Effects Of Enhanced Ultraviolet-B Irradiance On C,N Resource Allocation Of C₃ And C₄ Plants

The stratospheric ozone decrease has heightened concern about the ecological implications of increasing solar ultraviolet-B (UV-B region, between 280 and 320 nm) radiation on agricultural production and natural ecosystems. There is the osculatory relation between the response of plant resource allocation to enhanced UV-B radiation and the survival, evolution of population. Thus, it is impotant to reveal the response of plant resource allocation to enhanced UV-B radiation.In present study, spring wheat (Triticum aestivum) and millet (Setaria italica) were grown under ambient and enhanced ultraviolet-B radiation to determine the resource allocation and the energetic costs of tissue construction. Sample combustion in a bomb calorimeter combined with measurements of ash and nitrogen content provided the primary method of estimating tissue construction costs (W_G; g glucose g^-1 dry mass). The results may provide more theoretical foundation on global climate change and agriculture pratice. The results indicated that:(1) Pot cultivation experiments in greenhouse were conducted to study the effects of enhanced UV-B radiation on the diurnal variations of flavonoids in wheat leaves during grain filling stage. The results showed that the flavonoids content of wheat leaves increased significantly during enhanced UV-B radiation at daytime, and then declined at night when wheat seedlings released from the stress of UV-B radiation. Meanwhile, the difference of flavonoids content between control and UV-B treatment decreased gradually. The activity of phenylalanine ammonialyase also showed a similar trend with that of content of flavornoids. Simultaneously, the net photosynthetic rate of wheat was not affected by supplementary UV-B radiation, but the respiration rate was enhanced significantly at night. The results implied that the continuous fluctuation of flavonoids content might be another explanation why biomass was reduced significantly while plant net photosynthesis was not influenced by enhanced UV-B radiation.(2) Enhanced UV-B radiation significantly affected plant resource allocation in present study. The nitrogen content of wheat and millet leaves was much lower under greenhouse condition and field condition for plants at enhanced UV-B radiation compared with ambient radiation (P＜0.05). The carbon content of various plant parts for wheat and millet were not significantly affected by elevated UV-B radiation under field condition (P＞0.05). However, there was a decrease trend under greenhouse condition.(3) Estimates of W_G were substantiality higher for stems and leaves of wheat and for stems, leaves and spike of millet grown at elevated UV-B radiation compared with ambient UV-B radiation under field condition (P＜0.05). It may be that extra protective compounds induced by enhanced UV-B radiation need extra energy, consequently, increase the construction costs of plant tissue. The similar phenomenon was observed for wheat under greenhouse condition.(4) The results showed that supplemental UV-B resulted in significant change in biomass allocation under greenhouse and field condition (P＜0.05). The proportion of biomass partitioned to root increased significantly for wheat under enhanced UV-B radiation. However, more biomass proportion was allocated to root and spike for millet.(5) Comparisons between enhanced and ambient UV-B radiation wheat and millet individuals under greenhouse condition and filed comdition demonstrate profound effects of enhanced UV-B radiation on plant allometry as well as on the distributions aspects of size. The results showed that:①Enhanced UV-B radiation lead to a significant decrease of plant height and biomass for wheat and millet (P＜0.01). Meanwhile, their plant height and biomass frequency distributions under the condition of enhanced UV-B remained positively skewed.②The Gini coefficient of plant height and biomass were increased notably by enhanced UV-B radiation (P＜0.0001).③There were simple allometric relationships between plant height and biomass under enhanced UV-B radiation since these relationships were significantly linear (P＜0.05). But the slope of line changed by enhanced UV-B radiation. Consequently, the results implied that the depression effects of UV-B on individuals were not uniform. This allometric analysis may provide a much-needed link between individual plant behaviour to enhanced UV-B radiation and population-level processes.