This is because the photosynthesis require optimum pH to be slightly alkaline. Bases, such as sodium hydroxide (NaOH), are necessary to raise the pH to levels that are ideal for photosynthesis to occur.
Since, pH in soil, fresh water or groundwater becomes acidic, plant uptake and photosynthesis rates decrease. Sodium hydroxide (NaOH) raises pH levels and can affect photosynthesis rates.