RA 00048.pdf - OAR@ICRISAT

prepared by Dr. M o r g a n , have been trying to
duplicate t h e action of height and maturity g e n e s
of Milo. In t h e field, we have not d o n e as w e l l as
w e had h o p e d , apparently because auxins such a s
indole acetic acid (IAA) are d e s t r o y e d before they
have t i m e t o act w i t h i n t h e plant. But w e have
d o n e better w i t h gibberellin and o n e paper on t h e
effect of G A 3 on height and tillering has appeared
( M o r g a n et al. 1977). T r e a t m e n t s w i t h GA3 to
short g e n o t y p e s increased height by as m u c h as
t h e presence of o n e d o m i n a n t height g e n e .
In looking for n e w male-sterile-inducing cytoplasms,
I (and Dr. K. F. Schertz, as well) used t h e
first 62 tropical conversions that originated in the
tropical conversion p r o g r a m . Several F1 populat
i o n s w e r e male sterile, n u m e r o u s F2 populations
contained male-sterile plants, a n d 33 cultivars
w e r e involved as f e m a l e parents. It finally b e c a m e
evident that a Kafir could be male-sterilized in
t h r e e different cytoplasms, and four different
c y t o p l a s m s have n o w been recognized in sorg
h u m (Quinby, in press). A fifth c y t o p l a s m , that
should exist, is being sought.
Earlier w o r k to identify different c y t o p l a s m s has
b e e n d o n e in India and this w o r k has b e e n
r e v i e w e d by Tripathi (1979). It is difficult to w o r k in
Texas w i t h t h e varieties that have tropical adaptation
and are late in maturity in t h e t e m p e r a t e zone.
Nevertheless, it is apparent n o w that t h e Maldandi
source in male-sterile M - 8 5 - 1 is similar to " A 3 "
that w a s recognized recently in t h e United States
(Quinby, in press) and c a m e f r o m t h e Indian
variety Nilwa.
N o w that a single g e n o t y p e exists in four
c y t o p l a s m s , plants of o n e g e n o t y p e can be g r o w n
that can be analyzed for h o r m o n e levels. This
g e n o t y p e in its o w n c y t o p l a s m s has perfect,
hermaphroditic f l o w e r s but in t h e other t h r e e
c y t o p l a s m s exhibit slightly different t y p e s of
m a l e sterility in e a c h . Dr. M o r g a n intends to look
into h o r m o n e levels w i t h i n t h e t h r e e male steriles
a n d t h e one perfect f l o w e r e d g e n o t y p e and t h e
male and f e m a l e h o r m o n e s will probably soon be
identified.
It is n o w possible to produce s o r g h u m hybrids
of t h e s a m e g e n o t y p e in three different cytop
l a s m s and, if t h e hybrids turn o u t to be slightly
different, s o m e h o r m o n e physiologist m i g h t like
to look into this a s p e c t of t h e physiology of hybrid
vigor.
A s w o r k looking for n e w c y t o p l a s m s progress
e d at Plainview, Texas, it b e c a m e obvious that
a p o m i x i s w a s appearing i n s o m e segregating
populations. Y o u m a y r e m e m b e r that b o t h Drs. N .
G. P. Rao a n d U. R. M u r t y reported on a p o m i x i s at
t h e 1970 S y m p o s i u m . Dr. M u r t y has c o n t i n u e d t o
be active in studying apomixis in India, a n d in
Texas as w e l l , and reports of his w o r k in cooperation
w i t h Drs. E. C. B a s h a w and K. F. Schertz have
b e e n published (Murty et al. 1979).
The genetics of apomixis is not y e t c o m p l e t e l y
understood. R-473, t h e apomictic strain that Dr. N.
G. P. Rao w a s g o o d e n o u g h to s e n d . t o Texas, is
apparently a facultative apomict. It is my p r e s e n t
opinion that the g e n o t y p e of R473 in s o m e
different cytoplasm might w e l l be an obligate
apomict. A p o m i c t i c hybrids are probably not
needed in the United States because it is t o o easy
to produce and sell hybrid s e e d there. But w h e r e
only small a m o u n t s of seed are n e e d e d on small
farms, apomictic hybrids m i g h t be e x t r e m e l y
useful.
Also, hybrid vigor is not being u s e d in s u c h
crops as soybeans, and even w h e a t because of
t h e difficulty of producing hybrid s e e d . I think that
an understanding of the genetics of obligate
apomixis in s o r g h u m is likely to p r o m o t e t h e
recognition and use of apomixis in species w h e r e
hybrid vigor cannot be used at present.
At the present time, the testing of hybrids to
recognize m a x i m u m hybrid vigor at n u m e r o u s
locations is a major activity in m a n y breeding
p r o g r a m s and literally t h o u s a n d s of hybrids are
t e s t e d and all but a very f e w are t h r o w n a w a y .
This is an inefficient process but no suitable
substitute has been devised by c o m m e r c i a l s e e d
c o m p a n i e s t o identify hybrids w i t h m a x i m u m
hybrid vigor. Plant breeders in t h e public sector
can m a k e a real contribution if t h e y p r o d u c e
varieties or parents that carry t h e desirable g e n e s
that result in resistance to diseases or insects or
in better nutrition.
In countries w h e r e agriculture is m e c h a n i z e d
and s o r g h u m is a cash crop g r o w n on large
acreages, m a x i m u m hybrid vigor continues to be a
major requirement. Unfortunately, insect probl
e m s do not remain static as a t t e s t e d by t h e
e m e r g e n c e of biotype C of t h e g r e e n b u g , Schizaphis
graminum Rond. in t h e United States in
1968. This pest of w h e a t finally d e v e l o p e d a t a s t e
for s o r g h u m and, after 10 years, biotype E has
n o w e m e r g e d that develops w e l l o n hybrids
tolerant to biotype C. Also, experience has s h o w n
that hybrids that are resistant to certain diseases
in t h e United States are susceptible to s o m e o t h e r
races of t h e s a m e disease in M e x i c o , t h e Philip-
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