Photosynthesis and Zonation of Freshwater Macrophytes

SUMMARY Comparison is made of the photosynthetic response of leaves produced in a glasshouse in unscreened (sun) and screened (shade) conditions, from two species rooted in significantly different depths of water; the shallow‐water Potamogeton polygonifolius, a ‘sun’ species and the deep‐water P. obtusifolius, a ‘shade’ species. Their rates of net O2 production at various irradiances and of O2 uptake were estimated, and chlorophyll content (a + b), specific leaf area (SLA: cm2 leaf area per mg leaf dry weight) and leaf thickness were measured. Of the curves of rate of net O2 production/cm2 leaf area and light intensity, only that of shade obtusifolius was light‐saturated, and lines of best fit were applied to the remainder, which also had high values of r. Slopes were significantly steeper in the order sun obtusifolius shade polygonifolius sun polygonifolius. Under comparable growing conditions, the mean SLA of obtusifolius was twice that of polygonifolius, the range of the two species was mutually exclusive, and that of obtusifolius was narrower than that of polygonifolius, SLA was inversely correlated with the slope of the line of best fit of each leaf‐type. Only at 0.22 cal/cm2/hour did shade obtusifolius have a significantly higher net rate than sun polygonifolius. From calculation of relative rates of photosynthesis/cm2 leaf area, maximum energy yield, φ0, and total CO2 uptake/mg chlorophyll, at 0.22 cal/cm2/hour, of sun and shade leaves of both species, it is shown that P. obtusifolius had a rather similar photochemical capacity to P. polygonifolius; indeed its φ0 was lower. From these facts, increased SLA and reduced respiration and leaf thickness, we conclude that a higher net photochemical capacity/unit area of shade leaves and shade species of Potamogeton in low irradiances (c. 1% summer daylight) is achieved by lowered respiration rate/unit area which itself may result from reduction in leaf weight/unit area. Perhaps the limitation of net photosynthesis in shade species at higher irradiances is a function in a species like P. obtusifolius of its inability to reduce SLA, thicken its leaves and hence increase its capacity to absorb light.