The effects of drought on the growth and water balance of Dactylis glomerata and Lolium perenne
Simulated swards of Dactylis glomerata and Lolium perenne
were grown in containers of sufficient depth to permit largely
unrestricted root development.
Flowering and non-flowering plants, subjected to cut and
uncut treatments, were allowed to dry the profile and were
compared with watered controls. The effects of a drying cycle
on the growth, water balance and nutrient uptake were measured.
The rate of dry weight increase was reduced by drought
from an early stage. The cause appeared to be reduced leaf
expansion rather than a decline in net assimilation rate. Root
weight was particularly affected due to suppressed elongation of
new adventitious roots. There was some compensatory growth at
depth. Defoliation severely retarded root growth.
Leaf water potential fell during the day in treatments and
controls to levels which would be expected to affect growth
Defoliation reduced water stress and stomatal closure but
not drought susceptibility.
There was little relationship between leaf water potential
and stomatal diffusion.
Dactylis initiated water economy measures at a lower soil
water deficit than Lolium, possibly because of a less vigorously
extending root system. It was more sensitive to increasing
deficit in terms of leaf water potential, relative turgidity and
stomatal diffusion rate and so did not manifest the early abrupt
exhaustion of water supplies typical of Lolium.
Root density was adequate to allow current water requirements
to be met from a small volume of wet soil, The effect of
drying of a horizon was to shift the main uptake zone downwards,
with no corresponding fall in leaf water potential.
Calculated mean soil water potential was most closely
related to coil water potential in the few zones of maximum
Resistance to the movement of water from the soil to the
roots was 10 superscript2 -10 superscript4 times smaller than resistance to Movement
through the plant i.e. a major source of water stress lay
within the plant itself.
No evidence was found that droughted swards ceased growth
due to N shortage. Reduced P uptake was detected.