Increased illumination levels enhance biosynthesis of aloenin A and aloin B in Aloe arborescens Mill., but lower their per-plant yield

Leaves of Aloe arborescens Mill. are a relevant source of secondary metabolites of pharmaceutical relevance. Notwithstanding, specialized cultivations of A. arborescens are still rather limited, and a straightforward agronomical research addressed to the obtainment of high-quality material is lacking. With the purpose to fill this gap, from 2016 to 2018, a trial was arranged to evaluate the growth and development of A. arborescens, along with the production of four active metabolites (aloin A and B, aloenin A, and isoaloeresin D) with varying some growth conditions. Two growth substrates ("A"- a commercial substrate, and "B"- the same substrate + 20 % perlite), two durations of pre-transplant open-air storage ("stress", 7 and 14 days), and 3 illumination levels obtained by means of shadow nets with different mesh dimensions (SL: full sunlight; T50: 50 % shading; T70: 70 % shading) were tested, combined in a factorial experimental layout with 3 repetitions. In general, light intensity was the most crucial experimental factor, whereas the effects of growth substrate and pre-transplant stress were scarce and limited in time. The addition of perlite to the growth substrate gave the best results in terms of leaf size and root growth, without any significant effect on the yield of active metabolites. The increasing shading level caused a parallel increase of several biometrical characters of plants (height, number of leaves per plant and mean diameter of the stem), whereas the number of suckers per plant was positively affected by the increase of illumination level. The illumination level was also responsible for significant variations in the content of all secondary metabolites, except for aloin A, that resulted statistically not different among the illumination treatments (from 0.80 to 0.98 % in dry matter). The content of aloenin A and aloin B was higher in the plants exposed in full sun, statistically well differentiated from those exposed to 70 % shadow (2.0 vs. 1.4 % d.m. aloenin A, and 1.12 vs 0.86 % d.m. aloin B, in full sun and 70 % shadow, respectively). Contrastingly, the highest levels of isoaloeresin D were reached in the two shaded treatments (4.98 % in 50 % shadow and 4.89 % in 70 % shadow), whereas the full sun treatment reached the lowest value. The higher number of leaves in plants exposed at reduced illumination, however, brought to increased total amounts per plant of aloin (A and B) and aloenin A with increasing shadow levels.