Abstract
Changes in leaf length, width, area, weight, chlorophyll and carotenoids contents, and photosynthetic variables with different leaf positions were investigated in fruit cucumber. Plants were grown on rockwool slabs in an environmentally controlled greenhouse and irrigated by drip fertigation. Leaf measurements were conducted from the first to the 15th leaf (the oldest to the youngest). The results showed that fresh weight per unit leaf area decreased from the second to the 15th leaf. Changes in cucumber leaf length, width, and area followed quadratic models from the first to the 15th leaf. The quadratic models of leaf length, width, and area fit the measurements well, with R2 values of 0.925, 0.951, and 0.955, respectively. The leaf chlorophyll a and b and carotenoid contents increased from the oldest leaf (first leaf) to the youngest leaf and decreased after reaching the highest values. Changes in the net photosynthetic rate (Pn) also followed the quadratic model from the first to the 15th leaf, with R2 values of 0.975. The leaf transpiration rate (Tr) increased from the first to the 14th leaf. Our results revealed patterns in leaf growth and photosynthetic changes at different leaf positions in fruit cucumber and improved our understanding of the growth and development of fruit cucumber in the greenhouse production system.
Cucumber (Cucumis sativus) is among the most widely grown vegetables in the world and is native to the Indian subcontinent (Sebastian et al., 2010). It spread eastward to China ≈2000 years ago and westward to Europe 700 to 1500 years ago (Weng et al., 2015). Fruit cucumber is the fresh-eating type of cucumber that was recently introduced from Europe to China. Fruit cucumber is popular with consumers due to its crisp and tender taste and because it is convenient to eat.
A well-developed canopy with a high photosynthetic rate is the foundation of biomass accumulation and crop productivity. Recently, a few growth models, using leaf number, photosynthetically active radiation or temperature as variables have been established to predict the canopy growth of greenhouse crops (Fan et al., 2015; Zúñiga, 2014). In previous studies, models of leaf area, leaf number, growing degree days, and dry matter distribution (Gijzen et al., 1998; Kahlen, 2006; Marcelis et al., 1992, 1998; Uzun, 2006) were developed. Xu et al. (2010) integrated some of those parameters and demonstrated the precise prediction of cucumber and pepper growth in a greenhouse (Xu et al., 2010). All of the crop model studies mainly focus on increasing productivity along with quality and enriching crop management practices.
In crop production systems, the maximum photosynthetic capacity is usually difficult to achieve because of physiological and environmental limitations, such as varied leaf maturity, light deficiency caused by leaf shade, cloudy weather, sunlight incidence angle, greenhouse structure shade, and other environmental conditions, especially for crops with large leaf areas, such as fruit cucumber, in greenhouse vertical culture (Gunnlaugsson and Adalsteinsson, 2006; Lu et al., 2012; Matsuda et al., 2014). Leaf area and other growth indices at different leaf positions have been studied in tomato by Chang et al. (2011) and in purple yam by Hgaza et al. (2009), but the leaf growth and photosynthetic characteristics of fruit cucumber at different positions remain unclear.
We performed a comprehensive and systematic study on the leaf growth and photosynthetic characteristics of cucumber leaves at different positions. The information generated in this study will provide a reliable theoretical and practical basis for fruit cucumber cultivation in glass greenhouses.
Materials and Methods
Experimental facility and materials.
The experiment was conducted in a glass greenhouse at the Chongming base of the National Engineering Research Center of Protected Agriculture, Shanghai Academy of Agriculture Sciences, in Spring 2017. The utility systems of the greenhouse include groundwater sources (hot and cold water wells), groundwater energy exchange systems (high efficiency plate energy exchangers), heat pumps, end systems (greenhouse heating ducts and air treatment units), and energy storage systems (heat storage tanks and cold storage tanks). The operating modes of the utility and climate control systems can be referenced in Ding et al. (2019). Normally, the vents were opened during the day when temperatures exceeded 24 °C, and the heat system was turned on at night and maintained at 17 °C. The air temperature, relative humidity (RH), and CO2 concentration in the greenhouse were monitored and controlled automatically. Measurements were recorded automatically at 5-min intervals using a climate sensor (Priva, Netherlands) in the greenhouse.
