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Some physiological and growth responses of watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] grafted onto Lagenaria siceraria to flooding
Environmental and Experimental Botany 58 (2006) 1–8
Some physiological and growth responses of watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] grafted onto Lagenaria siceraria to flooding Halit Yetisir a,∗ , Mehmet E. C ¸ aliskan b , Soner Soylu c , Musa Sakar a a
Department of Horticulture, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey Department of Field Crops, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey Department of Plant Protection, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey b
c
Received 3 January 2005; accepted 15 June 2005
Abstract In this study, the effect of flooding on plant growth and photosynthetic activity of grafted watermelon were investigated. The watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] cv. ‘Crimson Tide’ was grafted onto Lagenaria siceraria SKP (Landrace). Grafted and ungrafted watermelon plants were flooded at the soil surface for 20 days. For every 5 days, three plants were sampled to determine plant fresh and dry weight, leaf number and main stem length. Leaf colour, single leaf CO2 exchange rate (CER), stomatal conductance (SC) and transpiration rate (Ts) were determined at 3 days interval. Flooding caused chlorosis on both grafted and ungrafted plants but such effect was more pronounced on ungrafted watermelon plants. CER, SC and Ts began to decrease from the 4th day of the flooding in both grafted and ungrafted plants as compared with non-flooded controls. However, grafted plants showed higher tolerance to flooding and had two-folds more CER, SC and Ts. Plant growth rate was also significantly lower in flooded plants than when compared to unflooded controls. Ungrafted plants had lower dry weight than grafted plants under flooding conditions. At the end of the experiment, decrease in fresh weight of plants was about 180% in ungrafted and 50% in grafted watermelons. Dry weight also decreased about 230% in ungrafted and 80% in grafted watermelons. Similar results were found in leaf number and main stem length. Adventitious roots and aerenchyma formation were observed in grafted watermelon but not in ungrafted watermelon under flooding. Adventitious root formation began from 3rd or 4th day of flooding and adventitious roots grew towards the soil surface. Flooding tolerance of watermelon could be improved by grafting onto L. siceraria. © 2005 Elsevier B.V. All rights reserved. Keywords: Flooding; Grafting; Lagenaria siceraria; Photosynthetic rate; Watermelon
1. Introduction Flooding and submergence are major abiotic stresses and are serious problems for the growth and yield of floodsensitive crops. Flooding conditions cause oxygen starvation, which arises from the slow diffusion of gases in water and oxygen consumption by microorganisms and plant roots. Flooded soil quickly becomes devoid of oxygen at depths below a few millimetres. In the floodwater itself, a broad unstirred boundary layer occurs around respiring tissue. This ∗
Corresponding author. Tel.: +90 326 2455845; fax: +90 326 2455832. E-mail addresses: [email protected] (H. Yetisir), [email protected] (M.E. C¸aliskan), [email protected] (S. Soylu). 0098-8472/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2005.06.010
formation can lead to tissue oxygen deficiency within a few hours. Respiring tissues such as root and rhizomes may die because of the oxygen deficiency and a rapid drop in energyrich adenylates causing a dramatic decrease in ion absorption and transport (Huang et al., 2003; Vartapetian et al., 2003). Water potential of flood-intolerant plants can decrease (Trought and Drew, 1980; Sav´e and Serrano, 1986; Smith and Ager, 1988; Liao and Lin, 1995) and leaf stomatal conductance can decrease, resulting in poor gas exchange rate (Andrews and Lorimer, 1987). Some plants have evolved a wide range of characteristic responses that appear to help withstand the effect of the stress. Several anatomical responses facilitate internal transport of oxygen by diffusion or sometimes by mass flow.
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This allows underground organs to avoid developing anaerobic interiors. Of particular importance is the development of aerenchyma, which can be described as gas-filled channels that can interconnect throughout much of the plant. This creates a low resistance network for the transport of gases from well-aerated aerial parts to organs engulfed by anaerobic surroundings (Aschi-Smiti et al., 2003; Colmer, 2003). Aerenchyma can be also developed in newly formed adventitious roots that emerge at the base of the shoot by many non-wetland herbaceous species in response to water logging or low oxygen concentration. Thus, these specialized roots are adapted anatomically to oxygen deficient media. It is widely assumed that aerenchyma cells functionally replaced the original root system, which is inhibited or killed by lack of oxygen (Jackson, 1955; Yu et al., 1969). Problems caused by flooding may be solved by growing flood-tolerant crops or grafting intolerant plants onto tolerant ones. This technique is widely used in fruit trees. Grafting has been performed in fruit-bearing vegetables against the soilborne disease and some other negative soil conditions since 1927 (Lee, 1994). Liao and Lin (1996) reported that bitter melon grafted onto Luffa had higher rubisco and photosynthetic activities than ungrafted bitter melon under flooding conditions. Watermelons are grafted in order to be protected from Fusarium wilt, to increase low soil temperature tolerance and to increase yield by enhancing water and plant nutrients uptake (Masuda et al., 1981; Jang, 1992; Heo, 1991; Oda, 1995). For these purposes, watermelons are grafted onto Cucurbita moschata, C. maxima, Benincasa hispida and Lagenaria siceraria. L. siceraria is a widely used species as rootstock for watermelon (Lee, 1994). Several studies on the effect of grafting on yield, quality and tolerance to some abiotic stresses such as low soil temperature and salinity have been conducted in watermelon (Balaz, 1982; Lee, 1994; Oda, 1995; Chouka and Jebari, 1999; Yetisir et al., 2003; Yetisir and Sari, 2003). To the best of our knowledge, the effect of flooding on grafted watermelon onto L. siceraria was not reported in previous studies. We observed that L. siceraria has a more vigorous root system and aerial parts than watermelon, and formed more adventitious roots. These observations led us to hypothesize that grafted watermelon plants would express greater biomass and photosynthetic activity than ungrafted watermelon under flooded conditions. To test this hypothesis, photosynthetic activity, growth rate and plant leaf colour change of grafted watermelon onto L. siceraria were investigated for different duration of flooding and compared with those of ungrafted watermelon of a similar period of flooding.
