POSTHARVEST QUALITY OF ROSES AS RELATED TO PREHARVEST CONDITIONS

A nursery comparison with four different rose varieties was carried out as a follow-up from a previous comparison with one variety. Some of these four rose varieties reacted in the same way to a parameter related to plant growth, but there were also exceptions. A high relative humidity was correlated with a shorter vase life in ‘Bianca’, but a longer life in ‘Red Berlin’roses. A high light level was positively correlated with vase life, in all varieties. A higher temperature corresponded with a longer vase life in ‘First Red’ but a shorter vase life in ‘Bianca’. A greenhouse experiment in which three varieties were grown at various vapour pressure deficits (VPD) showed that the higher the VPD (the dryer the air) the longer the vase life. The transpiration rate of the cut rose stem during the first day of vase life correlated well (negatively) with the length of vase life. We observed crispy (desiccated) leaves in many of the roses, as the experiment was done during winter. We checked the effects of the osmotic value of the vase solution on the number of desiccated leaves. In deionised water the problem was the least, but here also some leaves became eventually crispy, indicating that the rate of water uptake was unable to compensate for the very high rate of transpiration. INTRODUCTION Rose growers will usually try to choose the greenhouse climate conditions that are optimal for growth, production, externally visible quality parameters like the length and thickness of the stems and the size of the flower buds. Postharvest quality does not always feature as a priority in these choices. Continuous supplemental lighting (24 hours per day) is an example of a measure which influences growth of roses positively, but can have a very negative effect on the length of vase life because the stomata do not close in the dark period, leading to excessive transpiration. A low rate of ventilation in the greenhouse, in order to keep the warm air inside, results in high relative humidity and causes a high transpiration rate during vase life (Mortensen & Fjeld, 1995, 1998). Since the buyers of the product now often ask vase life guarantees, the growers need to know which preharvest conditions have a clear influence on postharvest quality. A detailed nursery comparison is a good method to investigate the influence of a multitude of growth factors and their interactions. Such a nursery comparison provides the growers with tools to actively improve the postharvest quality of their product. It also allows them to estimate the length of vase life, based on the growth conditions prior to harvest. These studies can single out important factors and thus be used to design further work on the effect of such single factors on post-harvest quality. Thus far it was unclear if the conclusions found for one variety were also valid for other varieties. In the winter of 1998 – 1999 a first nursery comparison was carried out for rose (Marissen & Benninga, 2001) with 35 nurseries growing the variety ‘First Red’. In the winter 2000 – 2001 a follow up was carried out with four varieties, with ten nurseries per variety. This was done in order to see whether the results found for ‘First Red’ would be corroborated in another year, and in order to test if other varieties would react the same. In the winter of 2001 – 2002 we also investigated the effects of relative humidity in the greenhouse on post-harvest quality of three rose varieties, in order to determine the threshold values for relative-humidity effects on vase life. This could be used as an Proc. VIII IS Postharvest Phys. Ornamentals Eds. N. Marissen et al. Acta Hort. 669, ISHS 2005 256 advisory tool for the growers, especially in winter, when relative humidity tends to be high in the greenhouses. In winter the occurrence of desiccated ‘crispy’ leaves is a serious postharvest quality problem. Often, the problem is not yet seen during transportation, but will appear when the product has reached the consumers, or even sometimes when it is still at the wholesalers. The use of additives in the water during the transport chain seems to aggravate the problem, and so do vase solutions containing sugars. Markhart and Harper (1995) described the problem and showed that the detrimental effects of sugars in the vase solution is caused by plasmolysis of the leaf cells, caused by an increase of the osmotic value in the intercellular space. Due to the high transpiration by the leaves the concentration of the chemicals in the intercellular space increases and causes plasmolysis. The growers tend to decrease the concentration of pre-treatment chemicals, in order to avoid this problem, but sometimes even without any pre-treatment (thus in tap water only) crispy leaves occur. We did some small-scale tests on the occurrence of crispy leaves, in relation to the type of vase solution. MATERIALS AND METHODS Nursery Comparison The nursery comparison was carried out as described before (Marissen and Benninga, 2001). A few alterations were made: four instead of one variety were used, ‘First Red’, ‘Sacha’, ‘Red Berlin’ and ‘Bianca’, with 10 growers per variety. Data were collected for 12 weeks, from week 42 in 2000, divided in three periods of four weeks, each period ended with the harvest of 20 stems for a postharvest test, and 10 more stems for mineral analysis of the leaves, and the ‘leaf drying test’ (LDT). This test, as described by Mortensen & Gislerod (1999) was slightly modified (1 and 2 hours of drying time, instead of three hours) and was included as a determination that has a good correlation with the transpiration rate during vase life. An excised leaf was left in a room of 20 °C, 35% RH and 13.8 μmol/m s light from fluorescent tubes, and weighed after one and two hours in order to determine the weight loss as a percentage of the initial fresh weight. From a defined area in the greenhouse the number and total weight of the stems was recorded during the experimental period. Vase life was tested under standard circumstances, without a transport simulation. Data were analysed with multivariate techniques (factor analysis, multiple regression and path analysis), as described before (Marissen and Benninga, 2001). Greenhouse Experiment In four greenhouse compartments three rose varieties were planted: ‘First Red’, ‘Orange Unique’ and ‘Vendela’. Planting date was October 2000. In three compartments air humidifiers were installed, and the relative humidity (RH) was set at 65, 77.5 and 85 %. From August 2001 until March 2002 regular postharvest tests were performed, in which vase life, transpiration, occurrence of crispy leaves, LDT and bud opening were measured. Transpiration rate was measured by weighing vase and stems, determining the leaf area and calculating the transpiration as ml water transpired per 100 cm leaf area per 24 hours. Crispy leaf occurrence was classified as 0 = no damage, 1 = one leaflet with small spots, 2 = several leaflets with spots of minimal 0.5 cm, 3 = all leaflets with large spots to completely dry leaves. Crispy Leaves as Dependent on Vase Solution Osmotic values of the vase solutions were measured with a Gonotec cryoscopy osmometer, using 300 μl solution per measurement. Crispy leaf damage was classified as described above.