Effects of Water Stress on in Vivo Photosynthetic Biochemistry (Maximum Photosynthesis, Quantum Yield and 77 K Fluorescence)

Abstract

The effects of water stress on photosynthetic biochemistry (non-stomatal effects) remain controversial. There is no doubt that mild water stress reduces CO2 and light saturated photosynthetic activity and extreme water stress in mesophytic plants impairs quantum yield (1, 2). In natural habitats, peak water stress at mid-day coincides with peak light intensity so the possibility of light dependent damage to the photosynthetic apparatus needs to be considered (3). In rapidly stressed He lianthus annuus (a mesophyte), the depression of quantum yield is not dependent on light intensity (4) and hence presumably does not involve photoinhibition. In a slowly stressed xerophyte Nerium oleander, the loss of photosynthetic activity and quantum yield is accompanied by light dependent changes in electron transport activity and 77K fluorescence, indicative of damage to primary photochemistry and photoinhibition (5). We have used new techniques for photosynthetic and quantum yield measurement based on the leaf disc O2 electrode (6) and 77K fluorescence analysis (7) to examine these relationships under chronic, mild water stress and acute water stress in Xanthium strumarium and Helianthus.