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Sand mulching and its relationship with soil temperature and light environment in the cultivation of Lilium longiflorum cut flower
Scientia Horticulturae 240 (2018) 453–459
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Sand mulching and its relationship with soil temperature and light environment in the cultivation of Lilium longiflorum cut flower
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Gabriel A. Lorenzo , Libertad Mascarini, Mariel N. Gonzalez, Elena Lalor Universidad de Buenos Aires, Facultad de Agronomía, Departamento de Producción Vegetal, Cátedra de Floricultura, Avenida San Martín 4453 (1417), Buenos Aires, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil cover Light quality Spectral reflectance Leaf area index Growth rate Commercial quality
Lilium longiflorum is one of the most important cut flowers both in the world and in Argentina. To plan a commercial crop, it is necessary to understand tools that relate environmental variables to crop growth and development. One of these tools is leaf expansion, which can be determined from the evolution of the leaf area index (LAI). The technique known as sand mulching substantially modifies the soil temperature but can also alter other environmental variables such as light quality. The objective of this work was to determine whether the application of sand mulching affects the growth rate and leaf expansion of a lilium cut flower crop. The experiment was carried out in a greenhouse at the Faculty of Agronomy, University of Buenos Aires, Argentina, where a lilium crop was planted in the soil, adding a 5-cm layer of fine sand over the entire soil surface to reduce soil temperature. Plant height and number of leaves were measured twice a week and the LAI was estimated by spectral reflectance once a week. The cycle length was also recorded, and the accumulation of dry matter was measured at the end of the experiment. The soil temperature was lower in the first month of cultivation, and there were differences in the light environment and in the water status of the crop. Although there were no significant differences in the cycle length or leaf appearance, sand mulching led to differences in the stem growth rate, LAI and final plant height, resulting in a better quality of the product.
1. Introduction Lilium (Lilium longiflorum Thunb.) constitutes the fourth largest cut flower industry (VBN, 2009) and one of the six most important genera of flower bulbs around the world (CBI, 2016; Hanks, 2015). Simulation models are efficient tools in crop management and planning. Models can help to understand genetic, physiological and environmental interactions, with interdisciplinary integration. They allow defining production strategies in the planning stage of a future crop or helping to take technical decisions during the crop cycle, including cultural practices, fertilization, irrigation and pesticide use. Soil temperature affects lilium growth, particularly during cultivation initial stages. Optimum soil temperature during the rooting of the bulb is 12–13 °C, whereas temperatures above 15 °C during the growing cycle decrease the quality of the final product (ibulb.org, 2017). Warmer temperatures also produce shorter stems and fewer flowers per stem (International Flower Bulb Centre (IFBC, 2002) and reduce growth and development of adventitious roots (Kim et al., 2007a). For hybrid L/A (longiflorum x asiatic), the best temperature after rooting is 14–16 °C, with a temperature of up to 20–22 °C being acceptable.
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Corresponding author. E-mail address: [email protected] (G.A. Lorenzo).
https://doi.org/10.1016/j.scienta.2018.06.025 Received 4 April 2017; Received in revised form 22 May 2018; Accepted 9 June 2018 Available online 21 June 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.
Similar optimal temperatures have been reported for other geophytes (Jennings and De Hertogh, 1977; Wilson and Peterson, 1982). Like many other physiological processes, root growth in geophytes is highly regulated by temperature (Kim et al., 2007b). Elevated temperatures in the first 30 days lead to stem length reduction by cycle shortening, abortion, and abscission and/or desiccation of flower buds, whereas low but non-lethal temperatures generally slow down the growth rate and therefore lengthen the crop cycle (Schiappacasse et al., 2006; Mascarini et al., 2007). In addition, light and temperature are the main climatic factors determining the leaf appearance and elongation rates (Repková et al., 2009). While the increase in temperature stimulates the development of leaf primordia, and could therefore accelerate the leaf appearance, light mostly affects the rate of elongation and the duration of leaves. Also, many studies have shown that temperature is the main factor controlling the phyllochron, or rate of leaf appearance, for example in wheat (Triticum aestivum L.). Experimental results in wheat suggest that the soil temperature at the crown level, rather than the air temperature at the canopy level, could better predict the rate of leaf emergence (McMaster et al., 2003). Another way to model the response of a crop to different
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typic Argiudolls, silt loam textural class, with 65% silt and 25% clay. In general, they are deep soils, with good drainage and high content of organic matter (> 4%). Plants were planted on May 6 and harvested on September 10. The treatments were: 1) bare soil control (S); an 2) sand mulching with a soil covered by a 5 cm fine sand addition, < 0.1 cm diameter, after planting the bulbs (M). Sand is widely used by growers due to its relatively low cost and availability, and is suitable to reduce the soil temperature. Xiaoyan et al. (2002) reported lower temperature in soils covered with fine sand than in those covered with a gravel-sand mix (0.5–2 cm in diameter), no differences with gravel (2–3 cm), and a higher evaporation rate. In addition, growers recover this material at the end of the crop cycle and reuse it several times. Irrigation water was applied daily with head drippers at a flow rate of 2 L h−1, and automatically controlled. The water volume applied depended on the evapotranspiration calculated by the PenmanMonteith method modified by FAO (Allen et al., 1998).
