Abstract
Trees growing in wetlands develop adventitious roots from the trunk during the rainy season and adapt to the flooded environment by forming primary (schizogenous or lysigenous) and secondary aerenchyma in the roots. Therefore, it is necessary to clarify the formation process of each type of aerenchyma in these adventitious roots. In this study, saplings of Syzygium kunstleri (King) Bahadur and R.C.Gaur were grown under four different treatments, and a total of 12 adventitious roots generated from trunks were used to clarify the distribution of each aerenchyma type in the roots using light or epi-florescence microscopy. Schizogenous aerenchyma was observed in the root tips where the root color was white or light brown, whereas lysigenous aerenchyma was found at some distance from the root tip where the root color gradually changed from light to dark brown. The secondary aerenchyma and periderm were observed in dark brown parts near the root base. None or only one layer of phellem cells was detected in the white roots near the root tip, but dark brown roots near the root base had at least three layers of phellem cells. Considering these results, oxygen transportation may occur between primary and secondary aerenchyma at the point where two or more layers of phellem cells are formed.
Highlights
Periodic or permanent flooding is a dominant environmental stress that critically impedes the growth, yield, and distribution of plants [1,2]
We aimed to examine how primary aerenchyma connects with secondary aerenchyma in adventitious roots using Syzygium kunstleri (King) Bahadur and R.C.Gaur, which is a woody plant distributed throughout Southeast Asia, especially in the peatlands of Thailand, peninsular Malaysia, and Borneo
In S. kunstleri, which is a known flood-tolerant species, both primary and secondary aerenchyma were observed in adventitious roots grown in hypoxic conditions
Summary
Periodic or permanent flooding is a dominant environmental stress that critically impedes the growth, yield, and distribution of plants [1,2]. This reduction in growth and yield is caused mainly by gas diffusion being approximately 10,000-fold slower in water than in air [3,4]. Wetland plants have adapted to hypoxic conditions through metabolic and morphological methods [5]. Switching from aerobic respiration to the anaerobic glycolysis of ATP induces a severe reduction in energy available for maintenance, growth, and nutrient uptake [6]. In continuously hypoxic conditions, morphological and anatomical adaptation is a more efficient response for plants.
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