Influence of Carbon Dioxide Enrichment on Postharvest Leaf Chlorosis of Miniature Roses
Nihal C. Rajapakse, John W. Kelly and William B. Miller Department of Horticulture, Clemson University

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Introduction

Leaf chlorosis and flower drop are common postharvest problems of potted miniature roses following shipment under dark conditions. High shipping temperatures can accelerate leaf chlorosis and flower drop thus, severely reducing postharvest quality. Our preliminary results with six potted miniature rose cultivars indicated that shipping at 60°F or 80°F increased leag chlorosis significantly compared to plants shipped at 45°F and that pre-treatment with benzyladenine (cytokinin, a plant hormone) at 50 to 100 ppm reduced shipping induced leaf chlorosis.

The reason for postharvest leaf chlorosis is not well understood. However, reduced sugar levels and water stress have been associated with postharvest problems such as leaf discoloration, bud abscission, and poor flower longevity in many plant species. With hibiscus, ot has been shown that low light levels increased flower drop and that removal of larger buds reduced subsequent drop of smaller buds. High light levels have been shown to reduce bud abscission of miniature roses during post-production evaluation, but had no effect on leaf yellowing.

Carbon dioxide (CO₂) enrichment can enhance the sugar and water status of plants due to its effedts on photosynthesis, respiration and stomatal characteristics. Therefore, we investigated the influence of CO₂ enrichment on the carbohydrate status of potted miniature roses to determine the relationship between sugar status and postharvest leaf chlorosis.

Materials and Methods

Uniformly rooted "Meijikatar" miniature roses were planted in 4.5 inch square plastic pots containing a commercial potting mix. Plants were grown in a greenhouse for two weeks before being subjected to CO₂ treatment. After the 2-week establishment period, plants were cut back to 3-4 inches above soil level and placed into three plexiglass CO₂ enrichment chambers and grown until plants reached flowering stage (buds showing color, approx. 28 days). The average CO₂ concentrations in the chambers were 350, 700, and 1050 ppm during the day. Plants were sub-irrigated once daily with 200 ppm nitrogen from Peter's 15-16-17 fertilizer. The experiment was conducted in March and was repeated in June.

At the time of harvest, leaf and stem sugar levels of 10 plants were evaluated. Postharvest quality evaluations were made on 10 representative plants grown in each of the CO₂ enrichment chambers. In a preliminary shipping and storage experiment, 40°F was shown to give the best quality while 60°F increased leaf yellowing. When plants in the CO₂ enrichment chambers reached flowering stage (buds showing flower color; approx. 28 days) paper sleeved plants were placed into cardboard boxes and subjected to simulated shipping at 40°F or 60°F for five days. At the end of the shipping, plants were removed from boxes and placed in an interior environment at 68°F with 30 µmol light from a cool white flourescent source with 24 h photoperiod for 5 days before postharvest evaluations were made. The number of etiolated shoots at the time of removal from storage was counted. The number of chlorotic and green leaflets were counted and percentage chlorotic leaves calculated after five days. Plant quality was evaluated using a scale of 1 to 5 where 1=unnacceptable quality with many chlorotic or dried leaves; 5=excellent quality without leaf chlorosis.

Results and Discussion

Leaf sucrose increased in response to CO₂ enrichment but no significant effect on stem sucrose was detected (Table 1). However, the increase in leaf sucrose content in response to CO₂ enrichment was small ( approx. 10% maximum increase at 1050 ppm). Increased CO₂ tended to decrease leaf and stem fructose levels, while it increased stem glucose. However, the overall changes were very small, usually less than 10%. Both leaf and stem starch pools were highly responsive to increased CO₂, showing about 4-fold increase as CO₂ level was increased to 1050 ppm. A slight increase (20%) in leaf starch content occurred as CO₂ increased from 700 to 1050 ppm. However, the stem starch content increased more than 40% as CO₂ increased from 700 to 1050 ppm. When plants were grown in 700 to 1050 ppm CO₂ both leaf and stem sucrose: starch ratios were reduced by about 70% compared to those of plants grown under low CO₂ level.

Carbon dioxide enrichment did not significantly affect the overall appearance of plant or percentage leaf chlorosis after 5 days in the interior environment of plants shipped at either temperature (Table 2). Although it was not directly monitored, there did not appear to be any obvious effect of CO₂ enrichment on flower or leaf abscission. Regardless of CO₂ treatment, plants shipped at 60°F exhibited a 3-fold increase in leaf chlorosis compared to plants shipped at 40°F, clearly indicating that post-production storage temperature had the most influence on leaf chlorosis. The number of etiolated shoots after shipping increased as the shipping temperature increased but the temperature effect on shoot etiolation was most pronounced in plants grown at the lowest CO₂ level. When grown at elevated CO₂, many etiolated shoots were present even at low shipping temperature. Growth of etiolated shoots on high-CO₂ -grown plants is undoubtedly supported by the higher stem starch levels in these plants.

Postharvest performance has been considered to be closely related to carbohydrate (sugars and starch) status of the plant at the time of harvest. During dark shipping or storage, plants can lose large amounts of carbohydrate reserves due to respiration, especially at elevated temperature. However, our results did not demonstrate a clear relationship between leaf chlorosis or over all plant quality of potted miniature roses and carbohdrate status. In our plants, elevated CO₂ mainly increased leaf and stem starch levels, with relatively little or no effect on sucrose and glucose. Despite the substantially greater starch content in CO₂ enriched plants, leaf chlorosis was similar in ambient and high CO₂ grown plants.

In our experiments, it was noted, however, that most of the chlorosis occurred in lower leaves, which were from previous growth prior to cutting back. It is possible that movement of carbohydrates to newly growing shoots from older leaves reduced the carbohydrate pools in lower leaves causing the leaf chlorosis. In the present study, no attempts were made to separate upper and lower leaves during sampling for carbohydrate analysis. In addition to carbohydrates, movement of nitrogen from lower leaves to growing shoots can induce nitrogen deficiency causing the leaf chlorosis. Our experiments indicated that high CO₂ slightly decreased the total shoot nitrogen content. However, no separation of lower and upper leaf nitrogen status was made.

Our results indicate that CO₂ enrichment increased the carbohydrate status (primarily starch) in leaves and stems of miniature roses. However, our results do not support the hypothesis that increases in carbohydrate supply play a role in improved postharvest quality and reduction in postharvest leaf chlorosis of potted miniature roses. Although high CO₂ did not affect overall plant quality and leaf chlorosis in the interior environment, it did increase the number of etiolated shoots after shipping which reduces the quality of plants immediately after removal from storage. Our results indicate that shipping temperature plays a major role in maintaining low leaf chlorosis and the development of etiolated shoots and that shipping of miniature roses at low temperatures is important in maintaining postharvest quality. Shipping of plants grown in CO₂ enriched environments at high temperature for extended period could lead to adverse quality due to shoot etiolation. Shipping temperature management is important when CO₂ enriched plants are shipped to long distance markets. Further research into carbohydrate and nitrogen status of lower and upper leaves are warranted to better understand the relationship.

Acknowledgements

We are greatful to Yoder Brothers for donating plant material and Clemson University Ornamentals Enhancement Program for financial support.

Last Updated 2/1/97