environmental situations is through the evolution of the leaf area index (LAI). Direct measurements of the leaf area by destructive methods are tedious and highly time-consuming, and the number of plants at the end of the experiment is usually lower than at the beginning. In contrast, remote sensors of the plant canopy are a non-destructive, simple and fast method that allows evaluating the temporary changes in the growth and development of the whole plants in situ (Holben et al., 1980; Asrar et al., 1985; Mascarini et al., 2006). The LAI accuracy obtained from the reflectance measurement depends on the contrast between the soil and the green leaves in a partially closed canopy. The highest contrast occurs in the near infrared region (700–1100 nm), where the transmission and reflectance of the leaves are maximal. Leaves show the highest absorbance in the visible region of the spectrum (400–700 nm). A combination between visible and near infrared reflectance can estimate the fraction of incident radiation that is absorbed by vegetation. Considering that spectral reflectance, radiation absorption and leaf area are interrelated, the LAI could be determined as a function of that fraction (Asrar et al., 1984). The way to estimate the LAI from reflectance has been successfully applied in several crops such as wheat (Asrar et al., 1985), soybean (Holben et al., 1980), and maize (Gallo et al., 1985). Sims and Gamon (2003) reported a normalized index as a LAI estimator for 23 species of shrubs, trees and grasses, and defined a specific range of wavelengths where accuracy was highest. In a previous study, we used this technique to estimate the LAI in rose cultures (Mascarini et al., 2006). To our knowledge, there are no studies of LAI modeling by means of spectral reflectance in lilium. In geophytic species, the effect of soil temperature on plant growth and development during the first 30 days is well-documented (Francescangeli et al., 2008; Khodorova and Boitel-Conti, 2013). The use of sand mulching or soil cover can contribute to the reduction of the temperature at the bulb level, positively affecting the growth and development of the plant, especially when the temperature is high for the crop, for example when grown in greenhouses (Lü et al., 2013; Wang et al., 2014). The soil temperature has a fundamental influence on the success of a commercial lilium crop. Such is the case of spring plantings, which, with increasing temperatures, often result in less development of adventitious roots, fewer leaves and shorter stems (Roberts et al., 1985). Cool temperatures affect hormonal balance, stimulating the synthesis of auxins and gibberellins (Khodorova and Boitel-Conti, 2013), which are related to the growth of stem roots (Inamoto et al., 2013). On the other hand, in Eucharis grandiflora, soil chilling increases the production of flowering stems, compared to the soil under uncontrolled temperature (Doi et al., 2000). Since the use of mulching modifies the soil temperature, this could trigger a positive response in the growth and development of lilium plants. The differential response in terms of soil temperature can be detected by the construction of simulation models considering environmental and crop variables. The objectives of this work were: 1) to compare soil temperature in a lilium crop with and without sand mulching, and 2) to model the effect of environmental factors on some variables of growth and development of lilium.
2.2. Measurements and data analysis During the experiment, air and soil temperature, relative humidity and incident solar radiation were recorded using a datalogger (Licor 1400, Lincoln, NE, USA). Plant height and number of leaf appeared leaves were recorded twice a week. Spectral reflectance was measured once a week and these data were used to estimate the LAI, light quality and water status, as previously mentioned (Mascarini et al., 2006). Crop spectral reflectance was measured with a handheld multispectral radiometer (MRS16C9, CROPSCAN® Inc.; Rochester, USA, 2000) at 450, 500, 550, 610, 660, 680, 710, 730, 760, 780, 810, 870, 950, 1080, 1220 and 1600 nm. Each band had a half peak band width between 5 and 15 nm. The sensor was mounted on an adjustable pole that was parallel to the ground surface with a field of view of 1.0 m diameter over the crop canopy. Measurements were taken in full-sun days, between 11:00 and 13:00 h. During each measurement, the substrate was covered with a black cloth to prevent undesirable reflection from the bare soil or mulching, and was removed after the measurement. Reference reflectance was measured over a white panel placed over the crop, which represented a value of 100%. These data were used to calculate several plant indices and assess the quality of the radiation reflected as described below: 1) Normalized difference vegetation index (NDVI, Sims and Gamon, 2003), calculated as NDVI = (R810 – R680) / (R810 + R680) This is the ratio between reflectance percent in the infrared region (800–1100 nm) and reflectance percent in the red region (600–700 nm). The wave lengths used in this study were 680 nm (reflection in R680) and 810 nm (reflection in near infrared, R810). 1) Water index (WI), which is a ratio between reflectance at a reference wavelength where water does not absorb, and a wavelength where water does absorb. The simple WI is a ratio between the reflectance at a reference wavelength not absorbed by water, and a wavelength absorbed by water. Sims and Gamon (2003) identified three wavelength bands (950–970, 1150–1260 and 1520–1540 nm) which produce the best correlation with the water content since water reaches maximum absorption in the infrared part of the spectrum. These authors suggested that the 1150–1260 nm and 1520–1540 nm bands could be the best to detect the water content with remote sensors. In this work, we used the 1220 nm band because it is strongly absorbed by the lilium plant, penetrates short distances in the canopy, and is sensitive to water in the upper layers or in a thin canopy.
