Scientia Horticulturae, 50 ( 1992 ) 219-228 Elsevier Science Publishers B.V., Amsterdam

219

Effects of temperature and irradiance level on ~. ~1~ -,1 ° ca buuyurate content and keeping quahty of

Christmas begonia (Begonia × cheimantha Everett )

Tove Fjeld Department of Horticulture, Agricultural University of Norway, P.O. Box 22, 1430 ,~as-NLH, Norway

(Accepted 29 October 1991 )

ABSTRACT

Fjeld, T., ! 992. Effects of temperature and irradiance level on carbohydrate content and keeping qual- ity of Christmas begonia (Begonia × cheimantha Everett ). Scientia ltortic., 50: 219-228.

The carbohydrate content and keeping quality of Christmas begonia were determined in a factorial experiment in which the plants were subjected to irradiance levels of 15, 30 and 60 pmol m -2 s- ' and to temperature levels of 15, 18 and 21 °C for the last part of the growing period. This period lasted from I 1 to 24 days depending on irradiance level and temperature. The contents of sucrose, glucose and starch were determined at the marketing stage. The keeping quality of the plants was determined after 3 weeks in a simulated interior climate. Increase in temperature and irradiance level both en- hanced the development of flowers during the later part of the growing period. The temperature did not have any effect on plant quality at the marketing stage nor on the carbohydrate content, but high temperature caused an increase h~ fower stem height after 3 weeks in interior climate. Increase in irradiancc level ~:auscd a significant increase in the content of starch at the marketing stage. Percent increase in new flowers during the interior holding period was highly correlated with the contents of sucrose and starch in the inflorescences and with the content of starch in the leaves. A large starch pool in the inflorescences and in the leaves at the marketing stage reduced the abscission rate during the interior holding period.

Keywords: abscission; Begonia × cheimantha; carbohydrates; Christmas begonia; irradiation; keeping quality; temperature.

Abbreviations: NT = night temperature; PAR = photosynthetic active radiation; SD = short day.

INTRODUCTION

Plant quality at the marketing stage and the keeping quality of flowering pot plants have been associated with the irradiance level during the later phase

Correspondence to: T. Fjeld, Department of Horticulture, Agricultural University of Norway, P.O. Box 22, 1430 Aas-NLH, Norway.

© 1992 Elsevier Science Publishers B.V. A!,I rights reserved 0304-4238/92/$05.00

220 T. FJELD

of the production period (Ba~vre, 1985; Valso, 1987; Field, 1990). Heins and Hanan (1984) demonstrated that a low irradiance level ( 50 ft candles) dur- ing the simulated interior holding period resulted in changes in fresh weight/ dry weight ratios, indicating a steady depletion of carbohydrates, followed by an early senescence of flowers in a number of crops.

This work was undertaken with a view to assessing the relationship between carbohydrate content and the postharvest quality of Christmas begonia. Dif- ferent growing temperatures and different irradiance levels were established to provide plants with a range of carbohydrate status at marketing stage, in order to determine to what extent this would influence the keeping quality of Christmas begonia.

MATERIALS AND METHODS

Young plants of Begonia × cheimantha cultivar 'Nova' propagated by means of leaf cuttings were potted in I l 0-mm pots containing fertilized peat. They were watered as needed and fertilized twice a week. After a vegetative period the plants were given short day (SD) of l0 h per day for 14 days to induce flowering. The night temperature (NT) was 22 °C ( +_ l °C) from potting till the end of the SD treatment. Thereafter the NT was lowered to 18°C (_~0.5°C).

The experiment was replicated in time. The plants of the first replicate were given SD from 15 September. From 21 December the NT was lowered to 14.5°C (+_0.5°C). On 27 January these plants were just starting to flower, and they were placed in three growth rooms at 15, 18 or 21 °C ( + 0.5 °C) until they were marketable. In each growth room three combinations of irradiance levels were established: 15, 30 and 60/lmol m- 2 s- t ( + 10%) photosynthetic active radiation (PAR). Plants grown at 15°C were kept in the growth rooms for 14-24 days depending on the irradiance level, while plants grown at higher temperature were kept there for l 1-18 days. Irradiation was given by means of fluorescent lamps (Phillips TL/33) for 18 h day -~. Irradiance level was measured by a Lambda LI-185B instn~ment with a quantum (400-700 nm) sensor. The relative air humidity was kept between 60 and 75%. It increased to 80-90% for a period of approximately 4 h after watering.

The second replicate was given SD from 23 February and grown at 18°C ( + 0.5 o C) until the growth room treatments started on 15 May.

