Effect Of Root-zone CO₂ Enrichment On Photosynthetic Physiology And Sugar Metabolism In Fruit Of Netted Muskmelon

In the culture condition of soil and substrate, High CO₂ concentration with low O2 concentration in root-zone always affects the growth and development of plants. Netted muskmelon is a kind of typical plants which is sensitive to root-zone gas. High CO₂ concentration in root-zone always affects growth and development of netted muskmelon. But under High CO₂ concentration in root-zone, Research on sugar accumulation mechanism in netted muskmelon fruit is less.By using aeroponics culture and root-zone CO₂ automatic control system, the netted muskmelon'zhong mi yi hao'was employed to study the effects of elevated root-zone CO₂ concentration on leaf photosynthetic physiology, fruit growth and development process and sugar mechanism of netted muskmelon during its fruiting period. The aim of this study is to provide theory reference for improving the root-zone gas environment and yield and quality of netted muskmelon. The main results showed as follows:1 Under the 2000?L·L-1 root-zone CO₂ treatment, there was no significant difference on the content of chlorophyll a and b with those in the control ?370?L·L-1?. When the root-zone CO₂ concentration reached to 3000?L·L-1, chlorophyll a and Chl. a/b were obviously lower than those in the control treatment during the period of root-zone CO₂ treatment. Growing in above 4000?L·L-1 root-zone CO₂ concentration for 30 days, including 4000?L·L-1, the content of chlorophyll b in netted muskmelon leaves was clearly reduced. The experiment revealed that growing in above 3000?L·L-1 root-zone CO₂ concentration for 10 days, including 3000?L·L-1, net photosynthesis rate?Pn?, intercellular CO₂ concentration ?Ci?, stomatal conductance ?Gs? and transpiration rate ?Tr? in netted muskmelon leaves are inhibited to some extent, and the impact on photosynthetic parameters grow with the increase of root-zone CO₂ concentration. But the stomatal limitation ?Ls? was clearly increased because of increased root-zone CO₂ concentration. Moreover time would be ahead with the increase of root-zone CO₂ concentration. So decline of net photosynthesis rate of netted muskmelon leaves was accelerated with the higher root-zone CO₂ concentration. And decline of net photosynthetic rate was also affected by the stomatal factors.2 The experiment showed that under the 2000?L·L-1 root-zone CO₂ treatment, there was no significant difference on the fluorescence parameters of PS? in netted muskmelon leaves with those in the control ?370?L·L-1?. Growing in above 3000?L·L-1 root-zone CO₂ concentration for 20 days, including 3000?L·L-1, the actual photochemical quantum yield, the electron transfer rate and photochemical quenching coefficient of PSII in netted muskmelon leaves were significantly lower than those in the control treatment. And its non-photochemical quenching coefficient also was significantly lower with the 3000?L·L-root-zone CO₂ treatment for 30 days. The impact fluorescence parameters of PSII grew with the increase of root-zone CO₂ concentration, time also would be ahead. Moreover maximal photochemical efficiency, minimal fluorescence and potential activity of PSII were decreased to some extent with the higher root-zone CO₂ concentration. So growing in high root-zone CO₂ concentration for long term, absorption of light energy, ability to capture and conversion of light energy of PSII in muskmelon leaves were decreased. Photosynthetic activity was decreased, and photosynthetic apparatus of PSII was damaged to some extent, which would results in the decrease of photosynthetic efficiency of PSII in netted muskmelon leaves.3 Growing in above 2000?L·L-1 root-zone CO₂ concentration environment for 25 days, including 2000?L·L-1, the diameter, vertical of netted muskmelon fruit were significantly restricted. There were no significant effects on its horizontal-vertical ratio among the all root-zone CO₂ treatments. That is to say its shape was not affected by root-zone CO₂ treatment. Growing in high root-zone CO₂ concentration environment for 40 days, the contents of Soluble solid, soluble protein, free amino acid and VC were decreased significantly, but the content of organic acid was increased with the rise of root-zone CO₂ concentration. And time would be ahead with the increase of root-zone CO₂ concentration. The results showed that the fruit development of netted muskmelon would be inhibited clearly when the root-zone CO₂ concentration elevated during its fruiting period. And this would results in the decrease of fruit yield and quality of netted muskmelon.4 When the root-zone CO₂ concentration reached to 3000?L·L-1 and lasted 40 days during the fruit development period of netted muskmelon, the contents of total sugar, sucrose were significantly reduced, but the contents of fructose, glucose had no clearly change. Meanwhile, growing in above 3000?L·L-1 root-zone CO₂ concentration environment for 15 days, The activities of acid invertase and neutral invertase reduced 17.22% and 16.44% respectively?P<0.05?. After 20 days of high root-zone CO₂ treatments, there were no differences for activities of acid invertase and neutral invertase among the all treatments. And there are also no differences for activities of Sucrose phosphate synthase and synthesis and cleavage of sucrose synthase among the all treatments within 25 days after the all root-zone CO₂ treatments. The activities of Sucrose phosphate synthase and sucrose synthase in direction of synthesis reduced 18.22% and 17.36% respectively ?P<0.05? in treatment 3000?L·L-1 root-zone CO₂, while the activity of sucrose synthase in direction of cleavage reduced 21.28%?P<0.05? in treatment 4000?L·L-1 root-zone CO₂. And the inhibition grow with the increase of root-zone CO₂ concentration. It was suggested that in the fruit development period of muskmelon, decreased of sucrose-metabolizing enzymes activities might lead to a decline in sugar content.5 The results of Real-Time PCR showed that within 20 days during fruit development period of netted muskmelon, expression of CmS-AIVl was restrained in the high root-zone CO₂ concentration environment. At the late stage of fruit development ?30-40d?, the expressions of Cm-SPS1 and Cm-SS1 would be depressed obviously with the high root-zone CO₂ concentration. But the expressions of Cm-SPS1 and Cm-SS1 had no significant changes among all root-zone CO₂ treatments before 30 days during fruit development. The expressions of enzyme genes related to sucrose metabolism among all root-zone CO₂ treatments were 370?L·L-1>2000?L·L-1>3000?L·L-1>4000?L·L-1>5000?L·L-1CO₂ treatment. It was suggested that in the fruit development period of netted muskmelon, the internal reason for decline in total sugar content in netted muskmelon fruit with the high root-zone CO₂ concentration might be the decreased of related genes expression.