2. Material and methods 2.1. Experimental design and plot set The research was carried out in the School of Agronomy of the University of Buenos Aires (34°35′S, 58°29′ W; 25 m above sea level), located within the Buenos Aires Metropolitan Area, the main commercial flower production area in Argentina. The experiment was performed in a greenhouse on soil, in 5 m × 1 m plots, and the crop was started with an L/A hybrid of Lilium longiflorum cv ‘Brindisi’, 14–16 cm in circumference (4.5–5 cm in diameter), with a planting density of 60 bulbs m−2 of cultivation bench. The soils of this region are classified as
WI = R870 / R1220. 454
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3.3. LAI, R:FR ratio and WI
1) The red:far red ratio (R:FR), which was calculated as the quotient between radiation reflected in the length of the R wavelength (680 nm) and the FR wavelength (730 nm). 2) Reflected blue light, which was measured as percentage of reflected radiation in the B (blue) wavelength (450 nm). 3) Leaf area, which was calculated from spectral reflectance with the NDVI by destructively measuring a separate plot of 1 to 60 plants with an area meter (LI-3100; LI−COR, Lincoln, NE, USA, 2000). Then, these measurements were used to build a model to estimate the LAI in a non-destructive way at any moment. This method has been previously and successfully used in roses (Mascarini et al., 2006).
The reflectance data and real leaf area measured were used to construct a model to estimate the LAI from the NDVI, at any time. The adjustment of this model was highly significant and with R2 = 0.69, resulting in a good index to monitor crop growth by a non-destructive method. This model allowed estimating the LAI, which showed significant differences, with a higher value in the plants grown with sand mulching. This would indicate that the plants grown with sand mulching had the same number of leaves but larger (Fig. 3). The sand mulching also modified the light environment at the level of the canopy of the crop, showing a significantly lower R:FR ratio during most of the crop, which is related to the higher LAI of that crop. A lower R:FR ratio would also contribute to obtaining significantly longer stems. On the other hand, the reflected radiation in the blue band (450 nm) showed significant differences, except at two times in June and July (Fig. 4), and therefore it could have effects on the vegetative growth, specifically regarding stem elongation and leaf expansion. The water index (WI) allows estimating the hydration status of the canopy from the ratio of the reflectance percentage in the bands of 870 nm and 1220 nm. At most evaluation times, sand mulching led to a significantly higher WI (Fig. 5).
The results of these measurements were related to the parameters of production quality to determine some of the eco-physiological causes that explain differences between treatments. At the end of the experiment, the flowering stems were harvested when the first flower bud showed colour, and the cycle crop, final plant height, and fresh weight were measured. Fresh material was dried in an oven at 65 °C until the weight was constant, and then dry weight was measured. A completely randomized design with five replications per treatment was used. We performed analysis of variance (ANOVA) and comparison of means by least significant difference (LSD) test and regression analysis, including tests of parallelism between the corresponding lines according to the case. The statistical program InfoStat / Professional V1.1 was used.
4. Discussion Lilium cut flower quality is determined by plant height, number of flowers per plant, and flower size.The results show that neither the cycle from planting to harvest (122 days) nor the number of leaves (M: 54.1 vs. S: 56.3) were affected. In contrast, stem length showed significant differences (M: 91.4 cm vs. S: 74.8 cm), with the soil temperature being on average 5 to 25% higher in S than in M treatment. This coincides with the results reported by Wang et al. (2014), who concluded that in melon (Cucumis melo) cultivated with sand mulching, soil temperature was slightly lower, but greatest effect was observed on available water, resulting in better product quality. In addition, a higher soil water content contributes to reducing soil temperature (AlKayssi et al., 1990). In geophytes, it is well documented that temperature plays a role in growth and development, affecting carbohydrate distribution, level of endogenous plant growth regulators and gene expression (Khodorova and Boitel-Conti, 2013). Low temperature plays a key role in the growth of stem roots, which are in turn related to nutrient uptake. Plants grown under low temperature conditions have longer stem and higher fresh weight of aerial parts, which are attributes related to high-quality cut flowers (Inamoto et al., 2013). Changes in light environment could be related to reflected light from mulching, resulting in a lower R:FR ratio, which in turn would influence the elongation of internodes, since number of leaves showed no significant differences. As a consequence, plants cultivated with sand mulching achieved higher stem length and LAI. In roses, the use of bending technique increased LAI, enhanced light environment, and produced longer stems (Mascarini et al., 2006). The same effect was observed when a fluorescent material was used as a roof covering of the greenhouse, with the particularity of increasing the proportion of FR to R in the environment, thus leading to a lower R:FR ratio (Mascarini et al., 2012). Light quality could affect the chemical composition of plant tissue, for example, by promoting accumulation of anthocyanins and carotenoids under a blue-enriched environment (Li and Kubota, 2009). There is strong evidence about the effect of blue light on plant height by inhibition of stem elongation (Liscum and Hangarter, 1991; Appelgren, 1991; Neff and Chory, 1998; Briggs and Huala, 1999). Wang et al. (2015) have recently reported that blue light could promote leaf expansion. In the present study, the reflectance in the blue band was
