Shoots

Senescence of the shoot axis takes many forms.

In species such as the hepatic Marchantia the thallus, likely evolutionary precursor of the axes and laterals of Spermatophyta, has the capacity to undergo a senescence-like process with many of the physiological and cellular features of the syndrome in angiosperms (De Greef et al. 1971, Maravolo 1980, Stanislaus and Maravolo 1994).

Senescence of the entire shoot is characteristic of monocarpic plants and of species that survive the quiescent season in below-ground perennating organs.

Crowding-dependent mortality in populations and individuals is referred to as self-thinning and is associated with differential shoot senescence (Westoby 1984, Asaeda et al. 2005).

In woody species senescence of the shoot axis in the sense of terminal differentiation of vascular and peridermal tissue is often independent of the senescence of leaves and other laterals.

Cladodes are stems that have adopted the morphology and function of leaves in some species. True leaves in these plants are vestigial structures. Cladodes are physiological leaf analogues to the extent of including a recognisable senescence phase in their development and environmental responses (Haase et al. 1999).

References

• Asaeda T, Lan NK, Manatunge J (2005) Effects of self-thinning of shoots on the nutrient budgets of Zizania latifolia. Hydrobiologia 537: 47-52.
• De Greef JA, Butler WL, Roth TF, Fredericq H (1971). Control of senescence in Marchantia by phytochrome. Plant Physiology 48: 407-412.
• Haase P, Pugnaire FI, Clark SC, Incoll LD (1999) Diurnal and seasonal changes in cladode photosynthetic rate in relation to canopy age structure in the leguminous shrub Retama sphaerocarpa. Functional Ecology 13: 640–649.
• Maravolo NC (1980) Control of development in hepatics. Bulletin of the Torrey Botanical Club 107: 308-324.
• Stanislaus RC, Maravolo NC (1994) The influence of polyamines on senescence in Marchantia polymorpha. The Bryologist 97: 162-165.
• Westoby M (1984) The self-thinning rule. Advances in Ecological Research 14: 167-226.