Seeds of cucumber (Cucumis sativus L. cv. Deltastar) were sowed directly in water-soaked rockwool blocks (10 × 10 × 6.5 cm) and covered with vermiculite substrate. Cucumber seedlings with uniform size and one fully unfolded leaf were transplanted onto rockwool slabs (100 × 20 × 7.5 cm) in the greenhouse and irrigated by drip fertigation. Plants were pruned to a single stem, and one flower was kept at each node from the sixth node onward. The stem density was 2.8 stems/m2 and the plants grew using a high-wire growing system.
The management of the irrigation nutrient solution depended on radiation accumulation, and the drain of nutrient solution was 20% to 30% of the applied solution every day. An electrical conductivity of 2.0 to 2.5 dS·m−1 and pH of 5.5 were maintained for the nutrient solution.
Experimental design.
The experimental greenhouse was divided into three plots, and 700 plants were planted in each plot. When the plants had 15 leaves (1 month after transplantation), harvesting of cucumber was started. The plant height was ≈1.5 m, and the first leaf at the bottom remained green at the time. At least three plants were selected in every plot at random and the length, width, and photosynthetic variables of all 15 leaves from bottom to top for each plant on sunny days were measured.
Leaf length, width, and area measurements.
The length and width of 15 leaves from the bottom to the top were measured with a caliper, and the mean of every leaf length and width was calculated. Leaf length was measured from the lamina tip to the intersection of the lamina and petiole along the lamina midrib. Leaf width was measured from tip to tip between the widest lamina lobes, as referenced in Cho et al. (2007) (Fig. 1). The measurement of leaf area was carried out according to the method of Cho et al. (2007), in which the leaf area (SA) = –210.61 + 13.358 * leaf width + 0.5356 * leaf length * leaf width.
Leaf length and width of cucumber plant, as used in the Cho et al. (2007) model. Leaf length was measured from the lamina tip to the intersection of the lamina and petiole along the lamina midrib. Leaf width was measured from tip to tip between the widest lamina lobes.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf length and width of cucumber plant, as used in the Cho et al. (2007) model. Leaf length was measured from the lamina tip to the intersection of the lamina and petiole along the lamina midrib. Leaf width was measured from tip to tip between the widest lamina lobes.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf length and width of cucumber plant, as used in the Cho et al. (2007) model. Leaf length was measured from the lamina tip to the intersection of the lamina and petiole along the lamina midrib. Leaf width was measured from tip to tip between the widest lamina lobes.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Chlorophyll and carotenoid measurements.
The methods of Lichtenthaler were adopted and optimized (Lichtenthaler and Wellburn, 1983), and leaf discs with particular diameters cut by hole punchers were soaked in 5 mL of acetone and ethanol, with a 2:1 ratio, until the leaves became completely white. The extraction solutions were measured with an ultraviolet-visible spectrophotometer (Shimadzu ultraviolet-2700; Japan) at wavelengths of 663, 654, and 470 nm.
Leaf photosynthesis measurements.
The net photosynthesis rate (Pn), stomatal conductance (gS), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of all 15 leaves were measured with a LI-6400 Photosynthesis System (LI-COR Inc., Lincoln, NE). The irradiance level was set at 1000 µmol·m−2·s−1. Air temperature, RH, and CO2 concentration were set at ambient conditions in the greenhouse (Jiang et al., 2017b).
Statistical analysis.
Statistical analysis was conducted using SAS software (SAS version 9.3, SAS Institute Inc., Cary, NC). The results are presented as the mean ± sd, with a minimum of three replicates. Figures were plotted using Origin 7.0 software (Origin Laboratory, Northampton, MA).
Results
Changes in leaf fresh weight per unit leaf area at different positions.
At the stage of complete unfolding of the 15th fruit cucumber leaf, the second leaf had the highest fresh weight per unit leaf area, and this value declined from the third to the upper leaves, and the 14th and 15th leaves had the lowest fresh weight (Fig. 2). There was no clear difference in fresh weight per unit leaf area between the first and third leaves (Fig. 2).
Leaf fresh weight per unit leaf area at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf fresh weight per unit leaf area at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf fresh weight per unit leaf area at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Changes in leaf chlorophyll and carotenoid contents at different positions.
