PHOTOSYNTHETIC ADJUSTMENTS OF AMARANTHUS VARIETIES IN RESPONSE TO GROWTH IRRADIANCE

Abstract

Increased concern of food security has created a demand for crops that exhibit high yields under conditions that require a minimal input of resources, such as light. Amaranthus is a C4 plant that has multiple uses, including that of a food source in many parts of the world. The objective of this research was to evaluate the photosynthetic performance of red and green vegetable varieties of Amaranthus blitum grown under high light (HL; 500 µmol m-2 s-1) photosynthetic photon flux density (PPFD) and low light (LL; 70 µmol m-2 s-1 PPFD). Under HL growth irradiance, the red variety accumulated more biomass than the green variety before flowering. This was accompanied by a 1.2-fold increase of photosynthetic efficiency, a 1.5-fold increase of photosynthetic capacity (maximal rates of O2 evolution), a 1.5-fold increase of dark respiration rate, a 1.3- fold increase in light compensation point and a 1.7-fold increase of light saturation point. These values were supported by a 2.5- and 2.6-fold higher content of chlorophyll and carotenoids, respectively. A 4.7-fold greater betalain content was also observed. In addition, exposure to a photoinhibitory irradiance (1450 µmol m-2 s-1 PPFD) at 2°C for 4 hours demonstrated the red variety exhibited increased tolerance to photoinhibition in comparison to the green variety when measured at the level of maximal photochemical efficiency of photosystem II. Further, this may be a result of the screening of light and shielding of chloroplasts by betalains that accumulate predominantly in the abaxial mesophyll and supported by a 1.2-fold increase in the red variety lower mesophyll cells thickness than in the green variety, leading to a thicker leaf. The granal index in bundle sheath cells under high light irradiance in the red variety was 2.0-fold greater than in the green variety, supported by 3.1-fold greater ratio of the length of appressed/non-appressed thylakoids and a 2.0-fold increase in thylakoids/granum. These results suggest that under HL growth irradiance, the granal index and stackness (thylakoids/granum) enhances photosynthetic efficiency and capacity. In contrast, under LL growth irradiance both varieties performed photosynthetically similarly. Collectively, these results indicate that the red variety possesses a greater photoacclimatory capacity than the green variety under HL growth irradiance, whereas neither variety expressed an advantage under the LL growth irradiance in this study.