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Effect of Arsenic on Different crop plants

 Image of leaf burning of V. sinensis

Abstract: In the present work effect of arsenic (As) on seed germination, growth activities, metabolic activities and internal structure was investigated in seven varieties of crop plants at three growth stages e.g; a) germination , b) vegetative and c) flowering.  The concentrations of As(III) was in the range of 10-50 ppm. Arsenic inhibited the total germination activity of the seeds of V. radiata  and V. unguiculata at 10 ppm, but comparatively less inhibitory effect was noted in T. aestivum and P. sativum even at 50 ppm As.  The sequence of germination (%) in control was: P. sativum > T. aestivum = V. radiata> O. sativa = V. unguiculata> A. esculentus and at 10 ppm As was P. sativum> T. aestivum> A. esculentus> O. sativa> V. sinensis.

The hightest tolerance index at germination was in P. sativum at 50 ppm As.  The root (R) growth was affected more than that of shoot (S) growth and consequently the R/S ratio was decreased due to As stress in most cases.  The R/S ratio of 6 days old seedlings for growth length was high in T. aestivum (at 10, 20 ppm As) and in A. esculentus (50 ppm As).  For dry weight the R/S ratio was high in T. aestivum at all concentrations As (10-50 ppm) where as tolerance index for shoot dry weight of the seedlings was high in A. esculentus at all As concentrations and only a few cases in root. At 35 days old stage T. aestivum and P. sativum had the highest root and shoot length respectively in control as well as treatment.  R/S ratio for length was high in V. radiata  in control and also in treatments except 50 ppm where it was high in V. sinensis.  At 70 days old stage plant height at 10 ppm As was high in V. sinensis, but the lowest value was in V. radiata at 10 and 50 ppm As. V. radiata revealed almost the highest R/S ratio and P. sativum had the lowest R/S ratio at 50 ppm.  Arsenic stress affected the leaf growth activities in number, length, breadth, leaf area, dry wt. as well as leaf pigments contents.  The leaf number was high in O. sativa in control, but leaf area was in V. sinensis in control and also at 50 ppm As.  Beside these, As induced burning and necrotic symptoms on leaf surface, turned it pale green, reduced root growth structurally and in dry mass production as well as root nodule formation (in Fabaceae).  At 70 days old stage the dry wt. was also low in treatments, the lowest dry matter was in P. sativum for root and its R/S ratio was low.  For root the tolerance index for dry wt. was high in T. asetivum and for shoot it was high in A. esculentus at 50 ppm As.  The pigments analysed at 35 days stage revealed that Chl. A had a general decreasing trend, but Chl. B and carotenoids in some cases, had higher concentrations under As stress (20 ppm) compared to control.

Arsenic stress had both inhibitory as well as stimulatory effects on root and shoot N, P, K+ and Ca2+ contents.  As stress increased the root N content in O. sativa and P. sativum, but in shoot it was high in O. sativa and V. sinensis. Under stress K+ was increased in roots, in most of the crops except P. sativum, but in shoot it was high in P. sativum and in some others.  Compared to K+ contents, Ca2+ contents in both root and shoot of the plant was relatively high.  The efficiency ratio for K+ in root and shoot was high in V. sinensis and P. sativum and for Ca2+ it was high in O. sativa and P. sativum for root and shoot respectively.  Stress due to As had both stimulatory as well as inhibitory effect on sugar and protein contents of the plant.  The sugar level under As stress was increased in root of O. sativa, P. sativum and A. esculentus, but in shoot it had inhibitory effect on sugar levels  in all crops.  In root protein content was increased in the root of O. sativa, P. sativum and in shoot of O. sativa along with V. sinensis under stress.  The highest root and shoot proteins were in P. sativum and V. sinensis respectively.  As stress revealed inhibitory effect on the internal structure of stem of some selected plants.  Extra stellar, stellar and pith reason become compact due to As stress. Xylem ray cells and metaxylem at endarch position became smaller, few in no. and septed in some cases.  Bast fibre patches were few and scantly due to As stress.

The tolerance levels of different crops were found to vary under different stress intensities and growth stages.  However, criteria like better growth activities, high R/S ratio, leaf number and area, nutrient absorption efficiency ratio, concentrations, sugar and protein contents under stress are to be considered important for stress tolerance efficiency of the crop plant.