In a 1998 paper, E.B. Jensen and B. Veierskov looked at the effect of photoperiod on photosynthesis in tomatoes. They found that increasing the length of the photoperiod from 8 hours to 16 hours caused the carbon dioxide assimilation rate (i.e., the rate of photosynthesis), but photosynthetic rates fell with a 23.5-hour photoperiod. (Open bars in the figures). Starch and sugar concentrations showed a similar pattern.
This is interesting - an increase (and then decrease) in the rate of photosynthesis. The paper provides a mechanism for this difference - changes in chlorophyll a, chlorophyll b and carotenoid levels probably explain these differences.
Protein levels increased sharply at a 23.5-hour photoperiod, as did ethylene production (ethylene is a plant hormone which is produced in response to stress). The transgenic pTOM13 plants (shaded bars) have a reduced ability to produce ethylene. Jensen and Veierskov concluded that ethylene production is likely to be the cause of the "chlorosis, leaf distortion, purpling of older leaves and growth reduction".
So what does this mean with regards to the original question of how long to leave the lights on in your aquarium? It's a start - the idea that overly long photoperiod can damage plants is reasonable. But you can't readily translate that into the specific requirements of even the most common aquarium plants. More importantly, saying that excessive day length harms plants isn't the same as saying that it encourages algal growth. While plant growth appears to suppress algal growth (or so says another piece of conventional wisdom), at this point it seems more likely that algal growth would either be an indirect effect (maybe increased nutrient availability driven by decreased plant growth?) or a direct effect unrelated to plant growth (for example, algal growth may increase continuously with photoperiod, while plant growth falls off once it exceeds some threshold, creating a relative advantage for algae).
I need to keep looking.
- Jensen, E.B. and B. Veierskov. 1998. Interaction between photoperiod, photosynthesis and ethylene formation in tomato plants (Lycopersicon esculentum cv. Ailsa Craig and ACC-oxidase antisense pTOM13. Physiologia Plantarum 103:363-368.