Effect of Light Intensity on Morpho-Physiological Traits and Flowering of Tagetes patula and T. erecta under Late Season Planting Conditions

Marigold species(Tagetes spp.) are ornamental plants which belong to Asteraceaefamily and their geographical dispersal occur mainly in temperate regions. Climate conditions, soil nutritional properties and water are considered as three important factors for plant growth. The majority of these plants are cultivated as bedding plant, flowerbed edging and mass planting from January through May in spring and early summer. During this period, plants are exposed to a wide range of temperature and light conditions. Light is an absolute requirement for plant growth and development. However, different plants have optimum requirements and both deficient and excessive light intensities are injurious. The aim of this study was to investigate the effect of different light intensity on the growth and flowering of two species of the Marigold under the late season planting dates, conditions in which young plants have to grow under high light intensity.
Material and In order to evaluate the effect of light intensity on the growth and flowering of two species of marigold (Tagetes erectaandTagetes patula), an experiment was conducted at the research station of Lorestan University (Khorramabad, Iran) in 2016. The experimental design was a split plot based on a completely randomized design with three replications. The treatments were consisted of three light intensity levels (600, 1200 and 1800 μmol m-2 s-1) as main factor and two species of marigold as a sub-plot factor. F1 seeds were growninto the pots containing equal amount of soil, sand and manure. Different levels of light intensities (1200 and 600 μmol m-2 s-1) were achieved by shading with one or two thin layers of green screen (Saran), respectively. Control plants were grown under natural light condition (no shading) with light intensity of about 1800 μmol m-2 s-1. The shading was applied at the two leaf stage until the end of the experiment. Plant height, stem diameter, numbers of axillary shoots, root length, internode length, leaf number, flower diameter, flower vase life, peduncle diameter, root, shoot and total dry and fresh weights, relative water content, chlorophyll and carotenoid content were measured at the flowering stage. The result of the present study showed that the effects of light intensity, species and their interaction effects were significant for plant height, leaf number, root length, flower diameter, leaf fresh weight, flower fresh weight, total fresh weight, leaf dry weight, stem dry weight, root dry weight, flower dry weight, chlorophyll a, carotenoids and total chlorophyll.The main effects of light intensity and species had also significant effects on stem diameter, flower vase life, stem fresh weight, root fresh weight, total dry weight and chlorophyll b. The highest mean of the most traits was found in plants grown under 600 μmol m-2 s-1, followed by that in 1200 μmol m-2 s-1. The mean comparison revealed that with increasing light intensity, plant height decreased in both cultivars. With decreasing light intensity, flower fresh and dry weight increased in T. erecta, while no differences were found in those of T. patula. The highest flower fresh and dry weight was found in T. erecta plants grown under 600 μmol m-2 s-1.In the present study, high light intensity had a detrimental effect on T.erecta as all plants died under the 1800 μmol m-2 s-1 level. However, under 600 or 1200 μmol m-2 s-1T. erecta performed much better than T. patulain most studied traits. The obtained results showed that by decreasing light intensity, plant height, internode length, stem diameter, leaf number, root length, number of axillary shoots, flower vase life and flower diameter increased which revealed that marigold is a sensitive plant to light intensity in late planting date, and so suitable planting date is very important for good performance of this plant. The responses of various species of marigold were different to light intensity and also toplanting date. Although the mean of most traits in T. erectawas higher compared to T. patula, T. erecta was more sensitive to light intensity compared to T. patulasince high light intensity could kill the T. erecta. Considering all the investigated traits, there is a possibility that high light intensity can affect marigold growth and development through oxidative stress. The results of present study suggest that, in late season planting conditions, T. patula and T. erectaare recommended for sunny and shaded area, respectively.