Nutrio-Physiological Evaluation of Drought Tolerance In Warm Season Forage Crops

湿潤気候にはいるわが国の西南暖地においても、暖地型飼料作物の生育が旺盛となる梅雨後は、高温寡雨のために土壌水分が減少し、干ばつが各地で頻発し易い。干ばつの被害を軽減し、作物の安定多収を測るためには、耐干性作物の生理生態額的特性を解明し、それを基礎にした栽培技術を確立することが必要である。本研究は、耐干牲の程度の異なるイネ科飼料作物を対象に、水ストレス下においても光合成、物質生産効率が比較的低下しにくい耐干性作物について、その生理生態的特性を比較栄養生理学的観点から研究し、作物における水ストレス耐性機構を解明したものである。 まず、数種の暖地型飼料作物に対する水ストレス実験から耐干性の大きい作物は水ストレス下でも葉の水ポテンシャルを常に高く維持し、光合成速度の低下が起こりにくい生理的特性を有することが明らかとなった。 耐干性作物の高い水分保持能力は水ストレス下でも(1)根系の発達がよく、(2)吸水能力が大きく、(3)吸収された水の葉への移送効率がよく、(4)クチクラ蒸散を抑える葉組織構造を有していることが14C及び3Hを用いたトレーサー実験などから立証された。 即ち、耐干性作物は水ストレス下でも14C同化産物を根へ効率よく転流させることによって根部組織を構成するタンパク質、セルローズ、リグニン等の合成を盛んに行っていること、及び吸水のための代謝活性を示す根圧、ATP含量、呼吸速度の大きいことが明らかになった。 3Hでラベルしたトリチウム水の吸収実験から耐干牲の大きい作物の根における吸水効率、葉への水移送効率は水ストレス下でも高いこと、また植物体内を水が通過する場合には、根と茎の移行部位、茎と葉の移行部位(節部)さらに葉鞘と葉身基部(カラー部)の3箇所で水の流れに対する抵抗、即ち通導抵抗が高く、その結果各器官への水の分配が阻害され、その程度は耐干性の弱い草種ほど大きいことが明らかとなった。 さらに耐干性作物は水ストレス下で葉表皮経胞にWaxを集積し、葉内における脂質合成能を高める事によって、クチクラ蒸散を抑え、水ポテシャルを高く保つ能力が大きいことが明らかになった。 耐干性は作物体内の栄養状態とくにリン栄養と密接に関係し、水ストレス下におけるリンの増施は作物の耐干性を強化する事が明らかになった。リン栄養状態が悪化すると根の生育が阻害され、根長、根表面積が著しく縮小することと、リン栄養状態が良好な葉に比べて高い葉の水ポテシャル値で気孔が閉じ、しかも水ポテンシャルの僅かな低下に対して光合成速度が著しく減少することが明確になった。Japan belongs to an area of much rainy and humidi climate with an yearly precipitation of 1500 to 3000mm, but the greater part of precipitation concentrate in Baiu period of June and July. In this period, plant grows vigorously, but their roots do not good well, compared to the tops, due to the high soil moisture condition. Precipitation after baiu period is very less which is about 10mm per month, and the evaporation rate is as much high as twice of precipitation due to high temperature and stong sunshine. Plant production is restricted by soil water deficits during dry season. Therefore, a wide study on physiological and morpological responses of crops to water stress in drought areas is important. The objectives of this study are (1) selection of drought tolerant crops by comparing growth rates under water stress conditions, and (2) evaluation of morphological and physiological adaptation of the crops to water stress condition to estimate their drought tolerant ability. 1) Effect of water stress on plant growth, leaf water potential, stomatal resistance and photosynthetic rate of forage crops. Seven species of warm season forage crops were grown in the field under irrigated and non-irrigated conditions. Ratio of dry weight of foliage in non-irrigated plants to irrigated plants was largest in Dallisgrass (Paspalum dilatatum Poir.), Rhodesgrass (Chloris gayana Kunth) and Bahiagrass (Puspalum notatum Flugge), smallest in Afirican millet (Eleusine coracana Gaerte), Corn(Zea mays L.) and Job's tears (Coix lacryma-jobi) and intermediate in Sorghum (Sorghum bicoior L.). Leaf water potential (Ψ1) was decreased, and stomata1 resistance(Υs) was increased in all crops under stress conditions. But tolerant plants were able to keep stomata open than sensitive plants under low Ψ1. In conclusion, the opening of stomata increases photosynthetic rate, growth, and yield under leaf water deficits conditions. The roots of rhodesgrass distributed in deeper layer of water stress soil. The development of roots in job's tears was severely lowered by stress and the roots could not distribute and/or penetrate in the deeper layer of soil under water stress. In sorghum and job's tears, 14C assimilate remained in leaves and stems and the translocation of 14C assimilates from shoots to roots was remakably reduced by water stress. And in job' tears, 14C incorporated into compartments of lignin-cellose and protein was apparently suppressed, but in rhodesgrass it was not so much as job's tears. The inhibition of root development in drought sensitive plants by water stress may be occured as a results of the lack of translocation of assimilate to roots from leaves. The results obtained show that drought tolerant palnts might maintain a higher water absorption ability under dry soil conditions by the development and penetration of roots. 