Fécondation

Résumé

thumb|300px|Un spermatozoïde rencontrant un ovule. La fécondation est l'étape de la reproduction sexuée où les gamètes, en général un gamète mâle et un gamète femelle, apportent leurs chromosomes et le reste de sa machinerie moléculaire à une cellule unique nommée zygote. Elle a été observée et décrite pour la première fois par Gustave Adolphe Thuret en 1854 chez l'algue brune Fucus. En 1876, Oscar Hertwig publie un article démontrant que la fécondation est l'union du noyau d'un gamète mâle avec un gamète femelle. Il avait alors travaillé sur l'oursin. La fécondation permet le passage de deux cellules haploïdes, c'est-à-dire les gamètes, en une cellule diploïde qui est le zygote. Fécondation chez les plantes vertes
Le processus complet de la fécondation est encore mal compris. Ce n'est que récemment qu'on a compris la manière dont le tube pollinique était chimiquement guidé vers l'ovule en s'enfonçant dans les tissus de reproduction féminin avant de cesser sa croissance

Source officielle

https://fr.wikipedia.org/wiki/F%C3%A9condation

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Soil nutrient availability alters tree carbon allocation dynamics during drought

Leonie Corine Schönbeck

Drought alters allocation patterns of carbon (C) and nutrients in trees and eventually impairs tree functioning. Elevated soil nutrient availability might alter the response of trees to drought. We hypothesize that increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. To test this hypothesis, we subjected three-year-old Pinus sylvestris saplings in open-top cambers during two subsequent years to drought using three different water treatments (100%, 20% and 0% plant available water in the soil) and two soil nutrient regimes (ambient and nitrogen-phosphorus-potassium (N-P-K) fertilization corresponding to 5 g N/m2/yr) and released drought thereafter. We conducted a 15N and 13C labelling experiment during the peak of the first-year drought by injecting 15N labelled fertilizer in the soil and exposing the tree canopies to 13C labelled CO2. The abundance of the N and C isotopes in the roots, stem and needles was assessed during the following year. C uptake was slightly lower in drought stressed trees, and extreme drought inhibited largely the N uptake and transport. Carbon allocation to belowground tissues was decreased under drought, but not in combination with fertilization. Our results indicate a potential positive feedback loop, where fertilization improved the metabolism and functioning of the roots, stimulating C allocation to belowground tissues. This way, soil nutrients compensated for drought-induced loss of root functioning, mitigating drought stress of trees.

2020

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The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root carbon exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root carbon depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged carbon replenishment in roots might eventually lead to carbon starvation and further mortality.

2021

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Drivers of soil carbon stabilization in oil palm plantations

Alexandre Buttler, Thomas Guillaume, Juan Carlos Quezada Rivera, Johanna Katharina Rüegg, Mathieu Raymond Santonja

Increasing soil organic carbon (SOC) in agroecosystems is necessary to mitigate climate change and soil degradation. Management practices designed to reach this goal call for a deeper understanding of the processes and drivers of soil carbon input stabilization. We identified main drivers of SOC stabilization in oil palm plantations using the well-defined spatial patterns of nutrients and litter application resulting from the usual management scheme. The stabilization of oil palm-derived SOC (OP-SOC) was quantified by delta C-13 from a shift of C4 (savanna) to C3 (oil palm) vegetations. Soil organic carbon stocks under frond piles were 20% and 22% higher compared with harvest paths and interzones, respectively. Fertilization and frond stacking did not influence the decomposition of savanna-derived SOC. Depending on management zones, net OP-SOC stabilization equalled 16-27% of the fine root biomass accumulated for 9 years. This fraction was similar between frond piles and litter-free interzones, where mineral NPK fertilization is identical, indicating that carbon inputs from dead fronds did not stabilize in SOC. A path analysis confirmed that the OP-SOC distribution was largely explained by the distribution of oil palm fine roots, which itself depended on management practices. SOC mineralization was proportional to SOC content and was independent on phosphorus availability. We conclude that SOC stabilization was driven by C inputs from fine roots and was independent of alteration of SOC mineralization due to management. Practices favouring root growth of oil palms would increase carbon sequestration in soils without necessarily relying on the limited supply of organic residues.

WILEY2019

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