Vergara - 1976 - Physiological and morphological adaptability of ri

24 cuivrxru AND rues
temperature. light. and nutritional conditions as the plants grew (Raper. 1971'.
Raper and Johnson. 1971). Progressive reductions in the nitrogen and potassium
supply are critically involved in determining the normal sequence of composition
and curing properties of leaves. A progressive increase in temperature is also
required. as Raper and Thomas (1972) showed that temperatures during the
first lt) days after seedling transplantation had a pronounced after effect on
the dimensions and shape of the uppennost leaves. These experiments constitute
the most thorough attempt yet made to reproduce in plrvtotrons the sequence
of conditions experienced in the field by’ a crop. The fact that the number.
size, shape, CUlnpOSlllUH. and curing behavior of leaves of field tobacco plants
have all been reproduced indicates that effective simulation of field conditions is
sometimes possible.
Perhaps the greatest problems in relating phytotron and field perfonnance
are posed by stress conditions, particularly water stress. Better adaptation of
many crops. even of rice. to dryland conditions is a major objective of much
agricultural research. and it is imperative that ways of improving phytotron
experimentation in this area be developed.
The regular provision of water and nutrients to phytotron plants in pots often
results in the development of very limited root sytstems. and may lead to atypical
results. Cotton plants behave in this way. for example, leading to their frequent
wilting late in the day". Applications of nitrogenous nutrients after squaring
may cause additional leaf growth. with consequent xvilting and ahscission of
bolls. a response not observed in the field.
Jordan and Ritchie (1971) found that the stoinatal resistance of leaves on
COIIDII plants grown in the field remained constant at about 2.5 s cm" even when
leaf-xvater potentials approached — 30 bars, whereas stomatal closure in phytotron-groxvn
plants occurred at — l6 bars. Comparable data for com, soybean.
and other crops grown under controlled conditions would suggest that stomata
must be closed for much of the day in the field. Clearly. the stomata of fieldgrown
plants can adapt their behavior to remain open at greater negative water
potentials. probably in response to preceding stresses. In fact, McCree (I974)
has now shown that “pre-stressing" of sorghum plants grown under controlled
conditions causes their stomata to become much less responsive to decreasing
leaf water potential, and to behave like those on field-grown plants. Thus. use of
such pre-stressing treatments to harden plants should make it possible for the
response of crop plants to water stress to be analyzed in a way more relevant to
field problems in the future.
CONCLUSION
I have reviewed some of the ways in which phyttotrons play‘ a role in agricultural
research. but there are many others not mentioned above. The provision of standard
conditions permits the growth of reproducible plants throughout the year
for experiments on the processes basic to agricultural production. Work on