2) Relationship between water use efficiency and cuticular wax deposition in forage crops grown under water deficit condition. In order to clarify the relationship between water use efficiency (apparent photosynthetic rate/traiispiration rate) and the synthesis and deposition of epicuticular wax and the synthesis and deposition of lipids in leaf internal tissues in various forage crops under different water stress conditions, the contribution of cuticular wax to the leaf cuticular resistance and water loss under stress was estimated. Under water stress condition, cuticular resistance of the drought tolerant plant rhodesgrass increased by about twofold compared with that of non-stressed condition, whereas the of the sensitive plant job's tears increased by about 10%.0 The amount of epicuticular wax deposited on the leaf surface in both plants increased by water stress. However, the amount in the tolerant plant was much higher than that of sensitive one. Radioactivity of 14C incorporated into leaf surface wax of stressed rhodesgrass and job's tears plants was 27 and 50-1.976868e+00ss than that of non-stressed plants, respectively, when they were treated with 14C-malonatc and 14C-acetate. Leaves of water stressed rhodesgrass assimilated 14c from 14C-malonate and 14C-acetate into the lipids of their internal tissues 5.7 times more than the leaves of non-stressed plants, while the leaves of water stressed job's tears assmilated 7815131060522f the amount recorded in non-stressed. On the other hand, the cuticular resistance of bloom line of sorghum with a high accumulation of wax on the epicuticular surface increased by about 90% by water stress, whereas the cuticular resistance of a bloomless line with a law amount of wax increased by about only 15%.0 Water use efficiency of bloom lines of sorghum increased remarkably by water stress, whereas that of the bloomless line of sorghum decreased. These date suggest that in the drought tolerant plants the water retention ability of the leaves increased due to the deposition of a larger amount of wax on their leaf surfaces, and a higher accumulation of lipids in internal tissues. And also these plants produced larger amounts of dry matter by increasing water use efficiency through the reduction of cuticular evaporation under water stress conditions. 3) Effect of water stress on root hydraulic conductivity, root pressure, and exudation from root system of forage crops. Rhodesgrass and job's tears were grown in pots in which the soil moisture was culture adjusted to pF1.7-2.0 and pF2.8-3.2. Tritium (3H) labelled water was injected to their rhizospheres and exudations from the cut surface of stem base were collected to determine radioacitivity of 3H. The total root length and surface area of roots in rhodesgrass were 1.5~2.0 times larger than in job's tears under water stress condition. 3H contents in the exuation of both crops which were well watered were higher than that of water stressed. In the exduation of rhodesgrass, 3H was obviously detected about 2 h after injection of 3 H2o and 3H content gradually under the water stressed condition, while 3H in exudation of job's tears delayed markedly and was about one-sixth of the content in rhodesgrass. The decrease of 3H in exudation found under water stress was due to the decrease of the exudation rate and the specific activity of 3H in xylem sap. Thus, rhodesgrass can transport water more effectivily from root to shoot, compared to job's tears under water stressed condition. The hydraulic conductivities of both plants were appreciably lowered by the water stress and estimated as 431 mg H2O/g root/h/bar for rhodesgrass and one third of that for job's tears under water stress condition solution culture in which the water potential was adjusted to -1.55 bar by additions of polyethylene glycol 6000. The root pressure of rhodesgrass and job's tears were estimated by using different water potential culture solution of 3.11 bar and 2.35 bar, respectively. From these results, it is assumed that the tolerance of plants to water stress is dependent on the root hydraulic conductivity and the root pressure, which are the main factors for the movement of water from roots to shoots under water stress condition as well as the root developing abilities. 