Yuki Sago

Advantages of pre-harvest temporal flooding in a catch crop field in relation to soil moisture and nutrient salt removal by root uptake

Catch crop cultivation coupled with subsequent flood activity is an environmental friendly method of removing nutrient salts from soil in greenhouse. However, in comparison with the usual fallow period in greenhouse horticulture in Japan, a longer time is required for cultivation and soil drying after flooding. To minimize such time while retaining catch crop performance, temporal flooding was conducted in an experimental catch crop field of corn before harvest (i.e., pre-harvest temporal flooding), when crops were growing well and most nutrient salts within the soil had been taken up by the roots. Results showed that pre-harvest temporal flooding enhanced crop growth and stomatal opening; hence, evapotranspiration (mostly transpiration) was increased to a high value (3.5 times that of bare soil plot in greenhouse). Therefore, compared with the bare soil field, there was a remarkable pronounced decrease in the soil water content due to evapotranspirational water loss in the catch crop field after temporal flooding. Furthermore, the total nutrient (nitrogen) uptake by crops was also significantly accelerated in relation to pre-harvest flooding owing to the increase in crop growth. It was also found that electrical conductivity and nitrate nitrogen concentration of soil solution (at a soil-water ratio of 1:5) decreased with time owing to root uptake, and were at a fairly low level when pre-harvest flooding was conducted. These results suggest that pre-harvest temporal flooding shortens the implementation time by accelerating soil drying, and increases salt removal by root uptake; thus, this method delivers considerable advantages for practical use in catch crop cultivation.

Development of an Android-tablet-based system for analyzing light intensity distribution on a plant canopy surface

We developed an Android application system for easy estimation of PPFD distribution.The system output a PPFD histogram on the canopy surface from a reflection image.The system can be applied for plant production under artificial lighting. An Android application system for the simple and real-time estimation of light intensity distribution on a plant canopy surface was developed, and its usefulness was tested under artificial lighting. The application system was designed to semi-automatically analyze the photosynthetic photon flux density (PPFD) distribution on the canopy from a reflection image acquired by the tablet. A single manual measurement by a quantum sensor at a point on the canopy was performed to build a regression model that estimated the PPFD on leaves from the pixel values of the image. Measured and estimated PPFD histograms, as well as parameters derived from histograms, were compared at three different growth stages of a plant canopy in a closed plant factory with artificial lighting. The measured and estimated histograms exhibited a similar pattern at each growth stage with close values of the parameters. The results suggested that the reflection-image-based estimation system developed in this study was a useful method for analyzing the light conditions under artificial lighting. Although the developed system will require additional improvements in automation and performance before it can be applied to actual cultivation-management procedures, this simple method for estimating the light intensity distribution is expected to help improve the efficiency and reproducibility of light control methods used for plant production.

Transpiration integrated model for root ion absorption under salinized condition

Salinization of crop fields is a pressing matter for sustainable agriculture under desertification and is largely attributed to root absorptive functions of the major crops such as maize. The rates of water and ion absorption of intact root system of maize plants were measured under the salinized condition, and the salt absorptive function of maize roots was analyzed by applying different two kinetic models of root ion absorption (i.e. the concentration dependent model and the transpiration integrated model). The absorption rates for salinization ions (Na+, Cl−, Ca2+ and Mg2+) were found to depend on ion mass flow through roots driven by the transpiration, and therefore the transpiration integrated model represented more accurately rates of root ion absorption. The root absorption of salinization ions was characterized quantitatively by two model parameters of Q′max and K′M involved in the transpiration integrated model, which are considered to relate to the potential absorbing power and the ion affinity of transport proteins on root cell membranes, respectively.

Effects of Light Intensity and Growth Rate on Tipburn Development and Leaf Calcium Concentration in Butterhead Lettuce