siceraria, SKP. Seeds of ‘Crimson Tide’ were sown on March 19, 2004 and seeds of rootstock (SKP) were sown on March 22, 2004 in peat and perlite 2:1 (v/v) mixture. The seeds were sown first in multipots, when seedlings reached the first true leaf stage (diameter of the leaf was about 2 cm) the grafting was performed. The hole insertion grafting technique was used and plants were grafted following the procedure described by Lee (1994) and Lee and Oda (2003). Seedlings were grown in an unheated greenhouse under plastic tunnel. After 20 days of grafting, surviving grafted plants and ungrafted plants were transplanted in 2 L pots filled with peat and perlite mixture. Growth medium was amended with 500 g/m3 of N, P2 O5 and K2 O. Grafted seedlings grown to the three to four true leaf stages were used for experiments. For flooding treatment, the potted grafted and ungrafted plants were submerged to the level of the soil surface for different periods of time (5, 10, 15 and 20 days) in three steel containers (2.5 m length, 1 m width and 0.3 m depth). For flooding, tap-water with EC = 0.5 dS/m and pH 7.00–7.40 was used. At the same time, grafted and ungrafted plants without flooding were also grown. Analysis and measurements of controls were carried out at the same time as with the treatments of various flooded plants. The experimental design was a completely randomized block design and each treatment was replicated three times with 15 plants in each replicate. 2.2. Measurement of leaf CO2 exchange rate, stomatal conductance and transpiration rate CO2 exchange rate (CER), stomatal conductance (SC) and transpiration rate (Ts) were collected with a portable photosynthesis analyzer (Model LCA-4, ADC Bioscientific Ltd., Hoddesdon, UK). Measurements were taken from the most recent, fully expanded terminal leaf of each plant. The measurements were conducted under full sunlight between the hours of 13:00 and 14:00 under clear sky conditions. Measurements was started on the first day of flooding then continued by 3 days interval for 15 days. 2.3. Biomass measurement For biomass measurements, three plants per replication were sampled at 0, 5, 10, 15 and 20 days after flooding. Total fresh weight, leaf number and main stem length were determined for each plant and means were calculated for each replicate. The plant material was dried at 68 ◦ C for 48 h and then weighted for dry weight. 2.4. Leaf colour measurement
2. Material and methods 2.1. Plant material and culture conditions The watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] cv. ‘Crimson Tide’ was grafted onto the landrace of L.
Plant leaf colour was measured as reflected in the CIELAB (L* a* b* ) colour space using a Minolta model CR-300 Colorimeter (Minolta, Osaka). Two readings (from young and old leaves) were performed from each plant by 2 days interval and continued for 14 days. L* represents lightness ranging
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from 0 = black to 100 = white. Chroma, C* represents colour saturation which varies from dull (low value) to vivid colour (high value) and was calculated by (a2 + b2 )1/2 . Hue angle h◦ represents colour wheel with red purple at an angle of 0◦ , yellow at 90◦ , bluish-green at 180◦ and blue 270◦ and was calculated by h◦ = tan−1 (b/a) (McGuire, 1992). 2.5. Tissue preparation for aerenchyma observation Light microscopic examination of samples was carried out using an Olympus microscope BX-50 (Tokyo, Japan), which was equipped with phase contrast. Stem tissues of watermelon from ungrafted plants and stem tissues of L. siceraria from grafted plant were detached after visual observations then decolorized in 100% methanol and cleared in chloral hydrate (2.5 g/mL) as described previously described (Shipton and Brown, 1962). The cleared and fixed stems tissues were sectioned with a sharp razor blade and sections of 0.1 mm thick were subsequently mounted in 50% glycerol on glass microscope slides for examination. 2.6. Statistical analysis Data were subjected to analysis of variance by SPSS (11.5 for windows) statistical program and means were compared by Tukey’s test at P ≤ 0.05 significance level. Correlation analysis was also done between the parameters obtained 15th day of flooding in SPSS program.
3. Results and discussions 3.1. Leaf gas exchange Flooding stress for 6 days significantly reduced CER, SC and Ts in both grafted and ungrafted watermelon plants (Figs. 1–3). However, there was no significant difference between the grafted and the ungrafted watermelon plants under unflooded conditions. After 6 days of flooding CER, SC and Ts continued to decrease in flooded plants and the differ-
Fig. 1. CO2 exchange rate of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
Fig. 2. Stomatal conductance of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
ence between the grafted and the ungrafted watermelon plants was significant. The CER of flooded–grafted plants was 38% that of the control after 15 days of flooding while the CER of flooded–ungrafted plants was 13.5% that of the control after 15 days of flooding. The SC of flooded–grafted plants was 33% that of the unflooded–ungrafted plants after 15 days of flooding whereas that SC of flooded–ungrafted plants was 14% that of unflooded–ungrafted plants after 15 days of flooding (Figs. 1 and 2). A similar trend with increasing duration of flooding observed for transpiration rate. Ts of flooded grafted plants was 42% that of the unflooded–ungrafted plants while Ts of flooded ungrafted plants was 14% of that of the unflooded–ungrafted plants after 15 days of flooding (Fig. 3). A decrease in leaf gas exchange parameters in flooded plants has been reported in a variety of species, including Momordica caharntia (Liao and Lin, 1994), Carya illinoensis (Smith and Ager, 1988), Lycopersicon esculentium (Bradford, 1983), Citrus spp. (Phung and Knipling, 1976), Pisum sativum (Jackson and Kowalewska, 1983) and Triticum aestivum (Trought and Drew, 1980). Stomatal closure in response to hypoxia (Jackson, 1990, 1991; Jackson and Drew, 1984; Zhang and Davies, 1987; Dat et al., 2004) and consequently, limitation of photosynthesis was reported (Liao and Lin, 1995; Smith and Ager, 1988; Vu and
Fig. 3. Transpiration rate of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Yelenosky, 1991). The results in the present study are in agreement with previous studies. 3.2. Leaf colour
Fig. 4. Change in leaf colour of grafted and ungrafted watermelon. (A) L values, (B) chroma values, (C) Hue values. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
L* values (lightness) were influenced by flooding. The plants under flooding conditions had higher L* values than non-flooded control plants. Ungrafted watermelon plants had the highest L* values while grafted plants showed intermediate values under flooding conditions. Leaf colour of flooded plants was lighter than unflooded plants (Fig. 5). Leaf colour changed from green (hue angle about 50◦ ) to yellow (hue angle about 60◦ ) in flooded plants while for unflooded plants (grafted and non-grafted) leaf colour remained green. Chlorosis in the leaves was more severe in ungrafted plants than in grafted plants under flooding conditions (Figs. 4B and 5). Chroma (C* ) values were also affected by both grafting and flooding. Ungrafted plants had the highest chroma values under flooded conditions. Grafted plants had more green leaves than ungrafted plants under flooded conditions but their leaves were light green as compared with unflooded–ungrafted plants. It has been reported that flooding modifies plant physiology and morphology and common symptoms of flooding are leaf chlorosis and wilting (Kawase, 1981; Kozlowski, 1985). Leaf greenness and duration of flooding was negatively correlated in Arabidopsis thaliana (Yetisir, 1997). Since nitrogen, magnesium and iron deficiency is highly correlated to chlorophyll content, responsible for leaf greenness, chlorosis under flooding conditions can be explained by deficiency of nitrogen and other elements active in chlorophyll synthesis (Schepers et al., 1992; Ma et al., 1995). Starch accumulation also caused yellow foliage colour in Brassica crops (Dauthery and Musgarave, 1994). 3.3. Plant growth
Yelenosky, 1991). Non-stomatal regulation of gas exchange rate is possible because of feedback-inhibition by starch accumulation in leaves due to the lack of transportation to roots under flooding conditions (Liao and Lin, 1994; Topa and Checseman, 1992; Wample and Davies, 1983; Vu and
Whole plant fresh weight of the all plants was significantly reduced by flooding. Reductions in plant fresh weight were more drastic in ungrafted plants under flooding while grafted watermelons onto L. siceraria were moderately
Fig. 5. Leaf colour of grafted (left) and ungrafted (right) plant after 15 days after flooding.
