3.1 Experiment 1 – U. brizantha cv. Marandu
The accumulation of Si in the shoot decreased in plants grown under deficiencies in N, P and Ca compared to the CS. However, the addition of Si to forage plants without and with nutritional deficiency studied in relation to treatments without Si, increased the accumulation of Si in the shoot of the plant (Fig. 1a).
The accumulation of Si in the plant roots decreased with deficiencies in N, P, independent of Si, and with Ca deficiency in the absence of Si in comparison with the CS. The addition of Si in plants under CS, and without addition of P and Ca, compared to the control treatment (without Si) increased the accumulation of Si in the root and shoot of the forages (Fig. 1b). Accumulation of Si in the absence of N addition did not differ in relation to the presence or absence of Si.
Plants that received a solution deficient in P and Ca, independent of Si, compared to plants under N deficiency only in the absence of Si, showed an increase in the electrolyte leakage index. The supply of Si in all treatments (-N, -P, -Ca and CS) compared to treatments that did not receive Si, decreased the electrolyte leakage index (Fig. 2a).
The levels of phenolic compounds in plants grown in CS and in solutions deficient of N and P were similar. But the content of these compounds increased in the solution deficient in Ca, independent of Si addition. Phenol’s content increased in the aerial part of the forage because of the addition of Si to both solutions (Fig. 2b).
The green color index decreased in forage cultivated with a solution deficient in N, P and Ca, compared to the CS, independent of Si. However, this index increased due to the increase of Si in plants deficient in N, P and Ca (Fig. 3a).
The forage grown with nutrient-deficient solution in relation to forage grown with CS, independent of Si, have decreased of the quantum yield of photosystem II (Fv/Fm). However, with the addition of Si to the nutrient solution, fluorescence quantum yield increased (Fig. 3b).
The N, P and Ca use efficiencies decreased in forage grown with solutions deficient in these nutrients, regardless of the +Si or -Si. However, use efficiencies increased in plants deficient in these nutrients because of the addition of Si (Fig. 4a, 4b and 4c).
Plant length decreased when plants were cultivated in solution deficient in N, P and Ca in relation to the CS, independent of Si. However, the addition of Si resulted in increased plant length, irrespective of the nutrient solution (Fig. 5a).
Tiller number of plants cultivated in solution deficient of N and P decreased in relation to the control group, independent of Si. However, under Ca deficiency, the number of tillers was higher than that of plants grown in CS. The addition of Si to the control and to the solutions deficient in N and P increased the number of tillers (Fig. 5b).
The shoot dry matter production was impaired in forage cultivated in solution deficient of N, P and Ca, irrespective of Si addition. In contrast, the addition of Si to the control group and the group deficient in N, P and Ca increased the shoot dry matter production (Fig. 5c).
3.2 Experiment 2 – M. maximum cv Massai
The accumulation of Si in the shoot decreased under deficiencies of N, P and Ca compared to the CS, in the presence of Si. The addition of Si resulted in increased Si accumulation in the shoot (Fig. 1c).
The accumulation of Si in the root was lower in plants cultivated in solution deficient of N and P and increased when Ca was deficient, independent of Si addition. The addition of Si to the control and to the solutions deficient in N, P and Ca increased accumulation of Si in the roots (Fig. 1d).
The electrolyte leakage index increased in plants deficient in N, P and Ca, irrespective of Si addition. However, the addition of Si decreased the electrolyte leakage index in all treatments regardless of if grown with CS solution or with -N, -P, and -Ca solution (Fig. 2c).
The levels of phenolic compounds decreased with Ca deficiency, independent of Si, and with N deficiency only in the presence of Si. Under P deficiency, the phenols content in the forage increased in relation to those grown in CS solution, albeit only in the absence of Si. The addition of Si increased the plant phenols content, regardless of whether it was grown in a control solution or under -N, -P and -Ca (Fig. 2d).
The green color index was lower when plants were grown under -N, -P and -Ca, independent of Si. However, the addition of Si increased the green color index in the forage grown in the control and in solutions lacking N, P and Ca (Fig. 3c).
The fluorescence quantum yield (Fv/Fm) was lower when plants were grown under -P and -Ca, independent of Si, and under -N in the absence of Si. The addition of Si increased the quantum yield of forage grown under -N, -P and -Ca in relation to the -Si but did not change this parameter for plants grown in the CS (Fig. 3d).
The N use efficiency was lower under nutrient deficiency, independent of Si. However, it increased when plants were deficient in N and Si was added (Fig. 4d). The P use efficiency was lower under P deficiency, both in the +Si or -Si in the nutrient solution. However, it increased when Si was added (Fig. 4e). The Ca use efficiency was higher when plants were grown without Ca, independent of Si. The addition of Si increased this parameter (Fig. 4f).
Plant length decreased in all treatments deficient in N, P and Ca compared to treatments receiving CS solution, independent of Si. However, the addition of Si had no positive impact on plant length (Fig. 5d). The number of tillers decreased under -N, -P and -Ca, regardless of Si supply. However, the addition of Si to the nutrient solution increased tiller number only when N and P were lacking (Fig. 5e).
The shoot dry matter decreased when cultivated under -N, -P and -Ca compared to the CS, regardless of the +Si or -Si. However, this parameter increased with the addition of Si to all treatments (Fig. 5f).