Photosynthesis and Respiration in Arctic Tundra Grasses: Field Light Intensity and Temperature Responses

Abstract

The influence of light intensity and temperature on the photosynthetic responses of intact leaves was studied in the principal grasses in the vicinity of Barrow. Alaska. Light saturated rates of young expanded leaves varied from 7.0 mg CO2 · dm-2 · hr-1 in Hierochloë alpina to 30.8 mg CO2 · dm-2 md hr-1 in Elymus arenarius although most species possessed rates in the range 10 to 20 mg CO2 · dm-2 · hr-1. Saturation was usually attained near 0.4 cal · cm-2 · min-1 (400 to 700 nm) suggesting that few leaves would be light saturated in situ. Light compensation was attained at 0.008 to 0.010 cal · cm-2 · min-1 which suggests that CO2 exchange could be positive for 24 hr in most leaves until late in July. The shape of the temperature response curves were similar in all species with optima usually around 15°C. All species possessed positive photosynthesis at the lowest temperatures (0 to 5°C) tested. Dark respiration was more temperature de-the range 10 to 20 mg CO2 · dm-2 · hr-1. pendent than the combined effect of light respiration and oxygen inhibition as shown by a comparison of CO2 exchange in 20% oxygen and 0% oxygen.All species utilized ribulose-1,5-diphosphate carboxylase as the principal carboxylating enzyme, possessed high CO2 compensation points (around 50 ppm at 15°C), and were inhibited by On. The absolute magnitude (mg CO2 · dm-2) of this inhibition was generally greatest at the light intensities and temperature regimes which supported the highest rates of photosynthesis. Inhibition, expressed as a percentage of the photosynthesis rate, increased with an increase in temperature and decreased with an increase in light intensity.All grasses in the Barrow area are C3 plants and possess relatively high rates of CO2 uptake at the low temperatures and light intensities characteristic of these high latitudes. On a daily basis photosynthetic rates should be high and should be closely coupled to radiation.