Der Stickstoffhaushalt immergrüner mediterraner Hartlaubblätter

Abstract

The seasonal variations of major nitrogenous components (total N, protein N) in sclerophyllous leaves of four mediterranean species - Ceratonia siliqua, Olea europaea var. sylvestris, Pistacia lentiscus and Quercus coccifera - were investigated throughout the whole lifetime of the leaves. The leaf-longevity of the first two species mentioned is about two yars, that of the latter two species is somewhat longer than one year. As reference values the raw fibre content and the total chlorophyll content of the leaves were also measured. The raw fibre content increase during leaf-growth, there-after it remains constant. In the leaves having a lifetime of two years, in the spring of the second year an additional increase, although weak, is observed. The chlorophyll content rises during the leaf growth, then remains unchanged for some time, but begins to decrease early. In the leaves of Ceratonia and Olea the shrinkage is recognized already in the spring of the second year. During the period of senescence, the decrease of chlorophyll a is more distinct than that of chlorophyll b. The total N content in all cases decreases during leaf-growth and then remains constant. In Ceratonia leaves it is reasonably high considering that it is an evergreen species. Rather early the N-content begins to decrease; which is intensified during the last months of the lifetime the leaves. This is due to the well-known export of N-compounds from senescent tissues. In Ceratonia leaves the N-content increases during the first winter; obviously they serve as N-storage organs. The content of protein-N in all species shows essentially the same variations as that of total N. Results of tests carried out in the months of January and August revealed that the proline content in the leaves of all species is higher during the drought period than during the cold period. Using the different reference-values, the variations of N-content during leaf longevity of the four species are discussed in connection with reflections on maintenance costs.

Bei unseren bisherigen Untersuchungen zur Ökophysiologie der Stoffspeicherung in mediterranen Hartlaubpflanzen wurde der Stickstoffhaushalt nicht einbezogen. Erstaunlicherweise gibt es bisher kaum Untersuchungen dazu. Sogar die Lebensdauer der Blätter von Hartlaubpflanzen des Mittelmeergebiets wurde am natürlichen Standort erst neuerdings genau festgestellt. Da der stickstoffhaushalt der Blätter in engem Zusammenhang mit Entwicklungs- und Alterungsvorgängen steht, wurde er nunmehr während der ganzen Lebensdauer der Blätter von 4 mediterranen Hartlaubarten untersucht. Als Bezugsgrößen wurden neben der Trockenmasse auch die Größen Rohfasergehalt und Chlorophyllgehalt der Blätter herangezogen. Es ist bekannt, daß der Stickstoffhaushalt von Blättern durch Wassermangel erheblich beeinflußt wird. Von besonderem Interesse war es daher, festzustellen, welche Auswirkungen die Sommertrockenheit am Standort auf den N-Haushalt mediterraner Hartlaubblätter hat.