The Reason Of Magnesium Deficiency In Tomato Grown In Calcareous Soil Under Solar Greenhouse

Magnesium?Mg?is one of the essential nutrients for plant growth and development,and play important roles in photosynthesis,assimilation transport,regulating enzyme activity,protein synthesis and so on.Mg deficiency in crops was common in south China due to low Mg contents and excessive leaching losses of Mg from the soil.In contrast,Mg deficiency is rare in calcareous soils in North China because of abundant Mg in the parent materials.However,in recent years Mg deficiency in vegetables?most frequently in tomato characterized by leaf chlorosis?was noticed in calcareous soil in the solar greenhouse in North China.As a result,tomato yield and quality were greatly affected.Mg nutrition disorder in horticultural crops has prevented the healthy and sustainable development of facility agriculture in China,so it was necessary to find out the reason why Mg deficiency occurs in tomato in calcareous soil under solar greenhouse in North China.The antagonism between potassium?K?and Mg may be an important reason for Mg deficiency in Mg content rich soil,many studies showed that high K concentration inhibited Mg uptake,but the transport of Mg at high K level remained unclear.It was found the chlorosis symptom was more serious at fruit enlargement stage in autumn-winter cultivated season.How Mg uptake was suppressed at low temperature and is there any interaction between low temperature and high K concentration,and whether Mg uptake and transport were affected by source-sink relationship,all of these doubts were not clarified well.We also found that the symptoms of Mg deficiency were significantly different among tomato cultivars,however the uptake,transport and utilization of Mg was less reported.For above reasons,a series of incubation experiments were conducted to study the effect or interaction effect of cultivars,high K,low temperature and transpiration on Mg uptake and transport from root to shoot.Two field experiments were conducted to investigate the effects of regulation of source-sink,foliar spraying of Mg fertilizer,and supply Mg fertilizer in soil on Mg uptake and utilization by tomato.The main results are as follows:1)The kinetics of nutrient uptake experiment was evaluated with three widely planted tomato cultivars?Zhongza No.9,Jingpenchaoguan and Gailiangmaofen?of North China at 15?and 30?temperatures to study the differences of Mg uptake among the cultivars.The results showed that the kinetics parameters?maximum uptake rate and the affinity coefficient?weresignificantly different among cultivars,but there was similar response to different temperatures.The three cultivars were incubated at Mg deficiency and optimum Mg level for45 days,we found that Mg uptake and flow rate in root were significantly different among three cultivars,Zhongza No.9 and Jingpenchaoguan performed a higher Mg uptake potential.2)Three tomato cultivars?Zhongza No.9,Jingpenchaoguan and Gailiangmaofen?were incubated at K/Mg_4:0,K/Mg_4:1,and K/Mg_8:1:1 levels for 15,30 and 45 days.The results showed that compared to optimum Mg condition(K/Mg_4:1),the Mg deficient treatment(K/Mg_4:0)showed slower growth rate,and the chlorophyll contents and photosynthetic rate decreased,the total biomass and shoot biomass both decreased significantly.Mg concentration was markedly reduced in shoot and root and less Mg was distributed in shoot.Mg deficiency symptoms was observed when Mg content in shoot was lower than 4 mg·g^-1 DW.At high K level(K/Mg_8:1)chlorophyll content and photosynthetic rate tended to decrease.The total biomass of shoots and roots decreased significantly;both concentration and uptake of Mg were decreased significantly.There were significant differences among three cultivars on Mg uptake potential in different K/Mg supply ratios and were ranked as Zhongza No.9>Jingpenchaoguan>Gailiangmaofen.3)Consider proper K concentration(K/Mg_4:1)and optimum temperature?25?day/18?night?as contrast,two tomato cultivars Jingpen 1 and Gailiangmaofen were incubated at high K concentration(K/Mg_12:1)and low temperature?15?day/8?night?for 30 days.The results showed that compared with optimum temperature,the stem diameter of tomato,total absorption area and specific surface area of root increased in low temperature,however,the