The chlorophyll a and b, and total chlorophyll contents increased rapidly from the first to the third leaf, maintained the high levels from the third to the sixth leaf, declined gradually from the seventh to the 10th leaf, and decreased drastically from the 11th to the 15th leaf (Fig. 3). The leaf carotenoid content increased rapidly from the first to the third leaf, increased slowly from the third to the fifth leaf, declined gradually from the sixth to the 10th leaf, and decreased drastically from the 11th to the 15th leaf (Fig. 3).
Leaf chlorophyll a and b, total chlorophyll, and carotenoid content at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf chlorophyll a and b, total chlorophyll, and carotenoid content at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf chlorophyll a and b, total chlorophyll, and carotenoid content at different positions. 1L through 15L indicate first through the 15th leaves.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Changes in leaf length, width, and area at different positions.
The leaf length, width, and area increased rapidly from the first to the third leaf, increased slowly from the fourth to the eighth leaf, and declined gradually from the ninth to the 15th leaf (Fig. 4). The relationship of leaf length, width, area and leaf position showed a quadratic equation, and the R2 determination coefficient values were 0.925, 0.951, and 0.955, respectively (P < 0.0001). The exponential model fit the measurements well. There was a large discrepancy between the measured values and the simulated values of the lower eight leaves, whereas one discrepancy of the upper seven leaves was small, indicating that the simulated values were close to the measured values from the ninth to the 15th leaf (Fig. 4).
Leaf length, width and area of cucumber at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf length, width and area of cucumber at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf length, width and area of cucumber at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Changes in leaf gas exchange parameters at different positions.
The Pn of the leaves increased gradually from the first to the seventh leaf under saturated light, remained steady from the seventh to the 10th leaf, with values all >21 μmol·m−2·s−1 CO2, and declined from the 11th to the 15th leaf. The change in Pn among different positions fit the quadratic model well with an R2 of 0.975 (P < 0.0001) (Fig. 5; Table 1). The gS increased from the first to the 12th leaf and decreased gradually from the 12th to the 15th leaf. The Tr increased from the first to the 14th leaf and decreased in the 15th leaf (Fig. 5).
Leaf photosynthetic variables at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation; Pn = net photosynthetic rate; Ci = intercellular CO2 concentration; gS = stomatal conductance; Tr = leaf transpiration rate.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf photosynthetic variables at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation; Pn = net photosynthetic rate; Ci = intercellular CO2 concentration; gS = stomatal conductance; Tr = leaf transpiration rate.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Leaf photosynthetic variables at different positions. 1L through 15L indicate first through the 15th leaves. RMSEA = root mean square error of approximation; Pn = net photosynthetic rate; Ci = intercellular CO2 concentration; gS = stomatal conductance; Tr = leaf transpiration rate.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Regression models of leaf length, width, area, and Pn with leaf position.
Comparison between simulated and measured values of leaf length, width, area, and Pn at different positions.
The R2 determination coefficient values of leaf length, width, area, and Pn of leaves at different position based on a 1:1 straight line between the simulated values and measured values were 0.900, 0.943, 0.946, and 0.973, respectively (Fig. 6), which means that the models fit well. Pn had the smallest discrepancy between the simulated values and measured values, whereas that of leaf length was the largest.
Comparison between simulated and measured values of leaf length, width, area, and net photosynthetic rate at different positions. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Comparison between simulated and measured values of leaf length, width, area, and net photosynthetic rate at different positions. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Comparison between simulated and measured values of leaf length, width, area, and net photosynthetic rate at different positions. RMSEA = root mean square error of approximation.
Citation: HortScience horts 55, 7; 10.21273/HORTSCI14637-19
Discussion
According to the results for unit fresh weight per leaf area, the second leaf from the bottom had the largest unit fresh weight per leaf area at the 15-leaf stage, and the values declined from the third to the 15th leaf. This might be related to the accumulation of photosynthetic products in different leaves, which mainly depends on the internal growth characteristics of fruit cucumber (Hikosaka, 2005; Poorter et al., 2009).
