Variation in photosynthetic response to temperature through time in a guild of winter annuals

Background/Question/Methods

Functional traits provide a filter through which the environment can affect fitness, and thus are expected to have important impacts on population and community dynamics.  Our previous work with Sonoran Desert winter annuals has identified a fundamental trade-off between relative growth rate (RGR) and water use efficiency (WUE) that translates into species differences in population dynamics across years, leading to coexistence via the storage effect.  Further investigation revealed that high WUE species have higher investment into light-driven RuBP regeneration capacity than high RGR species, which may allow these plants to have higher photosynthetic performance at lower temperatures.  In the present study, we use an experimental approach to explore the temperature sensitivity of photosynthetic processes across a subsample of the species in the winter annual community.  We chose six species of winter annuals based on their range of positions across the RGR-WUE tradeoff axis, and grew them in buried pots in the field, allowing them to experience seasonal changes in conditions.  We then moved plants to growth chambers and measured photosynthetic performance across a range of temperatures.  Measurements were repeated throughout the season, allowing for evaluation of both long- and short-term responses to temperature.

Results/Conclusions

Our results suggest that high WUE species have higher realized efficiency of photosystem II (ePSR) at low temperatures, relative to high RGR species.  This effect of temperature declined as the season progressed, suggesting that species responses to temperature were more important early in the season.  Though we found species responses to temperature varied for ePSR, we did not observe the same patterns for net photosynthetic rates.  Therefore, processes that occur downstream of light capture may also influence carbon assimilation rates, such as species differences in carbon allocation and phenology.  Our results suggest that species specific responses to temperature may be a mechanism underlying the WUE-RGR tradeoff that has been shown to promote coexistence in this community.  Further, our study suggests that even within a single functional group such as annual plants, species may partition the environment in complex ways.