Graduate Thesis Or Dissertation
 

The effect of drought on the rate of photosynthesis, respiration, and transpiration in two ecotypes of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)

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  • 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.
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