3. Results 3.1. Air and soil temperature During the growing season, the first stage coincides with the beginning of autumn, when high air temperatures can occur in Buenos Aires, as shown in Fig. 1a. In addition, the mean maximum daily irradiance was 300 W m−2, with 461 and 42 W m−2 as extreme values, and the mean relative humidity was 66%. The use of mulching allowed the temperature to be lower than that in uncovered soil in the order of 10–20%. This effect was much more pronounced in the first month, which is precisely the critical period when the temperature of the soil should not exceed 13 °C (Fig. 1b). 3.2. Leaf appearance and plant height The best initial thermal condition was what caused a differential response at the beginning of the crop in terms of leaf appearance although, at the end of the cycle, there were no significant differences in the number of leaves. On the other hand, the difference in plant height increased as the crop progressed, resulting in longer stem in the plants cultivated with sand mulching (Fig. 2). Although the cultivation in bare soil led to a higher number of leaves at the beginning of the experiment, the cultivation with sand mulching led to a higher rate of leaf appearance (data not shown), which would explain why no significant differences were found in the number of leaves at the end of the experiment. The final plant height of the flowering stem was significantly affected by the sand mulching, increasing by 15% vs. plants cultivated in uncovered soil. A higher stem growth rate was observed in the plants cultivated with sand mulching throughout the experiment, with significant differences at certain times (data not shown). The soil temperature closer to the optimum one for the crop would have allowed a better initial development of roots and, therefore, a greater use of water and nutrients. However, at the end of the experiment, no significant differences were detected in fresh or dry root weight or in the stem/root ratio (data not shown). 455
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Fig. 1. Maximum, minimum and average daily temperature of air, bare soil (S) and soil covered with 5 cm of sand (M) during the entire crop cycle (a) and the first month (b) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions.
plants with sand mulching. In the present study, during most of the growing season, plants cultivated under sand mulching showed a significantly higher WI, which indicates a better water condition of the crop. This could be due to a better developed root system as well as lower evaporation and, therefore, greater availability of water in the soil due to the mulching effect. Advantage of sand mulching over bare soil was well demonstrated by Li (2003), who reported a strong reduction of evaporation
higher in the treatment with sand mulching during most of the experiment, which would explain that despite having the same amount of leaves, the LAI was higher in the plants grown with sand mulching due to a larger leaf size. On the other hand, according to Hogewoning et al. (2010), the photosynthetic capacity could be increased under a blue light enriched environment. In our case, this behaviour could compensate the inhibition of stem elongation, therefore no significant differences were found in dry weight but longer stems were recorded in
Fig. 2. Change of plant height and number of expanded leaves of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05. 456
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Fig. 3. Change of leaf area index (LAI) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
Fig. 4. Change of the red:far red (R:FR) ratio and reflected blue light (B) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
cultivated in bare soil could cause a situation of water stress of short duration and low intensity, but sufficient to impose a limitation to the crop growth. Zhang et al. (2011) found that water stress in Lilium longiflorum reduced the rate of photosynthesis and transpiration, resulting in plants of lower height and with a lower leaf area, that is, of lower quality. Our main conclusion is that lower soil temperature at the beginning of the crop cycle allowed a better initial development of roots, which could lead to a better use of water, determining higher growth, both in height and leaf area.
and matric potential under gravel-sand cover, and thus increasing water availability. Another beneficial effect of mulching could be a greater brightness in the canopy due to the reflection from the sand cover, as demonstrated by Conforti et al. (2015), who reported higher reflectance from a loamy sand soil in comparison with other soil texture classes. Therefore, the better quality flowering stems obtained under sand mulching could be related to the greater plant height due to the closer R:FR ratio, added to a greater photosynthesis due to a greater LAI and better light environment. Although some authors have reported an increase in soil temperature under sand mulching (Li, 2003; Lü et al., 2013), there is also evidence of a reduction in temperature (Wang et al., 2014). One of the possible causes of these discrepancies could be the water regime of each particular experiment. According to Al Kayssi et al. (1990), the soil water content affects the thermal behaviour of the soil, decreasing extreme variations by reducing maximum temperature values and increasing minimum temperature values. The gradual drying of the soil can lead to a concomitant increase in temperature, as demonstrated by Lakshmi et al. (2003). In our case, an irrigation regime was applied to guarantee non-limiting water, keeping the water content between field capacity and water readily available, but a lower root mass of plants
Conflict of interest Authors have no competing interests to declare. Acknowledgement This work was supported by the Universidad de Buenos Aires scientific program 2014-2017, within the framework of the UBACyT project code 20020130200269BA (Techniques to increase the efficiency in the use of water and fertilizers for a sustainable production of 457
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Fig. 5. Change of the Water Index (WI) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
ornamental species).
