Some Physical Differences Between Sun and Shade Plants
AB - Photosynthetic efficiency is often quantified as the light-limited, maximum quantum yield in ecophysiological studies. Four published comparative studies report that photosynthetic efficiency varies little among plant species of widely diverse origins, and that quantum yields were near the maximum theoretically attainable value. However, many other published studies contradict this conclusion, reporting quantum yields as low as 30% of those found in the comparative studies. These studies have created the impression that certain plants, particularly wild plants growing outdoors, may have intrinsically low photosynthetic efficiencies. To investigate the validity of these differing interpretations, we compiled quantum yield data from a survey of 30 published studies and compared those with data from the two most comprehensive comparative quantum yield studies. We also included quantum yield observations that we made on ten species. While our data confirm the results of the comparative studies indicating that maximum quantum yield is high and invariant, the literature survey data showed a wide range of quantum yield values. To investigate whether low quantum yield values could be caused by data collection and analysis techniques, we analyzed photosynthetic light-response data. Substantial underestimation of quantum yield could result from including in the calculation data extending beyond the linear region of the photosynthetic light response. In some cases quantum yield measurements can be influenced by changing levels of intercellular CO2 during measurements. We conclude that many quantum yield values reported in the literature are affected by one or more of these errors, and the intrinsic efficiency of photosynthesis is mostly invariant among C3 plants. This emphasizes the importance of the measurement and data analysis protocols in obtaining accurate and reliable quantum yield data.
Comparative photosynthesis of sun and shade plants.
One consequence of generally low photosynthetic capacities in understorey plants is a limited ability to process the light energy they absorb during strong sunflecks. This limited ability can also be exacerbated by a low induction state. Under these conditions, understorey plants will need to dissipate excess energy if they are to avoid photodamage. Field measurements of chlorophyll fluorescence from A. macrorrhiza show a decline in the quantum yield of photosynthesis (measured as Fv/Fm) during saturating sunflecks, indicating that photoprotective mechanisms are probably being engaged. Simultaneous assessment of the xanthophyll pigments shows that interconversion of violaxanthin to zeaxanthin is also occurring. After the sunfleck has passed, conversion of zeaxanthin to violaxanthin is extremely slow in species such as A. macrorrhiza, perhaps allowing a more rapid photoprotective response for subsequent sunflecks. However, quantum yield increases more rapidly than xanthophyll reconversion on return to low light, demonstrating a requirement for both high ΔpH and zeaxanthin for internal photoprotection to occur (Watling et al. 1997b).
Sunflecks and sun patches are of potential use to understorey plants for photosynthesis, but is this potential realised? Growth of understorey tree seedlings has been shown to be correlated with the amount of direct light received in sunflecks, and up to 60% of carbon gain in such environments has been attributed to this source. However, when compared with expected values based on the known steady-state response of plants to light, sunfleck utilisation is often below predicted values (Pﬁtsch and Pearcy 1989). Moreover, species vary in their capacity to utilise sunflecks. Watling et al. (1997a) measured the growth of four Australian rainforest species under simulated sunfleck regimes and showed that sunflecks contributed to growth in two species (Diploglottis diphyllostegia and Micromelum minutum), whereas the other two species (Alocasia macrorrhiza and Omalanthus novo-guinensis) were unable to make effective use of sunflecks.
Comparative photosynthesis of sun and shade ..
Figure 12.13 Growth of (left) Toona australis (sun loving) and right Argyrodendron sp. (shade adapted). Plants are the same age (6 months) and grown in same size pots (15 cm) in high light and high nutrient supply. (Thompson et al., 1992a; photographs courtesy P.E. Kriedemann)
photosynthesis of sun and shade plants.
The differences in growth rate of early-successional fast-growing species versus later-successional and shade-adapted species is illustrated in Figure 12.13 by two rainforest species that are important in the timber industry: the sun-loving red cedar (Toona australis) and the shade-adapted tulip oak (Argyrodendron sp.).
Boardman NK (1977) Comparative photosynthesis of sun and shade plants
Emergent trees of tropical rainforests have to endure strong sunlight, and leaves comprising the crowns of such trees will have acclimated to full sun. In young-growth forests, canopy emergents are early-successional fast-growing species that are adapted for fast growth in full sun on large disturbances. Such species represent an initial phase in forest dynamics that might last 10–20 years. By contrast, in old-growth forests, early-successional species have long since completed their life cycles, and will have been replaced by later-successional species whose seedlings initially tolerated deep shade on the forest floor, but now endure full sun as canopy emergents. Such remarkable plasticity is an adaptive feature of late-successional species and involves sun/shade acclimation by individual leaves.
Dynamic Acclimation of Photosynthesis Increases Plant …
The rare species Magnolia wufengensis, which is adapted to the natural conditions of its native habitat in southern China, has shown poor growth in northern regions. We analyzed the photosynthetic and growth responses of M. wufengensis grown in northern China under three light levels (100%, 70%, and 40% sunlight) during one growing season. Under 70% sunlight, plants had a maximum net photosynthetic rate (Pmax), light saturation point (LSP), seedling height, basal diameter, root biomass and stem biomass. With decreasing light level, the dark respiration rate (Rd), light compensation point (LCP), specific leaf weight, leaf thickness and leaf density significantly decreased, and apparent quantum yield (AQY), maximum fluorescence (Fm), variable fluorescence (Fv), Fm/initial fluorescence (Fo), Fv/Fo, Fv/Fm, chlorophyll content, leaf area and petiole angle significantly increased. We concluded that 70% sunlight was the optimum light level for 1-year-old M. wufengensis seedlings grown in northern China. Poor growth responses were observed under full and 40% sunlight, resulting from excessive and insufficient light energy, respectively. For the successful introduction to northern China, microsites at forest edges or gaps should be favored to provide an optimal light environment for M. wufengensis.