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
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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)