plant height,petiole number and internode length decreased;Mg uptake and distribution in shoot were also decreased significantly.High K had no significant effect on plant growth or biomass compared with the proper K concentration,however,the concentration and uptake of Mg in shoot and root were significantly decreased.There was no significant interaction between temperature and high K on Mg uptake of tomato.Contrasted the two tomato cultivars,Jingpeng 1 showed poor growth at low temperature but it has higher potential in Mg uptake at high K concentration.4)Two tomato cultivars Zhongza No.9 and Gailiangmaofen were foliar sprayed with transpiration inhibitor and deionized water?contrast?for 24 hours.The results showed Mg uptake was decreased with transpiration inhibition,there was significant positive correlation between Mg uptake and transpiration rate?R²=0.684^*?.After incubation 24 days,the Mg,K,and calcium?Ca?uptake by tomato of the two cultivars were reduced by 17.1%,12.9%and44.1%,respectively.Then Zhongza No.9 was incubated at high humidity 80%and humidity50%,it was found Mg uptake was decreased with decrease in transpiration,and significant positive correlation was observed between Mg uptake and transpiration rate(R²=0.992^**).Mg uptake in tomato was double inhibited by high humidity and high K concentration.5)A field experiment was conducted to study the effect of foliar spraying of Mg fertilizer and three fruit setting styles in winter cultivated season on Mg uptake and utilization of tomato.The results showed without foliar spray of Mg,Mg distribution was different in source leaves and fruit by different fruit setting styles;the higher the fruit yield,the less the Mg accumulated in leaves,and the Mg concentration of leaves reached to the critical value which indicated that Mg nutrition was deficient.After spraying Mg fertilizer,Mg concentration in leaves and upper fruit,photosynthetic parameters,fruit yield,single fruit weight and hardness were all increased,and Mg uptake by tomato were increased by 45.3%in leaves and 14.7%in fruit,respectively.It was surplus in source-sink of tomato.6)In spring and winter cultivated seasons the growth and Mg uptake of tomato were measured in six solar greenhouses,respectively.The results showed that the biomass,Mg concentration in leaves and Mg uptake were lower in winter season than in spring.Another six selected solar greenhouses in winter cultivated season which were classified into two groups:three with soil available K in 600⁸00 mg kg^-1?K excessive soil?and others in1000¹400 mg kg^-1?K severe excessive soil?were supplied with four Mg application rates(0,15,30,45 kg hm^-2),respectively.The result showed K and Mg utilization efficiency were higher in K excessive soil than in K severe excessive soil;K and Mg concentration in tomato,yield and fruit quality were higher in excessive K soil.Mg concentration and yield were increased after applied Mg fertilizer in soil in K excessive soil,but it was not in K severe excessive soil.In summary,there was significant difference on Mg uptake between different tomato cultivars,Mg uptake was inhibited by low temperature and high K factors,respectively,and their interaction did further impact on Mg uptake.The transport rate was slower at low temperature,and low temperature with high K further inhibited Mg transport from root to shoot.In addition,Mg transport in tomato was inhibited by transpiration inhibition.High yield induced Mg deficiency in source leaves;otherwise,the Mg nutrition would be surplus in source leaves.Foliar spraying of Mg fertilizer could alleviate Mg deficiency with high yield.Thus,Mg deficiency was associated with tomato cultivars,excessive K input and low temperature in winter season,Mg nutrition of tomato were also affected by environmental stress in greenhouse;besides,pursuing for high yield induced Mg deficiency in fruit development period.In order to alleviate Mg deficiency of tomato in greenhouse,we should strictly control the input of K fertilizer and cooperate with Mg fertilizer to regulate the proportion of K/Mg in soil;regulating the temperature,humidity and light in the greenhouse especially during the winter season;control the tomato yield or adopt foliar spraying of Mg fertilizer;and besides,selection of highly Mg efficient cultivars is also necessary.