A great deal of importance has happened in research investigating photosynthetic response to environmental stress in the 25 years since the last anniversary issue of Plant Physiology. However, from my perspective, the importance of one set of discoveries stands out from the others for its far reaching influence on how we think about the photosynthetic response to a wide range on environmentally imposed limitations. As little as 15 years ago it was generally held that the success of plants in their environment was dictated by strategies that maximized the rate of photosynthesis. Further, maximum photosynthetic capacity was thought to be largely a static characteristic of individual leaves that was established during development. This view has now given way to the recognition that the regulation of photosynthesis in response to the environment is highly dynamic and dominated by a photoprotective process, the non-photosynthetic thermal dissipation of absorbed light, which was entirely unknown at the time of Plant Physiology's 50th Anniversary. This brief overview describes what is currently understood about this centrally important photoprotective process and highlights areas of current inquiry that may presage a detailed mechanistic understanding in the near future
Most days plants encounter light intensities that exceed their photosynthetic capacity. Exactly what constitutes excess light for a leaf depends on its instantaneous environmental conditions and can vary over an exceedingly wide range of irradiance levels. For example, irrigated field-grown sunflower is typical of C3 crop plants, exhibiting maximum photosynthetic capacity during mid-morning with photosynthesis declining throughout the afternoon as stomatal conductance declines in response to declining leaf water potentials . Thus even under conditions which may not generally be considered stressful, stomatal conductance can substantially restrict CO2 entry into leaves, rendering even moderate irradiances in the top of a crop canopy in excess of photosynthetic capacity.
When environmental conditions prevent the maintenance of a high capacity for photosynthetic and photorespiratory carbon metabolism to utilize absorbed light, the likelihood for the photosynthetic generation of biologically damaging molecules including reduced and excited species of oxygen, peroxides, radicals, and triplet state excited pigments increases dramatically. Although some plants can reduce the amount of incident light that is absorbed through strategic leaf and chloroplast movements, rapid reduction in light absorption appears to play only a minor role in the challenge of coping with excess light.
The development of the techniques and biophysical interpretation of pulse modulated fluorescence in the mid-1980s by Bradbury and Baker (2) bolstered by important additions and refinements by many others provided the basis for a new understanding about the dynamic trade-off between photosynthetic efficiency and photoprotection. A wide range of studies on many different species revealed that frequently over one-half of the light absorbed by photosystem II (PSII) chlorophylls in healthy, fully functional leaves can be redirected by a process that operates within the antenna ensemble of PSII, which harmlessly discharges excess photon flux energy as heat . This thermal dissipation process is measured and often called non-photochemical quenching, referring to the fact that the thermal dissipation of chlorophyll excited states competes with fluorescence emission as well as with photochemistry (i.e. photosynthesis).
not sure what this means but i found it interesting, seems like plants can get too much light..... this is just general plant info and not spacific to marijana
In photosynthesis, an ultraefficient charge transfer reaction converts virtually all incoming light energy into electrochemical energy. But such efficiency can be dangerous to the plant when there's too much light. "If the light gets too bright, then the reaction center works too fast and produces a lot of high-energy intermediates that don't have anywhere to go because the system is jammed up down the line," says Devens Gust, one of the leaders of the Arizona State team.
Plants keep themselves from burning out through nonphotochemical quenching, that is, by dissipating excess absorbed energy as heat. Inspired by this process, Gust and his coworkers designed a molecule that converts absorbed light to electrochemical energy but reduces the efficiency of the conversion, its quantum yield, as the light intensity increases.
The molecule consists of two light-gathering antennas, a porphyrin electron donor, a fullerene electron acceptor, and a control unit that reversibly photoisomerizes between a dihydroindolizine (DHI) and a betaine (BT) (Nat. Nanotechnol., DOI: 10.1038/nnano.2008.97). The DHI form doesn't effect the electron transfer between the porphyrin and the fullerene. In contrast, the BT form is "able to suck the excitation energy out of the porphyrin antenna system," thereby inhibiting electron transfer, Gust says.
At low light levels, most of the molecules are in the DHI form, and the system has a quantum yield of 82%. At higher light intensities, more of the molecules convert to the BT form, reducing the electron-transfer quantum yield to as low as 27%. When the light intensity goes back down, the molecule reverts to the DHI form.