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Short communication
Sand mulching and its relationship with soil temperature and light environment in the cultivation of Lilium longiflorum cut flower
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Gabriel A. Lorenzo , Libertad Mascarini, Mariel N. Gonzalez, Elena Lalor Universidad de Buenos Aires, Facultad de Agronomía, Departamento de Producción Vegetal, Cátedra de Floricultura, Avenida San Martín 4453 (1417), Buenos Aires, Argentina
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil cover Light quality Spectral reflectance Leaf area index Growth rate Commercial quality
Lilium longiflorum is one of the most important cut flowers both in the world and in Argentina. To plan a commercial crop, it is necessary to understand tools that relate environmental variables to crop growth and development. One of these tools is leaf expansion, which can be determined from the evolution of the leaf area index (LAI). The technique known as sand mulching substantially modifies the soil temperature but can also alter other environmental variables such as light quality. The objective of this work was to determine whether the application of sand mulching affects the growth rate and leaf expansion of a lilium cut flower crop. The experiment was carried out in a greenhouse at the Faculty of Agronomy, University of Buenos Aires, Argentina, where a lilium crop was planted in the soil, adding a 5-cm layer of fine sand over the entire soil surface to reduce soil temperature. Plant height and number of leaves were measured twice a week and the LAI was estimated by spectral reflectance once a week. The cycle length was also recorded, and the accumulation of dry matter was measured at the end of the experiment. The soil temperature was lower in the first month of cultivation, and there were differences in the light environment and in the water status of the crop. Although there were no significant differences in the cycle length or leaf appearance, sand mulching led to differences in the stem growth rate, LAI and final plant height, resulting in a better quality of the product.
1. Introduction Lilium (Lilium longiflorum Thunb.) constitutes the fourth largest cut flower industry (VBN, 2009) and one of the six most important genera of flower bulbs around the world (CBI, 2016; Hanks, 2015). Simulation models are efficient tools in crop management and planning. Models can help to understand genetic, physiological and environmental interactions, with interdisciplinary integration. They allow defining production strategies in the planning stage of a future crop or helping to take technical decisions during the crop cycle, including cultural practices, fertilization, irrigation and pesticide use. Soil temperature affects lilium growth, particularly during cultivation initial stages. Optimum soil temperature during the rooting of the bulb is 12–13 °C, whereas temperatures above 15 °C during the growing cycle decrease the quality of the final product (ibulb.org, 2017). Warmer temperatures also produce shorter stems and fewer flowers per stem (International Flower Bulb Centre (IFBC, 2002) and reduce growth and development of adventitious roots (Kim et al., 2007a). For hybrid L/A (longiflorum x asiatic), the best temperature after rooting is 14–16 °C, with a temperature of up to 20–22 °C being acceptable.
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Corresponding author. E-mail address: [email protected] (G.A. Lorenzo).
https://doi.org/10.1016/j.scienta.2018.06.025 Received 4 April 2017; Received in revised form 22 May 2018; Accepted 9 June 2018 Available online 21 June 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.
Similar optimal temperatures have been reported for other geophytes (Jennings and De Hertogh, 1977; Wilson and Peterson, 1982). Like many other physiological processes, root growth in geophytes is highly regulated by temperature (Kim et al., 2007b). Elevated temperatures in the first 30 days lead to stem length reduction by cycle shortening, abortion, and abscission and/or desiccation of flower buds, whereas low but non-lethal temperatures generally slow down the growth rate and therefore lengthen the crop cycle (Schiappacasse et al., 2006; Mascarini et al., 2007). In addition, light and temperature are the main climatic factors determining the leaf appearance and elongation rates (Repková et al., 2009). While the increase in temperature stimulates the development of leaf primordia, and could therefore accelerate the leaf appearance, light mostly affects the rate of elongation and the duration of leaves. Also, many studies have shown that temperature is the main factor controlling the phyllochron, or rate of leaf appearance, for example in wheat (Triticum aestivum L.). Experimental results in wheat suggest that the soil temperature at the crown level, rather than the air temperature at the canopy level, could better predict the rate of leaf emergence (McMaster et al., 2003). Another way to model the response of a crop to different
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typic Argiudolls, silt loam textural class, with 65% silt and 25% clay. In general, they are deep soils, with good drainage and high content of organic matter (> 4%). Plants were planted on May 6 and harvested on September 10. The treatments were: 1) bare soil control (S); an 2) sand mulching with a soil covered by a 5 cm fine sand addition, < 0.1 cm diameter, after planting the bulbs (M). Sand is widely used by growers due to its relatively low cost and availability, and is suitable to reduce the soil temperature. Xiaoyan et al. (2002) reported lower temperature in soils covered with fine sand than in those covered with a gravel-sand mix (0.5–2 cm in diameter), no differences with gravel (2–3 cm), and a higher evaporation rate. In addition, growers recover this material at the end of the crop cycle and reuse it several times. Irrigation water was applied daily with head drippers at a flow rate of 2 L h−1, and automatically controlled. The water volume applied depended on the evapotranspiration calculated by the PenmanMonteith method modified by FAO (Allen et al., 1998).