The plants were grown in an ordinary glasshouse before the treatments started. Average natural radiations (open-air) during the growing period were 5.01 tool m -2 day -~ PAR and 20.4 mol m -2 day -~ PAR for the lwo repli- cates, respectively. The radiance level inside the greenhouse was 55% of that of open-air radiation. When the plants were moved to the growth rooms for irradiation and temperatm~ treatments, all open flowers were removed from the plants.

CARBOHYDRATE CONTENT AND KEEPING QUALITY OF CHRISTMAS BEGONIA 221

When the plants had reached the marketing stage, they were moved to an interior holding room for 3 weeks and given the following conditions: 13.5 gmol m -2 s-~ PAR at plant level by means of warm white fluorescent lamps (Thorn) for 12 h day -l , a temperature of20°C (+_0.5°C) and a relative air humidity varying from 45 to 65% during night and most of the day except for a short period after watering, when relative humidity reached a maximum level of 80-90%.

In the first replicate each treatment comprised 10 plants. Marketing stage was considered to be reached when five of the 10 plants had at least 45 open flowers. In the second replicate each treatment comprised eight plants. Four of them were moved to a simulated interior climate when one-half of the plants had more than 25 open flowers (this represents marketing stage). The plants in this replicate were much smaller than the ones in the first replicate.

From each of the treatments five plants from Replicate 1 and four plants from Replicate 2 were analysed for carbohydrate content. After partitioning into leaves and flowers (including buds and flower peduncles) the material was dried and finely ground. The starch content was determined by means of the anthrone method (McCready et al., 1950; Yemm and Willis, 1954). The sucrose and glucose contents were determined by means of the enzymatic method developed by Hugger and Nixon (1957) for determination of blood sugar. Industrial enzyme preparations from Biochemica 'Boehringer', Man- nheim, FRG were used and the procedure used for plant material by Madsen (1968) was followed. Two or three parallel analyses were carried out. The differences between the values were less than 2.5%. The analyses were carried out by The Laboratory for Chemical Analyses at The Agricultural University of Norway.

The following observations were made: The numbers of days from removal of the flowers till the marketing stage were recorded. At the start and at the end of the interior holding period the numbers of flowers and flower buds were recorded. In addition, the height of the plant from soil level to the top of the inflorescence (flower stem height) and to the level of the upper leaves (leaf height) were measured. Flower stem height above the leaves was calcu- lated. During the interior holding period the numbers of abscised flowers and flower buds were recorded three times a week.

The numbers of flowers and flower buds after 3 weeks in the simulated interior climate were divided by the numbers of flowers and flower buds at the marketing stage and multiplied by 100 to give the percent increase in flow- ers and flower buds. The numbers of flowers and flower buds abscised during the interior holding period were divided by the numbers of flowers and flower buds at the marketing stage and multiplied by 100 to give the percent abscised flowers and flower buds.

The results were subjected to analysis of variance; least significant differ- ence and Duncan's mul~'~p~,e range test at 5% level were determined°

222 T. FJELD

RESULTS

Plant quali ty at m a r k e t i n g stage. - Leaf height, flower stem height and num- ber of flower buds increased significantly when the irradiance level increased from 15 to 60 #mol m -2 s -~ PAR (Table 1 ). The same tendency was ob- served with respect to number of flowers, but the increase was not significant. Increased irradiance level also significantly reduced the time needed to pro- duce marketable plants. The plants which received 60/zmol m -2 s-~ thus reached the marketing stage 10 days earlier than plants receiving 15/tmol m - 2 s- t (Table 1 ).

No significant effects of temperature were observed on leaf height, flower stem height or number of flowers or number of flower buds at the marketing stage. However, a rise in temperature from 15 to ! 8°C gave a small but sig- nificant shortening of the period from the time of removal of flowers till the marketing stage (Table 1 ).

Carbohydrate content . - No significant differences in sucrose content was found between inflorescences and leaves, but leaves contained significantly less glucose in percent of dry matter than inflorescences. The opposite was found for starch: the leaves contained two to three times more starch than the flowering part of the plant (Table 2 ). -.

No significant effect of temperature on carbohydrate content was observed (data not shown). The content of sucrose in inflorescences and leaves in- creased with increasing irradiation. However, because of only two replicates and some variability in the sucrose analyses this effect was not statistically

TABLE I

Main effects of irradiance level and temperature during the last part of the growing period on leaf height (cm), flower stem height (cm), number of flowers and flower buds at the marketing stage and days from removal of the flowers till the marketing stage of Begonia × cheimantha. The data are the average of two replicates. Mean separation by Duncan's multiple range, test, 5% level

Leaf Flower No. of No. of Days to height stem flowers flower marketing

height buds stage

lrradiancelevel(gmolm-2s -I) 15 15.9b 25.4b 342a 89.8b 22.8a 30 17.9a 26.5ab 35.7a 101.4ab 14.6b 60 17.3a 27.2a 40.9a 113.9a 12.1c

