The information and knowledge about the individual species of Paulownia spp., especially the artificially created hybrids, that should be available is very limited (Kumar et al. 1999; Guo-qiang et al. 2001). The growth parameters, the climatic and soil conditions and silvicultural measures for the individual hybrids are not generally available, which is confirmed by Sedlar et al. (2020). Internet shops often sell several hybrids together, without specifying which kinds they are (e.g. chinapaulownia.com or detail.en.china.cn) or they merely specify what purpose these plants serve best (e.g. saw-timber, biomass), what maximal thickness or volume they reach after how many years and what temperature range they can survive. Unfortunately, there is no specification regarding the optimal amount of water these plants should receive and the length of the vegetation period, or any technical reports containing the place of origin, the climatic and pedological conditions and the necessary silvicultural measures. Most authors of scientific articles only describe the hybrid in question, merely as a cross between two named species (e.g. “Structural characterization of the acetylated heteroxylan from the natural hybrid Paulownia elongata/Paulownia fortunei”(Goncalves et al. 2008), “EDTA and citrate impact on heavy metals phytoremediation using paulownia hybrids” (Miladinova-Georgieva et al. 2018) or “Metagenomic Analysis of Bacterial and Fungal Community Composition Associated with Paulownia elongata x Paulownia fortunei” (Wozniak et al. 2019). This could lead to people mistaking one hybrid for another, e.g. the cross between Paulownia elongata and P. fortunei can be called “P. Clon in vitro 112®” (Moreno et al. 2017) or “P. Cotevisa 2®” (ipaulownia 2018) or even “P. Bellissia®” (Bio Tree 2021), where each name is a registered trademark. All this (above) is why we were unable to find the optimal amount of water to provide the individual species with and, successively, to be able to perform a comparison with our result.
We found no significant differences in the total heights and lengths of the current-year shoots among the species that received the minimum amount of water. These results correspond with the results of Ayan et al. (2006), who investigated the influence of the amount of water on the growth of several species and did not reveal any differences among them, either. On the other hand, we found differences in the heights and lengths of the current-year shoots of the species when we used a greater amount of water. When given medium watering, Paulownia Shan Tong were the tallest and, when given the maximum amount of water, Paulownia tomentosa were the tallest of all species. These results are similar to those of Ptach et al. (2018), who described that when they applied a greater the amount of water, the plants grew taller. Also, the growth trend of Paulownia Hybrid 9502 shows the same results, as is stated by Ptach et al. (2018). We achieved the tallest plants with maximum watering, however, the total heights and lengths of the current-year shoots of these plants were ca. 1/3 shorter than those of Paulownia tomentosa. The growth trends of Paulownia Clon in vitro 112® and P. Bellissia® were similar to those of P. Shan Tong, however, the total heights and lengths of the current-year shoots of these plants were ca. 1/3 shorter than those of P. Shan Tong. The different reaction of Paulownia tomentosa and P. Hybrid 9502 to P. Shan Tong, P. Clon in vitro 112® and P. Bellissia® may have been due to the different amount of water necessary for optimal growth and their sensitivity to waterlogging. Paulownia tomentosa naturally occurs in places with precipitation from 500 to 1,500 mm (Zhao-Hua et al. 1986). This species may grow according to the rule: the more water, the greater the growth. Paulownia Hybrid 9502 may have a similar rule. On the other hand, Paulownia Clon in vitro 112® needs 750 mm (UCLM 2013) or 800 mm annual precipitation (Jabloński 2016). When the plants of this species occur in conditions with such precipitation, they should grow best. This amount of water corresponds to our medium watering. Paulownia Shan Tong and P. Bellissia® may need a similar amount of the water. Moreover, some of these five species may be affected by waterlogging. Jazireyi (2003) classified Paulownia as a plant demanding moisture, but our results show that not all species of Paulownia respond equally to soil moisture. Paulownia Shan Tong might be more intolerant to soil waterlogging, which occurred due to the large amount of water supplied in the case of maximum watering. This could correspond to the study conducted by Barickman et al. (2019), who state that excessive water slows down the growth. The same trend (as was in the case of Paulownia Shan Tong) was observed with Paulownia Clon in vitro 112® and P. Bellissia®.
