Physiological Response of Three Alfalfa Cultivars to One Chilling Night¹

Abstract

Most studies involving photosynthetic efficiency have compared species or cultivars that were grown under fixed day/night temperature regimes. Researchers attempting to improve photosynthetic efficiency and productivity of a crop may also need to consider genotype response to adverse factors, such as temporary chilling, which have been shown to reduce subsequent photosynthetic rates.Three alfalfa (Medicago sativa L.) cultlvars, representing a range of environmental adaptation, were grown in a greenhouse at 22/18 C (day/night), and then were exposed for one night to 5 or 18 C to determine the effect of low temperature on physiological processes the following morning. The CO2 exchange rate (CER) of all three cultivars was reduced following a 5 C night. A greater reduction in CER occurred in ‘Mesa‐Sirsa’ and ‘Saranac’ (69.1 and 60.2% less than 18 C, respectively) than in ‘Kane’ (36.7% less than 18 C). Transpiration rates were significantly reduced following chilling exposure of Mesa‐Sirsa only (75.5% less than 18 C). This reduction in transpiration was associated with a 50% increase in leaf diffusive resistance and a 4.3 bar decrease in xylem water p6tential. Increased leaf diffusive resistance was not observed in Saranac and Kane following a chilling night. A greater leaf carbohydrate concentration and reduced l4C‐photosynthate translocation from leaf tissue was noted in all cultivars following a 5 C night.These data suggest a feedback inhibition of CER in alfalfa following one 5 C chilling night. A water stress inducing an increased leaf diffusion resistance and decreased transpiration was found in Mesa‐Sirsa only.

Impairment of Photosynthesis by Chilling-Temperatures in Tomato

“…When the prechilling was extended to 96 h, the intercellular CO2 concentration was almost equal to that of the ambient atmosphere, and the photosynthesis was almost zero. In another study (20) employing three closely related alfalfa cultivars, only in one of the two cultivars which showed dramatic reduction in photosynthesis following a chilling night did an accompanying increase in leaf diffusive resistance occur. These observations indicate that, although stomatal aperture is sometimes reduced in thermophilic plants after prechilling, this reduction alone may not account for the observed inhibition of photosynthesis.…”

“…This plants, leaf diffusive resistance indeed increases (6,25) during chilling-induced inhibition of photosynthesis. In other cases, chilling directly produces a lesion in one or more of the enzymic steps of photosynthesis (5,20). A reduction in stomatal aperture could be an indirect result of chilling, a response to an increased intercellular CO2 concentration brought about by a direct impairment of the photosynthetic apparatus.…”

“…1 to 12 C for 12 to 24 h). Dramatic reductions in the rate of whole plant photosynthesis after exposure to chilling night temperatures have been documented in numerous plants including Phaseolus vulgaris (4,10), Sorghum vulgare (19), Oryza sativa (14), Medicago sativa (20), and Digitaria decumbens (3). The effect is persistent and can last for several days after plants are returned to optimal temperatures (17).…”