Wilted cucumber plants infected by Fusarium oxysporum f. sp. cucumerinum do not suffer from water shortage

Abstract

Background and aims: Fusarium wilt is primarily a soil-borne disease and results in yield loss and quality decline in cucumber (Cucumis sativus). The main symptom of fusarium wilt is the wilting of entire plant, which could be caused by a fungal toxin(s) or blockage of water transport. To investigate whether this wilt arises from water shortage, the physiological responses of hydroponically grown cucumber plants subjected to water stress using polyethylene glycol (PEG, 6000) were compared with those of plants infected with Fusarium oxysporum f. sp. cucumerinum (FOC).

Methods: Parameters reflecting plant water status were measured 8d after the start of treatment. Leaf gas exchange parameters and temperature were measured with a LI-COR portable open photosynthesis system and by thermal imaging. Chlorophyll fluorescence and chloroplast structures were assessed by imaging pulse amplitude-modulated fluorometry and transmission electron microscopy, respectively.

Key results: Cucumber water balance was altered after FOC infection, with decreased water absorption and hydraulic conductivity. However, the responses of cucumber leaves to FOC and PEG differed in leaf regions. Under water stress, measures of lipid peroxidation (malondialdehyde) and chlorophyll fluorescence indicated that the leaf edge was more seriously injured, with a higher leaf temperature and disrupted leaf water status compared with the centre. Here, abscisic acid (ABA) and proline were negatively correlated with water potential. In contrast, under FOC infection, membrane damage and a higher temperature were observed in the leaf centre while ABA and proline did not vary with water potential. Cytologically, FOC-infected cucumber leaves exhibited circular chloroplasts and swelled starch grains in the leaf centre, in which they again differed from PEG-stressed cucumber leaves.

Conclusions: This study illustrates the non-causal relationship between fusarium wilt and water transport blockage. Although leaf wilt occurred in both water stress and FOC infection, the physiological responses were different, especially in leaf spatial distribution.

Keywords: Cucumber (Cucumis sativus); chloroplast; fusarium wilt; injury; leaf regions; temperature; water balance.

Fig. 1.

Effect of water stress (WS) and FOC infection (PI) on relative leaf water loss per unit leaf area of cucumber plants during days (A) and nights (B). The relative water loss under stress was normalized through comparison with the water loss of unstressed control plant and represent the means of four replicates. Plants were supplied with half-strength Hoagland solution under the non-treated condition (CK, not shown), water stress was simulated by adding 2 % PEG (WS, solid circles) and FOC incubation was achieved by immersing the roots in a conidial suspension (10⁷ conidia mL⁻¹) (PI, open circles).

Fig. 2.

Effect of water stress (WS) and FOC infection (PI) on stem and root hydraulic conductivity of cucumber plants. Data represent means of four replicates; bars indicate the s.d. Significant differences (P<0·05) in each parameter between treatments are indicated by different letters. Plants were supplied with half-strength Hoagland solution under the non-treated condition (CK, open columns), water stress was simulated by adding 2 % PEG (hatched columns) and FOC incubation was achieved by immersing the roots in a conidial suspension (10⁷ conidial mL⁻¹) (black columns).

Fig. 3.

Effect of water stress (WS) and FOC infection (PI) on leaf temperature of cucumber seedlings. (A) Thermal images of cucumber leaves under light conditions after different treatments and durations of treatment. DAT: days after the start of treatment. (B) Continuous changes in temperature ratio between leaf centre and edge. (C) Thermal images of leaves under dark conditions 8d after the start of treatment. Environmental temperature was 28°C in the light and 32°C in the dark. Thermography was performed on one leaf per plant using four replicates per treatment. Plants were supplied with half-strength Hoagland solution under the non-treated condition (CK), water stress was simulated by adding 2 % PEG and FOC incubation was achieved by immersing the roots in a conidial suspension (10⁷ conidia mL⁻¹).

Fig. 4.

Effect of water stress (WS) and FOC infection (PI) on accumulation of proline (A), abscisic acid (ABA, B) and soluble sugars (C) in cucumber leaves. FW: fresh weight; DW: dry weight. Leaf samples were obtained by punching discs from the leaf centre and edge for four plants in each treatment 8 d after the start of treatment (for details see the Materials and methods section). Data represent means of four replicates; bars indicate the s.d. Significant differences (P<0·05) between treatments are indicated by different letters. The plants were supplied with half-strength Hoagland solution under the non-treated condition (CK), water stress was simulated by adding 2 % PEG and FOC incubation was achieved by immersing the roots in a conidial suspension (10⁷ conidia mL⁻¹).

Fig. 5.

(A) Relationship between water potential and proline content in cucumber leaves subjected to water stress (WS) and FOC infection (PI). FW: fresh weight. Lines represent linear regressions. The regression equation for WS treatment (solid line) is y=−26·67x+16·34, R²=0·487, P<0·05; that for PI treatment (dotted line) is y=0·521x+18·00, R²=0·000, P>0·05. (B) Relationship between leaf temperature and MDA content of leaves subjected to water stress and FOC infection. FW: fresh weight. Lines represent linear regressions; the regression equation is y=1·339x−23·98, R²=0·329, P<0·01.

Fig. 6.

(A) Changes in cucumber leaf appearance (left) and chloroplast fluorescence (right) in response to water stress (WS) and FOC infection (PI). Photographs of chloroplast fluorescence were obtained after a 20-min dark adaption period. (B) Effect of water stress and FOC infection on MDA content of cucumber leaves. FW: fresh weight. Leaves were photographed and sampled 8 d after the start of treatment. Data represent the means of four replicates, and the bars indicate the s.d. Significant differences (P<0·05) between treatments are indicated by different letters. Plants were supplied with half-strength Hoagland solution under the non-treated condition (CK), water stress was simulated by adding 2 % PEG and FOC incubation was achieved by immersing the roots in a conidial suspension (10⁷ conidia mL⁻¹).

Fig. 7.

Transmission electron micrographs of mesophyll cells at the centre and edge of cucumber leaves subjected to water stress (WS) or FOC infection (PI). (A) Chloroplast size and leaf structure, indicating chloroplast damage at the water-stressed leaf edge and FOC-infected leaf centre. Scale bars=2·0μm. (B) Granum structure in leaves was disordered at the edge of water-stressed leaves and the centre of FOC-infected leaves. Scale bars=500nm.