In vascular plants, vascular bundles are necessary structures for transporting substances, which consist of xylem and phloem. The xylem provides the mechanical support and stability that plants need to sustain themselves and expand their growth and carries substances such as water, minerals, inorganic salts, and countless signaling molecules over long distances from the roots to all above-ground parts [1–4]. The study of xylem has attracted many scholars for more than a century [5–7]. Some studies have found that the xylem of monocotyledonous plants has stronger plasticity, and the size and quantity of the xylem will change under different water conditions [8,9]. The high plasticity of xylem development is very important for the environmental adaptation of plant growth. To cope with the change in environmental conditions, the xylem will produce physiological changes accordingly, so it is very important to observe the transport of the xylem.

Lilies have high requirements for the physical and chemical properties of the growth environment, and the growth of lilies varies greatly in unsuitable acid-base environments. Under saline-alkali stress, it will be difficult to absorb water which reduces the absorption capacity of minerals, resulting in unhealthy growth and even death of plants [10]. The studies conducted by Carlquist have demonstrated that the xylem structure significantly influences water transport [8,9]. Moreover, it has been observed that xylem development varies among different plant species and even within the same plant under diverse stress conditions. Therefore, the study of the xylem structure and water transport of lily will be of great significance for the analysis of physiological changes in response to stress.

Some scholars have studied xylem transport and developed some methods to monitor it; the dye method is the most popular [11,12]. In the Arabidopsis xylem study, ink containing agar was applied to the roots to observe the shootward transport of ink in Arabidopsis seedlings, but it was not confirmed that the ink was only transported in the xylem [13]. Rhodamine B and fluorescein sodium salt have been applied to Arabidopsis roots to trace the transport of substances in the xylem from root to leaf, making it possible to measure the transport velocity [14]. Texas Red (sulforhodamine 101 acid chloride) has also been used to observe xylem connectivity in Arabidopsis [15]. Carboxyfluorescein diacetate (CFDA) is a fluorescent dye, which is broken down into CF by enzymes. The advantage of using CFDA is that it fluoresces strongly only after entering living tissue [16]. It was widely used in monitoring xylem transport in Arabidopsis, Populus, Aceraceous, and other plants [16,17], and widely used to observe the recovery of xylem transport capacity in plant grafting [18–21]. In addition, CFDA is often introduced through leaves as a phloem tracer [22]. However, CFDA introduced from the root can only be transported upward through the xylem [23].

In this study, the distribution and structure of the vascular bundle of L. lancifolium were observed using the method of semithin section, and CFDA was used as a fluorescent tracer to simulate the xylem transport of lily stems. The rate of xylem transport was measured, which provides new ideas for studying the growth and development of lily plants in the process of adapting to the environment. It also provides new ideas for understanding the response of xylem to fusarium wilt pathogens to prevent and control these diseases.

Understanding the structure of vascular bundles is fundamental to the successful introduction of fluorescein. There were no secondary structures in the vascular bundles in L. lancifolium, which was consistent with lilies such as Lilium Asiatic Hybrids [26]. The mechanical tissue gives the stem a certain hardness and lodging resistance [27]. The successful introduction of fluorescein is fundamental to the observation of xylem material transport. In this experiment, three fluorescent dye introduction methods were used: wound introduction method, solution immersion method, and medium introduction method. At present, wound introduction is widely used [28,29]. In our study, the use of the wound introduction method might cause some problems, such as the vascular bundle having no fluorescent signal, which might be due to the wound vascular bundle secreting callose and blocking the wound [30,31]. Then, we conducted a medium introduction and found that the medium introduction method affected the activity of CFDA due to the high medium temperature. This method is mainly used in cells [32] because the liquid medium is used in cells, the fluorescence agent will not be affected by temperature and produce instability, but this experiment can only use a solid medium, so this method is not suitable for this experiment. The solution soaking method was relatively more effective and stable and had been successfully applied to tomatoes [33].

In different species, the vascular structure is different, the number and size of vascular cells are different, and therefore, the transport rate of fluorescein is also different. Péron et al. found that fluorescent signals were found in the root cells of Phelipanche ramosa 2 h after the introduction of CFDA to the leaves of the host plant (Brassica napus) [34]. Zhang et al. found that it took 4 h for CFDA to reach the phloem of apple fruit when it was introduced from the carpopodium of apple, and at least 16 h for CFDA to reach apple fruit when it was applied to the surface of leaves [25]. Ren found that it took 5 h for CFDA to reach the stem vascular bundle after introducing it from the petiole of Neolamarckia cadamba [35]. When the ink was applied to the roots in Arabidopsis thaliana, ink was found in the shoots within a few hours, and Rhodamine B and fluorescein sodium salt were applied to the lower part of the taproot in 14-day-old Arabidopsis seedlings, and fluorescent signals were found in the upper root zone within a few minutes [14]. In our study, the transport rate of CF in the curly stems was 0.3 cm/h. It can be seen that different species, different application sites, and different fluorescent tracers may change the transport speed, the marking time can be reasonably selected according to the speed of species transport. Therefore, the study of xylem transport should continue in the future. In addition, we also found that the fluorescent signal of CF appeared in parenchyma cells, which indicates that there may be a symplastic mode in the transport of xylem substances in the L. lancifolium, which is consistent with the results of Katarzyna. In a study of Aceraceae plants, CFDA was used to trace xylem transport over long distances, and fluorescent signals were found in both xylem and parenchyma cells of branches, confirming that water and minerals transport in trees not only in the form of exosomes but also in the form of symplastic [17].

The finite vascular bundles of L. lancifolium consisted of phloem and xylem. The vascular bundles were smaller on the periphery of the stem, while those near the center were larger and dispersed. The most suitable method for CFDA labeling the xylem of isolated stem segments of L. lancifolium was solution soaking for 24 h. The running speed of CF in the isolated stem was 0.3 cm/h, which was consistent with the running speed of the material in the field. CF could be transported between xylem and parenchyma cells, and could not be transported to the phloem.