Abstract:
The subject of this study was the determination of the changes
in photosynthesis, respiration and transpiration in two ecotypes of
Douglas-fir as caused by decreasing soil moisture content. The photosynthetic
rate was also correlated with the relative turgidities of
needles.
All the above processes were greatly affected by decreasing
soil moisture. In general they decreased with the decrease in soil
moisture. The transpirational rate was affected relatively more
than photosynthetic rate, and the respiratory rate was affected least.
The photosynthetic rates of the two-months-old seedlings of
both sources were consistently higher under optimal soil moisture
conditions but they were significantly lower (at 1% level) at low soil
moistures than those of the three-months-old plants. There was no
significant difference found in the photosynthetic rates between the
two sources at two months of age, but significantly higher photosynthetic
rate at high soil moistures was found in the three-months-old
N. E. Washington seedlings when compared with the Valsetz plants.
There is an indication that at very low soil moistures the photosynthetic
rate of the N. E. Washington seedlings is higher also but this
could not be shown statistically because of the relatively small number
of observations.
A good correlation was found between the photosynthetic rate
and relative turgidities of needles in both ecotypes. For the same
soil moisture a higher relative turgidity was observed in the needles
of the N. E. Washington seedlings than in the Valsetz source.
The respiratory rates of the two-months-old seedlings of both
sources were consistently higher than those of the three-months-old
plants. While the difference in respiratory rates between the two
ecotypes was not significant at two months of age, it was significantly
higher in the Valsetz source at three months. It is suggested
that this difference was due to the relatively larger proportion of
the newly developing needles in this ecotype. Contrary to observations
made by some researchers no increase in the respiration was
determined at the onset of the drought or at the lowest soil moistures used in this study.
The relation between the soil moisture and transpirational
rate was, linear in both sources in the range below 12% of soil moisture.
In this range the transpirational rates of the Valsetz seedlings
were significantly higher (at 1% level) than those of the N. E.
Washington plants.
The top/root ratios were found significantly different between
the two- and three-months-old seedlings in both sources, and also
between the Valsetz and N. E. Washington seedlings at two and three
months of age. These ratios decreased in both sources between the
ages of two and three months. The ratio of the three-months-old
N. E. Washington seedlings was the lowest while that of the two-months-
old Valsetz plants the highest.
The lower transpirational rate in the N. E. Washington seedlings
is of definite advantage for survival on droughty sites. This
survival ability is further increased by their low top/root ratio.
There is some evidence that they would suit extreme sites much
better than the Valsetz source. It is suggested that the periodical
droughts of N. E. Washington exercised a strong selection pressure
In that direction while in the relatively mild climate of the Coastal
Range in Oregon another ecotype was created.
