Leaf senescence in a non-yellowing mutant of Festuca pratensis: implications of the stay-green mutation for photosynthesis, growth and nitrogen nutrition

Mutation of the nuclear gene sid disables chlorophyll degradation during leaf senescence in the pasture grass Festuca pratensis. This study investigated the effect of the mutation on photosynthesis and on leaf and whole plant growth under a range of nitrogen regimes. When plants were cultivated in a static hydroponic system, the chlorophyll content of fourth leaves of the stay-green mutant Bf993 remained virtually unchanged from full expansion to complete senescence, while tissue of the wild-type (cv. Rossa) became completely yellow. The retention of chlorophyll in Bf993 was not associated with maintenance of photosynthetic activity as shown by rates of light-saturated CO₂ fixation and apparent quantum efficiency. Higher levels of total N in senescing leaves of Bf993 than in Rossa indicated reduced nitrogen remobilization in the mutant. When using a range of NH₄NO₃], dry matter production and tillering Mere lower for Bf993 at all but the highest NH₄NO³, which was supra-optimal for the wild type. In contrast to the static system, where fluctuations in N supply occurred, growth and NO₃^?] uptake were similar in mutant and wild type when NO₃^?] was continuously maintained by a flowing solution culture system. The results are discussed in relation to the role of N supply and the effect of the stay-green mutation on N recycling.