The Role of Low Soil Temperature for Photosynthesis and Stomatal Conductance of Three Graminoids From Different Elevations

Abstract

In high-elevation grasslands, plants can encounter periods with high air temperature while the soil remains cold, which may lead to a temporary mismatch in the physiological activity of leaves and roots. In a climate chamber experiment with graminoid species from three elevations (4400, 2400, and 250 m a.s.l.), we tested the hypothesis that soil temperature can influence photosynthesis and stomatal conductance independently of air temperature. Soil monoliths with swards of Kobresia pygmaea (high alpine), Nardus stricta (lower alpine), and Deschampsia flexuosa (upper lowland) were exposed to soil temperatures of 25, 15, 5, and -2°C and air temperatures of 20 and 10°C for examining the effect of independent soil and air temperature variation on photosynthesis, leaf dark respiration, and stomatal conductance and transpiration. Soil frost (-2°C) had a strong negative effect on gas exchange and stomatal conductance in all three species, independent of the elevation of origin. Leaf dark respiration was stimulated by soil frost in D. flexuosa, but not in K. pygmaea, which also had a lower temperature optimum of photosynthesis. Soil cooling from 15 to 5°C did not significantly reduce stomatal conductance and gas exchange in any of the species. We conclude that all three graminoids are able to maintain a relatively high root water uptake in cold, non-frozen soil, but the high-alpine K. pygmaea seems to be especially well adapted to warm shoot - cold root episodes, as it has a higher photosynthetic activity at 10 than 20°C air temperature and does not up-regulate leaf dark respiration upon soil freezing, as was observed in the grasses from warmer climates.

Keywords: adaptation to cold soil; leaf respiration; photosynthesis; soil frost; stomatal conductance; transpiration.

FIGURE 1

Daily fluctuation of air and soil temperature at 1 cm depth in a Kobresia sward at Kema Research Station (Tibetan Plateau, 4400 m a.s.l.) on 2 days in August 2013.

FIGURE 2

Measurement setup. A water bath is cycling heated or cooled water through hoses wound tightly around each soil monolith with grass swards on top. The styrofoam insulation buffers against external temperature fluctuations.

FIGURE 3

(A) Photosynthesis at light saturation (1500 μmol photons m^-2s^-1), (B) leaf dark respiration, (C) stomatal conductance, and (D) transpiration at light saturation of Kobresia pygmaea, Deschampsia flexuosa, and Nardus stricta in the two air temperature (10 and 20°C) and four soil temperature (–2, 5, 15, and 25°C) treatments. Small letters indicate significant differences (p ≤ 0.05) between treatments for one species. Capital letters indicate significant differences (p ≤ 0.05) between species within a treatment. Error bars indicate standard errors, n = 6.