光合成速度における寒地型牧草と暖地型牧草との差異 On Difference between Tropical and Temperate Grasses in Photosynthetic Rate

本研究は寒地型牧草と暖地型牧草の光合成速度の差異を明らかにするために行なったものである。個葉の光合成速度は人工光下で同化箱を用い,赤外線ガス分析計を使用して測定した。なお,実験には寒地型牧草14種,暖地型牧草27種を供試した。光合成測定時の光条件は寒地型牧草に対しては45Klx,暖地型牧草については65Klxとし,呼吸作用は暗黒下で測定した。一方,気温は同化箱内のジャケットに冷・温水をとおして,寒地型牧草は20±0.5℃,暖地型牧草は35±0.5℃にそれぞれ調節した。実験は寒地型牧草については1975年の5月,暖地型牧草は1976年6月下旬から7月の期間にそれぞれ行なった。えられた結果を要約すると以下のとおりである。光合成速度は暖地型牧草と寒地型牧草とでは著しく異なり,寒地型牧草は低く,暖地型牧草の多くは寒地型牧草の二倍以上の能力を示した。特にダリスグラス,マカリカリグラス,グリーンパニック,ベィジィグラス,ローズグラス,バヒアグラスおよびグレインソルガムは50mg CO_2・dm^<-2>・hr・<-1>以上の高い光合成速度を示した。しかし,暖地型牧草の中には30mg CO_2・dm^<-2>・hr^<-1>前後の比較的低い光合成速度にとどまるものがあった。すなわちセントオーガスチングラス,センチペードグラス,カーペットグラス,ウィーピングラブグラスおよびキクユグラスであり,寒地型のトールフェスク,ブロームグラスとほぼ等しい光合成速度であった。以上の結果はすべての暖地型牧草が高い光合成を行うものでないことを示唆している。光合成速度と比葉面積との相関を求めた結果,寒地型牧草は負の高い相関係数が求められ,有意性を示したが,暖地型のそれは低く,有意性がえられなかった。つぎに光合成能力とイネ科牧草の分類との関係については,キビ亜科,あるいはスズメガヤ亜科の暖地型牧草はウシノケグサ亜科の寒地型牧草より光合成能力がすぐれており,キビ亜科またはスズメガヤ亜科の牧草にあっても,スズメノヒエ属,キビ属およびヒゲシバ属の牧草は概して他の牧草に比較して高い光合成能力をもっている。したがって牧草の光合成能力の種間差は個々の牧草の分類学上の位置や,進化発達と深いかかわりあいをもつものと推論される。

This study was undertaken to investigate the difference between tropical and temperate grasses in photosynthetic rate. The photosynthesis and respiration of an attached leaf were measured in an assimilation chamber (Fig.1) under artificial light condition. The concentration of CO_2 gas in the air was surveyed at the point of entering and leaving of the chamber by means of infra-red gas analyzer. Fourteen temperate and 27 tropical species (including sorghums, corn and African millet) were used. Photosynthesis was measured under light intensities being kept 45 Klx. for temperate and 65 Klx. for tropical species and respiration was measured in darkness. Air temperatures were controlled at 20±0.5℃ for temperate and 35±0.5℃ for tropical species by passing a stream of warm or chilled water through a jaket of the chamber. The experiments were carried out during May in 1975 for temperate species and late June to July in 1976 for tropical species, respectively. The results obtained were summarized as follows. It was quite evident that photosynthetic rate differed sharply between tropical and temperate grasses, being lower in the latter. Most tropical grasses were able to reach about twice as much CO_2 assimilation as the temperate grasses. Especially, dallisgrass, makarikarigrass, green panic, vaseygrass, Rhodesgrass, bahiagrass and grain sorghum showed higher photosynthetic rates (50-55mg CO_2・dm^<-2>・hr^<-1>). Tropical grasses such as Saint Augustine grass, weeping lovegrass, carpetgrass and kikuyugrass, however, showed no more than 27-31mg CO_2・dm^<-2>・hr^<-1>, which value was almost similar to that of smooth bromegrass and tall fescue of temperate grass. From these results, it is suggested that not all of tropical grasses show a high CO_2 assimilation. A significant negative correlation was observed between photosynthetic rate and specific leaf area in temperate grasses and no correlation was found in tropical grasses. Tropical grasses which belong to Panicoideae or Eragrostoideae are superior in photosynthetic activity to temperate grasses of Festucoideae. It is assumed that the species of Panicum, Paspalum and Chloris are superior to the other genera of Panicoideae or Eragrostoideae in photosynthesis. Therefore, it may be concluded that these specific differences in photosynthetic activity are closely associated with taxonomic and evolutionary group of each grass species.