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Fig. 6. Total fresh weight of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
affected by flooding. Decreases in fresh weight as compared to unflooded control ranged from 76 (5th day) to 143% (20th day) in flooded–ungrafted watermelon. On the other hand, grafted watermelon plants showed smaller decrease (46–34%) in fresh weight under flooded conditions. During flooding, ungrafted watermelon produced about 10 g fresh weight while grafted watermelon onto L. siceraria produced 30 g fresh weight per plant (Fig. 6). Results of dry weight were presented in Fig. 7. Dry weight of the plants was also significantly influenced by flooding. The lowest dry weight was found in ungrafted watermelon plants in flooded conditions. The grafted watermelon plants had moderate dry weight values between unflooded–ungrafted plants and ungrafted flooded watermelons. Flooding caused dry weight to decrease from 35 (5th day) to 228% (20th day) in flooded–ungrafted watermelon as compared to unflooded–ungrafted plants. Grafted watermelon plants were less affected than ungrafted plants by flooding and decrease in dry weight ranged from 43 to 88% based on duration of flooding. The number of leaves per plant was also significantly influenced by flooding. The least number of leaves per plant was determined in ungrafted watermelon plants in flooding for all sampling times. A gradual decrease in leaf number was observed in flooded plants as compared to unflooded–ungrafted plants and leaf number of flooded–ungrafted plants was severely affected by flooding while a moderate decrease in leaf number was observed in the
Fig. 7. Total dry weight of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Fig. 8. Leaf number of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
flooded–grafted watermelon plants in flooding with as compare to unflooded–ungrafted plants. The plants grafted onto L. siceraria had a greater number of leaves than ungrafted watermelon plants under flooded conditions for all sampling time (Fig. 8). Significant difference was found between treatments as regard to main stem length for all sampling times. Grafted and ungrafted plants showed similar main stem length at the all sampling time under unflooded conditions. Main stem elongation was negatively affected by flooding. The flooded–ungrafted watermelon plants produced shorter main stem than the flooded–grafted ones. During flooding about a 14 cm elongation was determined in flooded–ungrafted watermelon whereas flooded–grafted watermelon plants elongated around 18 cm. In addition, branching was also observed in flooded–grafted watermelon plants (data not presented) but it was not observed in flooded–ungrafted watermelon plants (Figs. 9 and 10). Correlation analyses were conducted between the measured parameters. Some of the correlation coefficients obtained were significant. Total dry matter was positively correlated with CO2 exchange rate (CER) (r = 0.984), SC (r = 0.977) and Ts (r = 0.960), while negatively correlated with L* values (r = −0.975), C* (r = −0.948), h◦ (r = −0.964). There was significant negative correlation between CER, and L* values, C* and h◦ , correlation coefficients were r = −0.980, −0.975 and −0.977, respectively. As expected, CER was significantly correlated with SC (r = 0.996) and
Fig. 9. Main stem length grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Fig. 10. Superficial adventitious roots produced by flooded–grafted plants (A). (B) Shows root and shoot growth by grafted and ungrafted watermelon. UU: Unflooded–ungrafted, UG: unflooded–grafted, FG: flooded–grafted, FU: flooded–ungrafted.