4) Effect of water stress on movement of tritiated water through rhodesgrass and job's tears plants. The movement of water in water stressed plants of rhodesgrass and job's tears was studied by following the uptake of 3H2O from a tritium labelled nutrient solution. The movement was less in stressed plants than non-stressed plants. Even after 24 hrs, in water stress condition, in rhodesgrass, radioactivity of water compared to that in the nutrient solution, did not reach 8, 10 and 48 2048n the expanding leaves, expanded leaves and stein tissues respectivily, with respect to that of 19-1073768464n the root tissues. In job's tears, the values did not reach 4, 7, and 10 1159735604n the expanding leaves, expanded leaves and stem tissues respectivily, and that of 35 1667589734n the roots tissues. The movement of water in stem tissues is less than in root tissues and least in the leaf tissues under water stress conditions. Decrease of radioactivity in leaves of both rhodesgrass and job's tears was considerably slower than in the other organs, when the plants were transfered from a labelled to a nonlabelled solution. Under water stress conditions, the expanding leaves may have difficulty in obtaining water required for its growth due to the strong competition by thr expanding leaves, stems and roots. And also, there are differances in resistance to water movement between plant organs. The main resistance to water movement in the plant is apparently in the base stem and the nodes. Due to the reisistance to water movement, water absorption can not catch up with transipiration during day time of a fine day, and consequently leaf water potential decrease considerably. Leaf water potential is generally dependent on the resistance to water movement and the sensitivity of the plant to metabolic process. This suggests that the resistance to water transport could have an effect on plant growth and ultimately the yield. 5) Effect of water stress on the growth and amount of phosphorus in plants. Plant production is effected by drought but the nutritional status of the plants may alter the extend of damage. Some researchers have concluded that the application of nutrients improve plant growth under water deficits condition. The role of pl1osphorus under to water stress condition is not yet well understood. Plants of rhodesgrass and job's tears grown under various leaves of phosphorus were subjected to drought. Leaf water potential and stomatal resistance were measured in the uppermost fully expanded leaves, and the root and shoot growth was determined at vegitative stage. In water stressed plants, phosphorus increased the rate of leaf area expansion, leading to an increased total leaf area. Leaf water potential and stomatal resistance were measured in the uppermost fully expanded leaves, and the root and shoot growth wad determined at vegitative stage. In water stressed plants, phosphorus increased the rate of leaf area expansion, leading to an increased total leaf area. Leaf water potential decreased and stomatal resistance increased under water stress condition. Assuming a maximum stomatal resistance of 10 sec/cm in rhodesgrass, 50% stomatal closure occurred at a leaf water potential of -8 ～ -10 bar for Pdefficient plants and -14 ～ -16 bar for P sufficient plants. These values of job's tears was -7 ~ -9 bar for P defficient plants and -12 ~ -14 bar for P sufficient plants. These results confirmed that P defficiency increased stomatal sensitivity and also decreased the photosynthetic rate under water stress condition. P sufficient plants maintained higher photosynthetic sate under low water potential condition than P defficient Plants