Abiotic Factors

The most important abiotic factors for plants are light, carbon dioxide, water, temperature, nutrients, and salinity. Their actions are interrelated in a functional network involving all major processes of plant physiology in the ecological performance of plants (Figure 3).

Light drives photosynthetic CO2 fixation and also photorespiration and excess light can lead to overenergi-zation of the photosynthetic apparatus and formation of reactive oxygen species (ROS) and oxidative stress. Light is heating up the leaves. Light may also have a signaling function. Day length (photoperiod) may have developmental consequences. Particularly, red and blue light can function as effectors in signaling networks (Figure 2) and in this way, light can also lead to the developmental formation of sun and shade leaves. Light affects the

PI-4,5-bis-P in membrane

Inactive cyclases in menbrane

PI-4,5-bis-P in membrane

Inactive cyclases in menbrane

GMP AMP

IP3 + DAG Ca2+ stores

GMP AMP

IP3 + DAG Ca2+ stores

Calmodulin Free calcium cGMP cAMP

Ca2+-calmodulin

Calmodulin Free calcium cGMP cAMP

Ca2+-calmodulin

proteins

Active phosphorylated proteins proteins

Active phosphorylated proteins

Figure 2 Molecular regulation network in the signaling cascade from effector to active phosphorylated proteins. An effector which can be any external factor including light, that is, mainly blue and red light, or any internal factor including metabolites and phytohormones reacts with an appropriate receptor (mainly in a membrane, for example, the plasma membrane). The activated receptor releases guanosine-triphosphate (GTP) from a GTB-binding so-called G-protein, which binds to and activates phospholipase C (PLC), active PLC hydrolyzes membrane-bound phosphatidyl-inositol-4,5-bisphosphate (PI-4,5-bis-P) to inositol-1,4,5-tris-phosphate (IP3) and diacyl glycerol (DAG) which elicit the release of the second messenger calcium from membrane-bound Ca2+-storing compartments, free Ca2+ ions bind to the small peptide calmodulin, and Ca2+-calmodulin activates protein kinases. GTP also can activate membrane-bound cyclases which from guanosine- and adenosine-monophosphates (GMP, AMP) form the respective cyclic derivatives cGMP and cAMP which then activate protein kinases. Activated protein kinases phosphorylate proteins, and the activated proteins can either directly affect metabolic processes or move into the nucleus and function as gene regulation factors.

movements of stomatal guard cells, opening and closing of stomatal pores in leaves which regulate gas exchange, that is, CO2 uptake and loss of water vapor by transpiration. This affects CO2 assimilation and also tran-spirational cooling of leaves. Temperature has important effects on metabolism. Heat, cold, and freezing are important stressors. Transpiration and the xylem flow of water affect uptake and distribution of nutrients. Nutrients and light interact, for example, in the formation of sun and shade plants, where the shade plants generally have a higher demand of nitrogen. CO2 assimilation and nutrients interact in the assimilation of inorganic nitrogen and sulfur and in carbon/nitrogen signaling functions in the whole plant. Salinity is one of the outstanding ecological challenges worldwide and, in particular, is a great problem in irrigation agriculture. Salinity affects plant-water relations due to osmotic consequences and has adverse effects on ion nutrient balances, and especially the sodium ions of NaCl have adverse effects if accumulated in plant cells and not sequestered by transport across the tonoplast into the central cell vacuole. These are but a few of the possible interactions and the reader may discover others by moving around in a scheme like that of Figure 3.

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