Temporal heterogeneity in photosystem II photochemistry in under a fluctuating desert environment.

Acclimation strategies in xerophytic plants to stressed environmental conditions vary with temporal scales. Our understanding of environmentally-induced variation in photosystem II (PSII) processes as a function of temporal scales is limited, as most studies have thus far been based on short-term, laboratory-controlled experiments. In a study of PSII processes, we acquired near-continuous, field-based measurements of PSII-energy partitioning in a dominant desert-shrub species, namely , over a six-year period from 2012-2017. Continuous-wavelet transformation (CWT) and wavelet coherence analyses (WTC) were employed to examine the role of environmental variables in controlling the variation in the three main PSII-energy allocation pathways, i.e., photochemical efficiency and regulated and non-regulated thermal dissipation, i.e., , , and , respectively, across a time-frequency domain from hours to years. Convergent cross mapping (CCM) was subsequently used to isolate cause-and-effect interactions in PSII-energy partitioning response. The CWT method revealed that the three PSII-energy allocation pathways all had distinct daily periodicities, oscillating abruptly at intermediate timescales from days to weeks. On a diurnal scale, WTC revealed that all three pathways were influenced by photosynthetically active radiation (), air temperature ( ), and vapor pressure deficit (). By comparing associated time lags for the three forms of energy partitioning at diurnal scales, revealed that the sensitivity of response was more acutely influenced by , declining thereafter with the other environmental variables, such that the order of influence was greatest for , followed by , and then soil water content (. PSII-energy partitioning on a seasonal scale, in contrast, displayed greater variability among the different environmental variables, e.g., and being more predisposed to changes in , and to changes in . CCM confirmed the causal relationship between pairings of PSII-energy allocation pathways, according to shrub phenology. is shown to have an innate ability to (i) repair damaged PSII-photochemical apparatus (maximum quantum yield of PSII photochemistry, with / > 0.78), and (ii) acclimatize to excessive , dry-air conditions, and prolonged drought. is relatively sensitive to extreme temperature and exhibits photoinhibition.