Influences of Seasonal Changes in Leaf Morphology on Water-Use Efficiency For Three Desert Broadleaf Shrubs

Smith, William; Nobel, Park

The influences of seasonal variations in leaf length, pubescence and the ratio of internal to external leaf area (Ames/A) on transpiration and photosynthesis were evaluated for three desert broad-leaved shrubs (Encelia farinosa [Grey], Hyptis emoryi [Torr.] and Mirabilis tenuiloba [Wats.]). Field data were incorporated into energy balance and gas exchange equations to determine water-use efficiencies (mass CO2 fixed/mass H2O transpired) at various times throughout a year. Results were compared with simulated values for leaves without the observed seasonal changes in morphology. For Encelia, leaf lengths were over 3-fold greater during the winter and spring than during the drier summer and fall (4.0 cm vs. 1.2 cm). Increases in pubescence during dry periods led to substantial decreases in the leaf absorptance to solar irradiation (from 0.61-0.43). Similar alterations occurred for Hyptis, but with less month-to-month variation, while Mirabilis had comparatively small changes in these leaf parameters. For Mirabilis Ames/A was about 75% greater (maximum values of approx/= 70 vs. 40 for Encelia and Hyptis) and varied less (9% variation) during the year compared with Encelia and Hyptis (46% and 24% variation, respectively). Minimum leaf resistances to H2O vapor diffusion (Rwv) were similar for Encelia and Hyptis throughout most of the year, while Mirabilis had much higher values. The optimal temperature for photosynthesis was several degrees higher for Mirabilis (34 deg. C vs. 30 deg. C for Encelia and Hyptis). The calculated cellular resistance for CO2 diffusion (RcellCO2) was lowest for Mirabilis (147 s/cm) and almost identical for Encelia and Hyptis (282 s/cm). Encelia and to a lesser degree Hyptis, had maximum photosynthetic rates at a lower water-use efficiency when water was available and greatest water-use efficiency at reduced photosynthetic levels during drier periods of the year. During a shorter growth period in the wet portion of the year, Mirabilis had a consistently high water-use efficiency (.016) and a moderate photosynthetic rate, reflecting its much higher Ames/A and Rwv combined with a lower RcellCO2. Simulations for leaves without the observed changes in morphology that occurred during the drier months showed that the mean water-use efficiency for the year would have been approx/= 48% less for Encelia, 41% less for Hyptis and only 3% less for Mirabilis. A decrease in leaf length, a decrease in absorptance, and an increase in AmesA contributed equally to the estimated seasonal increases in water-use efficiency for Encelia and Hyptis. Simulated transpiration and photosynthesis for leaves with morphology characteristic of drier periods resulted in water-use efficiencies similar to those for natural conditions, but annual photosynthesis was reduced approx/= 24% for Encelia, 17% for Hyptis and <2% for Mirabilis.

Influences of Seasonal Changes in Leaf Morphology on Water-Use Efficiency For Three Desert Broadleaf Shrubs

Series

Collections