Ts (r = 0.914). There was significant negative correlation between SC, and L* (r = −0.972), C* (r = −0.970) and h◦ (r = −0.963) (Table 1). There are numerous studies reporting reduction in dry matter accumulation in upland species by flooding. Leaf number was extremely sensitive to flooding and anoxia (Trought and Drew, 1980). Sojka et al. (1975) found an 83% decrease in leaf area of wheat after 25 days of root anoxia. Inhibition of stem elongation is almost always observed in waterlogged soil. In tomato, stem extension was inhibited by flooding until new adventitious roots emerge (Jackson and Campell, 1979). Flooded plants showed short stature, reduced shoot and root growth in Arabidopsis thaliana (Yetisir, 1997). The dry weight of flooded Ulmus armeniaca seedlings was
decreased to about 50% of the control plants (Newsome et al., 1982). In Betula papyrifera, flooded seedlings produced five times less dry weight than unflooded control plants (Tang and Kozlowski, 1982a). Flooding for 30 days dramatically reduced dry weight increment (50%) of Quercus macrocarpa seedlings (Tang and Kozlowski, 1982b), Eucalyptus camaldulensis and E. glabulus (Sena Gomes and Kozlowski, 1980a) and Fraxinus pennsylvanica seedlings (Sena Gomes and Kozlowski, 1980b). In this study, plant growth was reduced by flooding; reduction in plant growth can be seen on all measured parameters. Similarly, with studies cited above ungrafted watermelon was more affected than grafted watermelon plants. Adventitious roots and aerenchyma formation were observed in flooded–grafted plants but it was not observed in flooded–ungrafted watermelon plants. Flooding stress for 3 days induced adventitious root formation in grafted watermelon plants and adventitious roots grew towards the surface level of the floodwater (Fig. 10A). Flooded–grafted watermelon plants produced much more adventitious roots than flooded–ungrafted ones (Fig. 10B). Aerenchyma formation was not observed in unflooded–ungrafted, unflooded–grafted and flooded–ungrafted plants (Fig. 11A–C). Aerenchyma formation was found in flooded–grafted watermelon plants (Fig. 11D). Flooding is a major problem restricting the plant growth by leading to oxygen deficiency around roots and rhizomes, and consequently it can be fatal because aerobic respiration ceases and levels of energy-rich adenylates drop rapidly resulting in a dramatic decrease in ion uptake and transport (Huang et al., 2003; Vartapetian et al., 2003). When soil is saturated with water, gas diffusion is reduced. Consequently one of the main effects of flooding is a lower pool of available O2 in submerged plant parts. This decline in O2 is heightened by aerobic processes taking place in the root zone of plants. Accordingly anoxic conditions develop, leading to a reduction in ATP production and consequent decrease in root metabolism. The decline in available energy can subsequently reduce other active cellular processes such as nutrient uptake, osmotic adjustment or regulation of cytoplasmic pH regulation (Probert and Keating, 2000). CER also was significantly influenced by flooding and CER and dry matter production decreased in flooded plants. Adapted or tolerant plants to flooding conditions can compete with oxygen deficient condi-
Table 1 Correlation coefficients between observed characteristics in the experiment Variables
CER
L* value
Chroma
Hue
TDW CER L* value Chroma Hue SC
0.984**
−0.975**
−0.948**
−0.964**
−0.980**
−0.975** 0.988**
TDW, total dry weight; CER, CO2 exchange rate; SC, stomatal conductance; Ts, transpiration rate. ** Correlation is significant (P ≤ 0.01) n = 12.
−0.977** 0.995** 0.990**
SC 0.977** 0.996** −0.972** −0.970** −0.963**
Ts 0.960** 0.914** −0.899** −0.873** −0.907** 0.913**
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Fig. 11. Aerenchyma formation around phloem tubes in plant tissue of watermelon and L. siceraraia. No aerenchyma formation observed around phloem tubes in ungrafted (A) and grafted (B) plants under unflooding conditions. (C) Shows no aerenchyma formation observed around phloem tubes in flooded–ungrafted plant. (D) Shows typical aerenchyma formations (arrows) in flooded–grafted plant. UU: Unflooded–ungrafted, UG: unflooded–grafted, FG: flooded–grafted, FU: flooded–ungrafted. PT: Phloem tubes. Bars = 50 m.
tion by developing some organs such as adventitious roots and aerenchyma which connect oxygen rich medium with oxygen deficient medium (Aschi-Smiti et al., 2003; Colmer, 2003). Interestingly, Glaz et al. (2004) found that 7-days flooding did not affect or moderately enhanced CER, SC and Ts in sugarcane. Authors attributed this response to the presence of natural aerenchyma in sugarcane. Similarly to flooding tolerant plants, grafted watermelon onto L. siceraria produced adventitious root and aerenchyma to adapt to flooding conditions.
reduction in Ps. These findings suggested that grafted watermelon seedlings may be used in those areas where anoxic conditions occur due to excessive water in soil. However, our findings present limited physiological information regarding the positive effects on the flooding tolerance, and further works needs to be conducted to provide comprehensive data addressing the impact of flooding on yield and quality of watermelon.
References 4. Conclusion Flooding tolerance of flooding-sensitive watermelon plants can be improved by grafting watermelon onto a flood tolerant L. siceraria. CER, SC, Ts and plant dry matter accumulation were negatively affected by flooding in both grafted and ungrafted watermelon plants when compared with nonflooded controls; however, the reduction in these parameters were much less in grafted plants than ungrafted plants under flooding. Leaf colour was also affected by flooding, and the grafted plants onto L. siceraria remained greener under flooding compared with ungrafted watermelon. The reduction in dry matter appeared to be associated with stomatal closure and chlorophyll degradation which both caused