Tipburn is a severe problem in producing butterhead lettuce under artificial lighting and develops as a consequence of decreased calcium concentrations in leaves. Here, we investigated the effects of light intensity on tipburn development and calcium concentration in leaves by comparing their growth rates. Butterhead lettuce was grown in a plant factory under artificial light at photosynthetic photon flux (PPF) densities of 150, 200, 250, and 300 mmol·m·s. Fresh and dry weights of shoots, relative growth rate, the number of leaves, and the number of tipburned leaves significantly increased with light intensity. Associations existed between growth and tipburn occurrence. Calcium absorption rate per plant also increased with light intensity in association with increased water absorption rate. Consequently, calcium concentrations in the entire plant and outer leaves increased with light intensity. In contrast, calcium concentration in the inner enclosed leaves did not increase with light intensity. This pattern can be attributed to the higher mass flow of calcium to outer leaves than to inner leaves, driven by transpiration, under high light intensities. Thus, a lack of calcium in the inner leaves resulting from rapid growth may contribute to the frequent tipburn development. Tipburn, a leaf marginal apex necrosis, is a serious problem in vegetable production under controlled environments (Cox et al., 1976), such as in closed plant production systems equipped with artificial light (Son and Takakura, 1989). Tipburn is generally considered a calcium-associated physiological disorder (Bangerth, 1979; Thibodeau and Minotti, 1969). The necrosis results from the rupturing of laticifer cells (Olson et al., 1967), and develops as a consequence of decreased calcium concentrations in the leaves (Barta and Tibbitts, 1991; Struckmeyer and Tibbitts, 1965). Many researchers have reported that tipburn is associated with rapid growth rate and high calcium demand in leaves (Collier and Tibbitts, 1982; Gaudreau et al., 1994; Saure, 1998). The rate of plant growth can be increased by controlling the cultivation environment, and light intensity is an important environmental factor in such stimulation. In a plant factory, which is a closed system equipped with artificial light and a controlled environment system to produce high-quality crops all year-round (Kozai, 2013; Kozai et al., 2016), plants are continuously cultivated at a very high rate to maximize the total efficiency of the production process. The stimulation of growth is important for optimizing production (Higashi et al., 2015; Murase et al., 2015). In particular, extension of the lightning period to continuous lightning has been used to produce crops at a higher rate under artificial light (Goto and Takakura, 1988; Oda et al., 1989). However, rapid plant growth under favorable environmental conditions leads to decreased calcium concentrations in leaves. In plant factories, butterhead lettuce (Lactuca sativa var. capitata L.) is considered as an important vegetable because it can be produced with high yield and has a high market value. However, the rapid growth leads to calcium deficiency and tipburn in the enclosed leaves. Therefore, the development of tipburn is frequent in butterhead lettuce production in plant factories (Lee et al., 2013; Son and Takakura, 1989) and can result in severe economic loss. There are numerous reports of methods to prevent tipburn development, such as the use of tipburn-resistant varieties (Cox and McKee, 1976; Koyama et al., 2012), foliar spraying of calcium salts (Thibodeau and Minotti, 1969), supply of air to inner leaves (Goto and Takakura, 1992), reducing the length of the light period (Tibbitts and Rao, 1968), shortening the day/night cycle (Goto and Takakura, 2003), and altering the spectral characteristics of the light source (Kleemann, 2004). However, although many factors have been implicated in tipburn development, simultaneous measurement of the growth rate and calcium concentration in the leaves is rare. The objectives of the present study was to determine the relationship between light intensity and tipburn development, in relation to plant growth rate and leaf calcium concentration to minimize tipburn while maintaining rapid growth in a plant factory. Butterhead lettuce was grown in a plant factory equipped with artificial light and was subjected to different light intensities to change plant growth rate. In addition, the rate of calcium absorption by roots and calcium concentration in leaves were determined to characterize the dependence of tipburn development on leaf calcium concentration and plant growth rates at varying light intensities. Materials and Methods Plant cultivation. All experiments were performed in a plant factory equipped with artificial lighting at the Faculty of Agriculture, Yamaguchi University, Japan. The room had two air conditioners (SZYA50CAV; Daikin Industries Ltd., Japan), a dehumidifier (MJ-180JX; Mitsubishi Electric Corp., Japan), a carbon dioxide supply system, and a deep flow soilless cultivation system. We controlled the air temperature, relative humidity, and carbon dioxide concentration in the cultivation area at 20 C, 60%, and 1200 mmol·mol, respectively. We cultivated butterhead lettuce (Lactuca sativa var. capitata L.) ‘Pansoma’ (Syngenta Japan K.K., Japan) for 35 d for all experiments. We sowed the seeds in polyurethane foam installed in plastic trays (600 · 300 · 50 mm) and supplied with nutrient solution. Two days after sowing, we illuminated the seedlings with continuous light using white fluorescent lamps (FHF32EX-N-H; Panasonic Corp., Japan). We set the PPF density (PPFD) at the surface of the polyurethane foams to 200 mmol·m·s. Ten days after sowing, we transplanted the seedlings into 48-hole floating panels (900 · 600 · 50 mm) and installed in a soilless cultivation system containing nutrient solution at a depth of 100 mm. The cultivation system had two cultivation beds (39,000 · 600 · 200 mm), a solution tank (100 L), a pump (MD55-RM; Iwaki Co., Ltd., Japan), solution cooling unit (RKS-400FS; Orion Machinery Co., Ltd., Japan), solution heating unit (AWA1510; Hakko Electric Co., Ltd., Japan), and electrical conductivity (EC) and pH sensors (PCE-11M; Cem Co., Ltd., Japan). The pump circulated the nutrient solution continuously between the cultivation beds and the solution tank at 30 L·min. We controlled the EC, pH, and temperature of the nutrient solution with a computer at 2.0 dS·m, 6.2 pH, and 20 C, respectively. Four fluorescent lamps were mounted above each bed, and the PPFD at the surface of each panel was set at 150, 200, 250, and 300 mmol m·s by adjusting the distances between the panels and lamps to 350, 280, 175, and 150 mm, respectively. To cultivate the plants at a high rate, plants were exposed to continuous light for all treatments. Twenty days Received for publication 25 Feb. 2016. Accepted for publication 21 June 2016. This study was supported by a Grant-in-Aid for Scientific Research (No. 26850157) and ‘‘Program to Disseminate Tenure Tracking System,’’ MEXT, Japan. I thank Shoko Fukuoka for technical assistance with the experiments. Corresponding author. E-mail: sago@yamaguchi-u. ac.jp. HORTSCIENCE VOL. 51(9) SEPTEMBER 2016 1087 CROP PRODUCTION

Water and salt movement in soil driven by crop roots: a controlled column study

Water deficit and salt accumulation in soil presents serious problems to crop production in semi-arid regions. These problems depend on the active transpiration stream and the selective absorption of ions by crop roots. In this study, a large sized soil column system was used to examine the dynamics of water and ion transport and salt accumulation in soil layers. Special reference was placed on the effects of the active and selective absorption by roots of different crops (i.e., corn plants, sunflower plants and no plants). The column system was equipped with on-line systems for the control of groundwater level. Soil water content sensors enabled time-course evaluations of the volumetric water content and hence upward flux of the groundwater in the soils at different depths. Furthermore, the distribution and accumulation of ions in soil layers, plant organs and xylem sap were analyzed using ion chromatography. In this column experiment, diurnal and longer term changes in water movement and ion accumulation in soil, affected by root absorption characteristics of plants, were evaluated quantitatively. The results demonstrated that the column system was applicable for the quantitative analysis of the effects of root absorption by different crops on water deficit and salinization in soils.