environmental situations is through the evolution of the leaf area index (LAI). Direct measurements of the leaf area by destructive methods are tedious and highly time-consuming, and the number of plants at the end of the experiment is usually lower than at the beginning. In contrast, remote sensors of the plant canopy are a non-destructive, simple and fast method that allows evaluating the temporary changes in the growth and development of the whole plants in situ (Holben et al., 1980; Asrar et al., 1985; Mascarini et al., 2006). The LAI accuracy obtained from the reflectance measurement depends on the contrast between the soil and the green leaves in a partially closed canopy. The highest contrast occurs in the near infrared region (700–1100 nm), where the transmission and reflectance of the leaves are maximal. Leaves show the highest absorbance in the visible region of the spectrum (400–700 nm). A combination between visible and near infrared reflectance can estimate the fraction of incident radiation that is absorbed by vegetation. Considering that spectral reflectance, radiation absorption and leaf area are interrelated, the LAI could be determined as a function of that fraction (Asrar et al., 1984). The way to estimate the LAI from reflectance has been successfully applied in several crops such as wheat (Asrar et al., 1985), soybean (Holben et al., 1980), and maize (Gallo et al., 1985). Sims and Gamon (2003) reported a normalized index as a LAI estimator for 23 species of shrubs, trees and grasses, and defined a specific range of wavelengths where accuracy was highest. In a previous study, we used this technique to estimate the LAI in rose cultures (Mascarini et al., 2006). To our knowledge, there are no studies of LAI modeling by means of spectral reflectance in lilium. In geophytic species, the effect of soil temperature on plant growth and development during the first 30 days is well-documented (Francescangeli et al., 2008; Khodorova and Boitel-Conti, 2013). The use of sand mulching or soil cover can contribute to the reduction of the temperature at the bulb level, positively affecting the growth and development of the plant, especially when the temperature is high for the crop, for example when grown in greenhouses (Lü et al., 2013; Wang et al., 2014). The soil temperature has a fundamental influence on the success of a commercial lilium crop. Such is the case of spring plantings, which, with increasing temperatures, often result in less development of adventitious roots, fewer leaves and shorter stems (Roberts et al., 1985). Cool temperatures affect hormonal balance, stimulating the synthesis of auxins and gibberellins (Khodorova and Boitel-Conti, 2013), which are related to the growth of stem roots (Inamoto et al., 2013). On the other hand, in Eucharis grandiflora, soil chilling increases the production of flowering stems, compared to the soil under uncontrolled temperature (Doi et al., 2000). Since the use of mulching modifies the soil temperature, this could trigger a positive response in the growth and development of lilium plants. The differential response in terms of soil temperature can be detected by the construction of simulation models considering environmental and crop variables. The objectives of this work were: 1) to compare soil temperature in a lilium crop with and without sand mulching, and 2) to model the effect of environmental factors on some variables of growth and development of lilium.
2.2. Measurements and data analysis During the experiment, air and soil temperature, relative humidity and incident solar radiation were recorded using a datalogger (Licor 1400, Lincoln, NE, USA). Plant height and number of leaf appeared leaves were recorded twice a week. Spectral reflectance was measured once a week and these data were used to estimate the LAI, light quality and water status, as previously mentioned (Mascarini et al., 2006). Crop spectral reflectance was measured with a handheld multispectral radiometer (MRS16C9, CROPSCAN® Inc.; Rochester, USA, 2000) at 450, 500, 550, 610, 660, 680, 710, 730, 760, 780, 810, 870, 950, 1080, 1220 and 1600 nm. Each band had a half peak band width between 5 and 15 nm. The sensor was mounted on an adjustable pole that was parallel to the ground surface with a field of view of 1.0 m diameter over the crop canopy. Measurements were taken in full-sun days, between 11:00 and 13:00 h. During each measurement, the substrate was covered with a black cloth to prevent undesirable reflection from the bare soil or mulching, and was removed after the measurement. Reference reflectance was measured over a white panel placed over the crop, which represented a value of 100%. These data were used to calculate several plant indices and assess the quality of the radiation reflected as described below: 1) Normalized difference vegetation index (NDVI, Sims and Gamon, 2003), calculated as NDVI = (R810 – R680) / (R810 + R680) This is the ratio between reflectance percent in the infrared region (800–1100 nm) and reflectance percent in the red region (600–700 nm). The wave lengths used in this study were 680 nm (reflection in R680) and 810 nm (reflection in near infrared, R810). 1) Water index (WI), which is a ratio between reflectance at a reference wavelength where water does not absorb, and a wavelength where water does absorb. The simple WI is a ratio between the reflectance at a reference wavelength not absorbed by water, and a wavelength absorbed by water. Sims and Gamon (2003) identified three wavelength bands (950–970, 1150–1260 and 1520–1540 nm) which produce the best correlation with the water content since water reaches maximum absorption in the infrared part of the spectrum. These authors suggested that the 1150–1260 nm and 1520–1540 nm bands could be the best to detect the water content with remote sensors. In this work, we used the 1220 nm band because it is strongly absorbed by the lilium plant, penetrates short distances in the canopy, and is sensitive to water in the upper layers or in a thin canopy.