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McGuire, R.G., 1992. Reporting of objective colour measurement. HortScience 27, 1254–1255. Newsome, R.D., Kozlowski, T.T., Tang, Z.C., 1982. Response of Ulmus Americana seedlings to flooding of soil. Can. J. Bot. 60, 1688– 1695. Oda, M., 1995. New grafting methods for fruit-bearing vegetables in Japan. JARQ 29, 187–198. Phung, H.T., Knipling, E.B., 1976. Photosynthesis and transpiration of Cirtus seedling under flooded conditions. HortScience 11, 131–133. Probert, M.E., Keating, B.A., 2000. What soil constraints should be included in crops and forest models? Agric. Ecosyst. Environ. 82, 273–281. Sav´e, R., Serrano, L., 1986. Some physiological and growth response of kiwi fruit (Actinidia chinensis) to flooding. Physiol. Plant 66, 75–78. Schepers, J.S., Francis, D.D., Vigil, M.F., Below, J.D., 1992. Comparison of corn leaf nitrogen concentration and chlorophyll meters readings. Common Soil Sci. Plant Anl. 23, 2173–2187. Sena Gomes, A.R., Kozlowski, T.T., 1980a. Growth response and adaptation of Fraxinus pannsilvanica seedlings to flooding. Plant Physiol. 66, 267–271. Sena Gomes, A.R., Kozlowski, T.T., 1980b. Effects of flooding on growth of Eucalyptus camaldulansis and E. globules seedlings. Oecologia 46, 139–142. Shipton, W.A., Brown, J.F., 1962. A whole-leaf clearing and staining technique to demonstrate host–pathogen relationships of wheat stem rust. Phytopathology 52, 1313–1315. Smith, M.W., Ager, P.L., 1988. Effect of soil flooding on leaf gas exchange of seedling pecan trees. HortScience 23, 370–372. Sojka, R.E., Stolzy, L.H., Kaufman, M.R., 1975. Wheat growth related to rhizosphere temperature and oxygen levels. Agronomy J. 67, 591–596. Tang, Z.C., Kozlowski, T.T., 1982a. Some physiological and growth response of Quercus macrocarpa seedlings to flooding. Can. J. For. Res. 10, 308–311. Tang, Z.C., Kozlowski, T.T., 1982b. Some physiological and growth response of Betula papyrifera seedlings to flooding. Physiol. Plant 55, 415–420. Topa, M.A., Checseman, J.M., 1992. Effect of root hypoxia and low P supply on relative growth, carbon dioxide exchange rates and carbon partitioning in Pinus serotina seedlings. Physiol. Plant 86, 136–144. Trought, M.C.T., Drew, M.C., 1980. The development of waterlogging damage in wheat seedlings (Triticum aestivum L.). Part 1. Shoot and root growth in relation to change in the concentration of dissolved gasses and solute in the soil solution. Plant Soil 54, 77–94. Vartapetian, B.B., Andreeva, I.N., Generozova, I.P., Polyakova, L.I., Maslova, I.P., Dolgikh, Y.I., Stepanova, A.Y.U., 2003. Functional electron microscopy in studies of plant response and adaptation to anaerobic stress. Ann. Bot. 91, 155–172. Vu, J.C.V., Yelenosky, G., 1991. Photosynthetic reponses of Citrus trees to soil flooding. Physiol. Plant 81, 7–14. Wample, R.L., Davies, R.W., 1983. Effect of flooding on starch accumulation in chloroplast of sunflower (Helianthus annuus L.). Plant Phsyiol. 73, 195–198. Yetisir, H., 1997. Molecular responses of Arabidopsis thaliana to flooding. MS thesis. The Ohio State University Graduate School, 80 pp. Yetisir, H., Sari, N., Y¨ucel, S., 2003. Rootstock resistance to Fusarium wilt and effect on watermelon fruit yield and quality. Phytoparasitica 31, 163–169. Yetisir, H., Sari, N., 2003. Effect of different rootstock on plant growth, yield and quality of watermelon. Aust. J. Exp. Agric. 43, 1269–1274. Yu, P.T., Stolzy, L.H., Letey, J., 1969. Survival of plants under prolonged flooded conditions. Agronomy J. 61, 844–847. Zhang, J., Davies, W.J., 1987. ABA in roots and leaves of flooded pea plants. J. Exp. Bot. 38, 649–659.
Some physiological and growth responses of watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] grafted onto Lagenaria siceraria to flooding Halit Yetisir a,∗ , Mehmet E. C ¸ aliskan b , Soner Soylu c , Musa Sakar a a
Department of Horticulture, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey Department of Field Crops, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey Department of Plant Protection, Faculty of Agriculture, University of Mustafa Kemal, 31120 Hatay, Turkey b
c
Received 3 January 2005; accepted 15 June 2005
Abstract In this study, the effect of flooding on plant growth and photosynthetic activity of grafted watermelon were investigated. The watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] cv. ‘Crimson Tide’ was grafted onto Lagenaria siceraria SKP (Landrace). Grafted and ungrafted watermelon plants were flooded at the soil surface for 20 days. For every 5 days, three plants were sampled to determine plant fresh and dry weight, leaf number and main stem length. Leaf colour, single leaf CO2 exchange rate (CER), stomatal conductance (SC) and transpiration rate (Ts) were determined at 3 days interval. Flooding caused chlorosis on both grafted and ungrafted plants but such effect was more pronounced on ungrafted watermelon plants. CER, SC and Ts began to decrease from the 4th day of the flooding in both grafted and ungrafted plants as compared with non-flooded controls. However, grafted plants showed higher tolerance to flooding and had two-folds more CER, SC and Ts. Plant growth rate was also significantly lower in flooded plants than when compared to unflooded controls. Ungrafted plants had lower dry weight than grafted plants under flooding conditions. At the end of the experiment, decrease in fresh weight of plants was about 180% in ungrafted and 50% in grafted watermelons. Dry weight also decreased about 230% in ungrafted and 80% in grafted watermelons. Similar results were found in leaf number and main stem length. Adventitious roots and aerenchyma formation were observed in grafted watermelon but not in ungrafted watermelon under flooding. Adventitious root formation began from 3rd or 4th day of flooding and adventitious roots grew towards the soil surface. Flooding tolerance of watermelon could be improved by grafting onto L. siceraria. © 2005 Elsevier B.V. All rights reserved. Keywords: Flooding; Grafting; Lagenaria siceraria; Photosynthetic rate; Watermelon
1. Introduction Flooding and submergence are major abiotic stresses and are serious problems for the growth and yield of floodsensitive crops. Flooding conditions cause oxygen starvation, which arises from the slow diffusion of gases in water and oxygen consumption by microorganisms and plant roots. Flooded soil quickly becomes devoid of oxygen at depths below a few millimetres. In the floodwater itself, a broad unstirred boundary layer occurs around respiring tissue. This ∗
Corresponding author. Tel.: +90 326 2455845; fax: +90 326 2455832. E-mail addresses: [email protected] (H. Yetisir), [email protected] (M.E. C¸aliskan), [email protected] (S. Soylu). 0098-8472/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2005.06.010
formation can lead to tissue oxygen deficiency within a few hours. Respiring tissues such as root and rhizomes may die because of the oxygen deficiency and a rapid drop in energyrich adenylates causing a dramatic decrease in ion absorption and transport (Huang et al., 2003; Vartapetian et al., 2003). Water potential of flood-intolerant plants can decrease (Trought and Drew, 1980; Sav´e and Serrano, 1986; Smith and Ager, 1988; Liao and Lin, 1995) and leaf stomatal conductance can decrease, resulting in poor gas exchange rate (Andrews and Lorimer, 1987). Some plants have evolved a wide range of characteristic responses that appear to help withstand the effect of the stress. Several anatomical responses facilitate internal transport of oxygen by diffusion or sometimes by mass flow.
