High relative air humidity and continuous light reduce stomata functionality by affecting the ABA regulation in rose leaves

Plants developed under high (90%) relative air humidity (RH) have previously been shown to have large, malfunctioning stomata, which results in high water loss during desiccation and reduced dark induced closure. Stomatal movement is to a large extent regulated by abscisic acid (ABA). It has therefore been proposed that low ABA levels contribute to the development of malfunctioning stomata. In this study, we investigated the regulation of ABA content in rose leaves, through hormone analysis and β‐glucosidase quantification. Compared with high RH, rose plants developed in moderate RH (60%) and 20 h photoperiod contained higher levels of ABA and β‐glucosidase activity. Also, the amount of ABA increased during darkness simultaneously as the ABA‐glucose ester (GE) levels decreased. In contrast, plants developed under high RH with 20 h photoperiod showed no increase in ABA levels during darkness, and had low β‐glucosidase activity converting ABA‐GE to ABA. Continuous lighting (24 h) resulted in low levels of β‐glucosidase activity irrespective of RH, indicating that a dark period is essential to activate β‐glucosidase. Our results provide new insight into the regulation of ABA under different humidities and photoperiods, and clearly show that β‐glucosidase is a key enzyme regulating the ABA pool in rose plants.     

Occurrence of Inverted Water Potential Gradients between Soil and Bean Roots

Measurements with a pressure chamber were made of the xylem water potential of leaves, shoots and roots from bean plants (Pkaseolus vulgaris L. cv. Processor) grown with a 12 hour dark period and natural or artificial light conditions during the day. The water potentials were measured at the end of a dark period and during the light period. Measurements taken at the end of the dark period indicated normal potential gradients within the soil/plant system (leaf < shoot < root < soil), when the matric potential of soil water was relatively high (above ?0.02 bar), and the gradients then also remained normal during the day (natural light). When the soil water potential was ?1 bar or lower in the morning, however, the root xylem water potential was higher than the soil water potential; at very low soil water potentials (< ?4 bar) it remained higher during most of the day. In this case also leaf and shoot xylem water potentials were higher than the soil water potential in the early morning, although decreasing rapidly in daylight. Under artificial light, both leaf and root water potentials were higher than the soil water potential throughout the whole diurnal cycle when the latter potential was below ?4 bar. From measurements of stomatal diffusion resistance, transpiration, relative water content of leaves and of changes in the matric potential of soil water, it was concluded that when the matric potential of soil water was low, water could be taken up by the plant against a water potential gradient. Because leaf xylem water potential was always lower than root xylem water potential, the mechanism involved in the inversion of water potential gradient must be localized in the roots, and probably related to ion uptake. Symbols and abbreviations used in the text: Ψ: Plant water potential (thermocouple psychrometer); Ψx: Xylem water potential (pressure chamber); Ψs: Osmotic potential of xylem sap; Ψm: Matric potential of soil water; RWC: Relative water content.     

Landscapes of Settlement in Northern Iceland: Historical Ecology of Human Impact and Climate Fluctuation on the Millennial Scale

Early settlement in the North Atlantic produced complex interactions of culture and nature. The sustained program of interdisciplinary collaboration is intended to focus on ninth- to 13th-century sites and landscapes in the highland interior lake basin of Mývatn in Iceland and to contribute a long-term perspective to larger issues of sustainable resource use, soil erosion, and the historical ecology of global change.