Effects of the Coupling of Nitrogen and CO₂ on Leaf SPAD, Nitrogen Content, Temperature and Humidity of Purple Pak-choi Under Hydroponic Conditions

In order to investigate the effects of the coupling of nitrogen nutrition (N) and carbon dioxide (CO₂) on the leaf chlorophyll fluorescence value (SPAD) of purple pak-choi, nitrogen content (YN), temperature (YT) and humidity (YRH) under hydroponic conditions, six nitrogen coupling treatments including C0N0, C0N2, C0N4, C1N0, C1N2 and C1N4, were set with Nanjing local purple pak-choi as the experimental material, among which the nitrogen concentration in the hydroponic solution was set to be without exogenous nitrogen (N0), 0.2 g·L^-1 exogenous nitrogen (N2) and 0.4 g·L^-1 exogenous nitrogen (N4) in the diluent of hydroponic solution. Then, the changes of SPAD, YN, YT and YRH in leaves after transplanting were studied under the treatments of no CO₂ growth agent (C0) and CO₂ growth agent (C1) at each nitrogen concentration. The results showed that the application of CO₂ could significantly improve the leaf SPAD of purple pak-choi (P < 0.05). After transplanting, the leaf SPAD of purple pak-choi under C1N4 treatment was higher before November 16, and then it was the highest under C1N2 treatment. Increasing the N content in the hydroponic solution before November 11 was helpful to increase the YN content in leaves, but it was not conducive to the increase of YN content in leaves after November 11. The YN content in leaves was higher under the treatments of C1N0 and C1N2. Both N and CO₂ could significantly reduce the YT of leaves (P < 0.05). In addition, the coupling treatment of N and CO₂ significantly affected the YRH of leaves (P < 0.05), and high concentration of N could increase the YRH of leaves, and the effect was more obvious when increasing CO₂ after November 11. It should be noted that there was significant correlation among various indexes of leaves at the same time (P < 0.05). The Pearson analysis showed that the increase of YT at the early growth stage directly reduced the leaf SPAD, YN and YRH of purple pak-choi, and the increase of YT at the later growth stage was beneficial to the increase of SPAD, YN and YRH. YN and YRH were positively correlated with chlorophyll SPAD (P < 0.01). In conclusion, the reasonable coupling of nitrogen and CO₂ in hydroponic solution could increase the contents of chlorophyll and nitrogen in the leaves of hydroponic purple pak-choi, and the results could provide theoretical basis for the accurate management of nitrogen and CO₂ in the hydroponic purple pak-choi.