Common buckwheat (Fagopyrum esculentum) is well known for its weed-suppressive ability. This property is probably due to multiple factors such as resource competition, allelopathy and soil property modifications. A better understanding of the mechanisms of weed suppression could improve the development of new strategies for weed management using cover crops. In this review, the different factors that could be potentially responsible for weed suppression by common buckwheat are discussed. Special emphasis is put on the role of allelopathy.

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Circadian rhythms are synchronized by the external environment and are responsible for regulating many of physiological processes in organisms. In plants, a key component in the regulation of these physiological processes is referred to as circadian resonance (CR), which maximizes plant growth by entraining the periodicity of plant circadian rhythms to the external light-dark cycle. However, CR has only been extensively studied in Arabidopsis thaliana, and it is not known whether CR has a similar function in other plant species. In addition, the potential for manipulating CR in order to maximize agricultural production or plant growth has not yet been investigated. In this study, we sought to characterize the occurrence and role of CR in lettuce (Lactuca sativa L.), which is widely cultivated in closed plant factories. Our results showed that the variation in aerial weight is dependent upon both cultivar type and light quality, and the efficiency of CR is highest under red LED illumination and lowest under fluorescent light illumination. Furthermore, we demonstrated that LEDs may be highly beneficial for maximizing plant growth. Compared to FL illumination, LED illumination can be used to precisely control the circadian rhythm of plants and increase cultivation efficiency.

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Compared to sub-irrigated sweetpotatoes (Ipomoea batatas (L.) Lam.), periodic surface-irrigated plants, i.e., twice a week on root media, showed increased α-tocopherol content in their tuberous roots with no apparent changes in both of tuberous root development and oxygen concentration around the roots. We speculated that surface irrigation might temporarily cover the tuberous root surface with water and inhibit oxygen movement into the roots, thereby increasing the antioxidant α-tocopherol content, for coping with the slight oxidative stress occurring within the roots. Therefore, we performed 1–3 times instantaneous flooding, with different intervals, which perfectly covered the whole root surface with water, of sweetpotato plants grown in a phytotron glass room (25°C, 70%RH). Electrolyte leakage from the tuberous root flesh cells showed a temporal increase for the flooding treatment, while it immediately recovered within 24 h. Instantaneous flooding did not affect the storage root development in any of the experiments. Apparent increases in the α-tocopherol content were observed during the 3-time flooding at 3-day intervals. Our results suggest that more frequent root surface wetting is necessary for increasing the α-tocopherol content, which is released because of the oxidative stress that occurs within the roots.

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Night-break (NB) treatment inhibits flowering in short-day plants and is widely used in the flower production of cut chrysanthemum (Chrysanthemum morifolium Ramat.). Light-emitting diode (LED) lamps have been replacing incandescent lamps owing to energy savings. Previous studies on chrysanthemum under NB treatment with LED lamps raise the possibility of varietal difference in floral differentiation ability. Therefore, this study evaluated the effects of NB treatment using LED lamps with several wavelengths on 12 chrysanthemum cultivars. NB treatments at 630-, 660-, and a combination of 660- and 735-nm LEDs inhibited floral differentiation in all cultivars. However, the effects of NB treatments with 690- and 735-nm LEDs differed among cultivars. Some cultivars showed inhibited floral differentiation with both 690- and 735-nm LED NB treatment, whereas others were scarcely affected. Meanwhile, NB treatment with a combination of 660- and 735-nm LEDs had the most stabile inhibitory effect on floral differentiation among chrysanthemum cultivars.

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In the forced cultivation of asparagus (Asparagus officinalis L.), it is necessary to completely break the dormancy state. We investigated the effect of high temperatures on dormant asparagus (‘Welcome’) to develop an efficient method for the early breaking of dormancy. Dormant plants were treated at 20.0°C, 22.0°C, 24.0°C, 26.0°C, and 28.0°C for 21 d in incubators. After treatment, spear germination was observed in the plants treated at relatively high temperatures (26.0°C and 28.0°C), whereas those treated at 20.0°C, 22.0°C, and 24.0°C remained dormant. Dormant plants treated at 28.0°C for 2, 4, 8, 12, 15, and 20 d were moved to the greenhouse (average temperature 18.2°C) to evaluate the effects of the treatment period on dormancy breaking. Spear germination was observed in plants treated for 4, 8, 12, 15, and 20 d, while untreated plants and those treated for 2 d still appeared dormant. These results suggested that the dormancy of asparagus (‘Welcome’) could be broken by treatment for 4 d at 28°C.

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In this study, a system to measure leaf internal CO₂ (C_i) was incorporated into an open gas exchange system (LI-COR, Lincoln, NE, USA). The C_i was directly measured with a cup attached to the abaxial surface of sunflower (Helianthus annuus L.) leaves with open stomata while normal CO₂ and water vapor exchange through the same section of adaxial surface was simultaneously detected. The potential problems in the system, namely bulk air flow through the leaf, diffusion leaks, and change in the CO₂ gradient inside the leaf, were examined with the aim to apply the system to measure net photosynthesis at various C_i (i.e. A−C_i curves). A micro blower constantly circulated the air in a loop without pressure pulses or bulk air movement through the leaf. The measured C_i (C_i(m)) generally followed the external CO₂ as much as the calculated one (C_i(c)). There was close agreement between the C_i(m) and the C_i(c) particularly at low C_i, and the diffusion leak hardly affected the relationship between the two. Despite possible alterations of leaf properties by cup attachment, the direct measurement is expected to cast a new light on leaf gas exchange.

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Several studies have reported that the nutritional quality of spinach is enhanced by application of cold stress to the root area. However, the relationship of the nutritional value of spinach with the root area temperature and duration of root area chilling has not been investigated. Here, we examine the ascorbic acid, nitrate ion, and soluble solid content of spinach subjected to root area chilling in a nutrient solution for 2, 4, 5, 6, and 7 days at various temperatures (4, 6, 10, and 14°C). Under all temperature conditions, the nitrate ion concentration significantly decreased within 2 days of the onset of chilling. Ascorbic acid and soluble solid content showed similar trends; significant change in concentrations of both the compounds occurred after 4 days at 4°C, 5 days at 6°C, 6 days at 10°C, and 7 days at 14°C. On the basis of these results, we developed the relational expression for the chilling duration and solution temperature. The results of this study indicate that adjusting the chilling duration and solution temperature according to the relational expression would help plant factories produce high value-added spinach.

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Controlling weeds through allelopathy is one strategy to reduce dependency on synthetic herbicides. We investigated possible allelopathic effects of rattail fescue (Vulpia myuros (L.) C.C. Gmel.). Water extract of rattail fescue inhibited root and shoot growth of cress (Lepidium sativum L.). Powder of rattail fescue also inhibited the root and shoot growth of cress in a concentration dependent manner. The effectiveness of the water extract and powder of rattail fescue on the cress root and shoot growth was not significantly different. Allelopathic active substances may be leached from the powder into bioassay medium and those substances may inhibit the cress roots and shoots. In addition, the powder inhibited root and shoot growth of lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), Phleum pratense L., Digitaria sanguinalis L., Lolium multiflorum Lam. Lolium rigidum Gaund., Echinochloa crus-galli (L.) Beauv. and Echinochloa colonum L. in a concentration dependent manner. Therefore, rattail fescue could be useful for a weed suppressive residue or soil additive materials in the variety of agricultural settings to reduce dependency on synthetic herbicides, which should be investigated further in the field.

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