Effects Of Temperature Difference Between Day And Night On The Growth And Photosynthetic Characteristics Of Ulva Spp.

Ulva belongs to the genus Ulva of the Chlorophyta.Ulva prolifera,the dominant species of green tide in the coastal waters,has strong environmental adaptability..In order to explore the mechanism and the effect of a certain degree of the temperature difference between day and night on U.prolifera,two temperature gradients were set up in this study,including the culture temperature both in the light and dark cycle were 22℃（noted as 22℃）,and the culture temperature in the light and dark cycle were 22℃and 18℃respectively（noted as 22-18℃）.The results show that diurnal temperature can promote the growth of U.prolifera and the increase of the photosynthetic pigments.A total of 4232 differentially expressed genes were obtained from the transcriptome sequencing.These differentially expressed genes were mainly enriched in serine-type endopeptidase activity,membrane components,and nitrate reductase activity with NADH as a coenzyme.In the KEGG analysis,the differential genes were classified into 19 metabolic pathways,mainly including RNA transport,amino sugar and nucleotide sugar metabolism,phenylpropane biosynthesis,nitrogen metabolism and protein output.Although the differential genes related to photosynthesis and growth have not been observed in this study,the temperature difference between day and night affected the signal transduction of plant hormones.In order to study whether the influence of diurnal temperature on U.prolifera is mediated by light intensity,two light intensities（high light:250μmo L/m²/s,noted as:HL;low light:60μmo L/m²/s,noted as:LL）and two temperature gradients(the culture temperature in the light-dark cycle is 22℃（noted as 22℃）and the culture temperature in the light-dark cycle is22℃and 16℃（noted as 22-16℃）were set.The changes of the photosynthetic oxygen evolution rate,pigment content,SOD activity,soluble protein and soluble sugar of U.prolifera were studied.The results showed that:（1）For the first day,the growth of U.prolifera was significantly affected by light intensity and low night temperature with lowest and highest values for 22-16℃under 60 and 250μmo L/m²/s,respectively.After grown under different treatments for 4 days,the growth rate remained basically stable,with the higher values observed under HL at 22-16℃;（2）LL-grown thalli showed higher photosynthetic rate,F_v’/F_m’and Chl a content at 22-16℃;（3）SOD activity significantly decreased by HL at 22-16℃;（4）The contents of soluble protein and soluble sugar were significantly increased by low night-time temperature.That is,temperature difference between day and night plays an important role in the growth of this species,which is also affected by light intensity.In order to study the influence of diurnal temperature on the response to light intensity and temperature changes from the perspective of growth environment history of U.prolifera,the seaweed of Ulva genus such as U.lactuca,U.prolifera（sp1,normal temperature species;sp2,low temperature species）were chose as materials and were cultured at a constant temperature of22°C and a temperature difference between day and night of 6°C（22-16°C）with high light（250μmo L/m²/s）and low light（60μmo L/m²/s）conditions.When the algae adapted to the above light and temperature conditions,the thalli were treated with 5 temperature levels（10,15,20,25,30°C）and 5 light intensity levels（50,150,250,350,450μmo L/m²/s）,then the chlorophyll fluorescence efficiency and photosynthetic carbon fixation rate were measured.The results showed that:（1）The growth rate of U.prolifera sp1 was higher than that of sp2,while the growth rate of U.lactuca was the lowest;Under the same condition of the light intensity,the temperature difference between day and night promoted the growth of U.prolifera sp1 and sp2,while U.lactuca was inhibited.（2）sp1 grown under low light had a higher light saturation point,F_v’/F_m’,α,E_kand r ETR_maxunder 50μmo L/m²/s,while the higher carbon fixation rate was observed at 20,25 and 30°C.（3）When the light intensity was higher than 150μmo L/m²/s,the F_v’/F_m’,αand r ETR_maxof U.lactuca decreased;The higher carbon fixation rate was observed at20°C and 25°C,and U.prolifera with the life history of diurnal temperature difference was more sensitive to ambient temperature changes.（4）With a life history of temperature difference between day and night,U.prolifera sp2 had higherα,E_kand r ETR_max.That is,the growth temperature history such as the temperature difference between day and night affects the instantaneous stress ability of seaweed to light and temperature,and there are differences between species.In summary,diurnal temperature can promote the growth of U.prolifera,the effect of temperature difference between day and night can be affected by other environmental factors such as light intensity,and the growth temperature history of diurnal temperature could affect the influence of Ulva seaweed including U.prolifera on other environmental factors.However,the transcriptome data has not shown the significant effects of diurnal temperature on the pathways and key genes,so,further research is needed.