Print PageBased on the need for canopy gaps for successful regeneration, longleaf pine (Pinus palustris Mill.) has been classified as very shade intolerant. However, shade tolerance defined by leaf morphological and physiological response to light environment is not well understood in longleaf pine, especially given its persistent grass stage. We hypothesized that longleaf pine may shift from moderately shade tolerant when young to intolerant of shade with increasing age or size and thus exhibit plasticity in shade tolerance. Our specific objective was to gain a better understanding of shade tolerance in longleaf pine by examining phenotypic plasticity in shade tolerance in seedlings and young trees. Grass stage seedlings were exposed to ambient light and a 50% reduction in ambient photosynthetically active radiation (PAR) in a greenhouse. Entire branches of trees approximately 10 m in height were shaded to provide 0, 50 and 70% reductions in ambient PAR. Physiological and morphological responses to shade treatments were examined over one growing season.
Results/Conclusions
In seedlings, shading reduced leaf mass per unit area (LMA) from 49.9 to 38.5 g m-2, the light compensation point (LCP) from 13.1 to 8.9 µmol m-2 s-1 and leaf dark respiration (RD) from 0.32 to 0.19 µmol m-2 s-1. When expressed on a leaf dry weight basis, shading increased light-saturated photosynthesis (Amax) by 40% in July, chlorophyll b by 38% and total chlorophyll by 34%. In response to the deepest shade, trees exhibited smaller reductions in LMA, as much as a 65% reduction in Amax with no associated changes in RD, and reduced LCP from 20.4 to 12.4 µmol m-2 s-1. The 70% reduction in ambient light treatment increased N allocated to chlorophyll, chlorophyll a and b, and total chlorophyll in trees. Morphological and physiological factors contributing to shade acclimation when considered in the context of shoot and whole plant architecture suggest greater shade tolerance in seedlings than in trees.
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