Effects of irrigation management, mycorrhizal inoculation and humic acid application on color characteristics of roselle (Hibiscus sabdariffa L.) dried sepals

Introduction Roselle or Hibiscus sabdariffa L. is an annual herbaceous plant which grows properly in tropical and sub-tropical areas. Sepals are the main commercial part of roselle with unique brilliant red colour and flavor which are containing many nutritional compounds such as anthocyanin and vitamin C. Accordingly, sepals commonly are used in food industries for production of beverages, juices, jams and syrup (Duangmal et al., 2008; Sanjari-Mijani et al. 2015). Therefore, production of high quality sepals via the good agronomic practices is an important topic in roselle cultivation. So far, the effect of irrigation management (Rahbarian et al. 2011), mycorrhizal inoculation (Sembok et al. 2015) and humic acid application (Sanjari-Mijani et al. 2015) on some qualitative and quantitative indices of roselle has been evaluated in some studies. However, the impacts of mentioned factors on colour characteristics of dried sepals including hunter color values, colour intensity and hue angle has not been studied. Therefore, due to the impact of colour on marketability and drug quality, the aim of this study was to evaluate the effect of water availability, mycorrhizal symbiosis and humic acid application on colour indices of dried sepals in roselle. Materials and methods In this experiment, the influence of irrigation regimes [irrigation after 100 mm (normal irrigation) and 200 mm (deficit irrigation) pan evaporation], humic acid application (0 and 4 kg.ha-1) and mycorrhizal inoculation (Glomus versiforme, Glomus intraradices and control) were studied on colour characteristic of roselle sepals. For this purpose an experiment at the form of split-split-plot layout based on a randomized complete block design with three replications was carried out during 2015-16 at research field of Sarayan Faculty of Agriculture (33ºN, 58ºE, 1450 masl, annual precipitation of 150 mm and annual temperature of 17ºC.) in east part of Iran. Seed planting was done on 20 April using Sistan & Balochestan ecotype with 10×50 cm intra and inter-rows distances. Humic acid was used in irrigation water two times during vegetative growth of roselle i.e. 15 and 35 days after emergence. All plots were irrigated similarly two times during the first week after seed sowing and then irrigation treatments were exerted separately in each plot until November 15 when irrigation was stopped. After that, calyces belonged to each treatment were harvested and dried separately in ambient temperature and shade condition of laboratory and then were used for colour analysis. Color of dried sepals was evaluated using a colorimeter (TES 135, Shenzhen Youfu Tools Co., Ltd. - TAIWAN). Results were expressed as Hunter color values (l*, a* and b*). L is the brightness ranging from 0 (black) to 100 (white). The value a* is the redness ranging from negative values for green to positive values for red. The value b* is the yellowness ranging from negative values for blue and positive values for yellow. In addition, color intensity (Chroma), hue angle and browning index were calculated using equations presented by Duangmal et al., (2008). Finally, data were statistically analyzed using SAS 9.1 and means were compared by duncan test at 5% level of probability Results and discussion Results showed that the effect of experimental factors was significant on most of colour indices in roselle calyces. Droughts stress reduced a* value, hue angle and chroma by 22, 53 and 15% respectively. Humic acid application also reduced the amount of hue by 63% while increased the b/a by 25%. Both mycorrhizal species enhanced the hue index by 66% compared with control treatment. Results of interaction effects revealed that the highest amount of b/a (0.19) was obtained at droughts stress × humic application × mycorrhizal inoculation, while its lowest value (0.02) observed at no stress ×no-humic ×no-inoculation treatment. In addition, the maximum amount of hue (215 º) was obtained at no water stress × humic application × mycorrhizal inoculation using Glomus versiforme. The treatment of no stress × no humic × mycorrhizal symbiosis produced the maximum amount of redness index. Moreover, a significant correlation was observed between a* with chroma (+0.64**) and browning index (-0.71**) as well as between hue and redness index (+1.00**). Overall, colour quality of roselle sepals was more suitable under appropriate water supply. In addition, mycorrhizal inoculation partially reduced the negative impacts of droughts stress on calyxes colour characteristics. The reducing effect of sever water stress and the positive role of mycorrhizal inoculation on growth, yield and quality of roselle has been previously reported by other researchers (Rahbarian et al., 2011; Sembok et al. 2015). It has been reported that red anthocyanins in sepals of roselle are responsible of its brilliant red colour (Duangmal et al., 2008). Our qualitative analysis revealed that the amount of anthocyanins in drought stress condition was 12% lower than no stress treatment. Furthermore, mycorrhizal symbiosis using G. intraradices and Glomus versiforme improved the amount of this index by 20 and 5%, respectively (data are not presented). These observations are in accordance with the trend of redness change in dried sepals. Accordingly, it seems that more redness and a*, hue angle in no stress and inoculation treatments is because of more anthocyanin content in related plants. This finding is also understandable from positive regression relation between redness index and sepals anthocyanin content. Conclusions Considering to colour characteristics and regards to yield and qualitative indices such as anthocyanin and vitamin c content (data are not presented), it seems that appropriate water availability and use of mycorrhizal symbiosis are two good strategies for roselle production.