Synergistic Effect Mechanism Of Light And Temperature On Lettuce Growth And Light Regulation Strategy For Stereoscopic Cultivation

The light environment in the greenhouse is one of the key factors affecting the yield and quality of greenhouse crops.There is a related coupling relationship between light and temperature,and the growth of crops exists significant difference under different environment.In greenhouse production,insufficient indoor light is a common problem.For multi-layer stereoscopic cultivation in a greenhouse,this problem is particularly prominent due to the structural characteristics of the cultivation rack.Therefore,how to rationally supplement light according to the actual environment in the greenhouse,and ultimately achieve the purpose of increasing crop yield and quality,and reducing costs,has become a scientific problem that needs to be solved urgently in greenhouse production,especially in the production of multi-layer stereoscopic cultivation.Most of the existing research on light environment regulation methods are based on crop photosynthesis models to obtain light regulation target values.This method realizes the dynamic regulation of light environment,provides decision-making basis for scientific light regulation,and has important scientific significance and applicability.However,the dry matter accumulation characteristics of the whole crop,the influence of multi factor coupling on crop physiology and growth process in different growth periods,and the influence of crop growth time on light regulation target value are less considered in these studys,,the light regulation mechanism still needs to be further explored and improved.Therefore,this article takes Romaine lettuce as the test object,and uses a combination of three temperatures(T15,T23 and T30,unit:?)and five light intensities(P100,P200,P350,P500 and P600,unit:?mol m^-2 s^-1)to treat lettuces to study the growth of lettuces during the whole growth period.The changing laws of lettuce growth,physiology,morphology,and nutrient absorption during the whole growth period were studied.The influence mechanisms of temperature,light and interaction of light and temperature on the growth and development of lettuce were analyzed.The growth prediction models for lettuce during the whole growth period were establish,the light regulation target value based on the dry matter production model of lettuce was obtained,and a supplementary light strategy suitable for multi-layer cultivation mode was proposed.A light supplement strategy suitable for multi-layer cultivation mode was proposed and its economic efficiency was analyzed.The research content and conclusions of the thesis are as follows:(1)The analysis of the effects of different light and temperature environment combinations on the leaf chlorophyll,chlorophyll fluorescence parameters and gas exchange parameters during the growth of lettuce was carried out,and the physiological response of lettuce to changes in light and temperature environment was clarified.The contribution rate of light,temperature and the interaction of light and temperature on the physiological indicators of lettuce was clarified.The results showed that the lettuce plants treated with light intensity 600?mol m^-2 s^-1 under T15 and light intensity 100?mol m^-2 s^-1 under T23 and T30 showed photoinhibition.In comparison,lettuce plants grown in the light intensity range of 350?500?mol m^-2 s^-1in T15,350?600?mol m^-2 s^-1 in T23,and and 500?600?mol m^-2 s^-1 in T30 had higher chlorophyll content,higher light energy utilization efficiency and stronger photosynthetic capacity.(2)The effects of different light and temperature environment combinations on the number of leaves(LN),total leaf area of plant(LA),projected canopy area(PCA),single leaf morphology,dry mass(DM),fresh mass(FM)and nitrogen(N),phosphorus(P)and potassium(K)absorption of lettuce were carried out,and the effects of temperature,light,and the interaction of light and temperature on the growth indexes of lettuce were determined.The results show that in T15,the light intensity had little effect on the LN of lettuce in the early growth stage,but had a significant impact on the lettuce in the middle and late stages.The light intensity of 350?500?mol m^-2 s^-1 in T15 was beneifical to the expansion of lettuce leaves and plant canopy and the N,P and K uptake,while lettuce growed rapidly and had high photosynthetic nutrient utilization and light energy utilization.In T23,light intensity treatment had no significant effect on LN.During the entire sampling period,the range of 350?500?mol m^-2 s^-1 significantly increased the expansion of lettuce leaves and plant canopy,and promoted N uptake;the range of350?600?mol m^-2 s^-1 promoted the net assimilation ability,and was suitable for biomass accumulation and yield increase;the range of 350?600?mol m^-2 s^-1 promoted P and K uptake of lettuce.In T30,the range of 350?600?mol m^-2 s^-1 significantly increased the LN and LA of lettuce,while the range of 500?600?mol m^-2 s^-1 significantly increased the DM and FM,so the lettuce growed quickly and had a high utilization rate of light energy.In the early growth period(the first 24 days after transplanting),the range of350?600?mol m^-2 s^-1 significantly increased the PCA and SA.Afterwards,PCA of the lettuces treated with P600 decreased due to the entanglement of the top leaves.The range of 350?500?mol m^-2 s^-1 was conducive to the N uptake,and the range of 350?600?mol m^-2 s^-1 was conducive to P and K uptake.(3)The net photosynthetic rate model of lettuce was established based on the BP neural network algorithm.It was proposed to take the negative value of R_d calculated from the photosynthetic light response curve as the model input to characterize the P_nvalue of lettuce when the PPFD was zero.The model could simultaneously predict P_nand R_d values under different light and temperature environment.The model test results were R²=0.988,RMSE=0.867,MAE=0.625.The BP neural network algorithm was used to construct the lettuce plant leaf area prediction models NN1 and NN2.The NN1 model used the growth time,the average cumulative relative thermal effect and the average cumulative light radiation as the network input.In addition to the network input of the NN1 model,the NN2 model used PCA as the network input.In order to compare the performance of the BP neural network prediction model,the thermal effectiveness and PAR(TEP)method was used to construct the leaf area model.The results show that the prediction accuracy of the NN1 and NN2 models were both higher and better than the TEP model;the prediction effect of the NN2 model was better than that of the NN1model.Based on the analysis of the characteristics of the lettuce canopy structure at different growth stages,the leaf area correction factor was introduced into the existing dry matter production model to construct a dry matter production correction model suitable for lettuce growth.(4)Based on genetic algorithm,the target values of PPFD corresponding to the maximum dry mass of lettuce under different temperature conditions were obtained.The characteristics of the light and temperature environment distribution under the condition of stereoscopic cultivation in greenhouse were analyzed.Based on the lettuce growth prediction models constructed in Chapter 5,the leaf area and dry weight of lettuce in each layer of the cultivation rack were simulated and compared with the measured values to evaluate the simulation effect.The real-time temperature and lettuce growth time as input to obtain the PPFD target value were proposed,which was used as the light control strategy of the target value of supplement light for each layer of the cultivation rack.Comparing the cumulative dry weight change,light energy utilization efficiency,light regulation cost and output-input ratio of lettuce after light supplementation in each layer of the cultivation rack at the light regulation time interval of 10 min,30 min and 60 min in a certain light supplementation period,it was found that compared with the light regulation time interval of 30 min and 60 min,the light adjustment time was 10min,which not only ensured the output-input ratio and light energy utilization,but also improved the dry weight of lettuce.The daily constant light supplement intensity was calculated based on the total light supplement amount with a light adjustment time interval of 10min.Compared with the commonly used constant light supplement intensity method,the change of dry weight of lettuce in the upper,middle and lower layers of the cultivation rack under the PPFD regulation target light supplement(light adjustment time interval of 10min)increased by 13.95%,9.52%and 9.06%respectively,while the light supplement cost per 1kg yield increased by 14.15%,9.54%and 9.03%respectively,and the light energy utilization rate and output input ratio also increased.This research provides a scientific basis for the fine management and high-yield and high-efficiency industrialized production of lettuce,and lays a foundation for the research of light environment control system for lettuce production,and has certain reference value.