72 , 2017-03-24 , 国立研究開発法人 農業・食品産業技術総合研究機構 , National Agriculture and Food Research Organization (NARO)
近年のトマト施設生産において，多収化を目指した栽培や業務用需要の高まりに伴う周年安定生産技術の開発が望まれている．これを実現するためには，トマトの適温環境から外れる夏季の高温期および冬季の低温期の栽培が問題となり，冷暖房によって施設内気温を適温に近づける環境制御が実施されている．しかし，慣行的な冷暖房の手法は，装置の導入コストやエネルギー消費量の面で課題が残されているので，環境制御の有効性を維持しつつ，かつ導入コストの低減，消費エネルギーの削減が可能となる技術の開発が求められている．そこで本研究では，トマトの生育適温および障害の発生する温度が部位ごとに異なることに着目し，低温障害や高温障害を受けやすい部位を局所的に加温または冷却する方法を用いて，従来の施設全体を均一に温度制御する方法と同等の果実の収穫量を得ながら，導入コストおよび消費エネルギーの削減を達成することを目標にして，トマトの根域における夏季の高温期の局所冷却と冬季の低温期の局所加温，そして茎頂部における夏季の高温期の局所冷却と冬季の低温期の局所加温を行って，生育や果実の収穫量に及ぼす影響について検討することとした． まずⅡ章において，夏季の高温期にトマトをNFT水耕で栽培して，培養液を冷却（根域冷却）した場合の影響を調べした．その結果，根域冷却を行うと，一時的に根のインドール-3-酢酸（IAA）含量が培養液を冷却しない対照区より増加し，根の内部形態は木部特異的な肥大が認められた．さらに，養分の吸収や輸送および出液速度が増加し，根の相対成長速度（RGR）も増加した．また，根の内生IAA含量と根のRGRとの間に高い正の相関が認められた．これら地下部の特性の変化に続いて，地上部の乾物重も増加した． 次にⅢ章として，冬季の低温期にトマトをNFT水耕で栽培して，培養液を加温（根域加温）した場合の影響を調べた．その結果，Ⅱ章と同様に，養分の吸収と出液速度が培養液を加温しない対照区より増加し，根の乾物重とRGRも増加した．また，これら地下部の特性の変化に続いて，地上部の乾物重も増加した．一方，根の内生IAA含量と根のRGRとの間に相関が認められず，また，根の内部形態は皮層も中心柱も木部と同様に肥大し，Ⅱ章の結果と異なった．収穫期における部位ごとの乾物分配率には，根域加温の影響は認められなかったが，根域加温によって茎葉，根，果実を含む全体の乾物重が増加した結果，果実の収穫量と1果当たりの果実重は増加した． Ⅳ章では，夏季の高温期に茎頂部を局所冷却した場合の影響を調べた．その結果，茎頂部を局所冷却した区では，夜間の茎頂部の表面温度が局所冷却を行わない対照区と比べて低下した．一方，下位葉の表面温度に処理区間の違いは認められなかった．また，茎頂部の局所冷却によって，花粉稔性が対照区と比較して高くなった．さらに，収穫果実数と1 果当たりの果実重が増加し，果実の収穫量が多くなった． またⅤ章として，冬季の低温期における茎頂部の局所加温の影響について調べた．まず，1節において，冬季の低温期に小型の電気温風機を用いて茎頂部を局所加温した結果，茎頂部の表面温度が高くなるとともに，花粉稔性と着果率が高まり，花房間の開花間隔が短縮された．さらに，‘ 桃太郎はるか’においては，茎頂部の局所加温によって果実の収穫量が増加した．次に2節において，実用的な茎頂部の局所加温方法として，暖房機に接続する温風ダクトをトマト群落上に吊り下げる方法による局所加温の影響を調べた．その結果，茎頂部の局所加温によって，茎頂部の夜間の室温と，植物体の表面温度は慣行より高くなったが，下位葉の夜間の室温と，植物体の表面温度は慣行より低くなった．果実の収穫量に関して，可販果の割合は局所加温区の方が慣行区より高く，1果当たりの果実重も局所加温区の方が慣行区より大きくなったので，品種によって程度に差はあるものの，可販果の収穫量が多くなった．燃料消費量は，局所加温 区は慣行区と比較して26.2%の削減となったので，燃料の単価を 64.5 円 L-1とした場合，10a当たりで年間およそ13万円の暖房費が削減できると試算された． 以上の結果，本研究によって，トマトの施設栽培における夏季の高温と冬季の低温に対して，根域と茎頂部の局所冷却と局所加温が，それぞれ有効であると結論付けられた．これらの局所温度制御を適切に組み合わせることによって，温度障害を緩和し，周年安定生産に寄与する技術として体系化していくことが必要であると考察された． In recent greenhouse production of tomato（Solanum lycopersicum L.）, techniques to realize year-round and stable productions are desired for higher yield and commercial use. To achieve this, it is important to overcome suboptimal temperature conditions, such as high temperatures in summer and low temperatures in winter. Heating or cooling air is a practical solution to this. However, the initial cost and energy consumed are shortcomings that need to be addressed. Therefore, a new technique is needed to reduce energy consumption with low initial cost.The sensitivity of different parts of the tomato plant to temperature differs. In this study, therefore, we focused on controlling temperatures that resulted in temperature-related injury in the more sensitive parts of the tomato plants. If temperatures experienced by temperature-sensitive parts such as shoot apex, flowers, and roots are controlled locally, energy consumption can be reduced without loss of fruit yield when compared to conventional temperature management, under which ambient temperature in the greenhouse are controlled uniformly.Therefore, we cooled the root zone under high temperature condition in summer and heated this zone under low temperature condition in winter to reveal the influence of local temperature control on the root zone. Furthermore, shoot apexes and flowers were cooled or heated to clarify the effect of local temperature control around the shoot tips.In chapter II, we describe the cultivation of young tomato plants in a hydroponic system with a cooled nutrient solution by using nutrient film technique（NFT） under high temperature conditions. This cultivation method was used to acquire information about the physiological and morphological effects of root-zone cooling. We investigated plant growth, nutrient uptake, root activity（xylem exudation and root respiration rate）, root indole-3-acetic acid （IAA） concentration, and internal root structure. The root-zone temperature was maintained at 24.7℃ by cooling, which is considered the optimum temperature for tomato plants. The air and control temperatures were higher than this optimum（30.8 and 33.7℃, respectively）. Root-zone cooling increased the relative growth rate（RGR）of roots compared with the control, followed by increased RGR of shoots. Root IAA was positively correlated with root RGR. Root-zone cooling increased Ca and Mg uptake, as well as root xylem exudation and respiration.It also advanced the development of the internal structure of the xylem near the root tip. Thus, possibly by increasing root activity and root IAA, root-zone cooling promoted root growth and nutrient uptake mediated by the development of the root xylem, and thus, further improved shoot growth. These results suggest a physiological and morphological mechanism of growth enhancement by root-zone cooling under high air temperature conditions.These results are detailed in Chapter II. In Chapter III, we detail our study on the physiological and morphological effects of root-zone