Response of photosynthesis, growth, and acorn mass of pedunculate oak to different levels of nitrogen in wet and dry growing seasons

The objective was to examine the effects of optimal leaf nitrogen levels > 2.0% and suboptimal levels ˂ 2.0%, nitrogen nutrition on net photosynthetic rate, stem diameter increment, height growth increment and acorn mass of pedunculate oak during 2010 in the absence of drought stress and during 2011 under the impact of moderate drought stress. According to the results, moderate drought stress significantly reduced net photosynthetic rate, stem diameter increment and height growth increment, while acorn mass was not affected. Suboptimal nitrogen nutrition significantly reduced the net photosynthetic rate and stem diameter increment only in the wet year, acorn mass in both wet and dry years, while height growth increment was not significantly reduced by suboptimal nitrogen nutrition in either year. The results indicate that optimal nitrogen levels can stimulate photosynthetic rate and stem diameter increment of pedunculate oak only in the absence of moderate drought stress. Moreover, the results show that moderate drought stress is a more dominant stressor for photosynthesis and growth of pedunculate oak than suboptimal nitrogen nutrition, while for acorn development, it is the more dominant stressor.