Vergara - 1976 - Physiological and morphological adaptability of ri

332 cLix-L-vra AND RICE
and ‘water availability reduce the activity of the enz_vme (Beevers and Hageman,
1969'. Morilla et al._ 1973) largely because of an inhibition of protein synthesis.
Desiccation of the leaves resulted in markedly} lower (60 to 70%) levels of nitrate
reductase activity at leafwater potentials of — 6 to — 8 bars (Morilla et al., 1973).
Photosynthesis had declined only 10 to 20 percent at these water potentials,
however (Boyer, 1970b).
In addition to the importance of nitrate reductase as an indicator of protein
synthesis by the crop, the decline in nitrate reductase activity represents another
equally important effect during dry times. Nitrate reductase is the first enzyrme
involved in the enzymatic reduction of nitrate and its eventual incorporation
into proteins as the amino fonn. Because the enzyme may have low enough
activities to control the flux of reduced nitrogen for the plant, a lower activity"
of the enzyme may mean a lower llux of reduced nitrogen and consequently a
decreased capability for protein production (Becvers and Hageman. 1969). For
maize which gains most of its nitrogen during vegetative growth and anthesis.
a reduction in nitrogen content of the plant means a reduction in nitrogen
that can eventually be made available to the grain.
Hageman and his coworkers have shown correlations betitvcen the total frield.
the protein yield. and the seasonal activity of nitrate reductase in maize and
ivheat lines (Croy and llageman, 1970; Deckard et al.. 1973). Since nitrogen
uptake occurs primarily in the first half of the season in maize the protein of the
grain must have been derived primarily from nitrogen that had previously been
a part of vegetative proteins in this crop. Thus. it is to be expected that large
increases in grain carbohydrate occur early in the grain fill period, but increases
in grain protein would tend to be delayed and then continue while lcaf senescence
occurs.
In view of this behavior. selections for high nitrate reductase activity and high
vegetative protein could possibly be reflected in higher grain protein in certain
crops. The correlations shown by the Hageman group suggest that this avenue
of selection ought to be valuable to pursue. However. it might also be beneficial
to study the response of protein synthesis to drought in the grain itself. Since the
proteins of the vegetative portions of the plant probably must be dissembled
before transport to the seed, protein synthesis by the seed is a major activity
during grain fill.
EFFECTS OF DROUGHT ON FLORAL
DEVELOPMENT AND POLLINATION
There is abundant evidence ivhich shows that. in cereal grains, the most sensitive
portion of thc life cycle to drought is the stage of floral development and flowering
(Salter and Goodc. 1967). Dcsiccation at this time frequently causes a reduc-