Boron is injurious to plant when irrigational water contains . . . . . . . . concentration of boron.

Boron is injurious to plants when irrigation water contains more than 3 ppm concentration of boron.
When the concentration of boron in irrigation water is above the maximum level it causes havoc on the yield of the crop. There is various plant crop sensitive to boron. although boron is an essential nutrient for the plants it is required in small amounts.
Boron toxicity can cause conspicuous necrotic lesions in the interveinal areas of older leaves, leading to premature senescence and shredding of the leaves.