Biological processes such as metabolism and gene-related factors, environmental elements and physiological pathways work conjunctively in the process of seed development. The development process is offset by double fertilization which results in the development of the embryo and endosperm in a developmental cycle that involves asymmetric cell division, acytokinetic mitosis, mitotic cell division, and endoreduplication depending on the cell cycle type of the seed (Dante, Larkins & Sabelli, 2014). The complexity of these processes dictates seed development in phases. In basic terms these phases include cell development, cell enlargement, and cell differentiation and duplication. According to Dante, Larkins & Sabelli, the ultimate size of the seed depends on the rates of proliferation and multiplication and the timing of the changes that determine the development (2014). In addition, the cell cycle type is solely dependent on some important cell cycle genes which are a key determining factors of the growth processes of various seed types.
Water and temperature also affect seed germination. These factors can, independently or conjunctively, affect the extent of germination regardless of seed types (Shaban, 2013). While the effect of temperature on germination is known as thermal time the effect of water potential on seed growth is known as hydro-time. According to Shaban, thermal-time describes the effect of temperature on the germinability of seeds (2013). For example under low moisture the seeds of D. eriantha or finger grass can effectively germinate within a specified temperature range. However, greater water stress slackens the germination rate hence negatively affecting germination percentage (Shaban, 2013). The finger grasses response to moisture and water stress occurs as an survival adaptation because the plant is accustomed to high moisture environments. Penfield further states that with respect to dormancy, the interchanging seasons and temperatures highly affect the loss of primary dormancy and of the cycling of secondary dormancy (2017).
Soil salinity also adversely affects seed germination. Under saline conditions (salty) conditions, seed germination slackens. The higher the sodium chloride contents in the soil, the lower the germinability of seeds (Kolodziejek and Patykowski, 2015). For example, Rumex Confertus or Asiatic Dock seeds when subjected to 7% saline conditions germinate slower than the seeds placed in distilled water (control experiment) which indicates that soil conditions factor in germinability with regard to timing and progressive growth (Kolodziejek and Patykowski, 2015). The adverse effects of high salinity on crops are things like drought stress, ion toxicity, nutritional issues, oxidative stress, and alteration of metabolic processes, disorganization of internal and external membranes and reduced cell division and expansion. The graph below shows the effect of soil salinity on the germination of yellow sweet clover seeds.
The pH influences enzyme action in a germinating seed. Enzyme action is favored by neutral acidity whereby pH levels between 5 and 8 favor seed germination (Ghaderi-Far, Gherekhloo & Alimagham, 2010). However at lower pH value, germination does not occur. A pH value beyond 8 slackens germination progressively until pH 9 where the process is suddenly cut short. According to Kolodziejek and Patykowski, soil pH must be balanced, preferably at a constant value of 6 to ensure progressive growth, effective timing, and high yield in crops (2015).
Planting depth also affects seed emergency. The deeper the seed is placed in the soil, the longer it takes to emerge above the soil surface. For instance, burial depth shows a significant effect on the emergence of the Asiatic Dock (Kolodziejek and Patykowski, 2015). Hence, at a depth of 0.5cm dismal emergence is observable while a depth greater than 0.5cm the percentage of seedling emergence is almost zero. Decreasing in seedling emergence as a response to planting depth has been reported in several plant species and relates to light and seed size because “very little light is transmitted below a 4 mm depth in all soil types” (Ghaderi-Far, Gherekhloo & Alimagham, 2010). However, further research indicates that light has dismal effects on planting depth. Rather, seed reserves favor or disfavor the process. Larger seeds naturally have greater reserves compared to smaller ones. As such, large seeds are likely to emerge above the soil when planted at the same depth as smaller seeds with smaller seeds sometimes failing to germinate due to dismal food reserves (Ghaderi-Far, Gherekhloo & Alimagham, 2010). Nonetheless, seeds planted on the soil surface also show lower emergence than those sowed at the required two-centimeter depth. At the surface there is limited seed to soil contact hence low water availability for onset of germination.
Evidently, the germination processes of a seed depend on a variability of environmental factors. As such, in the face of radical environmental changes due to pollution, adapting effective agricultural practices requires ample understanding on favorable conditions for continuous g