Stomatal Movement and Water Relations in Crops : 2. Stomatal behavior of tobacco leaves of different ages and the influence of soil water shortage

Abstract

Diurnal stomatal behaviors were studied in three leaves (an upper immature, a middle immature, and a lower mature leaves) of a potted tobacco plant under green house condition using recording porometers, during three days. The effect of soil moisture stress (pF) upon stomatal behavior was also examined. 1. Every morning, pre-dawn opening was observed evidently in the upper and middle immature leaves, but hardly seen in the lower mature leaf (fig. 1). 2. The stomatal opening markedly increased in every leaf with increasing light intensity after sunrise even under high soil moisture stress (fig. 1). 3. The time of peak in the stomatal opening appeared earlier as the leaf became older. In the case of the immature leaf, the same phenomenon was seen under highest pF condition (table 1). These peaks showed sharped shape by the shortening of their width (opening period) due to the occurrence of remarkable midday closure (fig. 1). 4. The lower mature leaf showed severe midday closure of stomata at high light intensity even under mild soil moisture stress pF 3.10, but the upper immature leaf scarcely showed midday closure. 5. Soil moisture stress increment promoted the development of midday closure in depth and length in every leaf, especialy in the upper and middle immature leaves (table 2). 6. Averaged stomatal opening and opening period decreased with increasing leaf age and soil moisture stress (fig. 2). This resulted from that the leaf water deficit was produced by internal (leaf age) and external (pF) conditions, and that such deficit caused the midday closure and the suppression of reopening of tobacco stomata (table 2). 7. Marked difference was found between forenoon stomatal opening curve and afternoon stomatal closing curve within the middle and lower leaves. 8. The notable photoactive opening (including hydropassive opening) after sunrise and remarkable midday closure (hydroactive closing) under strong solar radiation occurred simultaneously in the tobacco stomata in three leaves. Such synchronism suggests that these both stomatal behaviors were associated with the variation of xylem hydraulic tension.