The Influence of Leaf-aging on the Heat-sensitivity and Heat-hardening of the Photosynthetic Apparatus in Phaseolus vulgaris

Abstract

The resistance of the photosynthetic apparatus of primary leaves from Phaseolus vulgaris to short-term heat-shock (5 min) and to long-term heat-stress (1 to 5h) is studied. A model system is used, in which the aging of primary leaves is retarded by decapitation (removal of the apex). This offers the opportunity to compare leaves from the same calendary age (25 to 32 d) but being in different physiological states (mature or senescing). Photosynthetic CO₂-uptake, chlorophyll fluorescence and the field-indicating absorbance change at 518 nm are measured with intact leaves after different heat-stress to characterize the pattern of heat-damage. It is shown, that there is no significant difference between the sensitivity of mature or aged leaves to short heat-treatments. However, the ability of leaves to increase their thermal stability during incubation at high, but not yet damaging temperatures (heat-hardening), clearly demonstrated in mature leaves, declines during aging and is almost absent in senescent leaves. This loss in adaptive changes may account for the decline in the overall resistance of the photosynthetic apparatus to longterm heat-stress observed during leaf-senescence.