Postharvest Treatments Improve Quality of Cut Peony Flowers

Peony is one of the most important ornamental plants in the international flower market, but has a relatively short vase life in water. This study tested the effects of 8-hydroxyquinoline citrate (8-HQC) and nanosilver (NS) in combination with sucrose, as well as two commercial preservatives, on the longevity and some physiological and biochemical aspects of senescence of cut flowers of 14 cultivars. Responses varied both by cultivar and treatment. The preservatives extended the vase life in only five cultivars; however, in nine cultivars, preservatives increased the flower diameter and improved the general flower appearance. Blockages in xylem vessels started to appear soon after harvest. Both NS and 8-HQC with sucrose prevented tylose formation, while bacterial blockages were reduced only by the NS solution. Reduction in stem blockages did not translate into better water balance or flower longevity. The highest carbohydrate accumulation in petals was observed in the NS solution. Preservatives mitigated the rise in free amino acids, including free proline. They did not prevent an increase in H2O2 content but flowers in preservatives generally had higher catalase activity than in the control. As solutions with NS produced comparable or even better results than 8-HQC, we recommend the latter as a component of a preservative for cut peony flowers. However, cultivar-specific responses indicate that postharvest treatments must be individually tailored to each cultivar.

Scanning Electron Microscopy Reveals Different Response Pattern of Four Vitis Genotypes to Xylella fastidiosa Infection

The xylem-limited bacterium Xylella fastidiosa causes Pierce's disease (PD), whose disease symptoms are primarily the result of xylem vessel blockage in susceptible grapevines. Stem internode and petiole tissues from infected and uninfected control plants of four grape genotypes (Vitis vinifera, V. rufotomentosa, V. smalliana, and V. arizonica/candicans) differing in PD susceptibility were examined using scanning electron microscopy (SEM). Tyloses, fibrillar networks, and gum plugs were observed in lumens of tracheary elements in petioles and internodes of both water-inoculated control plants and X. fastidiosa–inoculated plants of all genotypes. Bacteria were not observed in control plants. In both petiole and internode tissues, the greatest number of occluded xylem vessels were observed in V. vinifera and the smallest number in V. arizonica/candicans. The number of xylem vessels infested with X. fastidiosa was greatest in V. vinifera and did not differ among the other three genotypes. Systemic infection was found in all genotypes. The frequency with which X. fastidiosa infested vessels were observed using SEM corresponded well with bacterial levels estimated by enzyme-linked immunosorbent assay. Among infected plants, tylose formation in internodes was lowest in V. arizonica/candicans and did not differ among the other three genotypes. Infection with X. fastidiosa strongly induced tylose formation in V. vinifera and V. smalliana but not in V. arizonica/candicans. Analysis across tissues and genotypes indicated an induction of fibrillar networks and gum occlusions in response to X. fastidiosa infection, whereas treatment comparisons within genotypes were not significant except for V. vinifera petioles. Limiting the spread of X. fastidiosa infection by xylem conduit occlusions does not appear to be the mechanism conferring PD resistance or tolerance to V. arizonica/candicans, V. smalliana, or V. rufotomentosa. In contrast, the strong induction of tyloses may be detrimental rather than beneficial for V. vinifera survival after X. fastidiosa infection.

The Protective Layer as an Extension of the Apoplast

The protective layer between the cell wall and plasmalemma of xylem parenchyma cells has variously been suggested to be involved in protection of the protoplast from attack by autolytic enzymes from neighbouring, dying cells, tylose formation, deep supercooling of xylem, and strengthening of the pit. None of these ideas has universal application to all species in which parenchyma cells possess a protective layer. It is proposed instead, that the protective layer is primarily laid down in order to preserve apoplastic continuity around the protoplast of a lignified cell, bringing the entire plasmalemma surface, and not just that part of it in contact with the porous pit membrane, into contact with the apoplast. If this is so, then other functions may be coincidental, or have arisen secondarily.

Primary Function of the Protective Layer in Contact Cells: Buffer Against Oscillations in Hydrostatic Pressure in the Vessels?

In the literature it has been suggested that the protective layer, deposited along the wall between xylem parenchyma and vessels, is involved in tylose formation as part of an antipathogenic response. Yet, in a number of cases, the presence of a protective layer is not related with tylose development. It is proposed here, that the protective layer primarily acts as a buffer against hydrostatic oscillations in the vessels. As the hydrostatic pressure in the vessels becomes less negative, the xylem cells will increasingly withdraw water from the apoplast. Once the hydrostatic pressure has surpassed a certain limit, the protective layer is unable to withstand the osmotic pressure of the parenchyma cells and the latter will bulge into the vessels giving rise to the formation of tyloses.

Scanning electron microscope observations of internal symptoms of white elm following Ceratocystis ulmi infection and cerato-ulmin treatment

Internal vascular symptoms induced by Ceratocystis ulmi infection and cerato-ulmin (CU) in white elm (Ulmus americana) were observed under a scanning electron microscope. Symptoms caused by CU were indistinguishable from those caused by C. ulmi. The main symptoms observed were (i) edemalike surface wall alteration, (ii) granular deposit, (iii) pit membrane heaving, (iv) smooth coating, (v) bubble and (or) tylose formation, (vi) rough coating, (vii) droplet formation, and (viii) vessel plugging with calluslike material. Internal symptoms appeared earlier in CU treated elms (as early as 2 h after treatment) than in C. ulmi infected elms (10 h after infection).