(a) Explain the APG IV classification. State briefly how it differs from existing classifications. Discuss its merits and…

(a) APG IV classification, its difference from existing systems, and its merits and demerits (20 marks)

The APG IV classification (10 marks). The Angiosperm Phylogeny Group IV system, published in 2016, is a phylogenetic classification of flowering plants based on cladistics, in which every group must be monophyletic, that is, include an ancestor and all its descendants. It rests mainly on DNA sequence data, chiefly plastid genes (rbcL, atpB, matK), with nuclear ribosomal and mitochondrial genes and morphology in support. It is a consensus of many taxonomists and is revised as new evidence arrives. It recognises 64 orders and 416 families, and above the order it uses informal clades instead of ranks: the ANA grade (Amborellales, Nymphaeales, Austrobaileyales), magnoliids, monocots, and eudicots with core eudicots, superrosids and superasterids (rosids: fabids and malvids; asterids: lamiids and campanulids). APG IV added five orders to APG III (Boraginales, Dilleniales, Icacinales, Metteniusales, Vahliales) and re-arranged some families.

Difference from existing systems (4 marks). Earlier systems such as Bentham and Hooker, Engler and Prantl, and Cronquist were based on morphology and assumed phylogeny, and often placed groups on overall similarity, so they included paraphyletic groups. APG discards the dicot group, which is not monophyletic, and splits it into basal angiosperms, magnoliids and eudicots. It also re-defines many families, for example, Scrophulariaceae is broken up, many genera moving to Plantaginaceae, Asclepiadaceae is merged into Apocynaceae, Chenopodiaceae into Amaranthaceae, and Sterculiaceae, Tiliaceae and Bombacaceae into Malvaceae.

Merits and demerits (6 marks).

Merits:

• It rests on monophyly, so the groups reflect real evolutionary relationships.
• It uses objective, repeatable DNA evidence, is a consensus, and is open to revision.
• It resolves long-standing problems and predicts unexpected relationships, and gives a stable framework for research, herbaria and databases.

Demerits:

• Many groups have no simple morphological characters, so they are hard to identify or key in the field or classroom.
• Lumping of families (Malvaceae, Amaranthaceae) may hide useful diversity.
• It depends heavily on a few genes, mostly plastid, and changes with each revision.
• It covers only angiosperms and has no ranks above the order.
• Bentham and Hooker's system, being simple to use, is still followed in many Indian herbaria.

(b) Major strategies of plant disease control and management (15 marks)

Control rests on the principles of exclusion, eradication, protection, resistance and therapy, combined in an integrated approach.

• Exclusion: legal quarantine (India's Plant Quarantine Order, 2003), certified disease-free seed and planting material, and inspection, to keep pathogens out of an area.
• Avoidance: choose the site, sowing time, spacing and disease-free seed so that crop and pathogen do not meet at the vulnerable stage.
• Eradication: rouging of infected plants, crop rotation, destruction of crop residues, removal of weeds and alternate hosts, and soil solarisation.
• Protection: chemicals such as Bordeaux mixture, copper fungicides, sulphur, and systemic fungicides such as carbendazim as seed treatments, sprays and drenches; bactericides such as streptocycline; and cultural methods like irrigation and drainage.
• Host resistance: resistant or tolerant varieties bred conventionally or through genetic engineering, the cheapest and most durable method.
• Biological control: antagonists such as Trichoderma, Pseudomonas fluorescens and Bacillus, and Agrobacterium strain K84 against crown gall.
• Therapy: hot-water treatment of seed and setts, chemotherapy, and meristem culture to make virus-free plants.
• Integrated disease management: combining these, monitoring and forecasting, with the least reliance on pesticide.

(c) Comparison of the sporophyte of Psilotum and Equisetum (15 marks)

Both are homosporous pteridophytes, but they differ in almost every organ.

• Habit: Psilotum (whisk fern) is a small, bushy plant with forked green aerial stems; Equisetum (horsetail) is a larger plant with erect, jointed aerial stems and whorled branches.
• Underground part: Psilotum has a creeping rhizome with rhizoids and mycorrhizal fungus and no roots; Equisetum has a perennial rhizome, sometimes with tubers, bearing adventitious roots.
• Stem: in Psilotum it is dichotomously branched, ridged and green; in Equisetum it is differentiated into nodes and internodes, with ridges and furrows, a hollow centre and silica in the epidermis.
• Leaves: Psilotum has small, scale-like, vascular-less enations; Equisetum has small scale leaves in whorls at the nodes, united into a sheath.
• Stele: Psilotum has a protostele in the rhizome, changing to a siphonostele with pith in the aerial stem; Equisetum has a siphonostele broken into separate bundles, each with a carinal canal, and the cortex has vallecular canals.
• Sporangia: Psilotum has trilocular synangia at the tips of short forked branches; Equisetum has sporangia on peltate sporangiophores, in a terminal cone (strobilus), five to ten on each.
• Spores: Psilotum has spores without elaters; Equisetum has spores with four hygroscopic elaters.