Final answer:
Pollen grain viability decreases primarily due to dehydration, despite its resistant layer. Environmental stressors such as air and sunlight, and oxidative stress also play a role, impacting the moisture content and cellular components of the pollen grain. Hence, the correct answer is option (D).
Step-by-step explanation:
Despite the pollen grain having a highly resistant layer, its viability decreases primarily due to dehydration. Pollen is subject to environmental stressors such as air, sunlight, and dry periods, which can affect its moisture content and thereby reduce its viability.
Oxidative stress can also play a role, as reactive oxygen species produced within cells or due to environmental conditions can damage cellular components. Mechanical damage can occur due to abrasion, but generally, the robustness of the pollen grain's exterior offers some protection in contrast to the effects of dehydration.
Dehydration can cause a significant reduction in photosynthetic activity in plants, as seen in soybeans, where severe water stress causes a drop in photosynthesis by about 70% while not significantly affecting respiration. This illustrates how critical water balance is for plant cellular processes, including those of the pollen grain.