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This allows underground organs to avoid developing anaerobic interiors. Of particular importance is the development of aerenchyma, which can be described as gas-filled channels that can interconnect throughout much of the plant. This creates a low resistance network for the transport of gases from well-aerated aerial parts to organs engulfed by anaerobic surroundings (Aschi-Smiti et al., 2003; Colmer, 2003). Aerenchyma can be also developed in newly formed adventitious roots that emerge at the base of the shoot by many non-wetland herbaceous species in response to water logging or low oxygen concentration. Thus, these specialized roots are adapted anatomically to oxygen deficient media. It is widely assumed that aerenchyma cells functionally replaced the original root system, which is inhibited or killed by lack of oxygen (Jackson, 1955; Yu et al., 1969). Problems caused by flooding may be solved by growing flood-tolerant crops or grafting intolerant plants onto tolerant ones. This technique is widely used in fruit trees. Grafting has been performed in fruit-bearing vegetables against the soilborne disease and some other negative soil conditions since 1927 (Lee, 1994). Liao and Lin (1996) reported that bitter melon grafted onto Luffa had higher rubisco and photosynthetic activities than ungrafted bitter melon under flooding conditions. Watermelons are grafted in order to be protected from Fusarium wilt, to increase low soil temperature tolerance and to increase yield by enhancing water and plant nutrients uptake (Masuda et al., 1981; Jang, 1992; Heo, 1991; Oda, 1995). For these purposes, watermelons are grafted onto Cucurbita moschata, C. maxima, Benincasa hispida and Lagenaria siceraria. L. siceraria is a widely used species as rootstock for watermelon (Lee, 1994). Several studies on the effect of grafting on yield, quality and tolerance to some abiotic stresses such as low soil temperature and salinity have been conducted in watermelon (Balaz, 1982; Lee, 1994; Oda, 1995; Chouka and Jebari, 1999; Yetisir et al., 2003; Yetisir and Sari, 2003). To the best of our knowledge, the effect of flooding on grafted watermelon onto L. siceraria was not reported in previous studies. We observed that L. siceraria has a more vigorous root system and aerial parts than watermelon, and formed more adventitious roots. These observations led us to hypothesize that grafted watermelon plants would express greater biomass and photosynthetic activity than ungrafted watermelon under flooded conditions. To test this hypothesis, photosynthetic activity, growth rate and plant leaf colour change of grafted watermelon onto L. siceraria were investigated for different duration of flooding and compared with those of ungrafted watermelon of a similar period of flooding.
siceraria, SKP. Seeds of ‘Crimson Tide’ were sown on March 19, 2004 and seeds of rootstock (SKP) were sown on March 22, 2004 in peat and perlite 2:1 (v/v) mixture. The seeds were sown first in multipots, when seedlings reached the first true leaf stage (diameter of the leaf was about 2 cm) the grafting was performed. The hole insertion grafting technique was used and plants were grafted following the procedure described by Lee (1994) and Lee and Oda (2003). Seedlings were grown in an unheated greenhouse under plastic tunnel. After 20 days of grafting, surviving grafted plants and ungrafted plants were transplanted in 2 L pots filled with peat and perlite mixture. Growth medium was amended with 500 g/m3 of N, P2 O5 and K2 O. Grafted seedlings grown to the three to four true leaf stages were used for experiments. For flooding treatment, the potted grafted and ungrafted plants were submerged to the level of the soil surface for different periods of time (5, 10, 15 and 20 days) in three steel containers (2.5 m length, 1 m width and 0.3 m depth). For flooding, tap-water with EC = 0.5 dS/m and pH 7.00–7.40 was used. At the same time, grafted and ungrafted plants without flooding were also grown. Analysis and measurements of controls were carried out at the same time as with the treatments of various flooded plants. The experimental design was a completely randomized block design and each treatment was replicated three times with 15 plants in each replicate. 2.2. Measurement of leaf CO2 exchange rate, stomatal conductance and transpiration rate CO2 exchange rate (CER), stomatal conductance (SC) and transpiration rate (Ts) were collected with a portable photosynthesis analyzer (Model LCA-4, ADC Bioscientific Ltd., Hoddesdon, UK). Measurements were taken from the most recent, fully expanded terminal leaf of each plant. The measurements were conducted under full sunlight between the hours of 13:00 and 14:00 under clear sky conditions. Measurements was started on the first day of flooding then continued by 3 days interval for 15 days. 2.3. Biomass measurement For biomass measurements, three plants per replication were sampled at 0, 5, 10, 15 and 20 days after flooding. Total fresh weight, leaf number and main stem length were determined for each plant and means were calculated for each replicate. The plant material was dried at 68 ◦ C for 48 h and then weighted for dry weight. 2.4. Leaf colour measurement
2. Material and methods 2.1. Plant material and culture conditions The watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] cv. ‘Crimson Tide’ was grafted onto the landrace of L.
Plant leaf colour was measured as reflected in the CIELAB (L* a* b* ) colour space using a Minolta model CR-300 Colorimeter (Minolta, Osaka). Two readings (from young and old leaves) were performed from each plant by 2 days interval and continued for 14 days. L* represents lightness ranging
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from 0 = black to 100 = white. Chroma, C* represents colour saturation which varies from dull (low value) to vivid colour (high value) and was calculated by (a2 + b2 )1/2 . Hue angle h◦ represents colour wheel with red purple at an angle of 0◦ , yellow at 90◦ , bluish-green at 180◦ and blue 270◦ and was calculated by h◦ = tan−1 (b/a) (McGuire, 1992). 2.5. Tissue preparation for aerenchyma observation Light microscopic examination of samples was carried out using an Olympus microscope BX-50 (Tokyo, Japan), which was equipped with phase contrast. Stem tissues of watermelon from ungrafted plants and stem tissues of L. siceraria from grafted plant were detached after visual observations then decolorized in 100% methanol and cleared in chloral hydrate (2.5 g/mL) as described previously described (Shipton and Brown, 1962). The cleared and fixed stems tissues were sectioned with a sharp razor blade and sections of 0.1 mm thick were subsequently mounted in 50% glycerol on glass microscope slides for examination. 2.6. Statistical analysis Data were subjected to analysis of variance by SPSS (11.5 for windows) statistical program and means were compared by Tukey’s test at P ≤ 0.05 significance level. Correlation analysis was also done between the parameters obtained 15th day of flooding in SPSS program.
