Photosynthesis And Growth Of Seedlings Of Two Tree Species: Polysciasfulva And Warburgia Ugandensis Under Different Light Regimes.
Seedlings of two indigenous tree species, Warburgia ugandensis Spraque subsp. Ugandensis (Canellaceae) and Polyscias fulva (Hiem) Harms (Araliaceae) were grown for five months at four different light regimes. The light levels were: dense shade (25% of full sunlight), moderate shade (42% of full light), partial shade (65% of full light). The full (100%) sunlight acted as the control. Measurements were made of assimilation rate (A), transpiration rate (E), stomatal conductance (gs) and water use efficiency (WUE). Seedling height, leaf area changes of individual leaves, leaf survival and loss were determined after every fourteen days. Finally the seedlings were harvested and total dry weight (DW), leaf area (LA), specific leaf area (SLA) and shoot/root ratio (SIR) were estimated. As expected the photosynthetic photon flux density (PPFD) varied in all the four light levels, with the control recording the highest. The mean diurnal PPFD in the four light levels were: dense shade 347 umol m-2 s', moderate shade 588 umol m-2 S-I, partial shade 918 umol m-2 . S -1 and full sunli ght 1413 umol m-2 s-1. Leaf temperatures were , ... higher in the full sunlight and lowest in dense shade. The mean leaf temperatures were: dense shade 27°C, moderate shade 28 °C, partial shade 29°C and full sunlight 30 0c. There was no significant (P> 0.05) difference in tempejature for the two species in all the light regimes. The rate of photosynthesis was significantly (P< 0.001) higher for P. fulva than in seedlings of W. ugandensis when grown in full sunlight. The mean rate of photosynthesis in full sunlight grown seedlings of P. fulva was 1.4 umol m-2 s -1 as compared with 0.3 umol m-2 S-1 for W ~gandensis. Similarly in partial shade the rate of XlI photosynthesis for P. fulva was higher (P< 0.01) than in seedlings of W ugandens is. The mean rate of photosynthesis in seedlings of P. fulva grown under partial was 1.0 umolm" s -1, while those of W ugandensis recorded a mean value of 0.6 umol m-2 s -1. The rates of photosynthesis were not significantly (P> 0.05) different in seedlings of the two species when grown under moderate and dense shade. The mean rate of photosynthesis for P. fulva was 0.5 umol m-2 s-J and a similar value was observed for W ugandensis under moderate shade. Under dense shade the mean rate of photosynthesis was 0.4 umol m-2 s -J for the two species. In general, seedlings of P. fulva reduced their photosynthetic rates in response to increasing shade. However, in seedlings of W ugandensis photosynthetic rates increased under shade treatments. Stomatal conductance, transpiration and water use efficiency were reminiscent of photosynthetic rates, showing high values for shade grown seedlings of W ugandensis and full sunlight grown seedlings of P. fulva. Seedlings of P. fulva exhibited the highest stomatal conductances in full sunlight ranging from 0.007 to 0.02 mol m-2 s', while W ugandensis exhibited values ranging from 0.003 to 0.009 mol m-2 s'. Stomatal conductances in P. fulva seedlings ranged from 0.00) to 0.002 mol m-2 S-l, while W ugandensis exhibited values ranging from 0.007 to 0.01 mol m" S-1 under partial shade. Under moderate shade P. fulva seedlings showed stomatal conductance values ranging - from 0.002 to 0.01 mol m-2 S-I, while W ugandensis gave values ranging from 0.006 to 0.01 mol m-2 S-l. Stomatal conductance values under dense shade ranged from 0.001 to 0.01 mol m-2 S-l and .0.005 to 0.01 mol m-2 S-l for P. fulva and W ugandensis . - '.'.' respectively. Seedlings of P. fulva transpired more in full sunlight with values ranging from 0.3 to 0.8 mmol m-2 S-l, while W ~gandensis exhibit~d values ranging from 0.2 to Xlii OJ mmol m-z S-I. Transpiration rates ranged from 0.2 to 0.7 mmol m-z S-1 for P. fulva as compared with 0.2 to 0.4 mmol m-z S-l for W. ugandensis under partial shade. In moderate shade P. fulva recorded transpiration values ranging from 0.1 to 0.6 mmol m" S-1 and 0.2 to 0.4 mmol m-z S-1 for W. ugandensis. In dense shade P. fulva showed transpiration rates ranging from 0.1 to 0.5 mmol m-z soland 0.2 to 0.4 mmo1 m-2 S-1 for W ugandensis. Seedlings of P. fulva showed higher water use efficiency in full sunlight than W. ugandensis. The mean WUE for P. fulva in full sunlight was 2.3 umol COz mmol" H20 and 1.5 umol CO2 mmoCl H20 for W ugandensis. In partial shade P. fulva exhibited a mean WUE value of l.8 umol COz mmol -J HzO, while it was 2.0 urnol COz mmol" HzO for W ugandensis. The mean Wl)E values were 1.6 and l.3 umol CO2 mmol " H20 in moderate and dense shades for the two species respectively. Seedlings of P. fulva exhibited the highest dry matter accumulation under full sunlight while the shade grown conspecifics recorded reduced dry weights. Seedlings of P. fulva showed a mean dry weight of 8.9 g in full sunlight as compared to 4.0 g for the dense shade grown conspecifics. Seedlings of W. ugandensis showed higher dry weights in shade treatments than in full sunlight. The mean dry weight was 0.6 g in full sunlight and 1.8 g in shade treatments for W. ugandensis seedlings. In both species leaf number, leaf area, seedling height, shoot/root ratio and specific leaf areas were greater under shade treatments than in full sunlight. However, leaf loss was higher in the open than in the shade treatments for the two species. Leaf number was significantl~ (P< O.O}) higher in moderate and dense shade than in full sunlight and partial shade for the two species. The mean leaf number was 28 for P. fulva in full sunlight and 37 in dense shade. Seedlings of W. ugandensis gave a mean leaf XlV number of 21 in full sunlight and 29 under dense shade. Leaf area of seedlings was significantly (P< 0.001) higher under shade treatments than in full sunlight for the two species. Seedlings of P. fulva recorded a mean leaf area of 238 em' in the sunlight and 376 cm2 under dense shade. W ugandensis gave a mean leaf area of 33 cm2 in full sunlight and 221cm2 under dense shade. The mean shoot/root ratios for P. fulva were 0.97 and 1.8 gg" in full sunlight and dense shade respectively. For W ugandensis the mean shoot/root ratios were 3.0 and 6.0 ss' in full sunlight and dense shade respectively. In W ugandensis height increased by 36 mm in full sunlight as compared with 89 mrn in dense shade. For P. fulva height increment was 20 mm in full sunlight and 38 mrn under dense shade. Specific leaf area increased from 71 cm2 g-I in full sunlight to 198 em' g-l under dense shade for seedlings of P. fulva. For W ugandensis specific leaf area was 106 em' g-l in full sunlight and 221 crrr'g" under dense shade. Increasing PPFD in P. fulva enhanced growth and photosynthesis, while shade promoted higher growth and photosynthesis in W ugandensis . .p. fulva was better adapted to growth in high PPFD than W ugandensis because of its higher rates of photosynthesis at higher light intensities, above 1000 umol .m-2 s-l,while it was poorl,y adapted to growth at low light as a consequence of its lower rates of photosynthesis. P. fulva was also less well adapted to growth at low light intensity due to its relative inability to prevent a decrease in dry weight in low light intensity. W ugandensis on the, other hand was better adapted to growth at low light because of its significantly greater ability to prevent a decrease in its rates of photosynthesis and dry matter accumulation in low light intensity.