JIA-2019-11

2545 Slaven Jurić et al. Journal of Integrative Agriculture 2019, 18(11): 2534–2548 pore size is much smaller than the size of spores) and/or (ii) partial microsphere degradation. The release data showed good fit into equation (7). The values of k , n and intercept a are given in Table 3. The initial release of T . viride from microspheres prepared at higher calcium chloride concentration was quicker than those prepared at low concentration of calcium chloride. This may be ascribed to the smaller microsphere size (seen in Fig. 9) as was similary observed for active agents releasing from microspheres prepared with copper as gelling cations (Vinceković et al . 2016, 2017). After initial burst effect, the release constant decreased with increasing initial calcium ions concentration. Value n >0.43 would point to the controlling release mechanisms as an anomalous transport kinetics (combination of two diffusion mechanisms and the polymer swelling and relaxation). The increase in n value with calcium ion concentration indicates prevailing influence of the polymer swelling and relaxation on the rate of T . viride release, indicating that the transition of glassy structure to rubbery state is slower on less swelled microspheres (Fig. 7). This explanation is not entirely unambiguous because the amount of T . viride in the surrounding medium is closely related to two processes, one is the release from microspheres and the other is germination in the surrounding media. The release of calcium ions frommicrospheres prepared without and with T . viride with increasing concentration of calcium chloride is presented in Fig. 12-A and B. Calcium ions act as crosslinking cations and their slight release indirectly points to the stability of microspheres. All release profiles are characterized by rapid initial release followed by slower release obeying equation (6). To identify the kinetics and type of mechanism involved in the release, a semi-empirical Korsmeyer-Peppas model was applied (Korsmeyer et al . 1983). The values of the release constants k , and exponents n are listed in Table 4. Lower n values than 0.43 indicate that the release process is controlled by calcium ions diffusion through microspheres. In the case of the Fickian mechanism, the rate of calcium ions diffusion is much less than that of polymer swelling and relaxation. An increase in calcium concentration resulted in a decrease of calcium release rate for both types of Table 3 Variation of the y -axis intercept ( a ), release constant ( k ), exponent ( n ), and correlation coefficient ( R 2 ) of Trichoderma viride released from ALG/(Ca+ Tv ) microsphere prepared at various initial calcium chloride concentrations, c i (CaCl 2 ) c i (CaCl 2 ) (mol dm –3 ) a k (h –1 ) n R 2 0.5 0.124 0.105 0.46 0.98 1.0 0.227 0.059 0.52 0.99 1.5 0.507 0.025 0.64 0.99 2.0 0.550 0.025 0.70 0.99 0 100 200 300 400 500 0.5 1.0 1.5 2.0 2.5 c i (CaCl 2 ) (mol dm –3 ) 0.5 1.0 1.5 2.0 f Tv Time (t, h) ALG/(Ca+ Tv ) Fig. 11 The fraction of released Trichoderma viride spores ( f Tv ) with time (t) from ALG/(Ca+ Tv ) microspheres prepared at a various initial calcium chloride concentrations, c i (CaCl 2 ). The error bars indicate the standard deviation of the means. 0 100 200 300 400 500 0.0006 0.0009 0.0012 0.0015 0.0018 0.0021 0.0024 c i (CaCl 2 ) (mol dm –3 ) 0.5 1.0 1.5 2.0 Time (t, h) A B ALG/Ca 0 100 200 300 400 500 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 c i (CaCl 2 ) (mol dm –3 ) 0.5 1.0 1.5 2.0 f Ca f Ca Time (t, h) ALG/(Ca+ Tv ) Fig. 12 The fraction of released calcium cations ( f Ca ), with time (t) from ALG/Ca (A) and ALG/(Ca+ Tv ) (B) microspheres prepared at various initial calcium cation concentrations, c i (CaCl 2 ). The error bars indicate the standard deviation of the means.