Effects of Night Temperature and Water Stress on Photosynthetic Gas Exchange of Potentilla gracilis (Rosaceae)

Plants in stressful environments have evolved strategies to cope with fluctuating environmental conditions. Potentilla gracilis, also known as Alpine Cinquefoil, grows in alpine meadows of the Rocky Mountains (USA), and is subjected to wide ranges of temperature, light intensity and water availability on a time scale of minutes to days during the growing season. Leaves often freeze to a brittle state at night, are exposed to high radiation while still frosty, dehydrate to wilting during the following light period, and then repeat the cycle the following day. The main objective of this research was to determine the effect of night temperature on subsequent photosynthetic gas exchange in P. gracilis. We used a photosynthetic gas exchange system to compare assimilation and stomatal conductance from light response curves of cold-acclimated P. gracilis following warm and chilling nights, and for plants at different water potentials. From the light response curves, dark respiration, light compensation point, maximum assimilation, light saturation point, and inhibition of photosynthesis were determined and were compared among the same plants under varying conditions. Assimilation and stomatal conductance decreased with the fall in measurement temperature, following chilling nights, and with the severity of water stress. Low night temperature and high photon flux density during the daytime, which are very common during the growing season in the field, cause a reduction in photosynthesis of the plant. The probable underlying damage during inhibition is likely repairable indicating protection rather than damage. The cold nocturnal temperature, with its less efficient biochemical repair capabilities, may partly be responsible for the reduction in assimilation of the following day. P. gracilis species exhibited persistent acquired freezing tolerance; substantial photosynthetic productivity over a wide range of light intensity and temperature; and significant tolerance of, and rapid recovery from, severe drought; making a maximum use of often challenging resources.