Hydraulic properties of Zinnia elegans : from cellular development in vitro to performance in planta

P. Twumasi

Research output: Thesisinternal PhD, WU


The water status in plants is dependent on the xylem hydraulic conductance. In cut flowers, for example, the preservation of continuous hydraulic conductance is important for maintaining longer vase life, an important index for cut flower quality. Many factors, such as stomata performance, root water uptake efficiency, temperature, light quality and humidity, influence the efficiency of water transport in the soil-plant-atmosphere continuum. It has also been shown that the dimensions of the xylem conduits in the plant, such as the conduit length, diameter and density, are important for plant water transport. Therefore, the regulation of the xylem conduit size during xylogenesis is necessary for providing a particular hydraulic efficiency in a given environment. To understand the regulation of xylogenesis and the influence of the processes to produce xylem conduits of particular dimensions with specific hydraulic capacity, this thesis focuses on the Zinnia elegans tracheary element (TE) differentiation at the cellular level. It further considers the regulation of xylem conduit dimensions in whole plants by using drought stress and temperature as variable environmental factors.In Chapter 1, the mechanisms of plant water transport, as regulated by xylem conduit dimensions with consequent effect on plant quality, are reviewed. Also, the chapter provides general literature on xylem formation, Zinnia elegans as model for xylogenic research, and finally gives the outline of the thesis.In Chapter 2 , two cultivars of the Zinnia elegans cut flowers, 'Envy' and 'Purple Prince', were grown in soils with different water contents with increasing water stress levels: 70, 50 and 20 %(v/v) soil water content (SWC). Plant length increased with SWC. The hydraulic efficiency in the stems of cut flowers decreased with increasing water stress. The xylem conduit length was not different between Purple Prince cut flowers from the three different water stress levels. In the Envy cultivar, a significant difference in conduit length was found between plants from 20 and 50% (v/v) SWCs. The proportion of large-diameter conduits increased with decreasing drought stress, i.e., in cut flowers from both cultivars the proportion of large-diameter vessels was highest in the 70% (v/v) SWC and least in the 20% (v/v) SWC. This means that the decrease in hydraulic conductivity with water stress was largely due to the decrease in the diameter of the conduits making the conduit length less important. The vase life of the cut flowers correlated negatively with the diameter of the xylem conduits, confirming that small-diameter conduits are more efficient in overcoming embolism and therefore maintaining efficient water transport in the stems.In Chapter 3 , the role of average day temperature (ADT) and difference in day and night temperature (DIF) on the length of xylem elements and conduits, and fusion of the individual elements into conduits in planta were investigated. Plant length increased significantly with ADT between 17 and 25 o C. Opposite DIF treatments resulted in different plant length at 4 and 8 o C absolute DIF. The responses to temperature differences were similar in both Zinnia cultivars, although Purple Prince cultivar plants were always bigger. The maximal vessel length (L max ) and half vessel lengths (τ) both increased with increasing ADT, about 110-130% increase from 17 to 25 o C ADT. However, DIF had no effect on these conduit parameters. Measurements of average lengths of individual vessel elements from plants grown at average temperatures 17 and 25 o C showed about 10% and 20% increase in Envy and Purple Prince respectively. This change in vessel element length constitutes a small part of the overall change in whole conduit length, and therefore, it can be deduced that the amount of vessel element fusion is more important during formation of longer xylem conduits in the plant.In Chapter 4 , a method for establishing an in vitroZinnia elegans cell culture for differentiation into tracheary elements (TEs) is described. The xylogenic Zinnia system is rather unique for studying xylogenesis at the cellular level. It offers possibilities to study in detail the effects of certain factors important for regulating the xylogenesis process. The in vitro culture has been well standardized to eliminate the inconsistencies in TE differentiation that have been observed in the past. Additional methods for monitoring and measuring the important hallmarks of TE differentiation, including DNA laddering, vacuolar rupture, cellulose bands and lignin synthesis, have been treated in this section.In Chapter 5 , the effect of leaf osmolarity (LO) induced by different light intensities and electrical conductivity (EC) of soil-less plant culture, on the differentiation of TEs in cell cultures from the two cultivars, Envy and Purple Prince, was investigated. Both light intensity and EC correlated with LO with a similar response, although it was always higher in the Envy cultivar. By increasing the LO, the percentage of TEs (%TE) formed also increased, with a similar trend in cultures from both cultivars. About 60% increase in %TE was realized in the Purple Prince cultivar as compared to earlier reports by different authors. The LO of the starting material also correlated positively with the final size of the TE. This implies that the efficiency of the xylogenic Zinnia system can be improved by adaptations in the growth conditions for the starting material.In Chapter 6 , we investigated the effect of osmotic stress and timed application of NAA and BA (xylem differentiating phytohormones) on the TE differentiation in cultures from the two Zinnia cultivars. The %TE increased with increasing extracellular osmotic stress until 300 mOsm, after which any additional increase resulted in a diminishing %TE. The size of the TEs increased with decreasing extracellular osmolarity within the working range of 100 to 700 mOsm. These results were similar in the cultures from both cultivars. Application of NAA/BA induced a significant increase in the rate of cell division corresponding to a similar increase in TE differentiation. It is proposed that the increase in cell division may be beneficial in providing certain bioactive materials for the development of the TEs. Delayed application of NAA/BA reduced the %TE, but with a significant increase in TE size.In Chapter 7 , the role of programmed cell death (PCD) in determining the final size of the TEs and the %TE was investigated. Application of two protease inhibitors, a general caspase inhibitor (Zasp) and a cysteine protease inhibitor (E64), to xylogenic cultures from the two cultivars, caused a significant increase in the size of TEs but 65-300% decrease in %TE. There proved to be a significant delay or complete inhibition of the hallmarks of PCD, such as DNA laddering and vacuole collapse, in the cultures treated with the inhibitors. It was also found that the second stream of TE differentiation, common in non-inhibited xylogenic Zinnia cultures, was absent in cultures treated with 10nM or higher of each of the two inhibitors.The research experiments and their outcomes in this thesis range from fundamental xylogenesis at the cellular level to whole plant water transport performance. The overall discussion of the experimental approaches, results, and their practical implications is presented in Chapter 8 . This chapter also offers suggestions for future research.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Emons, Anne Mie, Promotor
  • van Kooten, Olaf, Promotor
  • van Ieperen, Wim, Co-promotor
  • Schel, Jan, Co-promotor
Award date8 May 2007
Place of Publication[S.l.]
Print ISBNs9789085046585
Publication statusPublished - 2007


  • zinnia elegans
  • water
  • xylem
  • hydraulic conductivity
  • cut flowers
  • plant water relations
  • temperature
  • osmolarity
  • osmosis
  • apoptosis
  • in vitro culture


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