Leaf chlorophyll and carotenoids are vital components of the plant photosynthetic system and play significant roles in light energy capture, maintenance of the stability of the thylakoid membrane, and energy transduction (Tanaka and Tanaka, 2006). We found that the overall variation in chlorophyll a, chlorophyll b, and carotenoid shared similar patterns from the first to the 15th leaf, meaning that the middle leaves possessed higher contents, and the bottom and upper leaves had lower contents. The most likely reason for this finding was that the bottom leaves had senesced, and the top leaves were not yet mature at this time (Hovi et al., 2004; Zou et al., 2011). A similar result was shown in research on the effects of interlighting on cucumber growth, which showed that the chlorophyll content and light absorptance reached maximum values in the seventh to eighth leaves in a greenhouse cucumber canopy (Trouwborst et al., 2010).
The results show that there were quadratic relationships between changes in leaf length, width, area, and position from the first to the 15th leaf. The R2 values were all >0.9, which indicates that these equations could accurately predict the changes at different positions. Basically, the maximal leaf length, width, and area were recorded at approximately the fifth through 10th leaf from the bottom. This result was partially consistent with the research of Xu et al. (2010), in which the leaf growth change was asymmetric between two sides of approximately the 10th leaf, where the maximum leaf area occurred. Leaf growth increases rapidly before that point and declines slowly from the 10th to the 40th leaf (Xu et al., 2010). One possible reason to explain this difference may be the duration of cultivation. With long-term cultivation, the cucumber plants have enough time to extend most of their middle leaves. The leaf area index (LAI), the total leaf area per unit land area, is a key parameter in the analysis of plant photosynthesis, transpiration, water use, and productivity. In this study, the LAI of fruit cucumber (from the first to the 15th leaf) was ≈2.69. Although previous studies have shown that in a winter greenhouse environment, a canopy LAI of 2 to 3 is optimal for cucumber production, an experiment in summer indicated that LAI of 3 to 4 provides the highest gross photosynthesis (Luo et al., 2005a, 2005b). Therefore, the proper LAI of fruit cucumber may be obtained when the 20th leaf begins to unfold in spring experiments, which requires further study according to the crop characteristics, growing season, and cultivation management (Xu et al., 2010).
Photosynthesis is the primary component of productivity that provides energy and assimilates for growth and reproduction in plants (Lawlor, 2009). The Pn of fruit cucumber followed the quadratic model from the first to the 15th leaf, and the change in Pn of leaves at different positions can be predicted well by this equation. Trouwborst et al. (2011) measured the photosynthesis of leaves at different layers and revealed similar results. The Pn of both bottom and top leaves was remarkably lower than that of middle leaves, which could be related to senescence (Ashraf and Harris, 2013) and lack of light (Wolff and Langerud, 2006) of bottom leaves and immaturity of top leaves (Cho et al., 2007). The results of our research showed that light conditions and Ci improved from the bottom to the top leaves, but gS declined from the 13th to the 15th leaf, which indicated that the main reason for the Pn reduction of bottom leaves was stomatal limitation and that nonstomatal limitation accounted for the Pn reduction of top leaves. This result was supported by research by Pettersen et al. (2010), in which a low gS, light compensation point and respiration rate were observed in the lower canopy. Ding et al. (2013) reported that weak light resulted in a remarkable decrease in the cucumber net Pn. Gunnlaugsson and Adalsteinsson (2006) found that lower leaves can also achieve a high accumulation rate under horizontal artificial light. This may indicate that the light deficiency of lower leaves accounts mostly for the observed low Pn. In actual cultivation, shading within plants or leaves is inevitable. Previous research has shown that shading of lower leaves not only weakens the real-time Pn and assimilation rate directly but also results in leaf damage, which in turn leads to additional photosynthetic decline (Lu et al., 2012; Steinger et al., 2003). Some studies in tomato culture were performed to adjust plant density dynamically among various growth periods (Jiang et al., 2017a), and this may also provide a feasible solution in cucumber production.
In conclusion, variations in leaf length, width, area, and Pn of fruit cucumber precisely followed the quadratic model, and the model fit very well with the measured value changes. The bottom leaf still had high Pn at the 15-leaf stage, so the time to remove aged leaves should be later, such as the 20-leaf stage, so that 18 leaves are maintained for plant photosynthesis (Ding et al., 2019). The middle leaves had the highest Pn, and the reasons for the decline in Pn of bottom leaves and upper leaves were stomatal limitations and nonstomatal limitations, respectively. The pattern of variation in the leaf photosynthetic variables at different positions improves our understanding of leaf properties and growth and provides guidance for proper cultivation of cucumber in the greenhouse.
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