EFFECT OF LIGHT AND GIBBERELLIC-ACID ON PHOTOSYNTHESIS DURING LEAF SENESCENCE OF ALSTROEMERIA CUT FLOWERS

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas- exchange measurements and analysis of in vivo chlorophyll fluorescence . Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA 3). The maximal photosynthesis of the leaves was approximately 6 mumol CO2 M-2 s-1 at 1 350 mumol M-2 s-1 (PAR) which is relatively low for intact C3 leaves. Qualitatively the gas-exchange rates followed the de cline in chlorophyll content for the various treatments, i.e. light an d GA3-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosy nthetic rates were already strongly decreased. In vivo fluorescence me asurements revealed that the decrease in CO2 uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analys is of the fluorescence data showed a high nonphotochemical quenching u nder all experimental conditions, indicating that the consumption of r educing power in the Calvin cycle is very low. The chlorophyll, remain ing after 9 days incubation of leaves with GA3 in the dark should be c onsidered as a 'cosmetic' pigment without any function in the supply o f assimilates to the flowers.