A cytomorphological investigation of three species of the genus Sonchus L . ( Asterales : Asteraceae ) from Punjab , India

Three species of the genus Sonchus L. (Sonchus asper, S. oleraceus and S. wightianus) were collected from the Malwa region of Punjab during 2019 to 2020. These species were studied for cytomorphological variations. The species under investigation were identified based on their morphological descriptions. Sonchus asper (L.) Hill and Sonchus wightianus DC. possess the same number of chromosomes (2n=2x=18) whereas Sonchus oleraceus (L.) L. is tetraploid with 2n=4x=32 chromosomes. Chromosome number of S. wightianus (2n=2x=18) was worked out for the first time from the state of Punjab. Sonchus olereceus has larger pollens than S. asper and S. wightianus. This study will be useful for researchers, taxonomists and cytologists for accurate identification of these three species.

ISSN 0974-7907 (Online); ISSN 0974-7893 (Print)  7367.13.11.19640-19644 OPEN ACCESS Sonchus L. is a member of the family Asteraceae with 95 species distributed throughout the world including western Morocco, Ethiopia, southern Sudan, South Africa, Canary Island, Europe, Iran, Iraq, Egypt, Afghanistan, and Turkistan (Boulos 1960;Cho et al. 2019). Sonchus species are annual to perennial herbs with a milky latex. The stem is clasping, toothed or pinnatifid, segmented leaves; terminal, umbellate, yellow, ligulatehomogamous heads; ovoid, ellipsoid, compressed, ribbed achenes with white hairy pappus which are the important features of the genus Sonchus L. (Quireshi et al. 2002;Rahman et al. 2008 Morphological parameters have been used for the identifications of plant species for a long time. It is one of the basic, simple and cost effective tools. Morphological features such as leaf shape and color; flower color and type; number, position and nature of androecium and gynoecium; shape and type of fruit and seeds are used for identification of species (Singh & Dey 2005). Chromosome number is also important in the identification of species because species, genera and families have their own unique chromosome numbers in general and basic chromosome number in particular. Variations in chromosome numbers are useful in taxonomic studies (Raven 1975;Jones 1979).
The present study is an attempt to differentiate between previously reported two (S. arvensis and J TT S. wightianus) species (Sharma 1990;Sidhu 1991). Therefore, it is important to look into the detailed morphology of the three species under investigation. Keeping this in view, the present study has been planned to characterize three species of Sonchus from the state of Punjab based on morphological and cytological observations.

Materials and Methods Collection of study materials
The present study has been undertaken in the Malwa region of the state of Punjab, India. The study material of three species of Sonchus was collected during 2019 to 2020. The collected plant specimens were cleaned thoroughly, pressed, and dried at room temperature. After this, the plant specimens were pasted on herbarium sheets. Herbarium specimens were deposited in the Herbarium, Department of Botany, Punjab University Chandigarh (PAN-21994, 21996 and 21997).

Morphological study
Morphological features of a leaf (arrangement, shape, type, color), stem (glabrous, hairy), flower (colour, type, shape), androecium (number, shape, nature), gynoecium (shape, number, nature) were examined to establish the identity of each of the three Sonchus species. The available literature (Hooker 1882;Bamber 1916;Turner et al. 1961;Walter & Kutta 1971;Boulos 1972;Hsieh et al. 1972;Nair 1978;Mejias & Andres 2004;Cho et al. 2019) have been looked into to describe the Sonchus species in question. The Herbarium, Department of Botany, Panjab University Chandigarh and online Herbaria have also been consulted for identification.

Meiotic and pollen study
Meiotic analysis has been carried out in three Sonchus species to examine their chromosome numbers. Young flower buds were collected and fixed in the fixative (ethanol 3: glacial acetic acid 1) for 24 hours then shifted to 70% ethanol till further use. Anthers were excised from young flower bud on the glass slide having a drop of acetocarmine and crushed with the help of a glass rod. The material was covered with a micro coverslip and pressed in two folds of filter paper after gentle heating. Slides were observed under the microscope. Photographs of the pollen mother cells containing countable chromosomes have been taken. For pollen study, mature anthers were taken on the slide and squashed in glycerol acetocarmine (1:1), covered with a cover-slip and observed under the microscope after 24 hours. Uniformly stained pollens (S.P.) were considered fertile whereas, poorly stained or unstained pollens as sterile. The percentage of pollen fertility was calculated using (Pollen fertility = S.P. / Total Pollens x 100) formula. Pollen size has been measured with the help of cameralucida technique.

Results and Discussion
Three species of the genus Sonchus, i.e., Sonchus asper, S. oleraceus, and S. wightianus were collected from the Malwa region of Punjab during 2019 to 2020. All the three species are annual with erect habit. Leaves of S. oleraceus are smooth, glabrous, and light green whereas they are dark green in the case of S. wightianus. In S. asper, leaves are spined and bluish-green. Leaves are elliptic-oblong, half amplexicaul with round auricles in S. asper and S. wightianus but auricles are spreading in the case of S. oleraceus (Image 1,2). Similarly, leaf auricles were found to be round in S. asper and pointed to acute in S. oleraceus (Barber 1941;Quireshi et al. 2002;Cho et al. 2019 Both Sonchus asper and S. wightianus are diploid and contain 2n=2x=18 chromosomes. Nine bivalents were observed at diakinesis and metaphase-I in S. asper and equal segregation of chromosomes (9-9) at anaphase-I in S. wightianus (Image 4.a,b,d). Razaq et al. (1994) also reported chromosome numbers 2n=18 in both Sonchus asper and S. wightianus and 2n= 32 in S. oleraceus from Pakistan.
Pollen size of S. oleraceus is 36.25 x 32.5 μm-40 x 33.75 μm followed by S. wightianus (33.75 x 32.5 μm-36.25 x 33.75 μm) and S. asper (31.25 x 28.75 μm-35   (Sharma 1990) and S. asper, S. oleraceus, & S. arvensis (Sidhu 1991) were documented from the state of Punjab, India. But according to available literature (Shumovich & Montgomery 1955;Mamgain 1998) S. arvensis grows exclusively in Europe and is likely confused with S. wightianus in India. In literature, from the state of Punjab third species of Sonchus was considered as S. arvensis but it is actually a S. wightianus.

Image 3. Morphological details of Sonchus species (e-h): e-flower | f-flower (black arrow showing stigma and blue arrows showing stamens) | g-achene with pappus | h-achene. © Rai Singh
Cytological details of Sonchus species are also incomplete from the state of Punjab, India. Previously, Gupta & Gill (1983)  Consequently, information about the chromosome number of S. wightianus is not known. Therefore, the present study has been carried out for cytomorphological characterization of Sonchus species from the state of Punjab India. The findings of the present study will be useful for researchers, cytologists, and taxonomists for correct identification of Sonchus species based on morphological, cytological, and palynological details.

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