[Defence of thesis] 20/05/2025 - Thomas Baron: "Study of the evolution of photosynthetic capacity and glycerolipidome in head lettuce (Lactuca sativa) in response to light during post-harvest preservation" (UMR QUALISUD)

Thomas BARON will publicly defend his thesis on Tuesday 20 May 2025, entitled: "Study of the evolution of photosynthetic capacities and the glycerolipidome of head lettuce (Lactuca sativa) in response to light in post-harvest preservation".

Laboratory

UMR_C95 QUALISUD - An integrated approach to quality food production

Composition of the jury

Ms Huguette SALLANON	Avignon University	Thesis supervisor
Christian CHERVIN	INP Toulouse	Thesis co-director
Ms Françoise CORBINEAU	Sorbonne University	Rapporteur
Mr Adnane HITMI	University of Clermont Auvergne	Rapporteur
Ms Anja KRIEGER-LISZKAY	CNRS	Examiner
Ms Yasmine ZUILY	University of Paris-Est Créteil	Examiner
Mr Mathieu LECHAUDEL	CIRAD	Examiner

Summary

The optimisation of post-harvest plant preservation conditions is a response to economic and environmental challenges, which have led to a great deal of work aimed at proposing technologies capable of limiting post-harvest losses. However, the physiological mechanisms of degradation in post-harvest storage are not well known. The chloroplast is cited as the first organelle to be degraded during senescence, whether induced or linked to development. It therefore seemed interesting to monitor its degradation through changes in the constituents of its membranes (glycerolipids) on the one hand and photosynthetic activity on the other. Over the last decade, at the QUALISUD research unit, the use of light as a conservation technology has been evaluated with a view to delaying and reducing the phenomena associated with senescence induced by exposure to darkness.

Surprisingly, analysis of the glycerolipidome showed that galactolipid levels remained stable after 14 days in the dark, unlike phospholipid levels, which fell rapidly. This phenomenon is accompanied by the maintenance of chlorophylls and proteins, all of which indicate that the integrity of the chloroplasts is maintained. The stability of galactolipid, chlorophyll and protein levels was confirmed during 24 days of storage, still in the dark. The beginning of chloroplast degradation is visible, following decreases in galactolipid content between 24 and 38 days of storage. Photosynthesis and fluorescence measurements were used as indicators of the condition of the leaves, as their values decreased during storage. This decrease is attributed to a drop in the stroma's ability to reoxidise in the dark. Two phases therefore appeared during storage: the first corresponds to a slowdown in metabolism up to 24 days of storage, during which the integrity of the chloroplast is maintained, and the second beyond 24 days, when degradation mechanisms appear. The perception of carbon and energy status could be the element that triggers the transition from one phase to the other.

The use of a conservation system with a constant flow of water-saturated air enabled lettuce leaves to be conserved for up to 38 days, demonstrating the importance of managing dehydration during conservation periods. In these leaves, which had been kept cold for 7 days, a transcriptomic analysis showed that many genes linked to chloroplast carbon metabolism were over-expressed following exposure to white light (50 µmol.m-2.s-1) compared with leaves kept in the dark after 2 days. White light (50 µmol.m-2.s-1) stimulated the degradation of phospholipids to produce phosphatidic acid and maintained photosynthetic capacity after 2 days. The application of red light to activate phytochrome reduced the extent of the degradation phase beyond 24 days. This is probably due to the improved reoxidation capacity of the stroma and the electron transport chain. To delay degradation, the use of light levels that activate phytochrome, followed by higher intensity light, could be beneficial.

Keywords Post-harvest, Senescence, Transcriptomics, Lipidomics, Photosynthesis, Glycerolipidoma