What are the name of the death members office xylem?
Answers
Maintenance of long distance water transport in xylem is essential to plant health and productivity. Both biotic and abiotic environmental conditions lead to embolism formation within the xylem resulting in lost transport capacity and ultimately death. Plants exhibit a variety of strategies to either prevent or restore hydraulic capacity through cavitation resistance with specialized anatomy, replacement of compromised conduits with new growth, and a metabolically active embolism repair mechanism. In recent years, mounting evidence suggests that metabolically active cells surrounding the xylem conduits in some, but not all, species are capable of restoring hydraulic conductivity. This review summarizes our current understanding of the osmotically driven embolism repair mechanism, the known genetic and anatomical components related to embolism repair, rehydration pathways through the xylem, and the role of capacitance. Anatomical differences between functional plant groups may be one of the limiting factors that allow some plants to refill while others do not, but further investigations are necessary to fully understand this dynamic process. Finally, xylem networks should no longer be considered an assemblage of dead, empty conduits, but instead a metabolically active tissue finely tuned to respond to ever changing environmental cues.
The success of vascular land plants is intimately tied to the maintenance and functionality of the xylem network. Hydraulic failure within the xylem network can result in tissue damage, decreases in gas-exchange, and ultimately plant death. Not surprisingly, plants have evolved mechanisms to both avoid embolism formation and restore xylem transport capacity once embolism occurs. Research over the past three decades has greatly advanced our understanding of the physiology and biophysics of these processes (Zwieniecki and Holbrook, 2009; Nardini et al., 2011). Here, we summarize recent and historical studies on the subject of embolism repair and evaluate the mechanisms utilized by species known to restore hydraulic conductivity from both ecological and anatomical perspectives.
Drought-induced embolisms form when xylem sap tension reaches a critical threshold, where air is aspirated through pit membranes separating adjacent conduits, or gas bubbles spontaneously nucleate from dissolved gas in the xylem sap (Tyree and Zimmermann, 2002). Because of the negative pressure inside the xylem conduits, the gas rapidly expands and forces water into connected, neighboring conduits (Tyree and Zimmermann, 2002). Embolisms can also form in xylem vessels following freeze-thaw events, where crystallization of liquid water forces dissolved gas out of solution. Once the ice melts gas bubbles remain and later expand to fill the conduit when tension is subsequently applied to the xylem sap (Sperry et al., 1988). Wounding and pathogen infections can also result in the entry of air into the xylem network (Dimond, 1970; Tyree and Sperry, 1989; McElrone et al., 2008), all of which lead to non-functional xylem conduits that disrupt the transport and distribution of water and nutrients throughout the plant. Despite the negative physiological impacts, most plant species live with some proportion of their xylem rendered non-functional due to embolism (Choat et al., 2012).
Hydraulic dysfunction caused by embolism is such a strong selective pressure that plants have evolved xylem anatomy specifically to prevent embolism formation and spread (Brodribb and Holbrook, 2004). Woody plants utilize multiple strategies to prevent hydraulic dysfunction due to embolism. Hydraulic capacitance (i.e., stored water reserves held in secondary xylem) buffers against extreme tension in the xylem sap (Meinzer et al., 2009). With the aid of specialized pit membrane structures (e.g., Pittermann et al., 2005; Brodribb et al., 2012) xylem anatomy is largely responsible for the cavitation resistance in many species, allowing them to thrive in a diverse range of habitats with low water availability (Johnson et al., 2012). Successful colonization of xeric habitats by both angiosperms and gymnosperms has been linked to xylem highly resistant to embolism formation (Brodribb et al., 2012).
Answer:
when tracheids are formed they die