Photosynthesis is the fundamental biological process by which light energy is converted into chemical energy. In plants, oxygenic photosynthesis uses light to oxidize water and reduce carbon dioxide into carbohydrates while releasing molecular oxygen, and it takes place in chloroplasts, where pigment-protein complexes are organized within the thylakoid membranes. The light-dependent reactions involve two functionally interconnected photosystems, Photosystem II and Photosystem I, together with the cytochrome b6f complex and ATP synthase. Linear electron transport begins with the oxidation of water in PSII and ends with the reduction of NADP+ to NADPH at PSI, and the sequential arrangement of these electron carriers is what scientists call the Z-scheme. Both ATP and NADPH produced along this chain are required for carbon fixation in the Calvin-Benson cycle, which is why photosynthesis is a key target for improving crop productivity and food security.

Dioecious plants are ideal subjects for this kind of comparison because they display secondary sexual dimorphism: sex-specific differences that extend beyond the reproductive organs into vegetative traits such as morphology, growth, physiology, water-use strategies, photosynthetic performance, hormones and phenology. These differences are thought to be linked partly to unequal reproductive investment. Female plants often invest more resources in reproduction than males because of the additional demands of fruit and seed production, and the costs vary with species, environmental conditions and developmental stage. Sex-related differences in photosynthetic performance have been reported under a wide range of stressful conditions, including drought, nutrient deficiency, extreme temperatures, ultraviolet radiation, wind erosion and heavy metal exposure. Yet the specific physiological mechanisms driving these differences remain largely unknown, which is why the review focuses on the electron transport chain itself.

Photosystem II is the natural starting point, because it performs what the authors describe as one of the most thermodynamically demanding reactions in biology: the light-driven oxidation of water. PSII is a multimeric pigment-protein complex in the thylakoid membranes whose reaction center is formed by the D1 and D2 proteins, with a peripheral antenna of light-harvesting complex II feeding excitation energy through the inner antenna proteins CP43 and CP47. Its oxygen-evolving complex contains the Mn4CaO5 cluster that catalyzes water splitting, and electrons then pass through pheophytin and the plastoquinones QA and QB. PSII is considered the most vulnerable component of the photosynthetic apparatus and a primary target of environmental stress. Photodamage occurs in two steps: excessive light first damages the Mn4CaO5 cluster, and then inactivates the reaction center. Crucially, the severity of photoinhibition is determined by the balance between photodamage and repair capacity rather than by photodamage alone.

Most of the evidence on sex differences comes from chlorophyll a fluorescence analysis, which partitions absorbed light energy into photochemistry, heat dissipation and re-emitted fluorescence. The most frequently used parameters are the effective quantum yield of PSII (ΦPSII), the maximum quantum efficiency (Fv/Fm), the photochemical quenching coefficient (qP), the electron transport rate (ETR) and non-photochemical quenching (NPQ), which reports on photoprotective energy dissipation. Finer detail comes from the fast chlorophyll a fluorescence induction transient known as OJIP and its JIP-test analysis, which yields parameters such as Wk, associated with the functional state of the oxygen-evolving complex, the quantum yield of electron transport beyond QA−, and the energy fluxes per excited cross-section, TRo/CS and ETo/CS.

What the compiled studies show is strikingly inconsistent across species. Under drought, male Populus cathayana maintained higher Fv/Fm and qP than females, while ΦPSII decreased only in the females, indicating better preservation of PSII photochemical function in the males. A similar male advantage appeared in Juniperus communis, where males showed higher ΦPSII, ETR and ETRmax at specific sampling dates, and in seed-derived Rumex thyrsiflorus exposed to short-term heat stress, where males were less affected and showed more stable PSII activity. In Amaranthus palmeri, the picture depended on both stress and developmental stage: under high light of 1300 micromoles of photons per square meter per second, males exhibited ΦPSII values 22.5 percent higher than females between 35 and 42 days after transplanting, males also held a 7.3 to 10.5 percent ΦPSII advantage under phosphorus deficiency, yet females showed higher maximum fluorescence under nitrogen deficiency at a later stage. In Litsea cubeba under lead stress, females suffered significant decreases in Fv/Fm and F′v/F′m at the lower lead concentration, while males increased NPQ across all lead concentrations, suggesting better adaptation.

