Nitrogen deficiency-induced leaf senescence in seedlings of tropical maize cultivars as a tool to characterize grain-yielding capacity at low nitrogen supply in the field

The selection process of nitrogen (N)-efficient cultivars during plant breeding can be simplified by a specification of secondary plant traits that are decisive for N efficiency (grain yield at low-N supply). N-efficient tropical maize cultivars were characterized by delayed leaf senescence during reproductive growth in field experiments in Kenya and Zimbabwe. The same cultivars were studied for leaf senescence under N deficiency in a short-term nutrient solution experiment. It was shown that leaf senescence in the short-term experiment was related to leaf senescence and N efficiency in the field experiments. However, photosynthesis rate during leaf senescence could explain only part of the cultivar differences in N efficiency. Therefore, we further investigated if a quantification of leaf- and plant-N flows could improve the evaluation of genotypic differences in leaf senescence in short-term experiments. Cultivars differed in leaf-N content prior to senescence; however, this had no significant impact on the development of leaf senescence. N export was enhanced by a high N availability in the leaves. N import into senescing leaves was not related to total plant N uptake, but was regulated by leaf-inherent factors. Photosynthesis rate during leaf senescence was decreased by carbon accumulation in the leaves rather than by changes in leaf-N content. Photosynthesis rate and leaf-N content during late leaf senescence were both related to leaf senescence at anthesis in field experiments. However, only photosynthesis rate reflected cultivar differences in leaf senescence during reproductive growth and N efficiency in field experiments. Possible implications of carbon and N flows in vegetative plants on N deficiency-induced leaf senescence are summarized