d e d in i n t e r p r e t i n g t h e s e r e s u l t s i s t h e t r i n u c l e a t e

g e r m i n a t e , p r o d u c e a p o l l e n tube and c o m p e t e with

condition of t h e m a t u r e m a i z e p o l l e n g r a i n and i t s r e -

P.L.

Pfahler

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130

and H.F.

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F i g . 2 . T h e e f f e c t ( i n % o f O e x p o s u r e l e v e l ) o f v a r i o u s e x p o s u r e l e v e l s o n t h e w e i g h t of t h e v a r i o u s k e r n e l t y p e s and the coleoptile length of the normal kernels obtained on the two hybrids. The 5 ~ significance range from O exposure level is indicated by the horizontal lines equidistant above and below 100~. Circles = W; stars = K

lationship to the double fertilization process and ker-

bryo, which is highly organized and differentiated,

nel abortion. Three nuclei, two s p e r m and one pollen

can tolerate little d a m a g e introduced by the male

tube, are present at the time of irradiation. Irradia-

g e n o m e before abortion occurs. On the other hand,

tion induces changes in these nuclei independently and

the triploid endosperm, which is less organized, can

at r a n d o m so that in the resulting kernel the e m b r y o

tolerate relatively large amounts of d a m a g e intro-

end endosperm contain male g e n o m e s that probably

duced by the s p e r m nucleus before abortion occurs.

have different degrees of damage. The diploid era-

Therefore, kernel abortion is m o r e c o m m o n l y as-

258

Theor. Appl. Genet. 49 (1977)

sociated with the embryo rather than the endosperm

T h e r e f o r e , selection for pollen r a d i o s e n s i t i v i t y w o u l d

(Brewbaker and E m e r y 1962). In the study reported

be possible.

here, the normal kernel weight and embryo weight in both hybrids followed the s a m e pattern with increasing e x p o s u r e , i n d i c a t i n g that t h e e f f e c t s of u l t r a v i o l e t i r r a d i a t i o n on the e m b r y o and e n d o s p e r m w e r e e s s e n t i a l l y the s a m e .

Acknowledgement J o u r n a l S e r i e s No. 6142, F l o r i d a A g r i c u l t u r a l E x p e r i m e n t S t a t i o n . Thanks a r e e x t e n d e d to H . S . A n s p a c h , J . C . H a r p e r and W . T . Mixon f o r t h e i r e x c e l lent t e c h n i c a l a s s i s t a n c e .

The m e c h a n i s m s r e s p o n s i b l e f o r t h e p r o n o u n c e d d i f f e r e n c e s b e t w e e n p o l l e n s o u r c e s a r e unknown at p r e s e n t . A p p a r e n t l y , the d i f f e r e n c e s w e r e p r e s e n t o r o c c u r r e d at t h e t i m e of i r r a d i a t i o n and r e s u l t e d in l e s s d a m a g e to t h e p o l l e n g r a i n s f r o m one s o u r c e than t h e o t h e r . Many f a c t o r s and t h e i r i n t e r a c t i o n s m a y b e i n v o l v e d . F i r s t l y , t h e n u c l e i in the p o l l e n g r a i n s f r o m one s o u r c e m a y be i n h e r e n t l y m o r e r a d i o s e n s i t i v e , and t h u s , m o r e e x t e n s i v e d a m a g e f r o m i r r a d i a t i o n o c c u r s . Secondly, the r e p a i r or r e a c t i v a t i o n m e c h a n i s m ( F u j u i 1969, I k e n a g a and Mabuchi 1966), in which t h e e f f e c t s of u l t r a v i o l e t i r r a d i a t i o n r e v e r t to n o r m a l , m a y be m o r e a c t i v e o r e f f e c t i v e in p o l l e n g r a i n s f r o m one s o u r c e than a n o t h e r . T h i r d l y , m o r p h o l o g i c a l and p h y s i o l o g i c a l d i f f e r e n c e s in p o l l e n g r a i n s would be v e r y i m p o r t a n t b e c a u s e of t h e low p e n e t r a n c e of u l t r a v i o l e t . In t h i s c a s e , d i f f e r e n c e s in s i z e , s h a p e , o p a c i t y , d e n s i t y , n u c l e a r o r i e n t a t i o n o r even e x t i n e s t r u c t u r e o r t h i c k n e s s would be c r i t i c a l . S u b s ta n t ia l d i a m e t e r d i f f e r e n c e s b e t w e e n pollen g r a i n s from various s o u r c e s have been r e p o r t e d ( P f a h l e r 1 9 6 5 ) . If a ll o r any of t h e s e f a c t o r s c o n t r i b u t e to r a d i o s e n s i t i v i t y d i f f e r e n c e s , then g e n etic variation for these factors is probably p r e s e n t .