3. Results and discussions 3.1. Leaf gas exchange Flooding stress for 6 days significantly reduced CER, SC and Ts in both grafted and ungrafted watermelon plants (Figs. 1–3). However, there was no significant difference between the grafted and the ungrafted watermelon plants under unflooded conditions. After 6 days of flooding CER, SC and Ts continued to decrease in flooded plants and the differ-
Fig. 1. CO2 exchange rate of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
Fig. 2. Stomatal conductance of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
ence between the grafted and the ungrafted watermelon plants was significant. The CER of flooded–grafted plants was 38% that of the control after 15 days of flooding while the CER of flooded–ungrafted plants was 13.5% that of the control after 15 days of flooding. The SC of flooded–grafted plants was 33% that of the unflooded–ungrafted plants after 15 days of flooding whereas that SC of flooded–ungrafted plants was 14% that of unflooded–ungrafted plants after 15 days of flooding (Figs. 1 and 2). A similar trend with increasing duration of flooding observed for transpiration rate. Ts of flooded grafted plants was 42% that of the unflooded–ungrafted plants while Ts of flooded ungrafted plants was 14% of that of the unflooded–ungrafted plants after 15 days of flooding (Fig. 3). A decrease in leaf gas exchange parameters in flooded plants has been reported in a variety of species, including Momordica caharntia (Liao and Lin, 1994), Carya illinoensis (Smith and Ager, 1988), Lycopersicon esculentium (Bradford, 1983), Citrus spp. (Phung and Knipling, 1976), Pisum sativum (Jackson and Kowalewska, 1983) and Triticum aestivum (Trought and Drew, 1980). Stomatal closure in response to hypoxia (Jackson, 1990, 1991; Jackson and Drew, 1984; Zhang and Davies, 1987; Dat et al., 2004) and consequently, limitation of photosynthesis was reported (Liao and Lin, 1995; Smith and Ager, 1988; Vu and
Fig. 3. Transpiration rate of mature leaves of ungrafted and grafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded– ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Yelenosky, 1991). The results in the present study are in agreement with previous studies. 3.2. Leaf colour
Fig. 4. Change in leaf colour of grafted and ungrafted watermelon. (A) L values, (B) chroma values, (C) Hue values. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
L* values (lightness) were influenced by flooding. The plants under flooding conditions had higher L* values than non-flooded control plants. Ungrafted watermelon plants had the highest L* values while grafted plants showed intermediate values under flooding conditions. Leaf colour of flooded plants was lighter than unflooded plants (Fig. 5). Leaf colour changed from green (hue angle about 50◦ ) to yellow (hue angle about 60◦ ) in flooded plants while for unflooded plants (grafted and non-grafted) leaf colour remained green. Chlorosis in the leaves was more severe in ungrafted plants than in grafted plants under flooding conditions (Figs. 4B and 5). Chroma (C* ) values were also affected by both grafting and flooding. Ungrafted plants had the highest chroma values under flooded conditions. Grafted plants had more green leaves than ungrafted plants under flooded conditions but their leaves were light green as compared with unflooded–ungrafted plants. It has been reported that flooding modifies plant physiology and morphology and common symptoms of flooding are leaf chlorosis and wilting (Kawase, 1981; Kozlowski, 1985). Leaf greenness and duration of flooding was negatively correlated in Arabidopsis thaliana (Yetisir, 1997). Since nitrogen, magnesium and iron deficiency is highly correlated to chlorophyll content, responsible for leaf greenness, chlorosis under flooding conditions can be explained by deficiency of nitrogen and other elements active in chlorophyll synthesis (Schepers et al., 1992; Ma et al., 1995). Starch accumulation also caused yellow foliage colour in Brassica crops (Dauthery and Musgarave, 1994). 3.3. Plant growth
Yelenosky, 1991). Non-stomatal regulation of gas exchange rate is possible because of feedback-inhibition by starch accumulation in leaves due to the lack of transportation to roots under flooding conditions (Liao and Lin, 1994; Topa and Checseman, 1992; Wample and Davies, 1983; Vu and
Whole plant fresh weight of the all plants was significantly reduced by flooding. Reductions in plant fresh weight were more drastic in ungrafted plants under flooding while grafted watermelons onto L. siceraria were moderately
Fig. 5. Leaf colour of grafted (left) and ungrafted (right) plant after 15 days after flooding.
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Fig. 6. Total fresh weight of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
affected by flooding. Decreases in fresh weight as compared to unflooded control ranged from 76 (5th day) to 143% (20th day) in flooded–ungrafted watermelon. On the other hand, grafted watermelon plants showed smaller decrease (46–34%) in fresh weight under flooded conditions. During flooding, ungrafted watermelon produced about 10 g fresh weight while grafted watermelon onto L. siceraria produced 30 g fresh weight per plant (Fig. 6). Results of dry weight were presented in Fig. 7. Dry weight of the plants was also significantly influenced by flooding. The lowest dry weight was found in ungrafted watermelon plants in flooded conditions. The grafted watermelon plants had moderate dry weight values between unflooded–ungrafted plants and ungrafted flooded watermelons. Flooding caused dry weight to decrease from 35 (5th day) to 228% (20th day) in flooded–ungrafted watermelon as compared to unflooded–ungrafted plants. Grafted watermelon plants were less affected than ungrafted plants by flooding and decrease in dry weight ranged from 43 to 88% based on duration of flooding. The number of leaves per plant was also significantly influenced by flooding. The least number of leaves per plant was determined in ungrafted watermelon plants in flooding for all sampling times. A gradual decrease in leaf number was observed in flooded plants as compared to unflooded–ungrafted plants and leaf number of flooded–ungrafted plants was severely affected by flooding while a moderate decrease in leaf number was observed in the
Fig. 7. Total dry weight of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Fig. 8. Leaf number of grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
flooded–grafted watermelon plants in flooding with as compare to unflooded–ungrafted plants. The plants grafted onto L. siceraria had a greater number of leaves than ungrafted watermelon plants under flooded conditions for all sampling time (Fig. 8). Significant difference was found between treatments as regard to main stem length for all sampling times. Grafted and ungrafted plants showed similar main stem length at the all sampling time under unflooded conditions. Main stem elongation was negatively affected by flooding. The flooded–ungrafted watermelon plants produced shorter main stem than the flooded–grafted ones. During flooding about a 14 cm elongation was determined in flooded–ungrafted watermelon whereas flooded–grafted watermelon plants elongated around 18 cm. In addition, branching was also observed in flooded–grafted watermelon plants (data not presented) but it was not observed in flooded–ungrafted watermelon plants (Figs. 9 and 10). Correlation analyses were conducted between the measured parameters. Some of the correlation coefficients obtained were significant. Total dry matter was positively correlated with CO2 exchange rate (CER) (r = 0.984), SC (r = 0.977) and Ts (r = 0.960), while negatively correlated with L* values (r = −0.975), C* (r = −0.948), h◦ (r = −0.964). There was significant negative correlation between CER, and L* values, C* and h◦ , correlation coefficients were r = −0.980, −0.975 and −0.977, respectively. As expected, CER was significantly correlated with SC (r = 0.996) and
Fig. 9. Main stem length grafted and ungrafted watermelon. FU: Flooded–ungrafted, FG: flooded–grafted, UU: unflooded–ungrafted, UG: unflooded–grafted. Bars represent standard error (n = 9).
