Abstract Detail

Xylem apoplast-symplast interactions

Ewers, Frank [1], Fisher, Jack [2], Schmid, Rudolf [3].

Xylem and phloem transport may be co-limiting in many cases.

The Hydraulic Limit hypothesis holds that hydraulic properties limit the maximum height a tree can achieve. In most cases, plant organs produce a proportional amount of xylem and phloem as the plants age, but here we look at a case where xylem and phloem production is not in synch, and a case where the production of xylem potentially limits the production of phloem. In the asynchronous case, leaves of Pinus longaeva produce only secondary phloem after their first year, and no secondary xylem. The leaf's length and area remain the same after the first year, thus there is not a continued increase in hydraulic demand to the leaf with advancing age. The leaves can persist for up to thirty or more years, dependent upon new phloem production each year for their longevity. Apparently phloem sieve cells have a finite life span, and the usefulness of the leaf as a source of carbon to the plant may depend upon renewed phloem production each year. Thus there appears to be an ongoing cost for maintenance of the phloem transport system as the leaves age, but no ongoing cost in xylem production. In the xylem limitation case, in comparing tree and liana species of Bauhinia, the liana stems are quite narrow relative to the leaf area they supply with water. The reduction in xylem area in liana stems is compensated by the production of a high frequency of wide vessels in the xylem. This allows for hydraulic sufficiency due to the fourth power relationship between vessel diameter and hydraulic conductivity. However, the reduced circumference of the vascular cambium in liana stems could well limit phloem production. If phloem transport, from the sources to sinks, is co-limiting with xylem transport, from root to shoot, one would predict the phloem to have adjustments in either the relative amount of, or type of, sieve tubes in liana species. Indeed, in Bauhinia the reduced phloem area in liana stems appears to be compensated by the production of extremely wide sieve tubes, which apparently allows phloem transport sufficiency in narrow stems. Considering those examples, given that there is a cost for the production and maintenance of living phloem tissues, it might be rare for only xylem transport to be limiting to assimilation and growth. With increased height of a tree, the total transport pathways lengthen. That should put increasing demands on both the xylem and phloem transport systems.

Broader Impacts:

1 - California State Polytechnic University, Pomona, Office of Research, 3801 West Temple Avenue, Pomona, CA, 91768, USA
2 - University of British Columbia, Botany (Honorary Professor), 2575 Mountain View Rd, Ferndale, WA, 98248, USA
3 - University of California Berkeley, DEPT OF INTEGRATIVE BIOLOGY, Berkeley, CA, 94720-3140, USA

Keywords:
xylem
phloem
apoplast
long distance sap transport.

Presentation Type: Symposium or Colloquium Presentation
Session: C3
Location: Belle-Chasse/Riverside Hilton
Date: Monday, July 29th, 2013
Time: 8:30 AM
Number: C3003
Abstract ID:202
Candidate for Awards:None