Not all leaves are created equal. Within a single plant, a spectrum of light availability exists- some leaves are positioned to receive maximum sunlight every day, while others, by luck of the draw, are primarily shaded and experience full sun only in short, erratic bursts. The sun leaves, by virtue of their superior position, are equipped with the capacity to capture the most light and perform the most photosynthesis. They tend to have more cell layers, larger numbers of the enzymes involved in capturing CO2 and converting it to sugar, and a larger volume of space to perform carbon-fixing reactions than their less-exposed neighbors.
Though it may seem that shade leaves get a raw deal, they are in fact uniquely adapted to a low light environment. Seemingly minor biochemical changes allow shade leaves to be far more efficient light harvesters, maximizing the amount of sunlight captured during those rare moments of exposure.
Chlorophyll is the main light-harvesting pigment in most plants, capturing light radiation primarily in the red and blue parts of the visible spectrum. These light waves are between 400 and 700 nanometers in length and are known as photosynthetically active raditaion (PAR). When PAR is absorbed by chlorophyllous tissue, radiation of longer wavelengths in the 680-760 nanometer range is re-emitted in a process known as fluroescence.
Measuring the kinetics of chlorophyll fluorescence has proved a useful tool in discerning the different photosynthetic capabilities of sun and shade leaves. To do so, a plant physiologist may incubate identical leaves in contrasting light conditions and measured fluorescence levels when a “saturating” pulse of white light is applied in each case. It turns out that the dark incubated leaves respond far more dramatically to a brief pulse of light than that already light-exposed leaves.
The pulses of light used in such laboratory experiments are in fact comparable to the erratic “sun flecks” that understory leaves may experience over the course of a day. Much as a starving animal will metabolize and release energy more slowly than a well-fed one, shade leaves take the limited light resources available and make far more out of these brief, shining moments than sun leaves would ever be able to.