Received September 30, 1976 Communicated by H .F. Linskens

Literature Brewbaker, J.L. ; Emery, G.C. : Pollen radiobotany. R a d i a t . B o t . 1 , 101-154 (1962) F u j u i , T. : P h o t o r e a c t i v a t i o n of m u t a t i o n s i n d u c e d by u l t r a v i o l e t r a d i a t i o n of m a i z e p o l l e n . R a d i a t . B o t .

9, 115-123 (1969) Harter, H.L. : Critical values for Duncan's multiple r a n g e t e s t . B i o m e t r i c s 16, 671-685 (1960) I k e n a g a , M. ; M a b u c h i , T. : P h o t o r e a c t i v a t i o n of a n d o s p e r m m u t a t i o n s i n d u c e d by u l t r a v i o l e t light in m a i z e . R a d i a t . B o t . 6 , 165-169 (1966) P f a h l e r , P . L . : F e r t i l i z a t i o n a b i l i t y of m a i z e p o l l e n g r a i n s . I. P o l l e n s o u r c e s . G e n e t i c s 5._22, 513-520

( 1965 ) P f a h l e r , P . L . : F e r t i l i z a t i o n a b i l i t y of m a i z e p o l l e n g r a i n s . III. G a m m a i r r a d i a t i o n of m a t u r e p o l l e n . G e n e t i c s 5 7 , 523-530 (1967) P f a h l e r , P . L . : In v i ~ o g e r m i n a t i o n and p o l l e n tube g r o w t h of m a i z e (Zec ma~js L . ) p o l l e n VII. E f f e c t s of u l t r a v i o l e t i r r a d i a t i o n . R a d i a t . B o t . 1_/3, 13-18

(1973) Stadier, L.J. ; Sprague, G . F . : Genetic effects of ultraviolet radiation in maize. Proc. Natl. Acad. Sci. U.S. 2__.22,572-591 (1936) Stadler, L.J. ; Sprague, G .F. : Contrasts in the genetic effects of ultraviolet radiation and X-rays. Science 8~5, 57-58 (1937) Stadler, L.J. ; Uber, F . M . : Genetic effects of ultraviolet radiation in maize. IV. Comparison of m o chromatic radiations. Genetics 2__~7,84-118 (1942) Uber, F . M . : Ultraviolet spectrophotometry of Zea rnays pollen with the quartz microscope. A m e r . J . B o t . 2-6, 799-807 (1939)

Prof. Dr. P.L. Pfahler Agronomy Department U n i v e r s i t y of F l o r i d a G a l n e s v i l l e , F l o r i d a 32611 (USA) P r o f . Dr. H . F . Linskens D e p a r t m e n t of Botany U n i v e r s i t y of N i j m e g e n Toernooiveld Nijmegen (the Netherlands)

Ultraviolet irradiation of maize (Zea Mays L.) pollen grains : I. Pollen genotype effects on kernel development.

Mature pollen grains from two single cross hybrids, Wf9 × H55 and K64 × K55, were exposed to eleven levels (0 to 6.80 erg/cm(2) × 10(5) at 0.68 interv...
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