Growth of the rose plant and the physiological and biological processes occurring in plant is influenced by the temperature, which in turn affect the vegetative growth, flower production and flower quality. Requirement of temperature varies with the cultivars to some extent; and the optimum night temperature ranges between 13.3°C to 21°C depending upon the cultivars for obtaining maximum flowers, heavier and stiffer flower stems of longest length and as well as better flower colour.
At low temperature flower production decreases but better quality flowers are obtained; along with better development of pigments in the leaves, stem and flowers. At higher temperature there is an enhancement of vegetative growth and flower production, but the flower quality deteriorates. The day temperature between 20°C to 28°C is optimum for roses, depending upon the cultivars. Temperature lower than 13°C and above 30°C adversely affects the production and quality of rose flowers.
The effective season of production decrease with the rise in temperature beyond 28°C. In India under natural conditions of Bangalore and Pune, good roses can be obtained for 5 months beginning from December to April. In high Tropics and North-East India the season is for 8 months from October to May, and in subtropical climate, roses can be obtained from November to April.
Shoots growing in short branches at 21°C and a photoperiod of 8 hours under florescent light aborted almost all of their flower buds. Low incidence of abortion occurred in shoots growing on long branches. At a photoperiod of 16h, the incidence of abortion wasiow at both temperatures and shoot positions. On the other hand under incandescent light, the rate-of abortion was high in all temperature, photoperiod and position treatments. The blackening of-rose petals is enhanced by the exposure of plants to sub-optimal temperatures and in absent in green houses covered with polyvinyl chloride (PVC) or polyethylene opaque to UV-B radiation shorter than 360 nm, even when grown at suboptimal temperature.
The promotive effects of supplementary lighting were more pronounced at 18°C night temperature than at 15°C and were facilitated by CO2 enrichment of the green house atmosphere. Reduction in light intensity extended duration of a growth cycle. This effect was influenced by temperature and was more pronounced at higher temperatures than at lower temperatures. A decrease of 65 per cent in light intensity increased the duration of one growth cycle of "Baccara" roses by 3 days at 15°C in comparison with 12 days at 21°C.
Higher temperature during field conditions decreases vase life and quality of rose cut flowers. At higher temperature, stored carbohydrates of flowers are quickly depleted during respiration and the plant transpires at a faster rate. But a decrease in temperature before harvest from 21-24°C to 15-21°C reduces the vase life of cut roses. The effect of temperature was attributed to an increase in the level of phenolic compounds produced in the leaves. The rose cultivars, "Dr Verhage", grown at lower temperature (15°C) develops a greenish tint because of incomplete conversion of chloroplasts to chromoplasts.
Exposure of rose plant to higher temperature (7 hours) reduces anthocyanin content and increases bluing. A strong reduction of temperature at 4 to 7 days before commercial stage of harvest of "Baccara" roses will increase anthocyanin content and blackening. Rose cv. "Carol" and "Dr. Verhage", grown at high temperature, yield very pale flowers, but produces normal colour when cut at bud stage and kept in sucrose solution.
Humid abnosphere may cause the development of fungal and bacterial diseases. Importance of humidity lies in the internal water conditions of the rose plants. Low humidity may cause browning of leaf edge of the plants with thin leaves or leaflets. High humidity can maintain water borne pollutants in a condition so that they can be more easily absorbed through the cuticles or stomatas. High humidity damage cut flowers during storage and transport. Damaged rose flowers remove water less quickly and emit ethylene. Certain diseases like mildews are associated with high relative humidity. Wilted plants grow very slowly and by maintaining very high amount of humidity, the internal deficiency of water will be less.
Enlargement of cells is caused by water, and therefore, if the loss from the plants could be less, leaves and flowers would be larger and stem longer. Environmental factors especially temperature and relative humidity, can have a strong influence on the entry of applied sprays. The most dramatic cause of humidity affecting entry is that of maleic hydrazide. It was also reported that low humidities also suppressed the entry of 2,4-D, amino triazole and some other leaf sprays.
Rose cut flowers show symptoms of Botnjtis infection after marketing and transportation. The air humidity inside the glass house is the factor determining the number of lesions during storage and transportation. So lowering of air humidity by heating and ventilating the glass house is essential. Some reported that increasing humidity from 65 to 85 per cent has no effect on the vase life of roses, while a further increase to 90 per cent significantly decreased it. This decrease in vase life closely related to an increase in the occurrence of the bent neck and leaf dying at the highest humidity.
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