A metanalytic study: does water deficit always increase soybean proline concentration?

Several studies have shown the role of proline in stressful conditions. However, no comprehensive compilation has been made addressing different categories such as the intensity of water deficit, tolerance and developmental stage of plants on the response of plants in relation to proline concentration. Here, we investigated through meta-analysis the influence of water deficit on the proline concentration in soybean plants at different drought intensities, plant developmental stages and cultivar susceptibility. A bibliographical search was carried out and 19 research articles were selected, resulting in 82 studies. Data on proline concentration were recorded along with the dispersion measures. The effect measure was calculated as the natural logistic ratio between the treatment group (water deficit) and the control group. Surprisingly we discovered that the proline concentration is not always increased. Under mild stressful conditions, only resistant soybean cultivars had significant effect with increase in proline concentration. Water deficit did not affect the proline concentration in susceptible cultivars, regardless the intensity. Soybean plants under mild drought conditions did not increase proline in the reproductive stage. These findings indicate the importance of performing highly specific experimental studies. Additionally, more complex approaches are required to analyze a larger set of data and different environmental conditions, for a more accurate understanding of proline metabolism in soybean plants under water deficit.