Abstract:
Completely climatized cuvettes were used to follow the CO₂
gas exchange of red fescue (Festuca rubra L.), growing on a fertilized
and an unfertilized plot, during a growing season from May
through October. Objective of the study was to determine the effect
of environmental factors on the seasonal CO₂ gas exchange.
Gas exchange rates were calculated on the basis of leaf dry
weight, surface area and chlorophyll. There was close correlation
between leaf dry weight and area. Photosynthetic rates differed
between the fertilized and unfertilized plants when based on leaf dry
weight or leaf surface area but were similar when based on chlorophyll.
Multiple regression analysis was used to relate photosynthetic
rates to radiation, temperature, vapor pressure deficit, chlorophyll content and time. A cubic regression equation based on daily radiation
alone explained 75% to 88% of the variation in total daily photosynthesis
for the season for the three reference bases.
During the growing season the unfertilized plants had a continual
decline in their photosynthetic rates until the end of the growing
season. On a dry weight basis the fertilized plants had 24% higher
photosynthetic rates for the growing season period; on a leaf area
basis the rates were only 16% higher.
Light response curves indicated greater photosynthetic rates
at light saturation as well as in the light limited portion of the photosynthetic
light curve for the fertilized plants. Photosynthetic rates
of fertilized plants were generally depressed during periods of warm
temperature and high light intensity in June and July.
Photosynthetic rates declined at temperatures above 24°C.
The decline was greater for the fertilized plants. A similar response
was noted to increasing vapor pressure deficit, although it was difficult
to separate from the temperature effect. A temperature increase
to 32°C decreased photosynthetic rates 50% and a decrease in temperature
to 12.5°C decreased photosynthesis by 12% for the fertilized
plants in July.
Maximum photosynthetic rates were found between 14° and
22°C, although there was considerable variation in the photosynthetic
rates. The effects of cutting (mowing) on the gas exchange were difficult
to determine due to the interaction of the environmental factors.
Chlorophyll content showed significant correlation with photosynthetic
rates.
