Vergara - 1976 - Physiological and morphological adaptability of ri

l8
CLIMATE AND RICE
MANIPULATING LIFE CYCLES
Perhaps the greatest sensitivity of plants to climatic conditions is displayed in
the control of their reproductive cycles by day length and temperature. especially
at the stage of flower induction. In many wild plants. control at this stage is
absolute and sensitive to changes of only a few minutes i11 day length, but with an
enormous variety of response suggestive of powerful adaptive value. The wide
range of response to day length and vemalization found even among local races
within a single species. such as Tlzenzeda auso-alis, implies that these responses
can be readily changed by natural selection much more readily, for example,
than growth responses to temperature.
Indeed. the domestication of plants as crops has often led to modification of
their environmental requirements for flowering. As crops which originally
required short days for flowering (cg. maize. rice. soybeans. and cotton) or
tuberization (eg. potatoes) have been selected for summer growth at higher
latitudes, their photoperiodic requirements have been relaxed, and some cultivars
have become indifferent to day length. In rice, for example, Katayama
(1971) found nearly all wild populations and only one quarter of the cultivars
to be sensitive to day length. insensitivity to day length has been an explicit
objective of both IRR1 and CIMMYT plant breeding programs. but there
remain many situations where close adaptation requires some degree of environmental
control of flowering time, such as in wheats at high latitudes to avoid
frost injury to young inflorescences.
Phytotrons have proved to be extremely powerful tools in the analysis of
day length and temperature requirements at all stages of the reproductive process.
in that the many pronounced interactions between day length and night
temperature are far more easily unravelled under controlled conditions. Because
of this analytical power. and because flowering time can be closely controlled
once the climatic factors influencing it are knotvn. phytotrons can
greatly extend the range of crossing in plant breeding programs. The genetic
base for many major crops is still narrow‘, and wider crosses will be needed
increasingly to broaden the basis of disease resistance and to maintain the pace
of advance in yield. One reason for the narrow genetic base of many crops has
been our inability to synchronize the flowering of many wild and cultivated
forms so that crossing is possible. A major objective of the Brisbane phytotron
was to develop an understanding of the control of flowering in wild sugar canes
so that they could be used more extensively in plant breeding programs, and
this has now been achieved (Daniels ct al.. 1967). Likewise. the factors which
control flowering in many races of tropical maize have been clarified (Duncan
and Hesketh, 1968; Stevenson and Goodman. 1972). Their flowering can now
be synchronized ‘with that of temperate maize cultivars so that crossing can
be carried out.
Controlled environment conditions may also be used to produce flotvering
in plants in which flowering in the field is undesirable, such as sugar cane and