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Fig. 10. Superficial adventitious roots produced by flooded–grafted plants (A). (B) Shows root and shoot growth by grafted and ungrafted watermelon. UU: Unflooded–ungrafted, UG: unflooded–grafted, FG: flooded–grafted, FU: flooded–ungrafted.
Ts (r = 0.914). There was significant negative correlation between SC, and L* (r = −0.972), C* (r = −0.970) and h◦ (r = −0.963) (Table 1). There are numerous studies reporting reduction in dry matter accumulation in upland species by flooding. Leaf number was extremely sensitive to flooding and anoxia (Trought and Drew, 1980). Sojka et al. (1975) found an 83% decrease in leaf area of wheat after 25 days of root anoxia. Inhibition of stem elongation is almost always observed in waterlogged soil. In tomato, stem extension was inhibited by flooding until new adventitious roots emerge (Jackson and Campell, 1979). Flooded plants showed short stature, reduced shoot and root growth in Arabidopsis thaliana (Yetisir, 1997). The dry weight of flooded Ulmus armeniaca seedlings was
decreased to about 50% of the control plants (Newsome et al., 1982). In Betula papyrifera, flooded seedlings produced five times less dry weight than unflooded control plants (Tang and Kozlowski, 1982a). Flooding for 30 days dramatically reduced dry weight increment (50%) of Quercus macrocarpa seedlings (Tang and Kozlowski, 1982b), Eucalyptus camaldulensis and E. glabulus (Sena Gomes and Kozlowski, 1980a) and Fraxinus pennsylvanica seedlings (Sena Gomes and Kozlowski, 1980b). In this study, plant growth was reduced by flooding; reduction in plant growth can be seen on all measured parameters. Similarly, with studies cited above ungrafted watermelon was more affected than grafted watermelon plants. Adventitious roots and aerenchyma formation were observed in flooded–grafted plants but it was not observed in flooded–ungrafted watermelon plants. Flooding stress for 3 days induced adventitious root formation in grafted watermelon plants and adventitious roots grew towards the surface level of the floodwater (Fig. 10A). Flooded–grafted watermelon plants produced much more adventitious roots than flooded–ungrafted ones (Fig. 10B). Aerenchyma formation was not observed in unflooded–ungrafted, unflooded–grafted and flooded–ungrafted plants (Fig. 11A–C). Aerenchyma formation was found in flooded–grafted watermelon plants (Fig. 11D). Flooding is a major problem restricting the plant growth by leading to oxygen deficiency around roots and rhizomes, and consequently it can be fatal because aerobic respiration ceases and levels of energy-rich adenylates drop rapidly resulting in a dramatic decrease in ion uptake and transport (Huang et al., 2003; Vartapetian et al., 2003). When soil is saturated with water, gas diffusion is reduced. Consequently one of the main effects of flooding is a lower pool of available O2 in submerged plant parts. This decline in O2 is heightened by aerobic processes taking place in the root zone of plants. Accordingly anoxic conditions develop, leading to a reduction in ATP production and consequent decrease in root metabolism. The decline in available energy can subsequently reduce other active cellular processes such as nutrient uptake, osmotic adjustment or regulation of cytoplasmic pH regulation (Probert and Keating, 2000). CER also was significantly influenced by flooding and CER and dry matter production decreased in flooded plants. Adapted or tolerant plants to flooding conditions can compete with oxygen deficient condi-
Table 1 Correlation coefficients between observed characteristics in the experiment Variables
CER
L* value
Chroma
Hue
TDW CER L* value Chroma Hue SC
0.984**
−0.975**
−0.948**
−0.964**
−0.980**
−0.975** 0.988**
TDW, total dry weight; CER, CO2 exchange rate; SC, stomatal conductance; Ts, transpiration rate. ** Correlation is significant (P ≤ 0.01) n = 12.
−0.977** 0.995** 0.990**
SC 0.977** 0.996** −0.972** −0.970** −0.963**
Ts 0.960** 0.914** −0.899** −0.873** −0.907** 0.913**
H. Yetisir et al. / Environmental and Experimental Botany 58 (2006) 1–8
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Fig. 11. Aerenchyma formation around phloem tubes in plant tissue of watermelon and L. siceraraia. No aerenchyma formation observed around phloem tubes in ungrafted (A) and grafted (B) plants under unflooding conditions. (C) Shows no aerenchyma formation observed around phloem tubes in flooded–ungrafted plant. (D) Shows typical aerenchyma formations (arrows) in flooded–grafted plant. UU: Unflooded–ungrafted, UG: unflooded–grafted, FG: flooded–grafted, FU: flooded–ungrafted. PT: Phloem tubes. Bars = 50 m.
tion by developing some organs such as adventitious roots and aerenchyma which connect oxygen rich medium with oxygen deficient medium (Aschi-Smiti et al., 2003; Colmer, 2003). Interestingly, Glaz et al. (2004) found that 7-days flooding did not affect or moderately enhanced CER, SC and Ts in sugarcane. Authors attributed this response to the presence of natural aerenchyma in sugarcane. Similarly to flooding tolerant plants, grafted watermelon onto L. siceraria produced adventitious root and aerenchyma to adapt to flooding conditions.
reduction in Ps. These findings suggested that grafted watermelon seedlings may be used in those areas where anoxic conditions occur due to excessive water in soil. However, our findings present limited physiological information regarding the positive effects on the flooding tolerance, and further works needs to be conducted to provide comprehensive data addressing the impact of flooding on yield and quality of watermelon.
References 4. Conclusion Flooding tolerance of flooding-sensitive watermelon plants can be improved by grafting watermelon onto a flood tolerant L. siceraria. CER, SC, Ts and plant dry matter accumulation were negatively affected by flooding in both grafted and ungrafted watermelon plants when compared with nonflooded controls; however, the reduction in these parameters were much less in grafted plants than ungrafted plants under flooding. Leaf colour was also affected by flooding, and the grafted plants onto L. siceraria remained greener under flooding compared with ungrafted watermelon. The reduction in dry matter appeared to be associated with stomatal closure and chlorophyll degradation which both caused
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