2. Material and methods 2.1. Experimental design and plot set The research was carried out in the School of Agronomy of the University of Buenos Aires (34°35′S, 58°29′ W; 25 m above sea level), located within the Buenos Aires Metropolitan Area, the main commercial flower production area in Argentina. The experiment was performed in a greenhouse on soil, in 5 m × 1 m plots, and the crop was started with an L/A hybrid of Lilium longiflorum cv ‘Brindisi’, 14–16 cm in circumference (4.5–5 cm in diameter), with a planting density of 60 bulbs m−2 of cultivation bench. The soils of this region are classified as
WI = R870 / R1220. 454
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3.3. LAI, R:FR ratio and WI
1) The red:far red ratio (R:FR), which was calculated as the quotient between radiation reflected in the length of the R wavelength (680 nm) and the FR wavelength (730 nm). 2) Reflected blue light, which was measured as percentage of reflected radiation in the B (blue) wavelength (450 nm). 3) Leaf area, which was calculated from spectral reflectance with the NDVI by destructively measuring a separate plot of 1 to 60 plants with an area meter (LI-3100; LI−COR, Lincoln, NE, USA, 2000). Then, these measurements were used to build a model to estimate the LAI in a non-destructive way at any moment. This method has been previously and successfully used in roses (Mascarini et al., 2006).
The reflectance data and real leaf area measured were used to construct a model to estimate the LAI from the NDVI, at any time. The adjustment of this model was highly significant and with R2 = 0.69, resulting in a good index to monitor crop growth by a non-destructive method. This model allowed estimating the LAI, which showed significant differences, with a higher value in the plants grown with sand mulching. This would indicate that the plants grown with sand mulching had the same number of leaves but larger (Fig. 3). The sand mulching also modified the light environment at the level of the canopy of the crop, showing a significantly lower R:FR ratio during most of the crop, which is related to the higher LAI of that crop. A lower R:FR ratio would also contribute to obtaining significantly longer stems. On the other hand, the reflected radiation in the blue band (450 nm) showed significant differences, except at two times in June and July (Fig. 4), and therefore it could have effects on the vegetative growth, specifically regarding stem elongation and leaf expansion. The water index (WI) allows estimating the hydration status of the canopy from the ratio of the reflectance percentage in the bands of 870 nm and 1220 nm. At most evaluation times, sand mulching led to a significantly higher WI (Fig. 5).
The results of these measurements were related to the parameters of production quality to determine some of the eco-physiological causes that explain differences between treatments. At the end of the experiment, the flowering stems were harvested when the first flower bud showed colour, and the cycle crop, final plant height, and fresh weight were measured. Fresh material was dried in an oven at 65 °C until the weight was constant, and then dry weight was measured. A completely randomized design with five replications per treatment was used. We performed analysis of variance (ANOVA) and comparison of means by least significant difference (LSD) test and regression analysis, including tests of parallelism between the corresponding lines according to the case. The statistical program InfoStat / Professional V1.1 was used.
4. Discussion Lilium cut flower quality is determined by plant height, number of flowers per plant, and flower size.The results show that neither the cycle from planting to harvest (122 days) nor the number of leaves (M: 54.1 vs. S: 56.3) were affected. In contrast, stem length showed significant differences (M: 91.4 cm vs. S: 74.8 cm), with the soil temperature being on average 5 to 25% higher in S than in M treatment. This coincides with the results reported by Wang et al. (2014), who concluded that in melon (Cucumis melo) cultivated with sand mulching, soil temperature was slightly lower, but greatest effect was observed on available water, resulting in better product quality. In addition, a higher soil water content contributes to reducing soil temperature (AlKayssi et al., 1990). In geophytes, it is well documented that temperature plays a role in growth and development, affecting carbohydrate distribution, level of endogenous plant growth regulators and gene expression (Khodorova and Boitel-Conti, 2013). Low temperature plays a key role in the growth of stem roots, which are in turn related to nutrient uptake. Plants grown under low temperature conditions have longer stem and higher fresh weight of aerial parts, which are attributes related to high-quality cut flowers (Inamoto et al., 2013). Changes in light environment could be related to reflected light from mulching, resulting in a lower R:FR ratio, which in turn would influence the elongation of internodes, since number of leaves showed no significant differences. As a consequence, plants cultivated with sand mulching achieved higher stem length and LAI. In roses, the use of bending technique increased LAI, enhanced light environment, and produced longer stems (Mascarini et al., 2006). The same effect was observed when a fluorescent material was used as a roof covering of the greenhouse, with the particularity of increasing the proportion of FR to R in the environment, thus leading to a lower R:FR ratio (Mascarini et al., 2012). Light quality could affect the chemical composition of plant tissue, for example, by promoting accumulation of anthocyanins and carotenoids under a blue-enriched environment (Li and Kubota, 2009). There is strong evidence about the effect of blue light on plant height by inhibition of stem elongation (Liscum and Hangarter, 1991; Appelgren, 1991; Neff and Chory, 1998; Briggs and Huala, 1999). Wang et al. (2015) have recently reported that blue light could promote leaf expansion. In the present study, the reflectance in the blue band was
