American Rose Annual 32:160-161 (1947)
The Photosynthetic Efficiency of Rose Leaves
DR. JOSEPH E. HOWLAND
Cornell University, Ithaca, New York

Wily should roses cut in the afternoon keep longer than those cut in the morning? The discovery in 1943‑44 that they did keep longer (1945 Rose Annual, p. 51) led to the thought that rose leaves used sunlight very efficiently to make sugar. The extra food produced during the day would give blooms cut in the afternoon an advantage in keeping quality.

No one had measured this difference in food content. No one knew the relative efficiency of rose leaves for using sunlight of different intensities. Dr. Kenneth Post and I had shown that flower production of greenhouse roses was correlated with the monthly differences in sunlight; we could only guess that food production by the leaves followed a similar pattern.

Accordingly, the amount of food produced in the leaves at different times during the year and also the amount of food lost from the leaves during the night by respiration and growth of the plant were measured. Experiments were made on greenhouse roses at Cornell University using 768 plants of Peter's Briarcliff (2‑year‑olds) and 576 plants of Better Times (3‑year‑olds). Twenty‑five leaflets were used each test day, and 25 leaflets each test night. The light intensity outside the greenhouse was measured by an automatic light recorder. The soil was maintained at a moisture capillary tension of less than 8 cm. of mercury, as measured by a soil tensiometer. Net dry weight changes, day and night, were measured on 25 dates with Peter's Briarcliff and 21 dates with Better Times. They were during July, August, September and December, 1944 and March, April and June, 1945.

The net gains in dry weight represent the gains available for renewal growth as the cut flowers were removed, assuming that a similar gain occurred in the leaves lower on the plants. Preliminary data collected by Miss Frances Helen Elliott at Cornell in 1944 indicate that this probably was true. She found old leaves were just as efficient in food synthesis as recently matured leaves. This conclusion is also supported by three 1945 Russian papers where a variety of plants were studied.

The greenhouse rose was surprisingly efficient over a wide range of light conditions. Net gains in dry weight for 4 hours were produced on all test days when there was more than 11,000 foot candle hours of light. The grand average net changes in dry weight for the days when both varieties were used, indicate for Better Times a 33% greater loss at night, a 5% greater gain during the day, and a 17% greater net gain in 24 hours.

In preliminary trials on mature leaves of Red Radiance made outdoors on September 6, 1944 (48,686 foot candle. hours of light), a net dry weight gain of 3.38% was found for the 24‑hour period. Data for greenhouse roses are not available for this date, but on September 5 (29,970 foot candle hours) there was a net gain of 4.40% by Peter's Briarcliff and 0.35% by Better Times. The latter variety gained 3.69% on September 7 (43,005 foot candle hours).

The data are, of course, insufficient to warrant strict comparison between varieties. Peter's Briarcliff was grown in the three southerly benches in an east‑west greenhouse, Better Times in the three northerly benches in the same greenhouse. Since Better Times thus received less light, its indicated greater efficiency in using light is supported. However, Peter's Briarcliff averaged 40 to 45 flowers per plant per year—5 more flowers per plant than Better Times.

It should also be noted that in December there was no difference in the net gains of the two varieties, and in April Peter's Briarcliff made a 39% larger gain in dry weight in 5 1/2 days.

The grand average daily gain, 8.8% for Peter's Briarcliff and 9.1% for Better Times, and the average net gain for 4 hours—3.9% for both varieties—are well within the ranges reported for other crops.