Spatial patterns and metabolic regulation of photosynthetic parameters during leaf senescence

To prevent premature cell death and to allow efficient nutrient mobilization from senescing leaves, the photosynthetic apparatus has to be dismantled systematically. This requires temporal, spatial and metabolic regulation of photosynthetic function and photoprotection. Conventional pulse-modulated fluorometry and chlorophyll fluorescence imaging were used to study age- and nutrient-dependent senescence patterns in Arabidopsis thaliana. Nonphotochemical quenching (NPQ) rose during leaf maturation, indicating increased energy dissipation. During later stages of senescence, overall plant NPQ declined, but NPQ remained high in the base of rosette leaves. Other fluorescence parameters also showed spatial patterns, for example minimum fluorescence (F-0) was temporarily increased in the tips of inner rosette leaves from where high F-0 spread to the base, in a zone preceding cell death. Senescence-dependent changes in chlorophyll fluorescence characteristics were accelerated by growth on glucose-containing medium in combination with low, but not with high, nitrogen supply. Our experiments revealed distinct spatial patterns of photosynthetic and photoprotective processes in senescing leaves and induction of these processes by high sugar-to-nitrogen ratios.