The decline and recovery of photosynthesis of ponderosa pine seedlings subjected to low, but above freezing temperatures

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Abstract
The daily course of photosynthesis (Ps) was determined for ponderosa pine (Pinus ponderosa Laws.) seedlings before and after their transfer from 23° to 11° or 3 °C and vice versa. Apparent Ps at 11 °C was greater than at 23 °C for up to 17 days after transfer. The increase was of the same order of magnitude as the decrease noted in dark respiration (Rs). Apparent Ps at 3 °C was little affected for the first 3 days after transfer, but declined to about 70% of pretreatment rate by day 10. If dark Rs at 3 °C is taken into account, actual Ps declined to about 50% of pretreatment rate by day 10. After this, the decline in Ps, with time, proceeds slowly. Recovery of Ps after transfer from the lower temperatures to 23 °C was rapid only if the length of time at the low temperature did not exceed 1 day. Return of apparent Ps to pretreatment rates was accomplished by day 7 for plants subjected to 17 days of 11 °C. Recovery of apparent Ps for plants subjected to 4 or more days of 3 °C reached 80% by day 7, and 90% by day 23. Dark Rs of plants brought from 3° to 23 °C was increased appreciably at first, this increase being of the same order of magnitude as the decrease in apparent Ps. Dark Rs then decreased with time at 23 °C, this decrease being correlated with an increase in apparent Ps. It is concluded that the initial chilling "injury," if any, occasioned by very low, but above freezing temperature, does not involve the physical or metabolic apparatus of Ps, but rather is associated with the respiratory apparatus as evidenced by increased rates of dark Rs during recovery at higher temperatures.

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