Cellular and Metabolite Changes in the Secondary Phloem of Chinese Fir (Cuninghamia lanceolata (Lamb.) Hook.) during Dormancy Release

Wood in the cold temperate zone is the product of the alternation of the growing season and the dormant period of trees, but our knowledge of the process of dormancy release in trees remains limited. Chinese fir (Cuninghamia lanceolata (Lamb.) Hook.) was used to investigate cellular and metabolite changes in the secondary phloem tissue during dormancy release. The sampling dates were 2 March, 28 March, and 13 April. The microsections of wood-forming tissue were prepared using the paraffin embedding technique to observe the formation of cambium cells; metabolites in secondary phloem cells were extracted using a methanol/chloroform organic solvent system. The results showed that the secondary phloem consists of phloem fibers, sieve cells and phloem parenchyma. The cells were regularly arranged in continuous tangential bands and were in the order of Phloem fiber-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Sieve cell-Phloem parenchyma-. The Chinese fir cambium was in dormancy on 2 March and 28 March, while on 13 April, it was already in the active stage and two layers of xylem cells with several layers of phloem cells were newly formed. The width of the cambium zone increased from 18.7 ± 5.7 μm to 76.5 ± 3.0 μm and the average radial diameter of sieve cells expanded from 15.4 ± 7.5 μm to 21.5 ± 7.4 μm after dormancy release. The cambium zone width and the average radial diameter of sieve cells before and after dormancy release were significantly different (p < 0.01). The phloem parenchyma cells without resin were squeezed and deformed by the sieve cells, and the width of the phloem during the active period was 197.0 ± 8.5 μm, which was larger than that during the dormant period. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based metabolomics was employed to analyze the secondary phloem of Chinese fir on 28 March and 13 April. Thirty-nine differential metabolites during dormancy release were detected. The results showed that the composition of Chinese fir metabolites was different before and after dormancy release. The relative increase in pyruvic acid and ascorbic acid contents proved that the rate of energy metabolism in Chinese fir increased substantially after dormancy release. Changes in cell development and the composition of metabolites revealed that the dormancy release of Chinese fir was at early April and the formation period of phloem tissue is earlier than xylem tissue.

The ultrastructure of dwarf mistletoe (Arceuthobium spp.) sinker cells in the region of the host secondary vasculature

Sinker cells showed ultrastructural similarities in three species of Arceuthobium on three different hosts despite differences in season of collection and fixation. All species had abundant osmiophilic lipid bodies, plastids with prolamellarlike bodies, mitochondria with large nucleoids, chromocentric nuclei, and peculiar saccules associated with plasmodesmatal fields. Xylem may not be continuous through sinkers. Apoplastic continuity between host and parasite is afforded by fused cellulosic cell walls. Pitlike wall thinnings and "half-plasmodesmata" are found between juxtaposed host and parasite cells. One-sided, imperforate sieve pores were noted between a sieve cell and a contiguous sinker cell. However, symplastic isolation of the host and parasite seems likely. Therefore, nutrients may be absorbed from the common host–parasite apoplast. Mobilization of nutrients out of the endophytic system to the aerial shoots is thought to be facilitated by differential starch storage in the parasite body.

Tissue volume changes in black spruce phloem

Cell dimensions and volume changes were studied in black spruce phloem by microscopic techniques.It was found that sieve cells measure 4 mm in length and 30 μm in diameter and constitute on the average 52.2% of the volume of the inner bark. However, sieve cell volume decreases gradually from 62.8% at the cambium to 38.7% in the outer dead phloem as a result of the collapse of these elements. In contrast, the volume of longitudinal parenchyma increases from 28.7% at the cambium to 43.7% in the outer dead bark.Sclereids possessing thick and multilayered walls make up on the average 3.4% of the inner bark. Sclereid volume increases from 2.8% at the cambium to 9.6% in the outer bark. On the average, rays constitute 6.4% of the inner bark, increasing from 5.7% at the cambium to 8% in the outer bark as a result of dilation.

Ontogeny of cambium and Phloem in the Epicotyl of Pisum sativum

The pattern of distribution and differentiation of the primary phloem, the cambium, and the secondary phloem, and the exact pattern of division of the initial cell and its derivatives have been studied in the epicotyl of pea plants by using electron microscopy. Three divisional patterns of the initial cell, in giving rise to the phloem cells, are recognized. The initial cell first divides periclinally to give rise to a transitional cell. This transitional cell then divides further (periclinally and/or anticlinally) to give rise to three sequences of phloem derivatives: (1) phloem parenchyma cells, (2) a companion cell and a sieve cell, and (3) a companion cell, a sieve cell, and a phloem parenchyma cell. The derived cells can all easily be distinguished from each other either by their position in the vascular bundle at low magnification or by the different types of plastids present in them. The general pattern of differentiation of the cytoplasm and the formation of the sieve plate and the sieve pores of the sieve element are essentially similar in the primary and the secondary phloem. However, the sieve element of the secondary phloem, unlike that of the primary phloem, possesses in its cytoplasm three kinds of inclusion bodies - an amorphous form, a "crystalline" form, and a tubular form; these are described and their nature discussed.