臺灣香蕉生理特性之研究（Ⅲ）香蕉光合作用、氣孔運動與葉片水分潛勢之日變化研究

Studies on the Physiological Characteristics of Banana in Taiwan (Ⅲ) A Study os Daily Variation in Photosynthesis, Stomatal Movement and Leaf Water Potential of Banana Plant

為瞭解香蕉光合作用、氣孔運動及葉片水分潛勢（leaf water potential; LWP）之日變化情形以及三者間之關係，曾於六十二年七月分別測定香蕉日間光合作用、氣孔運動及LWP之日變化。此外，LWP在香蕉全生育期之變化，亦加以探討測定。結果顯示，香蕉氣孔在早晨8：00時許即開始啟開，並急速增加，而於11：30〜12：00時左右，氣孔開度達到最高峰，但午後閉合極早，約在15：00時左右，即已完全閉合。而光合作用在上午則隨著氣孔開度之增加而增強，然而在12：00〜16：00時之間，卻發生午間下降（midday depression）之現象，使光合作用之日變化曲線呈雙峰狀，在18：00時以後，則只測出有呼吸作用而已。又LWP在早晨黎明時約-2.5 bars，然後隨著氣孔開度與光合作用之增加而降低，而於12：00～14：00時左右達到最低值（約為-4.5〜-4.8 bars.）。此時恰為氣孔開度開始急速下降與光合作用發生抑制減退之時刻，顯示LWP低於-4.5 bars時，氣孔開度與光合作用開始受到抑制。文文中並就三者之關係加以討論。在香蕉全生育期，LWP在營養生長期間，一般約-3.0 bars左右，但當香蕉生長進入花芽分化期時，LWP突然急速降低，而維持在-4.0～-4.3 bar之間，惟香蕉開始進入抽穗期時，LWP又急速增加（-2.4 bars），而於抽穗後又下降。顯示香蕉在花芽分化期至抽穗期，體內水分收支變化較激烈，用水管理上必需注意。此外，由於LWP在香蕉進入花芽分化期時突然降低，因此似可依此做為推測花芽分化期之指標。

A study was made of daily variation in photosynthesis, stomatal movement and leaf water potential of banana (Musa cavendishii Lambert cv. 'Rousta') under hot summer season in Taiwan. In the mornng, the stomata began to open at 8:00 a. m., then the aperture enlarged sharply and reached to maximum at 11:30~12:00 a. m., afterwards, the stomatal aperture commenced to shrink quickly and closed completely near at 15:00 in the afternoon irrespective of continueing fine sunlight condition. The rate of photosynthesis of banana plant in the morning increased concomitantly with stomatal aperture. However, a 'midday depression' occurred at 12:00 to 16:00 p. m., as a result, two peaks appeared on the curve of daily variation of photosynthesis. The rate of photosynthesis became unmeasurable after 18:00 in the afternoon. The daily changes in the leaf water potential varied from -2.3 to -4.8 bars, and at 12:00 to 14:00 p. m. the minimum potential (-4.5 to -48 bars) occurred. At this midday period, the stomatal aperture had shrinked sharply and photosynthesis was also suppressed. It implied that the stomatal opening and photosynthesis were depressed when leaf water potential decreased to below 4.5 bars. In this article, the relations of leaf water potential to stomatal opening and photosynthesis was discussed. Measurement of leaf water potential throughout the growth stage was also made. The changes in leaf water potential of banana plant varied among different growth stages. In vegetative growth, the leaf water potential remained at about -3.0 bars, but at time of flower formation stage the leaf water potential decreased abruptly to the minimum (-4,0 to -4.3 bars), thereafter, it raised again to maximum (about -2.4 bars) when the plant was shooting, and then the leaf water potential recovered and remained at near -3.0 bars after shooting. This suggested that a remark decrease in leaf water potential would signify the shift of banana growth from the vegetative to reproductive stage. The period for flower initiation to shooting stage is very important in water management. Therfore, to set up a sound water management program for the plantation of banana should take into consideration.