Vergara - 1976 - Physiological and morphological adaptability of ri

WATER DEFICITS IN CEREAL GRAINS 333
tion in the nulnber of seeds set by‘ the plant. and ifa subsequent improvement in
water availability occurs. yield remains depressed. An example is maize, and
experiments involving short exposures to desiccation resulted in the largest yield
reduction when desiccation occurred during pollen shed (Claasscn and Shaw",
1970a, 1970b). This was caused by a reduction in the number of seeds set and.
although the water status returned to normal after the desiccation period and
there was little overall effect on dry matter production by the plant. grain yield
was reduced and the stalk contained the dry matter that would have been
destined for the grain.
W'hy do fewer seeds develop when desiccation occurs during flowering? 1n an
excellent review; Slatycr (1969) raises this question and suggests that there are
at least three kinds of effects which seem to be significant. First. the development
of the floral primordia may be retarded (Husain and Aspinall. 1970).
Second, the egg cell within the embryo sac may abort (Moss and Doutney’, 1971)
or pollen development may be delayed (Salter and Gtvode, 1967). Third, the extension
of the stamens and styles of the flower or of the pollen tube may be
retarded. Any of these factors could prevent fertilization.
Little is knovrn about the physiological factors that cause these effects. The
floral primordia represent centers of intense metabolic activity and consequently‘.
they are large sinks for photosynthate. Perhaps the reduced supply of
photosynthetic products to the sink results in retarded cell division and/tor
eventual death of certain cells (Husain and Aspinall. 1970). On the other hand.
desiccation may have an effect on some essential hormonal or metabolic event
in the developing primordia themselves which leads to these kinds of effects.
Anthesis and pollination involve the process of cell elongation and this may
be an important determinant of the success of pollination. 1f elongation has a
sensitivity to desiccation that is similar to the sensitivity found in leaves, the
elongation of the stamens and stylar tissue might be affected by even moderate
desiccation of the plant. The growth of the pollen tube would he subject to the
same considerations, As pollen tube growth is taking place in the stylar tissue.
the water status of the style should be important for pollen tube growth.
One situation that could possibly affect the sensitivity of cell enlargement
during this phase of development is osmotic compensation. For roots and for
the hypoeotyls of germinating seeds (Greaeen and Oh, 1972; Meyer and Boy-er.
1972), osmotic compensation operates by causing the osmotic potential of the
tissue t0 keep pace xvith the water potential of the tissue. Turgor is preserved at
a relatively constant level. and cell elongation is affected much less than would
be the ease if osmotic compensation had not occurred. Leaves, although capable
of an accumulation of solutes during desiccation (Stewart. 1971). appear
to show this compensation to a lesser degree. Whether such a mechanism could
be involtted in cell enlargement during anthesis and pollen tube growth cannot
be predicted at this time.