Promoting pepper (Capsicum annuum) photosynthesis via chloroplast ultrastructure and enzyme activities by optimising the ammonium to nitrate ratio

Optimal plant growth in many species is achieved when the two major forms of N are supplied at a particular ratio. In this pot experiment, the effects of five different ammonium: nitrate ratios (ANRs) (0: 100, 12.5: 87.5, 25: 75, 37.5: 62.5, and 50: 50) on photosynthesis efficiency in chilli pepper (Capsicum annuum L.) plants were evaluated. The results showed that an ANR of 25: 75 increased the contents of chl a, leaf area and dry matter, whereas chl b content was not affected by the ANRs. Regarding chlorophyll fluorescence, an ANR of 25: 75 also enhanced the actual photochemical efficiency, photochemical quenching and maximum photosynthetic rate. However, the 0: 100 and 50: 50 ANRs resulted in higher values for nonphotochemical quenching. An inhibition of maximal photochemical efficiency was found when 50% NH₄⁺ was supplied at the later stage of plant growth. The addition of 25% or 37.5% NH₄⁺ was beneficial for gas exchange parameters and the 25% NH₄⁺ optimised the thylakoid of chloroplasts. Compared with nitrate alone, 12.5-50% NH₄⁺ upregulated glutamate dehydrogenase (GDH), the large subunit and the small subunit of Rubisco. It can be concluded that the 25: 75 ANR accelerated N assimilation through active GDH, which provides a material basis for chloroplast and Rubisco formation, resulting in the increased photosynthetic rate and enhanced growth in chilli pepper.