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2 Polish Journal of Ecology

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Early-season defoliation of Sorbus aucuparia L. and Acer platanoides L. can induce defense mechanisms against the spider mite Tetranychus urticae (Koch)

Giertych M., Karolewski P., Oleksyn J.

Polish Journal of Ecology

2008

Vol. 56, nr 3

443-452

Artificial defoliation is often used in studies of induced defense reactions of plants to damage by arthropods. However, little is known about the role of various external factors that may determine the nature of inducible defenses. Here we tested whether light level under which plants grow could affect the natural invasion of two-spotted spider mite (Tetranychus urticae Koch) with a broad range of host plants. For this study we used two host species that differ widely in shade tolerance: shade-adapted Norway maple (Acer platanoides L.) and sun-adapted European mountain ash (Sorbus aucuparia L.). The purpose of this study was to explore mite invasion and to test whether prior simulated defoliation, light conditions and host species differences in secondary defense metabolites (soluble phenolics) play a role in the pattern of invasion. One-year-old maple and ash seedlings growing in pots were placed into two shade houses that produced a treatment with 5% light transmittance, and the second group of seedlings was placed in full sunlight. The experiment was carried out in a location chronically affected by spider mite presence. The defoliation was performed in mid-May by manual removal of 0 (control), 25, 50, 75 and 100% of leaves. Natural two-spotted spider mite invasion took place in mid-July and was observed only on seedlings growing in the shade. Mites were found on leaves of almost all seedlings. However, the intensity of feeding damage to leaves was related to the level of earlier defoliation. Control and slightly to moderately (25.50%) defoliated seedlings of both species were most affected by spider mites. Mite feeding behavior was directly linked to changes in concentration of leaf carbon and phenolic compounds. Carbon content was positively correlated with the percent of seedlings damaged by spider mites in both tree species (R[^2]> 0.80; P <0.05). The highest concentration of phenolic compounds was observed in leaves of seedlings subjected to 50 and 75% defoliation. Our results indicated that two-spotted spider mites preferred plants grown in a shade environment and within these plants favored leaves which are high in carbon and low in phenolic compounds. Both plant species were able to survive early season leaf damage inflicted by spider mites. Inducible phenolic compounds were among defensive secondary metabolites contributing to low spider mite success in plants recovering from artificial spring defoliation.

Ecophysiology of horse chestnut (Aesculus hippocastanum L.) in degraded and restored urban sites

Oleksyn J.

Polish Journal of Ecology

2007

Vol. 55, nr 2

245--260

We explored changes in growth, phenology, net CO[2] assimilation rate, water use efficiency, secondary defense compounds, substrate and foliage nutrient concentration of a degraded urban horse chestnut (Aesculus hippocastanum L.) site restored for three years using mulching (tree branches including foliage) and fertilization (primarily nitrogen addition). Prior to restoration, this site was characterized by high pH (ca. 8), low foliage and substrate N, and high Na and Cl concentration. Our data indicated that in untreated plots NaCl used for road deicing is the decisive factors that may be responsible for the decrease of foliar N concentration (via a reduction in NO[3] - uptake), for the decrease in photosynthesis (through high concentrations of Na and Cl in the leaves) and for increased senescence of the leaves. After three years of treatment, total nitrogen concentration in substrate increased by 3- to 4-fold and calcium concentration decreased by more than 50% in relation to pretreatment levels. Treatment significantly increased seed production (from less than 12 to more than 100 seeds per tree), individual leaf mass (from 1.8 to 3.3 g/leaf), CO[2] assimilation rate (by 21 to 30 %), improved leaf C:N ratio, and increased foliage life span by as much as six weeks. The beginning of leaf fall in untreated control trees started in mid-July and those of mulched and fertilized trees in late October. Applied treatment also eliminated visible symptoms of leaf damage due to high sodium and chlorine levels, indicating the possible role of other factors in the development of necroses. After three years of treatment, pH of most degraded plots declined from 8.2 to 7.8. That decline was accompanied by an increase in foliar Zn, Cu, and Pb concentration in the mulched and fertilized plants. In addition, treatment lowered foliage phenolics making these plants potentially more vulnerable to insect herbivory. Our study indicates that stable carbon isotope discriminationis of little value as an indicator of cumulative salinity and urban environment stress in A. hippocastanum due to pronounced differences in leaf phenology and ontogeny. The results of our study show that street tree recovery can take as little as two to three years after application of fertilization and mulching.