Question

Which of the following pigments is essential for nitrogen fixation by leguminous plants?
(a) Anthocyanin
(b) Phycocyanin
(c) Phycoerythrin
(d) Leghaemoglobin

Answer 1

Hint: It is an oxygen carrier and hemoprotein that is found in leguminous plants' nitrogen-fixing root nodules. These plants are developed as part of the symbiotic relationship between plants and bacteria in response to the roots that are colonised by nitrogen-fixing bacteria, called rhizobia.

Complete answer:
Leghaemoglobin is the important pigment in the root nodules of leguminous plants for nitrogen fixation. It protects the nitrogen fixing enzyme system from oxygen, which is harmful to the atmospheric nitrogen fixing enzyme system. Pigments present in cyanobacteria and red algae are phycocyanin and phycoerythrin, and anthocyanin is a red purple pigment found in fruits, flowers, and other parts of angiosperms.

Additional information:
Anthocyanins are water-soluble vacuolar pigments that can appear in red , purple, blue or black, depending on their pH. Anthocyanin-rich food plants include, among many others, blueberry, raspberry, black rice, and black soybean, which are red , blue, purple, or black.
Phycocyanin is, along with allophycocyanin and phycoerythrin, a pigment-protein complex from the light-harvesting phycobiliprotein family. It is a chlorophyll accessory pigment. Both phycobiliproteins are water-soluble, so they do not live like carotenoids do within the membrane.
Phycoerythrin is a complex of red protein pigments from the light-harvesting family of phycobiliproteins found in red algae and cryptophytes, an accessory to the main photosynthesis-responsible chlorophyll pigments.
So, the correct answer is ‘(d) Leghaemoglobin’.

Note: Leghemoglobin has similar chemical and structural comparisons to haemoglobin and is red in colour, like haemoglobin. It was originally assumed that within symbiotic root nodules, the heme prosthetic community for plant leghemoglobin was supplied by the bacterial symbiont. Subsequent work, however, shows that the plant host strongly expresses genes of heme biosynthesis within nodules, and that activation of those genes correlates with expression of the leghemoglobin gene in nodules growing.