Short- and long-term modulation of the lutein epoxide and violaxanthin cycles in two species of the Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana Mill.).

Short- and long-term responses of the violaxanthin (V) and lutein epoxide (Lx) cycles were studied in two species of Lauraceae: sweet bay laurel (Laurus nobilis L.) and avocado (Persea americana L.). The Lx content exceeded the V content in shade leaves of both species. Both Lx and V were de-epoxidised on illumination, but only V was fully restored by epoxidation in low light. Violaxanthin was preferentially de-epoxidised in low light in L. nobilis. This suggests that Lx accumulates with leaf ageing, partly because its conversion to lutein is limited in shade. After exposure to strong light, shade leaves of avocado readjusted the total pools of alpha- and beta-xanthophyll cycles by de novo synthesis of antheraxanthin, zeaxanthin and lutein. This occurred in parallel with a sustained depression of F(v)/F(m). In Persea indica, a closely related but low Lx species, F(v)/F(m) recovered faster after a similar light treatment, suggesting the involvement of the Lx cycle in sustained energy dissipation. Furthermore, the seasonal correlation between non-reversible Lx and V photoconversions and pre-dawn F(v)/F(m) in sun leaves of sweet bay supported the conclusion that the Lx cycle is involved in a slowly reversible downregulation of photosynthesis analogous to the V cycle.

Plasticity of photoprotective mechanisms of Buxus sempervirens L. leaves in response to extreme temperatures.

Summer 2003 was unusually hot in Western Europe, with local droughts and an intense heatwave, that led to a massive damage in vegetation. Since high temperatures are supposed to generate photooxidative stress, we analysed photoprotective responses in leaves of the evergreen boxtree (Buxus sempervirens L.) during summer 2003. All the photoprotective compounds analysed (alpha-tocopherol, beta-carotene, and xanthophylls cycle pigments) were simultaneously induced in parallel with a reduction in photochemical efficiency (Fv/Fm). To characterise these responses, we compared these data with other data obtained during cold stress periods (2003, 2005) and with an unstressful summer (2002). Photoprotective responses observed during the heatwave were also induced by low temperature stress, and in both situations, this effect was exacerbated by light. In parallel with such induction the accumulation of red retro-carotenoids and xanthophyll esters was also observed under unfavourable conditions, suggesting a photoprotective role for both groups of carotenoids. This is the first report showing that in any species (Buxus sempervirens L.), the same retro-carotenoids can be induced in response to winter and summer stress. Present results demonstrate that the same mechanisms are induced as response to sub- and supraoptimal temperatures and the plasticity of such responses plays a critical role in plant acclimation to extreme temperatures, an ability that is specially important in the context of any future climate warming.

Role of red carotenoids in photoprotection during winter acclimation in Buxus sempervirens leaves.

The red leaf coloration of several plant species during autumn and winter is due to the synthesis of phenolic compounds such as anthocyanins or red carotenoids. The latter occur very rarely and are non-ubiquitous and taxonomically restricted compounds. The present study shows that the leaves of common box ( Buxus sempervirens L.) accumulate red carotenoids (eschscholtzxanthin, monoanhydroeschscholtzxanthin, anhydroeschscholtzxanthin) as a response to photoinhibitory conditions during winter acclimation. These compounds are produced in a coordinated manner with the operation of other photoprotective systems: accumulation and sustained deepoxidation of VAZ pigments with a concomitant decrease in maximal photochemical efficiency, accumulation of alpha-tocopherol and a gradual decrease on chlorophyll content. All these processes were reversed when the photosynthetic tissues were transferred from photoinhibitory winter conditions to room temperature for 9 days. Buxus leaves showed a large degree of phenotype variation in the degree of reddening, ranging from green to orange. The differences in colour pattern were mainly due to differences in the accumulation of red carotenoids and xanthophyll esters. Red pigments were mainly anhydroeschscholtzxanthin and esters of eschscholtzxanthin. Conversely to fruit or petal chromoplasts, the plastids of red leaves in this species are not the terminal differentiated state but are able to redifferentiate again to chloroplasts. Their photoprotective role during winter as a light screen system or as antioxidants, in a similar way to other red pigments, and their implications on the wide ecological tolerance of this evergreen species are discussed.