Erinea formation on Quercus ilex leaves: anatomical, physiological and chemical responses of leaf trichomes against mite attack

Structures on the surfaces of leaves, such as dense layers of non-glandular trichomes, strongly affect phylloplane mite activities. On the other hand the feeding of eriophyoid mites on leaf surfaces can cause hyperplasia of leaf trichomes (erinea formation). In many cases the hyperplasia is accompanied by the accumulation of pigments within trichome cells, causing an impressive red-brown colouration of the erineum. There is no information, however, on the structure of these pigments as well as on the chemical alterations in the phenolic content of plant trichomes in response to mite attack. Erinea formation on the abaxial surface of Quercus ilex leaves upon Aceria ilicis (Acari: Eriophyoidea) attack provides an excellent model on this topic. Differences in the structure and chemical composition of isolated trichomes derived either from healthy (normal trichomes) or mite attacked (hypertrophic trichomes) leaves were examined. Carbon investment was comparable between the two different trichome types, but the cell walls of the hypertrophic trichomes appeared thinner and did not contain microcrystalline cellulose. Observations under the fluorescence microscope showed that the emitted fluorescence was different between the two trichome types, indicating a different composition in fluorescencing phenolic compounds. The chemical analyses confirmed that hypertrophic trichomes contained higher concentrations of the feeding deterrents proanthocyanidin B3 and catechin, as well as of quercetin-3-O-glucoside, but lower concentrations of acylated flavonoid glycosides, than the normal ones. The results showed that the structural and functional changes in leaf trichomes upon mite attack may be an effort of the leaf to compensate the damage caused by the pest.

Guard cells in albino leaf patches do not respond to photosynthetically active radiation, but are sensitive to blue light, CO2 and abscisic acid

Stomatal openings can be stimulated by light through two signalling pathways. The first pathway is blue light specific and involves phototropins, while the second pathway mediates a response to photosynthetically active radiation (PAR). This second pathway was studied with the use of albino Vicia faba plants and variegated leaves of Chlorophytum comosum. Treatment of V. faba with norflurazon (Nf) inhibits the synthesis of carotenoids and leads to albino leaves with guard cells that lack functional green chloroplasts. Guard cells in albino leaf patches of C. comosum, however, do contain photosynthetically active chloroplasts. Stomata in albino leaf patches of both plants did not respond to red light, although blue light could still induce stomatal opening. This shows that the response to PAR is not functioning in albino leaf patches, even though guard cells of C. comosum harbour chloroplasts. Stomata of Nf-treated plants still responded to CO2 and abscisic acid (ABA). The size of Nf-treated guard cells was increased, but impalement studies with double-barrelled microelectrodes revealed no changes in ion-transport properties at the plasma membrane of guard cells. Blue light could hyperpolarize albino guard cells by triggering outward currents with peak values of 37 pA in albino plants and 51 pA in green control cells. Because of the inhibition of carotenoid biosynthesis, Nf-treated V. faba plants contained only 4% of the ABA content found in green control plants. The ABA dose dependence of anion channel activation in guard cells was shifted in these plants, causing a reduced response to 10 microM ABA. These data show that despite the dramatic changes in physiology caused by Nf, the gross responsiveness of guard cells to blue light, CO2 and ABA remains unaltered. Stomata in albino leaf patches, however, do not respond to PAR, but require photosynthetically active mesophyll cells for this response.

Nondestructive assessment of leaf chemistry and physiology through spectral reflectance measurements may be misleading when changes in trichome density co-occur

Reflectance indices are frequently used for the nondestructive assessment of leaf chemistry, especially pigment content, in environmental or developmental studies. Since reflectance spectra are influenced by trichome density, and trichome density displays a considerable phenotypic plasticity, we asked whether this structural parameter could be a source of variation in the values of the most commonly used indices. Trichome density was manipulated in detached leaves of three species having either peltate (Olea europaea and Elaeagnus angustifolius) or tubular (Populus alba) trichomes by successive removal of hairs. After each dehairing step, trichome density was determined by light or scanning electron microscopy and reflectance spectra were obtained with a diode-array spectrometer. Although species-specific differences were evident, most of the indices were considerably affected even at low trichome densities. In general, the less-affected indices were those using wavebands within the visible spectral region. The index that could be safely used even at very high hair densities in all species was the red edge index (lambda(RE)) for chlorophyll. The results indicate that changes in reflectance indices should be interpreted cautiously when concurrent changes in trichome density are suspected. In this case, the red edge for chlorophyll content may be the index of choice.

Exposed red (anthocyanic) leaves of Quercus coccifera display shade characteristics

Young leaves in some plants are transiently red due to the presence of anthocyanins, which disappear upon maturation. We investigated the hypothesis that light attenuation by anthocyanins may lead to a shade acclimation of the photosynthetic machinery in red leaves. We took advantage of the intra-species variation in anthocyanin levels of young, exposed leaves of Quercus coccifera. Thus, photosynthetic and photoprotective characteristics were compared in young green and red leaves of the same age, sampled from the corresponding phenotypes occupying the same habitat. Red leaves displayed several shade attributes like thinner laminae, lower Chl a/b ratios and lower levels of the xanthophyll cycle components and β-carotene. In addition, although both leaf kinds had the same area based levels of chlorophylls, these pigments were excluded from the sub-epidermic anthocyanic cell layers, leading to a further reduction of effective mesophyll thickness and an increase in chlorophyll density. Accordingly, red leaves had higher absolute chlorophyll fluorescence signals. In spite of these apparent shade characters, red leaves were less prone to photoinhibition under mild laboratory conditions and displayed slightly but significantly higher PS II photochemical efficiencies at pre-dawn in the field. No differences in all the above measured parameters were found in mature green leaves of the two phenotypes. The results confirm the light acclimation hypothesis and are also compatible with a photoprotective function of anthocyanins.

Improved flower advertisement, pollinator rewards and seed yield by enhanced UV-B radiation in the Mediterranean annual Malcolmia maritima

•  The effects of enhanced UV-B radiation on reproductive and pollination success were investigated in the Mediterranean annual Malcolmia maritima. •  Plants were exposed in the field to ambient or ambient plus supplemental UV-B radiation (biologically equivalent to a 15% ozone depletion over Patras, Greece, 38°14' N, 21°44' E) up to leaf senescence and fruit maturation. •  UV-B radiation had no effect on stem and fruit biomass, anthesis time and duration and flower number. However, flower diameter, nectary volume and nectar amount per flower (but not nectar concentration) were significantly increased by supplemental UV-B radiation. In addition UV-B treated plants showed higher reproductive success (i.e. lower abortion rates and higher fruit to flower ratio) and a trend to higher pollination success (i.e. increased number of seeds per fruit). As a result, the seed yield was increased. Seed mass, seed germination and early seedling growth were not affected by UV-B treatment of mother plants. •  It is suggested that the UV-B induced changes in flower attributes might have affected pollinators' behaviour in a way that improved the fitness of M. maritima.