Do soil fertilization and forest canopy foliage affect the growth and photosynthesis of Amazonian saplings?

Most Amazonian soils are highly weathered and poor in nutrients. Therefore, photosynthesis and plant growth should positively respond to the addition of mineral nutrients. Surprisingly, no study has been carried out in situ in the central Amazon to address this issue for juvenile trees. The objective of this study was to determine how photosynthetic rates and growth of tree saplings respond to the addition of mineral nutrients, to the variation in leaf area index of the forest canopy, and to changes in soil water content associated with rainfall seasonality. We assessed the effect of adding a slow-release fertilizer. We determined plant growth from 2010 to 2012 and gas exchange in the wet and dry season of 2012. Rainfall seasonality led to variations in soil water content, but it did not affect sapling growth or leaf gas exchange parameters. Although soil amendment increased phosphorus content by 60 %, neither plant growth nor the photosynthetic parameters were influenced by the addition of mineral nutrients. However, photosynthetic rates and growth of saplings decreased as the forest canopy became denser. Even when Amazonian soils are poor in nutrients, photosynthesis and sapling growth are more responsive to slight variations in light availability in the forest understory than to the availability of nutrients. Therefore, the response of saplings to future increases in atmospheric [CO2] will not be limited by the availability of mineral nutrients in the soil

When growing tall is not an option : contrasting shade avoidance responses in two wild Geranium species

Plants can deal with shade in different ways. Sun-adapted species express a set of growth traits to reach for light; the so-called shade avoidance syndrome (SAS). However, shade-tolerant species from the forest understory are not able to outgrow surrounding trees and adopt a tolerance strategy including suppression of shade avoidance. All plants sense the drop in the ratio between red (R) and far-red (FR) light via specialized photoreceptors, the phytochromes, but unlike the molecular pathways promoting SAS, virtually nothing is known about the regulation of shade tolerance. We selected two wild Geranium species with opposite growth responses to FR light enrichment; the shade avoiding G. pyrenaicum and shade tolerant G. robertianum. RNA sequencing of these species revealed contrasting transcriptomic changes treated upon FR light and in the first place showed how differential regulation of phytohormone synthesis and –perception could correlate to the species growth patterns in shade. Analysis of the hormone levels and growth responses to auxin, gibberellic acid and brassinosteroid application or –inhibition confirmed a role for these hormones in the Geranium shade responses. Secondly, the transcriptome analysis revealed a striking difference in FR induced regulation of pathogen defences. G. pyrenaicum showed far-red-mediated down-regulation of defence genes and increased susceptibility to the fungal pathogen Botrytis cinerea, similar to model species Arabidopsis. Interestingly, G. robertianum seems to boost its defences and becomes more resistant when exposed to FR-enriched light. Furthermore, the RNAseq analysis identified genes of which the expression patterns match growth data and give an insight into novel possible players and pathways responsible for the opposite light responses; two receptor-like kinases and a bHLH transcription factor. The importance of these novel candidates was verified in heterologous functional studies in Arabidopsis

Shade tolerance: when growing tall is not an option

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Perception of low red:far-red ratio comprises both salicylic acid- and jasmonic acid-dependent pathogen defences in Arabidopsis

In dense stands of plants, such as agricultural monocultures, plants are exposed simultaneously to competition for light and other stresses such as pathogen infection. Here, we show that both salicylic acid (SA)-dependent and jasmonic acid (JA)-dependent disease resistance is inhibited by a simultaneously reduced red:far-red light ratio (R:FR), the early warning signal for plant competition. Conversely, SA- and JA-dependent induced defences did not affect shade-avoidance responses to low R:FR. Reduced pathogen resistance by low R:FR was accompanied by a strong reduction in the regulation of JA- and SA-responsive genes. The severe inhibition of SA-responsive transcription in low R:FR appeared to be brought about by the repression of SA-inducible kinases. Phosphorylation of the SA-responsive transcription co-activator NPR1, which is required for full induction of SA-responsive transcription, was indeed reduced and may thus play a role in the suppression of SA-mediated defences by low R:FR-mediated phytochrome inactivation. Our results indicate that foraging for light through the shade-avoidance response is prioritised over plant immune responses when plants are simultaneously challenged with competition and pathogen attack