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Qing Xu, Shi-Rong Guo, He Li, Nan-Shan Du, Sheng Shu and Jin Sun

The use of grafted seedlings in vegetable crops has increased in recent years to enhance the resistance to biological and abiotic stresses, and improve yields. However, incompatibility restricts the wide application of grafting. In this study, two pumpkin (Cucurbita) cultivars, with great differences in grafting affinity and symbiotic affinity, were used as rootstocks and cucumber (Cucumis sativus) seedlings were used as the scion. The effects of compatibility or incompatibility on histological aspects, antioxidant enzyme activities, phenylpropanoid contents, and chlorophyll fluorescence were studied. The results showed that compatible graft combinations present a stronger resistance to the oxidative damage resulting from grafting and had relatively weak phenylpropanoid metabolisms. The results also indicated that the chlorophyll fluorescence levels of incompatible combinations were lower, except compared with the original fluorescence. Finally, a necrotic layer existed earlier in compatible graft combinations. These differences at the morphological, physiological, and cellular levels may govern compatibility and incompatibility, and may provide valuable information for determining the symbiotic affinity of grafted seedlings at an early stage.

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Jing Tian, Li-Ping Wang, Yan-Juan Yang, Jin Sun and Shi-Rong Guo

Heat tolerance is considered to be an essential feature for cucumber (Cucumis sativus) production, and it has been suggested that higher antioxidant ability could prevent the oxidative damage in plants caused by high-temperature stress. We aimed to investigate whether the application of exogenous spermidine (Spd) increases antioxidant activities and, therefore, elevates the heat tolerance of cucumber. Cucumber seedlings (cv. Jinchun No. 4) showing moderate heat tolerance were grown in climate chambers to investigate the effects of exogenous Spd (1 mm) foliar spray treatment on the activities and isozyme levels of antioxidative enzymes under both high-temperature stress 42/32 °C (day/night) and normal temperature 28/18 °C (day/night). On high-temperature stress, the activities of superoxide dismutase and ascorbate peroxidase were significantly reduced; the catalase activity was initially lower and then increased, whereas the peroxidase activity was initially higher and then decreased. The levels of these isozymes also changed differently. On treatment with exogenous Spd, the activities of these antioxidant enzymes were noticeably enhanced, and the isozyme zymogram expression had some changes. It was concluded that foliar spray with Spd effectively improved the total antioxidant ability of cucumber seedlings and, therefore, enhanced the tolerance of the plants to high-temperature stress.

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Ming Liu, Aijun Zhang, Xiaoguang Chen, Rong Jin, Hongmin Li and Zhonghou Tang

Potassium (K+) is an essential nutrient element for the growth and development of sweetpotato [Ipomoea batatas (L.) Lam.]. To investigate growth and physiological responses to K+ deficiency during early growth stage of sweetpotato, two representative cultivars with different tolerance to K+ deficiency were chosen. The seedlings of ‘Xushu 32’ (tolerance to K+ deficiency) and ‘Ningzishu 1’ (sensitive to K+ deficiency) were cultured in three different K+ concentrations (K0: 0 mmol·L−1 K+; K1: 5 mmol·L−1 K+; and K2: 20 mmol·L−1 K+, the control) of nutrient solution. Results showed that the extreme K+ deficiency (K0) significantly reduced the total dry weight, leaf number, root length, and chlorophyll content (CCI) compared with K2. However, the growth traits of ‘Xushu 32’ were less suppressed than those of ‘Ningzishu 1’. The net photosynthetic rate (P n), stomatal conductance (g S), and transpiration rate (T r) of ‘Ningzishu 1’ were significantly decreased in K0 and K1 (low K+), whereas ‘Xushu 32’ showed no significant change in K1 treatment. Increasing minimal fluorensence (F 0) of ‘Ningzishu 1’ comes with decreased maximum quantum efficiency of photosystem II (PSII) photochemistry (F v/F m) and photochemical quenching (q P) at K0 treatment. However, all the chlorophyll fluorescence parameters in ‘Xushu 32’ were nonsignificantly changed by K+ deficiency (K0 and K1). These results suggest that ‘Xushu 32’ could maintain a better growth state to adapt to K+ deficiency stress, which may be mainly because of a lighter affected photosynthesis and a less damaged PSII reaction center.