Progressive Decline in Xylem Inflow into Developing Plums

Recent evidence suggests xylem functionality may decline in developing European plums. Loss of xylem function may have negative consequences for fruit quality. The aim of this study was to establish and localize the loss of xylem functionality, both spatially and temporally using detached fruit. Fruit were detached from the tree under water and fed through a capillary mounted on the cut end of the pedicel. The rate of water movement through the capillary was recorded. Fruit were held above dry silica gel [≈0% relative humidity (RH)] or above water (≈100% RH) to maximize or minimize transpiration, respectively. Water inflow rate depended on developmental stage. It increased from stage I to a maximum at early stage III and then decreased until maturity. Feeding acid fuchsin to developing fruit revealed a progressive decline in dye distribution. The decline progressed basipetally, from the stylar end toward the stem end. At the mature stage III, only the pedicel/fruit junction was stained. The same pattern was observed in four further plum cultivars at the mature stage III. The inflow into early stage III fruit decreased as the RH increased. In contrast, the inflow was less dependent of RH at the mature stage III. Abrading the fruit skin cuticle had no effect on water inflow during early and mature stage III but did markedly increase fruit transpiration rate. Decreasing the osmotic potential (more concentrated) of the feeding solution decreased the water inflow. Our results indicate a progressive loss of xylem functionality in European plum. Transpiration and osmotic pull are the main drivers of this xylem inflow.

Mucilage production by wounded xylem tissue of maize roots — time course and stimulus

As a reaction to invasion by pathogens, plants block their xylem conduits with mucilage, restricting pathogen advance. Wounding soil-grown roots of maize revealed that pectinaceous mucilage could be found in the vessels after 6 h, and abundantly filled most vessels up to 3 cm proximal to the wound after 1 d. Phenolics increased in the mucilage at later times. The same reactions occurred in vessels following mechanical wounding of axenically-grown roots, showing that the presence of microbes is not necessary for the response. The xylem mucilage is similar to root-cap mucilage in mode of extrusion from the periplasmic space of living cells through primary wall, apparent phase transition, and staining indicative of acidic polysaccharides. Whether other known properties of root-cap mucilage which might alter vessel functioning, such as reduction of surface tension and increased viscosity produced by dissolved solutes, are also common to xylem mucilage requires further investigation. However, our results indicate that possible influence of wounding-induced mucilage in xylem vessels should be considered in all experimental investigations of xylem function.