Just as many studies favor the females. In Pistacia lentiscus, females exhibited higher Fv/Fm during spring at all three Mediterranean sites studied, with differences in ΦPSII depending on season and site, and no consistent patterns in qP or NPQ. In Taxus baccata, females maintained higher Fv/Fm, although nutrient availability influenced PSII performance more strongly than sex. Under wind erosion, Salix gordejevii males showed a significant drop in Fv/Fm while females remained stable and kept higher ΦPSII. In Ginkgo biloba during natural leaf senescence, monitored from day of year 188 to 332, males showed an earlier and more pronounced decline in PSII function after day 268, with stronger acceptor- and donor-side limitations, lower oxygen-evolving complex stability and higher QA− accumulation, while females maintained higher TRo/CS and ETo/CS and a slower decline in Fv/Fm. Reproductive investment also matters: in Chamaerops humilis, females were more vulnerable to photoinhibition during the summer reproductive period, but the effect disappeared in winter and the damage was reversible. Under waterlogging, female Morus alba increased NPQ and upregulated the PSII proteins PsbB and PsbQ, whereas males downregulated PsbD, PsbB and PsbH. Female Salix paraplesia, especially from high altitude, kept higher net photosynthetic rate and chlorophyll content under drought, and in Urtica dioica no sex differences in Fv/Fm appeared at all.

Evidence for Photosystem I is scarcer but points in interesting directions. PSI acts as a light-driven plastocyanin-ferredoxin oxidoreductase, and although PSII is usually the primary photoinhibition target, PSI can be damaged when electron pressure exceeds the capacity of its acceptor side, and its recovery is slower than that of PSII. PSI activity is assessed through the redox state of P700, using absorbance changes around 820 nanometers measured by modulated reflection at 820 nm (MR820), which can be combined with prompt and delayed chlorophyll fluorescence. In Ginkgo biloba, males showed higher PSI electron transport activity during early senescence, but females maintained higher activity at later stages, and the OJIP-derived parameter REo/CS — the electron flux reducing the terminal PSI acceptors per cross-section — declined by 67.94 percent in females versus 73.11 percent in males. In Torreya grandis after 20 days of progressive drought, the efficiency of electron transport to the PSI end acceptors fell by 27.1 percent in males but only 12.7 percent in females, with larger changes in MR820 kinetics in the males. Molecular studies add nuance: male Litsea cubeba expressed more PSI-related genes under control and lead-stress conditions, male Populus deltoides upregulated PsaN and showed higher Lhca3 abundance after Alternaria alternata infection, and under waterlogging male Morus alba downregulated PsaA, PsaB and Lhca3 while females kept Lhca1-6 stable.

The review’s conclusion is that sex modifies how the photosynthetic apparatus responds to environment rather than determining performance on its own. No consistent advantage of either sex emerges for PSII or PSI, and information about the cytochrome b6f complex and ATP synthase — the other major Z-scheme components — remains limited, so the authors call for further studies at both physiological and molecular levels, including direct PSI-specific measurements.

Subject of Research: Sex-related differences in the photosynthetic electron transport chain of dioecious plants

Article Title: Sex-related differences in the photochemistry and in the photosynthetic electron transport chain (Z-scheme) of dioecious plants: a short review

Article References: Angelopoulou, E., Govindjee, G., & Korres, N. E. (2026). Sex-related differences in the photochemistry and in the photosynthetic electron transport chain (Z-scheme) of dioecious plants: a short review. Frontiers in Photobiology, 4, Article 1970295. https://doi.org/10.3389/fphbi.2026.1970295

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1970295

Keywords: photosynthesis, dioecious plants, Photosystem II, Photosystem I, chlorophyll fluorescence, Z-scheme, photoinhibition, sexual dimorphism, environmental stress, electron transport, Ginkgo biloba, OJIP