The relationship between photosynthesis and respiration is not primarily regulated through carbohydrates in a desert shrub (Larrea tridentata [L])

The intermediary role of carbohydrate levels is assumed to be of key importance in regulating net photosynthetic rates and respiration rates. A shading experiment was conducted in the Chihuahuan Desert in Big Bend National Park in October 2005 to examine the role of carbohydrates in regulating CO₂ flux in the desert shrub (Larrea tridentata L.). To generate differences in light levels, and hence, photosynthetic rates we used natural conditions (full sun 100%, natural shade 40%) and artificial shade cloth (green shade cloth 5%, natural shade cloth 30%). Leaves exposed to sun had the highest net photosynthetic rates and therefore, highest CO₂ uptake (192 mmol CO₂ m^-2) over a four-day period. In contrast, leaves beneath the green shade cloth only showed net respiration throughout the same period, which resulted in a CO₂ loss of 105 mmol CO₂ m^-2). Leaves in natural shade and beneath tan shade cloth had similar net photosynthetic rates and similar CO₂ gain (87 and 42 mmol CO₂ m^-2, respectively). Non-structural carbohydrate levels were only significantly different in the afternoon and were, unexpectedly, highest in leaves beneath tan shade cloth, while no differences were found among the other three treatments. No significant relationships were found between carbohydrate levels and daily CO₂ gain or loss. However, there were significant relationships between CO₂ uptake during the day with CO₂ loss during the following night and vice versa. This suggests that in this desert plant, photosynthesis and respiration are primarily regulated via other mechanisms than carbohydrate regulation.