Researchers from the department of agricultural sciences at the University of Naples Federico II in Italy looked at ten leafy vegetables: chicory, green lettuce, lamb’s lettuce, mizuna, red chard, red lettuce, rocket, spinach, Swiss chard and tatsoi.
They examined the impact of low and high Photosynthetically Active Radiation (PAR) at harvest on leaf dry matter, nutritional composition and antioxidant activity.
PAR refers to the range of solar radiation that photosynthetic organisms are able to use in photosynthesis. The low level of PAR corresponded to light intensity typical early in the morning around 8 o’clock while the high PAR corresponded to the afternoon at 2 o’clock.
The study, published in the journal Food Chemistry, showed that harvesting under lower PAR could enhance the nutritional value of leafy vegetables in terms of protein, potassium, calcium and magnesium contents and hydrophilic antioxidant (HAA) and lipophilic antioxidant activity (LAA).
While this also led to higher nitrate accumulation, this remained within safe limits established in Europe meaning the “advantages of harvesting leafy vegetables at low light intensity outweigh the disadvantage of higher nitrate accumulation”.
Leafy vegetables harvested at low PAR were higher in dry matter, protein, potassium, calcium and magnesium, HAA and LAA by 12.5%, 10.0%, 12.6%, 23.7%, 14.1%, 11.9%, and 18.5%, respectively, compared to the high PAR harvest.
Meanwhile the highest total phenolic contents for chicory, green lettuce, lamb’s lettuce, mizuna, red chard, and red lettuce, were seen under high PAR conditions.
“The health-promoting compounds of rocket and mizuna (higher dry matter, Ca and protein contents, and higher HAA), spinach (higher P, K, and Mg contents) and also red lettuce (higher ascorbic acid and total phenolic) fortified by low light intensity harvest, confer these baby leaf species candidacy as constituents of a healthy diet,” the researchers wrote.
Source: Food Chemistry
Vol 199, pp. 702–710, doi:10.1016/j.foodchem.2015.12.068
“Nutritional quality of ten leafy vegetables harvested at two light intensities”
Authors: E. Colonna, Y. Rouphael, G. Barbieri and S. De Pascale