The effect of photosynthesis parameters on leaf lifespan

Resumo

Leaf longevity (LL) varies widely among higher plants. LL is a sum of the functional component LL_f (the duration of active photosynthesis) and the nonfunctional component LL_n (duration of the period during which photosynthesis does not occur). LL_n corresponds to the period of winter dormancy in the case of evergreen boreal species. The photosynthetic potential of the leaf (PPL)—that is, the maximal possible amount of CO₂ fixed during the leaf lifespan—is inferred from the dynamics of the maximal rate of photosynthesis (P_a) during LL_f. P_a reaches a peak value (P_{a max}) during leaf “maturation.” The photosynthetic potential depends on the functional lifespan to a greater degree than on the maximal rate of photosynthesis. The PPL/LL_f ratio determines the rate of realization of the photosynthetic potential during the lifetime of the leaf. LL_f is strongly and positively correlated to LL, and LL can therefore be considered a parameter determining the rate of PPL realization, along with LL_f (for a first approximation). The prolonged LL_f characteristic of evergreen species gives them an advantage—a higher photosynthetic potential than that of deciduous species. Consequently, PPL is realized more slowly in evergreen species than in deciduous species. An increase of LL_f and LL is accompanied by an increase of leaf construction cost (LCC_a) and a decrease of the photosynthesis rate, with the decrease per unit dry weight (P_m) being much more pronounced than that per unit leaf area (P_a). This points to a much higher contribution of cell wall weight to the total dry weight of longlived leaves of evergreen species than to that of short-lived leaves of deciduous species. Unidirectional changes of PPL and LCC_a stabilize leaf payback (LP). Species with a short (long) LL_f and high (low) rate of PPL realization are characteristic of early (late) succession and habitats with favorable (unfavorable) environmental conditions for photosynthesis and growth, since these species are more competitive under said conditions. Species with a high rate of PPL realization have an advantage in competition under conditions favorable for photosynthesis and growth, since they use environmental resources for rapid growth and expansion. The rate of photosynthetic potential realization characterizes the aging rate of the leaf.