Decreased Photochemical Efficiency of Photosystem II following Sunlight Exposure of Shade-Grown Leaves of Avocado: Because of, or in Spite of, Two Kinetically Distinct Xanthophyll Cycles?  

Abstract

This study resolved correlations between changes in xanthophyll pigments and photosynthetic properties in attached and detached shade-grown avocado (Persea americana) leaves upon sun exposure. Lutein epoxide (Lx) was deepoxidized to lutein (L), increasing the total pool by DL over 5 h, whereas violaxanthin (V) conversion to antheraxanthin (A) and zeaxanthin (Z) ceased after 1 h. During subsequent dark or shade recovery, de novo synthesis of L and Z continued, followed by epoxidation of A and Z but not of L. Light-saturated nonphotochemical quenching (NPQ) was strongly and linearly correlated with decreasing [Lx] and increasing [ΔL] but showed a biphasic correlation with declining [V] and increasing [A+Z] separated when V deepoxidation ceased. When considering [DL+ΔZ], the monophasic linear correlation was restored. Photochemical efficiency of photosystem II (PSII) and photosystem (PSI; deduced from the delivery of electrons to PSI in saturating single-turnover flashes) showed a strong correlation in their continuous decline in sunlight and an increase in NPQ capacity. This decrease was also reflected in the initial reduction of the slope of photosynthetic electron transport versus photon flux density. Generally longer, stronger sun exposures enhanced declines in both slope and maximum photosynthetic electron transport rates as well as photochemical efficiency of PSII and PSII/PSI more severely and prevented full recovery. Interestingly, increased NPQ capacity was accompanied by slower relaxation. This was more prominent in detached leaves with closed stomata, indicating that photorespiratory recycling of CO 2 provided little photoprotection to avocado shade leaves. Sun exposure of these shade leaves initiates a continuum of photoprotection, beyond full engagement of the Lx and V cycle in the antenna, but ultimately photoinactivated PSII reaction centers.

Genotype Specific Photosynthesis x Environment Interactions Captured by Automated Fluorescence Canopy Scans Over Two Fluctuating Growing Seasons

“…It did not fit the response measured under controlled steady-state conditions (Figure 1B). In agreement, the curvature factor and the light saturation point were reported to be reduced under natural light conditions compared to control conditions probably caused by higher NPQ levels (Rascher et al, 2000;Jia et al, 2013;Meacham et al, 2017). In relation to that, the response of F q '/ F m ' to temperature was negligible under fluctuating conditions in contrast to lab conditions (Supplementary Table 1).…”

“…Photosynthesis under field conditions is complex not only because of the fluctuating environment but also due to the canopy structure. Sunlight flecks which occur by canopy movement and light penetration into the canopy alter the photosynthetic status (Jia et al, 2013;Kono and Terashima, 2014;Townsend et al, 2018;Kaiser et al, 2018b). Consequently, photosynthetic light responses differ in controlled versus field conditions (Rascher and Nedbal, 2006;Meacham et al, 2017;Vialet-Chabrand et al, 2017).…”

Our Eclectic Adventures in the Slower Eras of Photosynthesis: From New England Down Under to Biosphere 2 and Beyond

“…Themes included the C 4 and Crassulacean acid metabolism (CAM) cycles of CO 2 assimilation (106-109, 112, 114, 128), the C 2 and N cycles of photorespiration (15,124,157,178), and photoinhibition in relation to C and N metabolism, touching on the D1 protein cycle during photoinactivation and recovery of photosystem II (PSII) (38,123,149) and, currently, on two potentially photoprotective xanthophyll cycles (33,39,62,(90)(91)(92)(93). Access to phytotrons in Canberra; Madison, Wisconsin; Reno, Nevada; and Durham, North Carolina, has been important, and we shared a unique opportunity to explore some of these processes in the larger context of carbon cycling in model ecosystems in the controlled environments of B2L (117).…”

“…In retirement, I received my first and last ARC grant, which continued support for Britta Förster's work on naturally selected very-highlight-resistant mutants of Chlamydomonas discovered a decade earlier at Duke (38). This grant also allowed us to pursue work on lutein-based photoprotection in avocado shade leaves (33,39,62), confirming Shizue Matsubara's discoveries with Inga in B2L (93); in experiments at the Eden Project, Bodelva, Cornwall, United Kingdom (92,101); and at the Smithsonian Tropical Research Institute, Panama (91). This additional mechanism of lutein-dependent photoprotection in leaves of tropical trees actually presaged evidence for this role of lutein in targeted mutants of Arabidopsis (79).…”

“…Jan's inspiring support and sympathetic mentoring of students and colleagues were celebrated recently (90). Fred's in vivo assays of photochemical processes continue to be invaluable for our pursuit of enigmatic processes of photoinactivation (56,61,62,122,149), and he has trained a steady stream of students and postdocs, especially from East Asia.…”