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  • Author or Editor: Bing Zhao x
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The effects of different relative air humidity (RH) levels under high temperature and accompanying vapor pressure deficits (VPDs) on the physiology, photosynthesis, and anatomy of Rhododendron need to be better understood to help in reducing damage to leaves caused by high temperature. In this study, two Rhododendron cultivars were exposed at 45%, 55%, 65%, 75%, and 85% RH to a treatment of constant temperature at 38 °C for 14 days, resulting in a VPD of 3.64, 2.98, 2.32, 1.66, and 0.99 kPa, respectively. The results showed the least reduction of the net photosynthetic rate (Pn) under 75% RH treatment in R. ‘Fen Zhenzhu’ (decreased by 79.8%), and under the 85% RH treatment in R. ‘Zhuangyuan Hong’ (decreased by 75.4%). The decline in relative water content (RWC) was less under the 75% and 85% RH treatments, and electrolyte leakage showed a slight decrease under the 75% RH treatment in the two Rhododendron cultivars. The appearance of the two cultivars under greater RH showed less damage, probably because plants can avoid damage by increasing total chlorophyll content, decreasing stomatal area, stomatal density, and opened stomata ratio, enhancing enzymatic activity and osmoregulation substances, and improving leaf structure. The findings show that greater RH can alleviate damage caused by heat stress and improve thermostability.

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Rhododendrons (Rhododendron) are ornamental plants that exhibit poor thermotolerance. Salicylic acid (SA) and Ca2+ regulate the physiological and biochemical mechanisms in plants adapted to adverse environmental conditions. This study investigated the role of SA and CaCl2 in managing heat tolerance of Rhododendron ‘Fen Zhen Zhu’. Plants of the triennial Rhododendron ‘Fen Zhen Zhu’ were pretreated with SA and CaCl2, alone and combined. Following this pretreatment, the plants were subjected to 38/30 °C (day/night) incubation for 6 days, and then allowed to recover for 20 days under 25/17 °C (day/night) in a chamber. Changes in morphology were observed and recorded. Data were collected on plant chlorophyll content, malondialdehyde (MDA) content, H2O2 level, antioxidant enzyme activity, and total soluble protein content. The results revealed that the plant growth was considerably affected by heat stress, the leaves became brown and withered, and the plant defoliated. Under heat stress, chlorophyll content and total soluble protein levels decreased. Peroxidase (POD) activity and superoxide dismutase (SOD) also decreased, whereas the H2O2 and MDA content increased. Individual or combined application of SA and CaCl2 had a positive effect on plant growth, chlorophyll content, total soluble protein levels, and enzymatic antioxidant activity under heat stress. In general, the effect of the combined application of SA and CaCl2 was superior to individual application. In addition, treatment with high CaCl2 concentrations effectively alleviated the decrease in chlorophyll content. However, at low SA and CaCl2 concentrations, SOD and POD activity and total soluble protein accumulation increased whereas MDA and H2O2 levels decreased. These results suggest that SA and CaCl2 may interact to alleviate heat stress.

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