Leaf life span plasticity in tropical seedlings grown under contrasting light regimes

Abstract

Background and aims: The phenotypic plasticity of leaf life span in response to low resource conditions has a potentially large impact on the plant carbon budget, notably in evergreen species not subject to seasonal leaf shedding, but has rarely been well documented. This study evaluates the plasticity of leaf longevity, in terms of its quantitative importance to the plant carbon balance under limiting light.

Methods: Seedlings of four tropical tree species with contrasting light requirements (Alstonia scholaris, Hevea brasiliensis, Durio zibethinus and Lansium domesticum) were grown under three light regimes (full sunlight, 45 % sunlight and 12 % sunlight). Their leaf dynamics were monitored over 18 months.

Results: All species showed a considerable level of plasticity with regard to leaf life span: over the range of light levels explored, the ratio of the range to the mean value of life span varied from 29 %, for the least plastic species, to 84 %, for the most. The common trend was for leaf life span to increase with decreasing light intensity. The plasticity apparent in leaf life span was similar in magnitude to the plasticity observed in specific leaf area and photosynthetic rate, implying that it has a significant impact on carbon gain efficiency when plants acclimate to different light regimes. In all species, median survival time was negatively correlated with leaf photosynthetic capacity (or its proxy, the nitrogen content per unit area) and leaf emergence rate.

Conclusions: Longer leaf life spans under low light are likely to be a consequence of slower ageing as a result of a slower photosynthetic metabolism.

Fig. 1.

Survival functions of leaves of tropical seedlings grown under three light levels; no data is presented for Lansium and Durio in full light, due to high levels of mortality. Symbols: circle + continuous line, 100 % sunlight; triangle + dashed line, 45 % sunlight; diamond + dotted line, 12 % sunlight; cross, censored observation.

Fig. 2.

Leaf N content per unit area (a proxy for photosynthetic potential) of tropical tree seedlings growing under three light levels. Error bars represent standard error of the mean. ‘Plasticity’ is defined as the range of variation of the parameter monitored across light treatments relative to the mean; no data is presented for Lansium and Durio in full light, due to high levels of mortality.

Fig. 3.

Maximum photosynthetic rate (A_max, continuous line) and dark respiration (R_d, dashed line) of Hevea brasiliensis (left) and Alstonia angustiloba (right) seedlings growing under three light levels. Plasticity is defined as the range of variation of the parameter monitored across light treatments relative to the mean value over all light treatments. Error bars represent 95 % confidence interval of the mean.