Contribution of keeping more stable anatomical structure under high temperature to heat resistance of Rhododendron seedlings

High temperature has been an obstacle to grow Rhododendron plants, and there are few reports on anatomical changes of Rhododendron plants under heat stress. To identify the mechanism of anatomical adjustment under high temperature, three Rhododendron species with different sensitivity to high temperature stress were selected to investigate the effect of heat stress on the anatomical structure. Heat stress resulted in stomatal closure and the decrease of stomatal density, however, the limited stomatal adjustment reached by R. fortune and R. mariesii may not prevent water loss effectively, of which the leaf thickness was reduced and obvious plasmolysis was observed in the mesophyll cells. R. simsii with smaller stomata and higher stomatal density had greater heat resistance than R. fortune and R. mariesii. Heat stress injured cell membrane structure seriously, and it was found that the nuclear membrane was digested and the nucleolus disappeared. The response of chloroplasts to high temperature was most sensitive, in which thylakoid lamellas became blurred, even degraded in heat-sensitive Rhododendron species. The thermal endurances were sequenced as follows: R. simsii, R. mariesii and R. fortunei. Greater heat resistance of R. simsii may be associated with stabilizing anatomical structure under high temperature. The results offered cytological evidence of adaptation for heat stress in heat resistant Rhododendron species.