Temperature ( ° C) ! 5 ! 7.3a 26.0a 38.6a 103. la 17.4a 18 17. la 26.3a 37. la 94.0a 16.3b 21 16.9a 26.8a 35.2a 108.0a 15.8b

CARBOHYDRATE CONTENT AND KEEPING QUALITY OF CHRISTMAS BEGONIA 223

TABLE 2

Effect of irradiance level on the content of sucrose, glucose and starch in inflorescences and leaves of Begonia × cheimantha. Contents given as percentage of dry matter. The data are the average of two replicates. Mean separation by Duncan's multiple range test, 5% level

Irradiance level Inflorescences Leaves Mean (#mol m-2 s- t )

Sucrose

Glucose

Starch

15 1.6 2.5 2.1a 30 5.3 3.1 4.2a 60 11.5 8.5 10.0a

Mean 6. I a 4.7a

15 15.4 9.9 12.7a 30 l 5.5 9.1 12.3a 60 15.8 9.1 12.5a

Mean 15.6a 9.4b

15 2.3 5.2 3.8c 30 3.1 8.1 5.6b 60 3.5 10.2 6.9a

Mean 3.0b 7.8a

significant (Table 2). The increase in sucrose content is nevertheless dis- cussed as an effect of increasing irradiance level.

The irradiance level had no significant effect on glucose content, whereas starch content increased significantly with increasing irradiar:ce levels. In the leaves starch content increased almost 100% when the irradianee level i,- creased from 15 to 60 pmol m-2 s-~ (Table 2 ).

K e e p i n g quali ty . - Increasing irradiance levels significantly affected the de- velopment of flowers and flower buds during the period in interior climate, and hence affected the quality of the plants (Table 3, Fig. 1 ). There was a small reduction in number of buds during the interior holding period at high irradiation (5%) and a considerably larger reduction at low irradiation (Ta- ble 3). Final growing temperature did not ~ignificanfly affect development of new flowers during the interior holding period (Table 3 )o A growing temper- ature of 15 °C however caused a reduction in number of flower buds of ap- proximately 50% during the interior holding period.

Abscission of flowers and flower buds was significantly higher at low irra- diation than at the two higher irradiance levels. Temperature had no signifi- cant influence on the rate of abscission during the interior holding period (Table 4 ). Flower stem height was not significantly influenced by irradiance level, but increased with increasing temperature.

224 T. FJELD

TABLE 3

Effect of irradiance level and temperature during the last part of the growing period of Be- gonia × cheimantha on the relative increase in flowers and flower buds during a 3 week interior hold- ing period. Number of flowers or flower buds at marketing stage equals 100. Mean separation by Duncan's multiple range test, 5% level

Flowers Flower buds

Irradiancelevel(#molm-2s -~ ) 15 184.3b 74.7b 30 255.2a 65.9b 60 230.1a 95.7a

Temperature ( ° C) 15 209.9a 52.9b 18 243.8a 83.1a 21 215.9a 100.5a

Fig. 1. Effects of irradiation during the last 3 weeks of the growing period on the plant quality of Begonia × cheimantha after 3 weeks in a simulated interior climate. Temperature during the growing period was 18 ° C. I rradiance levels (from left ): 15, 30 and 60/~mol m - 2 s- i PAR.

Correlations between carbohydrate content and keeping quality. - Correlation coefficients between glucose content and the keeping-quality observations are not calculated, due to the non-significant effects of irradiation on glucose contents.

The sucrose and starch content of the inflorescence, and the starch content of the leaves showed a strong positive correlation with the increase in flowers and buds in the interior holding period and a strong negative correlation with percent abscission (Table 5).

CARBOHYDRATE CONTENT AND KEEPING QUALITY OF CHRISTMAS BEGONIA 225

TABLE 4

Effect of irradiance level and temperature during the last part of the growing period of Be- gonia×cheimantha on flower abscission (%) and flower stem height above the leaves (cm) during a 3 week interior holding period. Mean separation by Duncan's multiple range test, 5% level

Leaves Percent abscission of flowers and buds

Flower s tem height above the leaves

Irradiancelevel(gmolm-2s -~ ) 15 35.8a 10.2a 30 20.6b 10.9a 60 18.5b l l . l a

Temperature(°C) 15 28.2a 10.1b 18 22.4a 10.5b 21 24.4a l l .7a

TABLE 5

C~,~l~qon coefficients demonstrating the relationship between sucrose and starch contents of the inflorescences (I) and of leaves (L) at the marketing stage of Begonia × cheimantha with percent increase in new flowers and buds and with percent abscised flowers and buds during a 3 week interior holding period

Sucrose Starch ( I ) (L) (1) (L)

Increase (%) 0.~3 O~3 0.97 0.93 Abscission (%) - 0.85 - 0.66 - 0.98 - 0.95

DISCUSSION

The effect of irradiance levels on the numbers of buds and the rates of in- florescence development till the marketing stage corresponded well with re- sults obtained in other species, such as Azalea indica (Bodson, 1983) and Rosa cultivar 'Baccara' (Moe and Kristoffersen, 1969 ), who argue that high light level, and presunmbly thus a high carbohydrate content, is of great im- portance for the flower develepment. The effect of irradiance level on the increase in new flowers and flower buds during the interior holding period, corresponds with earlier findings in Christmas begonia (Field, 1990).

