The earth is very dynamic and has undergone dramatic changes in its history. All of the elements, including those common in living things, were synthesized from primordial hydrogen in the interior of stars. Supernovas and other stellar instabilities dispersed many elements into space. Because hydrogen and the noble gases are greatly depleted on earth as compared with their cosmic abundances it is likely that the chunks of matter giving rise to the protoplanet did not carry with them gaseous shells of their own. As a result of contraction and redistribution of materials in the developing planet, a hydrosphere and atmosphere developed that were highly reduced . The surface of the earth today is much more oxidized, even to 21% O2 in the atmosphere . Was this oxidation linear or have there been fluctuations several times during the history of the earth resulting in major species extinctions
Plants grow by the process of cell division or mitosis followed by cell enlargement and maturation. Cells then differentiate into tissues that make up the organs of the plant. Mitosis includes replication of organelles, synthesis of nuclear material, enzymes, etc. Cell enlargement consists largely of water uptake to form a large vacuole. Growth may be measured as change in mass, volume, or length of shoot or root. Crop productivity is often expressed not in biomass but in yield of the desired product: flower, fruit, seed, root, oil, protein, or specific chemical.
CATABOLISM AND ANABOLISM
Photosynthesis transforms energy from sunlight into energy-rich organic matter, i.e., carbohydrates. This organic matter then serves as the energy source for all life on earth. The energy is partially liberated in glycolysis (fermentation) or in the oxidative pentose phosphate cycle, both in the cytoplasm. Substrate-level ATP and reduced pyridine nucleotides are produced. This may have been the extent of energy conservation in anoxic early earth.
Once oxygen began to increase, mitochondrial activity provided a much higher rate of energy turnover, resulting in explosive adaptive radiation. The key to rapid expansion of life on earth as well as growth of a single plant is rapid turnover of ATP/ADP perhaps as much as 50% of the dry biomass of active tissues every 24 hrs. If an inhibitor blocks the cytochrome oxidase pathway or an uncoupler destroys the proton gradient across the inner mitochondrial membrane, there is a rapid increase in oxygen uptake and CO2 production in response to the drop in ATP production.
Plants are subject to many forms of environmental stress. Some are abiotic, physicochemical, or density independent, such as temperature, drought, fire, and air pollution. Other sources of stress are biotic or density dependent, such as competition, herbivory, disease, and parasitism . For each of these environmental factors there is a range or life zone that the plant can tolerate. If the tolerance range for a given stress factor is exceeded, the plant will suffer stress, and if the stress is severe enough, the plant may die. Short-term acclimation may be possible, and given enough time, natural selection may result in adaptation to the stress.