Losciale et al - Light energy management in fruit species: utilization, photo-protection and photo-damage. What implications for productivity?

Pasquale Losciale*, Emanuele Pierpaoli e Luca Corelli Grappadelli
Dipartimento di Colture Arboree, Università di Bologna, via Fanin 46, 40127 Bologna

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Key words: Photosynthesis, photo-damage, Non Photochemical Quenching, electron alternative transports, light excess.

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Abstract

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The energetic basis of orchard productivity lies in the interaction between the tree and sunlight. The light intercepted by a plant is linearly related to the amount of dry matter it produces. This concept drew the evolution of the new, intensive orchard planting systems, although this dependence seems to be more subordinate to planting system rather than light intensity. At whole plant level not always the increase of irradiance determines productivity improvement. One of the reasons can be the plant intrinsic un-efficiency in using energy. Generally in full light only the 5–10% of the total incoming photosynthetic photon flux density (PPFD) is allocated to net photosynthesis. Therefore preserving or improving this efficiency becomes pivotal for scientist and fruit growers. Net photosynthesis increases with light until the saturation point and additional PPFD doesn’t improve carboxylation. In several parts of the world, under clear sky the PPFD reaches commonly 2,000 µmol photons m-2 s-1 or above, and about 50% of the incoming light is enough for reaching the saturating point in most plant species. On the other hand, about half of the available light may be in excess. Even tough a conspicuous energy amount is reflected or transmitted, plants can not avoid to absorb photons in excess. The chlorophyll over-excitation promotes the reactive oxygen species (ROS) production increasing the photoinhibition (photo-damage) risks. The dangerous consequences of photoinhibition forced plants to evolve a complex and multilevel machine able to dissipate the energy excess quenching heat (Non Photochemical Quenching), moving electrons (waterwater cycle, cyclic transport around PSI, glutathioneascorbate cycle and photorespiration) and scavenging the generated ROS. The price plants must pay for this equipment is the use of CO2 and reducing power with a consequent decrease of the photosynthetic efficiency, both because some photons are not used for carboxylation and an effective CO2 and reducing power loss occurs. The wide photo-protective apparatus, although is not able to cope with the excessive incoming energy, therefore photo-damage occurs. Each event increasing the photon pressure and/or decreasing the efficiency of the described photo-protective mechanisms (i.e. thermal stress, water and nutritional deficiency) can emphasize the photoinhibition. Likely in nature a small amount of not damaged photo-systems is found because of the effective, efficient and energy consuming recovery system. Since the damaged PSII is quickly repaired with energy expense, it would be interesting to investigate how much PSII recovery costs to plant productivity. This review purposes to improve the knowledge about the several strategies accomplished for managing the incoming energy and the light excess implication on photo-damage in plants. Furthermore the chlorophyll fluorescence measure technique is described. This is the most useful method, particularly because it can be used in vivo as well and it is possible to quantify and discriminate the contribution of pathways in which the incoming photon pressure is engaged. Finally some cases of light excess linked with abiotic stresses and particular physiological condition on fruit species are reported.

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