Changes in leaf area, nitrogen content and canopy photosynthesis in soybean exposed to an ozone concentration gradient

Authors: OIKAWA, SHIMPEI - Ibaraki University; Ainsworth, Elizabeth - Lisa

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2016
Publication Date: 7/1/2016
Citation: Oikawa, S., Ainsworth, E.A. 2016. Changes in leaf area, nitrogen content and canopy photosynthesis in soybean exposed to an ozone concentration gradient. Environmental Pollution. 215:347-355.

Interpretive Summary: This study investigated the effects of elevated ozone concentrations on soybean canopy photosynthesis. Many studies have investigate changes in leaf-level photosynthesis with elevated ozone, but few have modeled the impact of elevated ozone concentrations on carbon gain of the whole canopy, or attributed changes in canopy photosynthesis to leaf-level properties versus changes in canopy structure. This study measured leaf photosynthesis, light profiles and canopy structure in soybean canopies exposed to 9 different ozone concentrations across a gradient from ambient (37 ppb) to very high ozone concentrations (116 ppb). Canopy photosynthesis decreased by 10% with each 10 ppb increase in ozone, and lower canopy photosynthesis was primarily associated with changes in leaf -level physiology, not canopy structure. This information is important for modelers and experimentalists aiming to understand and improve soybean responses to ozone, a damaging air pollutant.

Technical Abstract: Influences of ozone (O3) on light-saturated rates of photosynthesis in crop leaves have been well documented. To increase our understanding of O3 effects on individual- or stand level productivity, a mechanistic understanding of factors determining canopy photosynthesis is necessary. We used a canopy model to scale photosynthesis from leaf to canopy, and analyzed the importance of canopy structural and leaf ecophysiological characteristics in determining canopy photosynthesis in soybean stands exposed to 9 concentrations of [O3] (37 to 116 ppb; 9-h mean). Light intensity and N content peaked in upper canopy layers, and sharply decreased through the lower canopy. Plant leaf area decreased with increasing [O3] allowing for greater light intensity to reach lower canopy levels. At the leaf level, light-saturated photosynthesis decreased and dark respiration increased with increasing [O3]. These data were used to calculate daily net canopy photosynthesis (Pc). Pc decreased with increasing [O3] with an average decrease of 10% for an increase in [O3] of 10 ppb, and which was similar to changes in above-ground dry mass production of the stands. Absolute daily net photosynthesis of lower layers was very low and thus the decrease in photosynthesis in the lower canopy caused by elevated [O3] had only minor significance for total canopy photosynthesis. Sensitivity analyses revealed that the decrease in Pc was associated with changes in leaf ecophysiology but not with decrease in leaf area. The soybean stands were very crowded, the leaves were highly mutually shaded, and sufficient light for positive carbon balance did not penetrate to lower canopy leaves, even under elevated [O3].