Temperature did not have any significant effect on the carbohydrate status at the marketing stage. This suggests that the treatment period might have been too short for the temperature to influence the net photosynthesis. Irra- diation, on the other hand, had a significant effect on the carbohydrate status. It is generally accepted that photosynthesis results in formation of hexoses,

226 T. FJELD

which in turn is converted to starch, sucrose, cell-wall components or fructo- sans (Salisbury and Ross, 1978 ).

The positive correlation of sucrose content in inflorescences and the keep- ing quality of Christmas begonia may be explained by the fact that sucrose is a non-reducing molecule and is the main translocation form of carbohy- drates. It is also efficient in energy conservation in the plant, and serves as a useful component in regulation of osmotic pressure and phloem flow of water (Avigad, 1982). The importance of sucrose for the keeping quality of cut flowers is well documented (Halevy and Mayak, 1979), and it is thus not surprising that the sucrGse content plays an important role in the post-pro- duction behavior of Christmas begonia.

In Christmas begonia irradiance levels providing high sucrose contents of leaves and inflorescences, resulted in the largest increase in numbers of flow- ers and flower buds during the interior holding period. Light has been shown to promote the translocation of assimilates both to vegetative (Thrower, 1964) and to reproductive plant parts (Mae and Vonk, 1974). Mot and Halevy (1980) showed that the amoont of ~4C assimilates translocated out of the leaves to rose shoots held in darkness was 50% lower than that of rose shoots held in light. Number of flowers and growth rate were both reduced when plants were held in darkness. They concluded that carbohydrate translocation to the shoot tip was reduced in darkness, thus promoting its atrophy.

In Christmas begonia low irradiance level resulted in slower development of flowers and buds, indicating that the inflorescences probably are weak sinks, as pointed out for rose shoots kept in darkness by Mot and Halevy (1980).

The sucrose content also influenced the abscission rate. The abscission is, however, not influenced by the carbohydrate status alone. The abscission pro- cess is induced by ethylene (Reid, 1985). On the other hand, lack of carbo- hydrates might induce increased endogenous ethylene, and hence induce the abscission.

Starch is the predominantly 'non-mobile' storage polysaccharide in plants (Avigad, 1982 ). During the photosynthesis the CO2 fixation results in depo- sition of starch. Degradation of starch during darkness provides a flow ofpre- cursors for biosynthesis of sucrose molecules. The starch content is conse- quently affecting the sucrose content of the plant. A low starch content at the marketing stage may hence result in a lower increase in flowers and buds dur- ing the interior holding period. The strong negative colTelation between starch and abscission rate shows that plants with a high content of stored carbohy- drates tend to abscise less flowers and flower buds in the interior holding pe- riod than plants with lower starch content.

The glucose content was about five times higher than the starch content in the inflorescence. Nichols ( 1973 ) and Kaltaler and Steponcus (1974) found that the main constituent of the sugar pool of mature petals isreducing sugars. This seems to be the case for Christmas begonia as well.

CARBOHYDRATE CONTENT AND KEEPING QUALITY OF CHRISTMAS BEGONIA 227

D-Glucose is evaluated as a key 'building block' in plants and a m~or por- tion of this hexose is transferred to different plant products after cycling as a sucrose molecule (Avigad, 1982 ). Being the main 'building block' of cellular structures and a major part of the carbohydrate pool of the inflorescence in Christmas begonia, glucose certainly serves as an important component in the development of new flowers and flower buds.

The present study clearly shows that the carbohydrate content may play an important role in the keeping quality and decorative value of Christmas be- gonia. It also points out ~ha~ ihe light conditions under which the plant is kept, are important. Further investigations on carbohydrate content and postharvest behavior, including the light acclimatization process, are how- ever necessary in order to get a broader understanding of the relationship between carbohydrate content and keeping quality of flowering pot plants.

ACKNOWLEDGMENTS

The authors wants to thank Prof. emer. E. Stramme, Prof. R. Moe and Prof. O. Heide for helpful comments on this paper, and Vigdis Revhaug for her excellent technical assistance.

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