Effects of Low Soil Temperature on Transpiration, Photosynthesis, Leaf Relative Water Content, and Growth Among Elevationally Diverse Plant Populations

Abstract

The effects of low soil temperature on photosynthesis and water relations were examined in 17 populations of 12 vascular plant species from native elevations ranging between 10 m and 3,170 m. Root permeability to water was sufficient in the majority of populations studied so that neither transpiration nor net photosynthesis was reduced at 3 degrees C soil temperature compared to the rates at 20 degrees C soil temperature. There was no evidence of differential natural selection along altitudinal gradients for ability to maintain photosynthesis rate when roots were chilled. Leaf relative water content was typically reduced in response to soil cooling. There is a critical relative water content above which both transpiration and photosynthesis are insensitive to water content reductions. Root chilling failed to reduce relative water content below this level in most populations studied. Low soil temperatures which had no adverse effects on transpiration or photosynthesis were found to significantly retard plant growth. The observed relative water content reductions might limit growth despite their failure to affect photosynthesis. Evidence for natural selection for ability to maintain turgor upon root chilling in high elevation populations was provided by an inverse relationship between the magnitude of depression in relative water content and native elevation. Thus, growth reduction at low soil temperatures must result from impaired turgor, decreased root growth and metabolism, or impaired cytokinin synthesis and translocation rather than direct limitation of carbon assimilation.