Photosynthetic capacity in middle-aged larch and spruce acclimates independently to experimental warming and elevated CO2.

Photosynthetic acclimation to both warming and elevated CO2 of boreal trees remains a key uncertainty in modelling the response of photosynthesis to future climates. We investigated the impact of increased growth temperature and elevated CO2 on photosynthetic capacity (Vcmax and Jmax) in mature trees of two North American boreal conifers, tamarack and black spruce. We show that Vcmax and Jmax at a standard temperature of 25°C did not change with warming, while Vcmax and Jmax at their thermal optima (Topt) and growth temperature (Tg) increased. Moreover, Vcmax and Jmax at either 25°C, Topt or Tg decreased with elevated CO2. The Jmax/Vcmax ratio decreased with warming when assessed at both Topt and Tg but did not significantly vary at 25°C. The Jmax/Vcmax increased with elevated CO2 at either reference temperature. We found no significant interaction between warming and elevated CO2 on all traits. If this lack of interaction between warming and elevated CO2 on the Vcmax, Jmax and Jmax/Vcmax ratio is a general trend, it would have significant implications for improving photosynthesis representation in vegetation models. However, future research is required to investigate the widespread nature of this response in a larger number of species and biomes.

Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures.

Warming shifts the thermal optimum of net photosynthesis (ToptA) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO2) conditions. It is unclear whether shifts in ToptA of field-grown trees will keep pace with the temperatures predicted for the 21st century under elevated atmospheric CO2 concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO2 levels, we show that ToptA of mature boreal conifers increased with warming. However, shifts in ToptA did not keep pace with warming as ToptA only increased by 0.26-0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO2 spruce, while remaining constant in ambient CO2 spruce and in both ambient CO2 and elevated CO2 tamarack with warming. Although shifts in ToptA of these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures.

The roles of photochemical and non-photochemical quenching in regulating photosynthesis depend on the phases of fluctuating light conditions.

The induction and relaxation of photochemistry and non-photochemical quenching (NPQ) are not instantaneous and require time to respond to fluctuating environments. There is a lack of integrated understanding on how photochemistry and NPQ influence photosynthesis in fluctuating environments. We measured the induction and relaxation of chlorophyll a fluorescence and gas exchange in poplar and cotton at varying temperatures under saturating and fluctuating lights. When the light shifted from dark to high, the fraction of open reaction centers in photosystem II (qL) gradually increased while NPQ increased suddenly and then remained stable. Temperature significantly changed the response of qL but not that of NPQ during the dark to high light transition. Increased qL led to higher photosynthesis but their precise relationship was affected by NPQ and temperature. qL was significantly related to biochemical capacity. Thus, qL appears to be a strong indicator of the activation of carboxylase, leading to the similar dynamics between qL and photosynthesis. When the light shifted from high to low intensity, NPQ is still engaged at a high level, causing a stronger decline in photosynthesis. Our finding suggests that the dynamic effects of photochemistry and NPQ on photosynthesis depend on the phases of environmental fluctuations and interactive effects of light and temperature. Across the full spectra of light fluctuation, the slow induction of qL is a more important limiting factor than the slow relaxation of NPQ for photosynthesis in typical ranges of temperature for photosynthesis. The findings provided a new perspective to improve photosynthetic productivity with molecular biology under natural fluctuating environments.