Lilian Schmidt

Soil temperature and intermittent frost modulate the rate of recovery of photosynthesis in Scots pine under simulated spring conditions

An earlier onset of photosynthesis in spring for boreal forest trees is predicted as the climate warms, yet the importance of soil vs air temperatures for spring recovery remains to be determined. Effects of various soil- and air-temperature conditions on spring recovery of photosynthesis in Scots pine (Pinus sylvestris) seedlings were assessed under controlled environmental conditions. Using winter-acclimated seedlings, photosynthetic responses were followed after transfer to different simulated spring conditions. Recovery rates for photosynthetic electron transport and net CO(2) uptake were slower in plants from cold or frozen soil compared with controls. In addition, a greater fraction of light absorbed was not used photochemically, but was dissipated thermally via xanthophyll cycle pigments. Intermittent frost events decreased photosynthetic capacity and increased thermal energy dissipation. Within a few days after frost events, photosynthetic capacity recovered to prefrost levels. After 18 d under spring conditions, no difference in the optimum quantum yield of photosynthesis was observed between seedlings that had been exposed to intermittent frost and control plants. These results show that, if air temperatures remain favourable and spells of subfreezing air temperatures are only of short duration, intermittent frost events delay but do not severely inhibit photosynthetic recovery in evergreen conifers during spring. Cold and/or frozen soils exert much stronger inhibitory effects on the recovery process, but they do not totally inhibit it.

Local and systemic effects of two herbivores with different feeding mechanisms on primary metabolism of cotton leaves

Caterpillars and spider mites are herbivores with different feeding mechanisms. Spider mites feed on the cell content via stylets, while caterpillars, as chewing herbivores, remove larger amounts of photosynthetically active tissue. We investigated local and systemic effects of short-term caterpillar and spider mite herbivory on cotton in terms of primary metabolism and growth processes. After short-term caterpillar feeding, leaf growth and water content were decreased in damaged leaves. The glutamate/glutamine ratio increased and other free amino acids were also affected. In contrast, mild spider mite infestation did not affect leaf growth or amino acid composition, but led to an increase in total nitrogen and sucrose concentrations. Both herbivores induced locally increased dark respiration, suggesting an increased mobilization of storage compounds potentially available for synthesis of defensive substances, but did not affect assimilation and transpiration. Systemically induced leaves were not significantly affected by the treatments performed in this study. The results show that cotton plants do not compensate the loss of photosynthetic tissue with higher photosynthetic efficiency of the remaining tissue. However, early plant responses to different herbivores leave their signature in primary metabolism, affecting leaf growth. Changes in amino acid concentrations, total nitrogen and sucrose content may affect subsequent herbivore performance.

Jasmonic acid does not mediate root growth responses to wounding in Arabidopsis thaliana

Jasmonic acid (JA) is a crucial plant defence signalling substance that has recently been shown to mediate herbivory-induced root growth reduction in the ecological model species Nicotiana attenuata. To clarify whether JA-induced reduction of root growth might be a general response increasing plant fitness under biotic stress, a suite of experiments was performed with the model plant Arabidopsis thaliana. JA bursts were elicited in leaves of A. thaliana in different ways. Root growth reduction was neither induced by foliar application of herbivore oral secretions nor by direct application of methyl jasmonate to leaves. Root growth reduction was observed when leaves were infected with the pathogen Pseudomonas syringae pv. tomato, which persistently induces the JA signalling pathway. Yet, high resolution growth analyses of this effect in wild type and JA biosynthesis knock-out mutants showed that it was elicited by the bacterial toxin coronatine that suggests ethylene- but not JA-induced root growth reduction in A. thaliana. Overall, the results demonstrate that the reaction of root growth to herbivore-induced JA signalling differs among species, which is discussed in the context of different ecological defence strategies among species.

Leaf wounding or simulated herbivory in young N. attenuata plants reduces carbon delivery to roots and root tips.

AbstractMain conclusionInNicotiana attenuataseedlings, simulated herbivory by the specialistManduca sextadecreases root growth and partitioning of recent photoassimilates to roots in contrast to increased partitioning reported for older plants. Root elongation rate in Nicotiana attenuata has been shown to decrease after leaf herbivory, despite reports of an increased proportion of recently mobilized photoassimilate being delivered towards the root system in many species after similar treatments. To study this apparent contradiction, we measured the distribution of recent photoassimilate within root tissues after wounding or simulated herbivory of N. attenuata leaves. We found no contradiction: herbivory reduced carbon delivery to root tips. However, the speed of phloem transport in both shoot and root, and the delivery of recently assimilated carbon to the entire root system, declined after wounding or simulated herbivory, in contrast with the often-reported increase in root partitioning. We conclude that the herbivory response in N. attenuata seedlings is to favor the shoot and not bunker carbon in the root system.

Caterpillar regurgitant induces pore formation in plant membranes

Formation of channel‐like pores in a plant membrane was induced within seconds after application of an aqueous solution containing regurgitant of the insect larvae Spodoptera littoralis. Gated pore currents recorded on the tonoplast of the Charophyte Chara corallina displayed conductances up to several hundred pS. A voltage‐dependent gating reaction supports the assumption that pore‐forming molecules have amphipathic properties. Regurgitant samples separated into masses smaller or larger than 3 kDa were evaluated by patch–clamp and mass spectroscopy. Fractions containing peptides larger than 3 kDa constituted pores of large conductances, peptides smaller than 3 kDa constituted pores of small conductances. Peptide‐free eluates did not constitute conducting pores, indicating that pore‐forming components in regurgitant are membrane‐spanning oligopeptides.

Root growth is affected differently by mechanical wounding in seedlings of the ecological model species Nicotiana attenuata and the molecular model species Arabidopsis thaliana

In the ecological model plant Nicotiana attenuata, leaf wounding or herbivory lead to a reduction of root growth via jasmonic acid (JA) signaling. A single wounding treatment is sufficient to induce this response; multiple wounding does not increase the plant growth reaction. in a recent study, in which JA bursts were elicited in leaves of the molecular model species Arabidopsis thaliana in different ways,1 we tested whether JA induces the same response there. Root growth reduction was neither induced by foliar application of herbivore oral secretions nor by direct application of methyl jasmonate to leaves. Root growth reduction was observed when leaves were infected with the pathogen Pseudomonas syringae pv. tomato, which persistently induces the JA signaling pathway. Yet, growth analyses of this effect in wild type and JA-signaling mutants showed that it was elicited by the bacterial toxin coronatine which suggests ethylene—but not JA-induced root growth reduction in A. thaliana. Moreover, the growth effects were somewhat masked by a light-induced diurnal decrease of root growth. Overall, we conclude that the reaction of root growth to herbivore-induced JA signaling differs among species, which is related to different ecological defence strategies that have evolved in different species.