3. Results 3.1. Air and soil temperature During the growing season, the first stage coincides with the beginning of autumn, when high air temperatures can occur in Buenos Aires, as shown in Fig. 1a. In addition, the mean maximum daily irradiance was 300 W m−2, with 461 and 42 W m−2 as extreme values, and the mean relative humidity was 66%. The use of mulching allowed the temperature to be lower than that in uncovered soil in the order of 10–20%. This effect was much more pronounced in the first month, which is precisely the critical period when the temperature of the soil should not exceed 13 °C (Fig. 1b). 3.2. Leaf appearance and plant height The best initial thermal condition was what caused a differential response at the beginning of the crop in terms of leaf appearance although, at the end of the cycle, there were no significant differences in the number of leaves. On the other hand, the difference in plant height increased as the crop progressed, resulting in longer stem in the plants cultivated with sand mulching (Fig. 2). Although the cultivation in bare soil led to a higher number of leaves at the beginning of the experiment, the cultivation with sand mulching led to a higher rate of leaf appearance (data not shown), which would explain why no significant differences were found in the number of leaves at the end of the experiment. The final plant height of the flowering stem was significantly affected by the sand mulching, increasing by 15% vs. plants cultivated in uncovered soil. A higher stem growth rate was observed in the plants cultivated with sand mulching throughout the experiment, with significant differences at certain times (data not shown). The soil temperature closer to the optimum one for the crop would have allowed a better initial development of roots and, therefore, a greater use of water and nutrients. However, at the end of the experiment, no significant differences were detected in fresh or dry root weight or in the stem/root ratio (data not shown). 455
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Fig. 1. Maximum, minimum and average daily temperature of air, bare soil (S) and soil covered with 5 cm of sand (M) during the entire crop cycle (a) and the first month (b) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions.
plants with sand mulching. In the present study, during most of the growing season, plants cultivated under sand mulching showed a significantly higher WI, which indicates a better water condition of the crop. This could be due to a better developed root system as well as lower evaporation and, therefore, greater availability of water in the soil due to the mulching effect. Advantage of sand mulching over bare soil was well demonstrated by Li (2003), who reported a strong reduction of evaporation
higher in the treatment with sand mulching during most of the experiment, which would explain that despite having the same amount of leaves, the LAI was higher in the plants grown with sand mulching due to a larger leaf size. On the other hand, according to Hogewoning et al. (2010), the photosynthetic capacity could be increased under a blue light enriched environment. In our case, this behaviour could compensate the inhibition of stem elongation, therefore no significant differences were found in dry weight but longer stems were recorded in
Fig. 2. Change of plant height and number of expanded leaves of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05. 456
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Fig. 3. Change of leaf area index (LAI) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
Fig. 4. Change of the red:far red (R:FR) ratio and reflected blue light (B) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
cultivated in bare soil could cause a situation of water stress of short duration and low intensity, but sufficient to impose a limitation to the crop growth. Zhang et al. (2011) found that water stress in Lilium longiflorum reduced the rate of photosynthesis and transpiration, resulting in plants of lower height and with a lower leaf area, that is, of lower quality. Our main conclusion is that lower soil temperature at the beginning of the crop cycle allowed a better initial development of roots, which could lead to a better use of water, determining higher growth, both in height and leaf area.
and matric potential under gravel-sand cover, and thus increasing water availability. Another beneficial effect of mulching could be a greater brightness in the canopy due to the reflection from the sand cover, as demonstrated by Conforti et al. (2015), who reported higher reflectance from a loamy sand soil in comparison with other soil texture classes. Therefore, the better quality flowering stems obtained under sand mulching could be related to the greater plant height due to the closer R:FR ratio, added to a greater photosynthesis due to a greater LAI and better light environment. Although some authors have reported an increase in soil temperature under sand mulching (Li, 2003; Lü et al., 2013), there is also evidence of a reduction in temperature (Wang et al., 2014). One of the possible causes of these discrepancies could be the water regime of each particular experiment. According to Al Kayssi et al. (1990), the soil water content affects the thermal behaviour of the soil, decreasing extreme variations by reducing maximum temperature values and increasing minimum temperature values. The gradual drying of the soil can lead to a concomitant increase in temperature, as demonstrated by Lakshmi et al. (2003). In our case, an irrigation regime was applied to guarantee non-limiting water, keeping the water content between field capacity and water readily available, but a lower root mass of plants
Conflict of interest Authors have no competing interests to declare. Acknowledgement This work was supported by the Universidad de Buenos Aires scientific program 2014-2017, within the framework of the UBACyT project code 20020130200269BA (Techniques to increase the efficiency in the use of water and fertilizers for a sustainable production of 457
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Fig. 5. Change of the Water Index (WI) of a Lilium longiflorum cv ‘Brindisi’ cut flower crop cultivated under greenhouse conditions, in bare soil (S) or soil covered with 5 cm of sand (M). Vertical bars indicate least significant difference at p ≤ 0.05.
ornamental species).
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