Periodic Nocturnal Stomatal Opening of Citrus in a Steady Environment

In a steady environment, leaf stomates of sour orange (Citrus aurantium L.) remained closed in the dark, whereas those of rough lemon (Citrus jambhiri Lush) underwent several 20-minute periods of pronounced opening at intervals of 60–85 minutes. Peak nocturnal opening occurred about 10 minutes after the cycle began, as shown by a change in leaf diffusion resistance from 60 s cm^?1 (closed) to 2 s cm^minus;1 (open). This minimal leaf resistance equals that obtainable in rough lemon in strong illumination (26 kilolux). The transitory stomatal opening detected by leaf resistance measurements was corroborated by measurements of leaf temperature and leaf thickness. During nocturnal opening, leaf temperature was 2°C below the value representing non-cycling periods. Also, minimal leaf thickness coincided with maximal stomatal opening. The triggering mechanism for nocturnal stomatal opening appears to originate within the plant, since the environmental factors of air temperature, humidity, carbon dioxide concentration, and substrate (aerated water culture) were held steady.