The lack of a significant growth response under reduced PAR is not totally unexpected in the high light environment of Mauna Kea where clear sky ambient PAR levels can routinely exceed 2,200 pmol m s . Light saturation would be expected for many of the leaves of the experimental plants even in early stages of canopy development. Furthermore, reductions in total available PAR have little effect in the nearly linear portion of the photosynthetic light response curve often observed for C3 photosynthesis at high levels of PAR (e.g., Lambers et al., 1998). In addition, these results are consistent with those reported by Stanhill and Cohen (2001) for many crops in high-radiation, arid climates. They concluded that there would be minimal negative effects of global dimming on crop productivity due to shade tolerance and anticipated reductions in water stress. Thus, our results did not agree with previous research indicating increased productivity associated with increased diffuse radiation in light-limited canopies (Roderick et al., 2001; Black et al., 2006) under global dimming, and likely reflect a nearly light saturated canopy within the experimental setup. Our results further suggest that reduced leaf photosynthesis resulting from high light exposure (e.g., photoinhibition, Demmig-Adams, 1990) did significantly reduce productivity (Werner et al., 2001).
Many plant responses to a global dimming environment are not necessarily linked to overall plant productivity. In experiments with wheat, Fischer (1975) found that shading reduced dry matter production approximately in proportion to the percent shade, even at the lowest (13%) shade levels used. However, grain production was less influenced by shading, most likely because shading reduced tillering, but enhanced tiller survival (Fischer, 1975). The response of wheat to shade was considerably greater than the growth responses we observed in fava bean, but may relate to higher leaf area and greater canopy self-shading in the wheat experiment. Although not significant, our data suggested a possible increase in branching rate with reduced PAR and UV-B exclusion. Over an entire growing season, such morphological differences might affect productivity as more energy put into branch structures likely results in less energy and carbon allocated to other plant parts.
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