Functional anatomy of the water transport system in cut chrysanthemum

Abstract

Cut flowers show a wide variance of keepability. The market demands more and more a guaranteed quality. Therefore, methods must be developed to predict vase life of cut flowers. Chrysanthemum ( Dendranthema x grandiflorum Tzvelev) and some other cut flowers suffer from unpredicted early leaf wilting during vase life. Researchers from Wageningen University and from the Research Station for Floriculture and Glasshouse Vegetables started a joint project to investigate the problem of early leaf wilting and to come to a better prediction of vase life of cut flowers. Preliminary experiments pointed out that early leaf wilting is caused by a decrease of the water uptake due to embolisms that are induced at the cut surface. This thesis reflects a part of the project on early leaf wilting and is focussed on the anatomical aspects of the stem water transport system.Chrysanthemums are propagated by stem cuttings, which grow in about 12 weeks to commercial maturity. Flowering is induced by a short day treatment. Cut chrysanthemums have an erect stem and the leaves are helically arranged along the stem. The primary vessel bundle network was elucidated, revealing that leaves have their direct water supply from different vascular bundles, which are positioned around nearly half of the circumference of the stem (2.1). The xylem water transport system consists of primary xylem and secondary xylem. The older the stem part (i.e. the lower in the stem) the higher the relative amount of secondary tissue.Digital image analysis procedures were constructed to enable the quantification of large amounts of anatomical data. Xylem vessel characteristics along the chrysanthemum stems were thus quantified and a mathematical description of the vessel characteristics was developed (2.2). Hydraulic conductivity, amount of vessels, average diameter of vessels, and vessel length all showed a gradual exponential decrease from the base to higher up the stem. The hydraulic resistivity calculated from vessel lumina was 30% lower than the experimentally measured resistivity, irrespective of the position in the stem. The remaining 30% is at least partly caused by the resistance of the intervessel pits.A new theory was developed to explain the regulation of vessel lengths in plants (2.3). Vessel length depends on the amount of fused tracheary elements. The only assumption in the theory is that each element has the same chance to be the end of the vessel during vessel formation. This results in an exponential vessel length distribution, which indeed is always found in our chrysanthemum stems. The plant can thus determine its vessel length distribution by just steering the chance factor. This theory provides the most simple mechanism that enables plants to regulate the length of xylem vessels. Stochastic regulation of biological processes might be widely present in nature.We reviewed and refined a method to obtain flat planes in all desired directions through frozen hydrated (biological) specimens (Chapter 3). A combination of proper sample preparation, trimming with a circular diamond saw, tight mounting using indium, and planing with a diamond knife proved to be a useful method to prepare stem xylem tissue for observation in a cryo-scanning electron microscope (cryo-SEM). This planing method is useful for a wide range of other applications of cryo-scanning electron microscopy.We used cryo-SEM to study the dynamics of emboli in stem xylem vessels (Chapter 4). In accordance with our theoretical assumptions, wide vessels that were cut open at the cut surface appeared to embolise at a lower hydraulic tension than narrower vessels (4.1). The tension needed to embolise all vessels is easily reached in cut flowers, and it therefore can be assumed that under normal postharvest conditions all cut open vessels embolise. Our cryo-SEM results agree with the hypothesis that refilling of embolised vessels is needed to restore the fresh weight of cut chrysanthemums on vase after a dehydration treatment (4.2). Under normal post-harvest conditions air only enters cut open xylem vessels (4.3). The blockage of the xylem water flow due to emboli in cut chrysanthemums is therefore located in the base of the stem in the cut open vessels.We developed a physical explanation of the mechanisms of induction and removal of emboli in cut open vessels of cut flowers (5.1). The refilling of embolised vessels after dehydration takes place in two phases:initial rise of vase water into the vessels, resulting in redistribution and compression of the thus entrapped air, anddissolution of the entrapped air into the surroundings.It was concluded that the anatomy of the xylem vessels plays an important role in rehydration capability of cut flowers after air aspiration. A compact vessel system (narrow, short and well connected vessels) restores better from emboli than a vessel system consisting of wide, long and loosely connected vessels.Knowing that the stem anatomy is important with respect to embolism repair, we tried to find an accurate, but easy method to test the stem anatomy on its sensitivity to early leaf wilting (5.2). Within our experiments we found that plants with wider vessels were more sensitive to early leaf wilting, but no absolute thresholds were found at comparing over different experiments. It was confirmed that the anatomy of the stem water transport system is important with respect to vase life quality. However, the combination with several other factors ultimately determines the vase life quality. Xylem thickness and some other stem tissue dimensions seem to be good indicators of the quality of the xylem vessel system and its sensitivity to embolism. A more extensive study is needed to prove the practical value of the use of these indicators. Breeders and growers may use our findings and optimise genotype and growing conditions to obtain chrysanthemums with narrower and shorter vessels at the cut surface in order to prevent the problem of early leaf wilting.</p