2019 ASHS Annual Conference

Responses among cultivars within a species to supplied N can vary drastically. Limited research exists on the physiological mechanisms associated with growth differences to supplied N among different cultivars within a species. This is useful for developing fundamental knowledge on plant-N responses, new cultivars and fertilizer guidelines. Two experiments were conducted using poinsettia (Euphorbia pulcherrima) cultivars in a greenhouse. In the first experiment, four cultivars were evaluated for differential N responses under optimal (375 ppm N) and sub-optimal (112.5 ppm N) fertilizer treatments. Two cultivars, i.e., Christmas Glory White (CGW) and Christmas Tradition (CT), were selected for further testing based on their growth differences to supplied N in experiment I. Shoot dry weight (SDW) was significantly higher in CT than CGW under optimal treatment whereas it was not different between cultivars under sub-optimal treatment. There were no differences in root dry weight (RDW), root weight ratio and tissue N content between cultivars in both treatments. To further understand the physiological bases for differential responses, we conducted a second experiment in which we measured SDW, leaf area (LA), leaf photosynthesis (A) and chlorophyll fluorescence responses to photosynthetic photon flux density (PPFD) and chlorophyll concentration (CC). Statistical analysis confirmed that SDW responses between two cultivars were similar in both experiments. Relative decrease in SDW and LA between sub-optimal and optimal treatment were significantly higher for CT than those of CGW. This indicated that CGW performed better than CT under sub-optimal compared to optimal treatment. These responses were not likely associated with differences in N uptake in the sub-optimal treatment as there were no differences in root growth and tissue N (based on experiment I). In addition, there were no differences in spectrophotometer based chlorophyll content indicating no differences in N partitioning to chlorophyll between cultivars. A-PPFD analysis indicated that light saturated photosynthesis (A_max) was higher in CT than CGW under optimal but it was not different under sub-optimal treatment. Observed differences in A_max may indicate a relatively higher RuBisCO content in CT than CGW under optimal and no differences in RuBisCO between cultivars under sub-optimal N conditions. Based on these results, we conclude that observed differential shoot growth responses between CGW and CT under sub-optimal treatment are likely associated with differences in N partitioning to RuBisCO between cultivars.