Question

7. Consider the evolution of land plants. a. According to the phylogeny of land plants, pores are an ancestral trait and stomata are derived. Explain why this is logical, in terms of the mutations that had to occur for these traits to evolve. (2 sentences max) (2 pts) b. Despite the advantages of stomata, Liverworts still thrive today and only have pores. Why? (2 sentences max) (2 pts)

Answer 1

a) Mutants deficient in the phytohormone abscisic acid (ABA) is important for physiological role in land plants. The wilty mutant of Pisum sativum is best example of ABA-deficient mutants. The Phylogenetic analysis of the ABA2 gene from diverse land plants has shown that an ABA biosynthesis specific short-chain dehydrogenase (ABA2) evolved in angiosperms and its role is to close the stomata. With this mutant, the role of ABA biosynthesis in the rapid stomatal response to changes in humidity or vapour pressure deficit can be determined. Wilty mutant plants do not show increase in foliar ABA levels, and as a result of this stomata do not close. The stomata might have first evolved on the gametophyte and subsequently been co-opted by the sporophyte. The production of stomata on the sporophytes of mosses require much simpler cellular processes than that of dicots.

b)  Liverworts do not have stomata that is needed for photosynthesis. Its main function is to open and close itself to get carbon dioxide in dry weather. Since the pores cannot open and close like stomata, liverworts are more susceptible to drying out than other bryophytes. The air pores of liverworts remain open all the time and cannot control their water loss through these pores. But each pore can pass into an air chamber which has columns of photosynthetic cells and thereby facilitating in gases exchange. So, Liverworts can still thrive today.