Synergistic Effects Of Light And Water Regulation On Growth And Water Use Of Alfalfa

Alfalfa（Medicago sativa L.）,with good palatability,high yield,and nutritional value as well as good environmental adaptability,is one of the high-quality forage grasses widely grown in China.However,alfalfa consumes a lot of water,so how to maintain alfalfa yield while minimizing water consumption is an urgent problem in alfalfa production in arid and semi-arid areas.In this study,we used"Longdong alfalfa"as the experimental material and conducted light and water control pot experiments in the daylight greenhouse of Qingyang Grassland Agroecosystem National Field Scientific Observation Research Station from 2021 to 2022.The experiment was set up with three water supply levels（high water treatment W1:70%to 90%of field water holding capacity;medium water treatment W2:50%to 70%of field water holding capacity;low water treatment W3:30%to 50%of field water holding capacity）and three light patterns（natural light treatment L0;light redistribution pattern L1:daily 6:00 to 9:00,17:00～20:00 with LED full-spectrum supplemental lights to supplement 40%of natural light,10:00～16:00 with shading net to block 40%of natural light;shading treatment L2:daily 10:00～16:00 with shading net to block 40%of natural light）to study the photosynthetic characteristics,growth dynamics,yield,and soil nutrient changes of alfalfa,aiming to elucidate the synergistic effects of light and water treatments on the growth and water utilization of alfalfa,and to provide some basis for optimizing the cultivation management of alfalfa.The main research results are as follows:（1）The daily changes of photosynthetic rate of alfalfa mainly showed a"double-peak"trend,with peaks at 12:00 and 16:00,respectively.Among the water treatments,the high water treatment significantly increased the net photosynthetic rate,transpiration rate and light energy utilization;among the light treatments,the light redistribution treatment significantly increased the light energy utilization,net photosynthetic rate and instantaneous water utilization of leaves,and decreased the transpiration rate.Under the light redistribution treatment,medium water treatment reduced transpiration rate and water consumption compared with the high water treatment.（2）Light and water treatments affected the aboveground biomass of alfalfa by changing the number of leaves,leaf area,and plant height,and crude protein yield of alfalfa by changing the stem-to-leaf ratio and crude protein content of each part.the average leaf area of each crop in 2021 and 2022 was the largest under the light redistribution high water treatment,566.6 and 1912.1 cm²,respectively,and the average plant height of each crop was the largest under the light redistribution medium water treatment,71.5 and 1912.1 cm²,respectively.The low water and shade treatments increased leaf crude protein content and whole-plant crude protein content,with the maximum values for both shade and low water treatments（21.3%and 25.5%of whole-plant crude protein content in 2021 and 2022,respectively）.Both light pattern and water treatment had highly significant effects on aboveground biomass and crude protein yield,and high water treatment and light redistribution pattern significantly increased aboveground biomass and crude protein yield.The aboveground biomass and crude protein yield were the highest in the high water treatment with light redistribution in two years,165.2 and 29.6 g·pot^-1in 2021 and468.7 and 91.1 g·pot^-1in 2022,respectively.Crude protein yield in the medium water treatment under the light redistribution treatment was not significantly different from the high water treatment,and aboveground biomass was lower than the high water treatment in 2021,but not significantly different from the high water treatment in2022.（3）With the decrease of soil moisture content,root biomass of alfalfa increased and then decreased,root crown ratio gradually increased,and root length,root surface area and root volume significantly decreased.Among different light patterns,root biomass of natural light treatment was the largest and root crown ratio of light redistribution treatment was the smallest.Among the nine treatments,the treatments with the smallest root crown ratio in 2021 and 2022 were the high and medium water treatments under light redistribution.In terms of water use,water consumption increased as soil moisture content increased;light redistribution treatments increased soil moisture content and reduced water consumption.Water treatment and light pattern and their interaction had significant effects on dry matter water use efficiency and crude protein water use efficiency.The low water treatment and light redistribution treatment significantly increased the dry matter water use efficiency and crude protein water use efficiency of alfalfa.The dry matter water use efficiency and crude protein water use efficiency were highest in the light redistribution low water treatment with 2.57 and 0.25 kg·m^-3in 2021 and 6.18 and 1.26 kg·m^-3in 2022,respectively.Under the light redistribution treatment,the dry matter water use efficiency and crude protein water use efficiency of the medium water treatment were higher than those of the high water treatment,with a significant increase of 25.5%and33.4%respectively in 2022.In conclusion,reduced water supply under light redistribution（L1W2）reduced transpiration rate and transpiration water consumption compared with the conventional treatment（L0W1）and light redistribution high water treatment（L1W1）,which improved the water use efficiency of dry matter and crude protein while ensuring higher dry matter and crude protein yields.Therefore,light redistribution treatments under arid and semi-arid conditions have the potential to reduce water consumption,improve water use efficiency,and maintain dry matter and crude protein yields in alfalfa.The study is a guide for water management in alfalfa under irrigation and drought conditions.