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 O 2 production at various irradiances and of O 2 uptake were estimated, and chlorophyll content (a + b), specific leaf area (SLA: cm 2 leaf area per mg leaf dry weight) and leaf thickness were measured. Of the curves of rate of net O 2 production/cm 2 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/cm 2 /hour did shade obtusifolius have a significantly higher net rate than sun polygonifolius . From calculation of relative rates of photosynthesis/cm 2 leaf area, maximum energy yield, φ 0 , and total CO 2 uptake/mg chlorophyll, at 0.22 cal/cm 2 /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 φ 0 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.