Assessment of nutrient remobilization through structural changes of palisade and spongy parenchyma in oilseed rape leaves during senescence

Abstract

Differential palisade and spongy parenchyma structural changes in oilseed rape leaf were demonstrated. These dismantling processes were linked to early senescence events and associated to remobilization processes. During leaf senescence, an ordered cell dismantling process allows efficient nutrient remobilization. However, in Brassica napus plants, an important amount of nitrogen (N) in fallen leaves is associated with low N remobilization efficiency (NRE). The leaf is a complex organ mainly constituted of palisade and spongy parenchyma characterized by different structures and functions concerning water relations and carbon fixation. The aim of the present study was to demonstrate a specific structural evolution of these parenchyma throughout natural senescence in B. napus, probably linked to differential nutrient remobilization processes. The study was performed on 340 leaves from 32 plants during an 8-week development period under controlled growing conditions. Water distribution and status at the cellular level were investigated by low-field proton nuclear magnetic resonance (NMR), while light and electron microscopy were used to observe cell and plast structure. Physiological parameters were determined on all leaves studied and used as indicators of leaf development and remobilization progress. The results revealed a process of hydration and cell enlargement of leaf tissues associated with senescence. Wide variations were observed in the palisade parenchyma while spongy cells changed only very slightly. The major new functional information revealed was the link between the early senescence events and specific tissue dismantling processes.

Leaf Development Monitoring and Early Detection of Water Deficiency by Low Field Nuclear Magnetic Resonance Relaxation in Nicotiana tabacum Plants

“…Thus, the variation of the T 2 in LR 8 and LR 4 at 57 DAS is logical. However, the older leaves studied (LR4) of stressed plants had begun senescence at 57 DAS as shown by the increase in T 2-3 and LWW 3 whereas T 2-2 remained stable, which is typical of the progression of senescence previously reported in oilseed rape leaves [35]. For these leaves, most of the vacuolar water was associated with the spongy cells vacuoles at 42 DAS, but this fraction decreased after 57 DAS to the benefit of the water fraction associated with the vacuoles of the palisade cells.…”

“…This study proved the ability of low field NMR to monitor these developmental processes in tobacco leaves. Our team [35] already demonstrated that NMR relaxation spectra are a powerful developmental indicator to characterize the physiological age of the leaf in Brassica napus. The similarity of the tobacco leaf NMR signal and its agreement with physiological measurements confirms the potential of NMR to provide a universal index of development whatever the plant species.…”

“…The NMR spectra of oilseed rape leaves have been extensively studied and an attribution of the different water peaks has been proposed [30]. In young oilseed rape leaves, the water NMR signal consists of three peaks: the first one characterized by a T 2 around 2 ms was associated with the water interacting with polysaccharides (cell wall, starch), the second one characterized by a T 2 around 15 ms was associated with the water in plastids and other small vacuoles, and the last one, the highest in terms of intensity and with the longest T 2 around 150 ms, was associated with the water inside the cell vacuoles [35]. It has been shown in Brassica napus that vacuoles of palisade cells evolve differently than the spongy ones with leaf age.…”

“…Applied on a wide leaf panel collected from plant grown in both controlled and field conditions, the low field NMR relaxometry was shown to provide precise and robust markers of development of oilseed rape leaves [30]. NMR relaxation is thus a powerful tool to monitor changes in water relations inside the leaf caused by development or different stress as Nitrogen deficiency [35] or thermal stress [36]. Only a few studies have focused on the impact of water stress on the NMR relaxation signal of the leaf [37,38].…”

Nitrogen use efficiency in rapeseed. A review

“…The inversion of the relative expression levels of these two genes defines the onset of leaf senescence. More recently, it was demonstrated that cellular structural changes associated with leaf senescence can be monitored by noninvasive 1 H-NMR relaxometry (Musse et al 2013;Sorin et al 2015). In addition, the NMR signal captured chloroplast dismantling and was therefore judged to be a precise indicator of leaf senescence (Sorin et al 2015); it has been tested on two genotypes showing contrasting responses to N depletion (Sorin et al 2016).…”