heating. For this, as an economical option at low air temperatures, we grew tomato plants on an NFT hydroponic system in a heated nutrient solution. We investigated the effects of short-term root-zone heating after transplanting and long-term heating until harvest. We measured short-term plant growth, nutrient uptake, root activity, IAA concentration, internal root structure, and long-term fruit weight and dry matter distribution. The minimum rootzone temperature was maintained at 16.6℃ while the minimum air temperature（5.9℃） and minimum rootzone temperature（5.8℃） in the control were lower than optimal. Similar to the results reported in Chapter II, root dry weight and RGR increased after 7 days of root-zone heating, compared with those of the control. This was accompanied by increased mineral nutrient uptake and xylem exudation. These changes may explain the increased shoot growth after 21 days of heating. In roots, development of the epidermis and stele, including the xylem, was promoted by heating, in contrast to the effects of root-zone cooling, which promoted only xylemspecific development. This could be explained by a lack of correlation between root IAA concentration and root RGR. Although the proportion of dry matter distributed to the fruit was not changed by root-zone heating, individual fruit size and total yield were higher than in the control because of a higher total dry weight in the heating treatment. Our results suggest that root-zone heating is an effective low-cost heating technology at low air temperature because of its effect on root activity, growth, and fruit yield, but that the mechanisms may differ from those in root-zone cooling at high air temperatures.In Chapter IV, we detail the experiment where we tested a local cooling system that cools the air around flowering trusses and shoot apexes at night by using a heat pump with a hanging air duct. In order to reveal the effect of the local cooling system, temperature conditions, and characteristics related to fruit yield were compared with non-cooling treatment. Local cooling reduced the night air and surface temperatures around the flowering trusses by ~2℃, but did not change those lower down the plant. It resulted in increased pollen viability（measured as the proportion of pollen that accepted an acetocarmine stain）, fruit number, and individual fruit weight, and thus marketable fruit yield. These results show that local cooling alleviated the effects of high-temperature on tomato during summers in greenhouses.In Chapter V, we detail our investigation on the influences of local heating around flowering trusses and shoot apexes under low air temperature conditions. First, we used a small electric heater to provide local heating around flowering trusses and shoot apexes, and examined differences in surface temperatures and fruit yield. We measured the surface temperature of tomato plants using thermography, and examined some characteristics related to fruit yield. Tomato plants were grown using two local heating treatments（“high,” with an average night temperature of 13.0℃ at the surface around the shoot apex, and “low,” with a surface temperature of 11.5℃） under the lowest night temperature of 8℃, as well as a control condition, with a surface temperature of 9.6℃. The surface temperatures of shoot apexes were increased by the local heating, although the lower leaf temperatures were not.Pollen viability, fruit set, and intervals between flowering days of adjacent trusses were improved by local heating, although cultivars appeared to differ in their responses.Second, in order to develop a more useful local heating system, we examined the system with air ducts hung above the culture beds supplying warm air to shoot apexes and flowering trusses, and clarified the economic and physiological effects. Differences in the vertical distributions of surface temperature at night, fruit yield, and fuel consumption were investigated. As a result of the local heating, the temperature of air and plant surface in the upper part of the plant at night were slightly higher than the corresponding temperatures recorded in the control, where conventional air duct on the ground was used. However, the air and plant surface temperatures in the lower part of the plant were considerably lower. The commercial fruit yield when this local heating system was used tended to be greater than those in the control because of a higher ratio of intact fruit and fruit weight after exposure to local heating. Fruits grown by local heating had a higher ratio of locule tissue. The local heating system used 26.2% less fuel per unit area than the control.In this study, we concluded that local heating in winter and cooling in summer on root-zone and around shoot tips were effective for greenhouse tomato production. If these techniques are combined adequately, temperature injuries can be alleviated with lower costs, and this will contribute to year-round and stable tomato production in greenhouses.