Cell elongation in plants is caused by

The plant hormone produced by $Rhizobium$ for nodulation is

Who among the following discovered the $Avena$ curvature test to find out the concentration of auxins?

$A$ potted plant placed near the window bends outwardly due to

Auxins are growth hormones capable of promoting cell elongation. They have been used in horticulture to promote growth,flowering,and rooting. Write a line to explain the meaning of the following terms related to auxins:
$(a)$ Auxin precursors
$(b)$ Antiauxins
$(c)$ Synthetic auxins

Which phytohormone promotes falling of old leaves whereas inhibits falling of young leaves?