The thickness of each newly-formed terminal shoot was measured 10 cm from the stem. Unlike the observations of Ayan et al. (2006), we did not find any differences among the species – only in the case of minimum watering, because when we applied medium and maximum watering, we found differences which do not correspond to the results of Ayan et al. (2006). The response of the shoot thickness to the medium and higher amount of water supplied varies, depending on the species. The plants of Paulownia tomentosa and P. Shan Tong were thicker than others in these irrigation groups. We expected the plants of individual species to react similarly to the amount of irrigation and, with an increase in the amount of water, their thicknesses to increase. The same is described by Ptach et al. (2018) who used Paulownia Shan Tong, where they applied two types of watering. It was confirmed only in the thicknesses of the shoots of Paulownia tomentosa. This may be due to the fact that Paulownia tomentosa naturally occur in places with precipitation from 500 to 1,500 mm (Zhao-Hua et al. 1986). The thickness (as well as the height) of this species can increase according to the statement: the more water, the greater the growth. This species manifests the same trend in the stem thickness and the height. Paulownia Shan Tong is a hybrid of P. tomentosa and P. fortunei, which naturally occur in places with precipitation of 500-1,500 mm and 1,200-2,500 mm, respectively (Zhao-Hua et al. 1986), therefore we expected the growth trend to be similar to that of P. tomentosa. However, the plants of Paulownia Shan Tong had the thickest shoots when we applied medium watering, which can partially correspond with the results of Ptach et al. (2018). This trend was the same as the trend of the height. Therefore, we assumed that Paulownia Shan Tong is intolerant to intense waterlogging, which occurred due to the great amount of water supplied, thus reducing the height and thickness increment. On the other hand, Paulownia Clon in vitro 112®, P. Hybrid 9502 and P. Bellissia® had similar thicknesses when we applied all three types of watering. This trend is similar to that in the study by Rad and Mirkala (2015), where they did not find any differences between the thicknesses of the plants grown with a different amount of water.
The leaf area and the leaf biomass showed similar trends. According to Ptach et al. (2019), the more watering there was, the greater the leaf area and the amount of biomass. Our results, similarly, show that the amount of the leaf biomass and leaf area were usually the smallest with the minimum amount of water. A study conducted on Pinus ponderosa Douglas ex C.Lawson (Maherali and DeLucia, 2001) describes similar results. These studies, however, examined the effect of only two types of watering on the leaf biomass.
Our results indicate that the greatest amount of leaf biomass and leaf area were gained with medium watering. On the one hand, little water could bring about a drop in the level of cytokinin and auxin and, subsequently, lead to limited growth (Seeley 1990), on the other hand, a large amount of water could cause waterlogging, which also reduces growth (Barickman et al., 2019). Our results show that Paulownia tomentosa and P. Shan Tong gave the most leaf biomass and leaf area with medium watering, whereas the plants of Paulownia Hybrid 9502 had the smallest amount of biomass and leaf area in all types of watering.
We did not find differences in the LAI within each species after the three different types of watering, which does not correspond to the results of, for example, Devakumar et al. (1999), who investigated the effect of water supply on Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg. They found that the differences in the LAI between the non-irrigated and the irrigated plants were almost 50%. We found differences in the LAI only among certain species receiving the same amount of water. With the minimum water, we found a difference between Paulownia Shan Tong and P. Hybrid 9502 (i.e. between the highest and lowest LAI, respectively). In this case, the difference in LAI was evident due to their amount of the leaf biomass and leaf area, because the plants of Paulownia Hybrid 9502 had a fraction of the leaf biomass of P. Shan Tong, but the projection areas of their crowns were similar. There was a difference between Paulownia bellissia® and P. Clon in vitro 112®, which had the same maximum watering, i.e. between the highest and lowest value of LAI. A comparison of the individual parameters indicated that the projection area of the crown of Paulownia Clon in vitro 112® was almost 50% smaller than that of P. bellissia®; Paulownia Clon in vitro 112® was taller and also its leaf area was smaller. This means that Paulownia Clon in vitro 112® was taller (with fewer leaves that were closer to the stem) than P. bellissia®, whose leaves were more spread out. In the case of P. bellissia®, there may have been changes in the distribution of the biomass (Lei 2006), where it is the roots and leaves that grow first and then all the other parts (Zhang et al. 2004; Duan et al. 2005). With the medium amount of the water, Paulownia Shan Tong had the highest LAI value. There were differences in the LAI between Paulownia Shan Tong and P. Clon in vitro 112®, P. Hybrid 9502 and P. bellissia®, due to the greater amount of leaf biomass and larger leaf area in Paulownia Shan Tong plants, compared to those of the above-mentioned species (whereas the projected area of the crowns was similar). In contrast, the difference between Paulownia Shan Tong and P. tomentosa was probably in the distribution of their crowns because the amount of the leaf biomass and size of leaf area of these species were similar, but the projected area of the crown of Paulownia tomentosa was nearly 100% larger, compared to that of P. Shan Tong.