Alteration of the anatomical and morphological structure of kiwifruit roots subjected to soil anoxic conditions

Italy, the third largest producers of kiwifruit producing in the world, has lost 10% of its production in recent years because of the spread of the Kiwifruit Vine Decline Syndrome (KVDS / moria), which leads to a gradual deterioration of the plant that starts from the roots. KVDS symptoms have been observed in different areas and have often been associated with water stagnation and anoxic soil conditions, resulting in root rot. Kiwifruit is a species adapted to hot-humid environments and requires large amounts of water. At the same time, it is also extremely sensitive to waterlogging, whose deleterious effects can arise if a rapid drainage of excess water does not occur in the soil, with the consequent establishment of soil anoxic conditions. Kiwifruit roots, and in particular meristems, have a high oxygen consumption and are therefore more sensitive to anoxia. Therefore, at low oxygen concentrations, they are the first tissues to be damaged. Early symptoms associated with KVDS are root browning, disappearance of absorbent roots, morphological and anatomical alterations, and tissue rupture and decomposition, resulting in blockage of xylem vessels, followed by late symptoms, such as leaf necrosis, shoot wilting and, in its acute phase, plant death. Unfortunately, the causes of KVDS are still unknown. On this basis, an experimentation was started in a kiwifruit orchard (Actinidia chinensis (A. Chev.) CF Liang & AR Ferguson, 1984, var. deliciosa) affected by KVDS and located Latina (Lazio Region, Italy), in order to investigate the potential causes and suggest solutions to counter this physiopathy. Root samples from healthy plants were collected and compared with samples from plants with KVDS. Macroscopically, the roots affected by KVDS were rotting and with reddish sections, showing a loss of rhizodermis and cortical parenchyma (Fig. 1). For microscopic analysis, the roots were fixed in 10% formalin, dehydrated in alcohol, cleared and finally embedded in paraffin. Each individual sample was sectioned into 5 μm thick sections and stained with Safranin-Fast Green (Fig. 2) and PAS (Fig. 3) for light microscopy, or prepared for fluorescence microscopy (Fig. 4). Damage to the root system was observed, with tissue breakdown and decomposition, rhizodermis flaking, cortical area with loss of cell turgor, initial decay of the stele, and clear detachment of the cortex from the central conducting tissues. In the control samples, the roots exhibited a 13 μm thick rhizodermis and a mean parenchymatous cell size of 44.5 μm, unlike the KVDS sample, where the rhizodermis was 8.3 μm thick and the average cell size 34.7 μm. Waterlogging and the resulting decrease in oxygen around the roots had a negative, rapid and significant effect on the physiological state and on kiwifruit growth. To ensure optimal growth of kiwifruit plants, an innovative soil management will be applied to reduce compaction and increase aeration.