|Abstract or Summary
- Leaf elongation is a major indicator of plant growth. It is
controlled by complex interactions of many factors including light
intensity, photoperiod, water availability, atmospheric conditions,
and plant water stress. In this study, the response of leaf elongation
to length of photoperiod and to plant water stress as indicated by leaf
water potential was investigated.
Fifteen day old seedlings of Sudan grass (Sorghum vulgare)
were subjected to soil water potentials ranging from -0.35 bars to
-2.50 bars using the polyethylene glycol (carbowax) semi-permeable
membrane technique of controlling soil water potential. Leaf
elongation rates and leaf water potential changes were measured in
response to photoperiods of 5, 6, and 8 hours, in response to the
initiation of lighting, and in response to continuous lighting for several
days. The elongation rate appeared to increase in a sigmoid curve as
the photoperiod was increased up to 8 hours. The sigmoid relationship
indicated that the increase in the elongation rate as the photoperiod
was increased from zero to 2 hours would be much smaller
than the increase in elongation rate as the photoperiod was increased
from 2 to 4 hours or from 4 to 6 hours. This was believed to be due
to a larger portion of the total photosynthate produced during the
shorter photoperiods being used for respiration.
For one hour after the initiation of lighting, the plant experienced
mild water stress, as the leaf water potential decreased,
increased, and decreased again. The elongation rate was depressed
during this period of water stress. It was suggested that earlier
reports of light inhibition of leaf elongation should at least partly be
attributed to light-induced water stress.
At a carbowax osmotic potential of -2.50 bars around the soilroot
slabs, and under continuous lighting, the total leaf water potential
and leaf elongation rate gradually decreased until the experiments
were discontinued because of microbial decomposition of the membranes.
Extrapolation from these data indicated that leaf elongation
ceases at a total leaf water potential of -24.0 bars. This was compared
to a value for maize, obtained in an earlier experiment, of
-9.2 bars. It was suggested that the ability of Sudan grass to continue leaf elongation at a lower leaf water potential than maize is due to its
being able to maintain a turgor pressure sufficiently high for growth
at a lower leaf sap osmotic potential.