Effects Of Root Illumination On The Growth And Photosynthetic Characteristics Of Leaf Beet

Leaf beet（Beta vulgaris L.cv.Baby leaf）,as a special health-care vegetable,has become a new type of leafy vegetable produced in plant factories with high nutritional value,heanthcare significance,and unique flavor.There are a variety of photoreceptors in plant roots,such as phytochrome,cryptochrome and photosynthetic protein,which give the roots with the ability of direct perception and response to light.In this study,the root illumination intensities of PPFD 0,30μmol·m^-2·s^-1,60μmol·m^-2·s^-1,90μmol·m^-2·s^-1 were designed by taking PPFD 0 as the control.The effects of root illumination with different light intensities on the growth,photosynthetic characteristics and nutritional quality of leaf beet were studied.The main results were listed as follows:1.The root illumination treatments had significant effect on the growth index of leaf beet.The dry weight and fresh weight of shoot,leaf area were increased with the increase of root illumination intensity.They were significantly higher under root illumination at PPFD 60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1than those of the control and reached the maximum at PPFD 90μmol·m^-2·s^-1.The plant height and the number of lateral roots were significantly increased and the root length was significantly decreased under root illumination at PPFD 30μmol·m^-2·s^-1,60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1 than those of the control.The root/shoot ratio was significantly increased under root illumination at PPFD 60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1.Therefore,the yield per plant of leaf beet was increased by the root illumination.In the range of PPFD 0-90μmol·m^-2·s^-1,the greater the root illumination intensity,the more the plant biomass.2.The photosynthetic characteristics of leaf beet leaves under different root illumination intensities were measured by Li-COR photosynthesis system.The net photosynthetic rate（P_n）of single leaf was significantly higher under root illumination at PPFD 90μmol·m^-2·s^-1 than that of the control,and there was no significant difference between other treatments and the control.Compared with the control,the transpiration rate（T_r）of leaves was significantly higher and the intercellular CO₂concentration（C_i）was significantly reduced under root illumination at PPFD 60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1,respectively.And there was no significant difference in stomatal conductance（G_s）between root illumination treatments and control.The results of photosynthetic parameters showed that the effect of root illumination on photosynthetic characteristics of leaf beet was mainly induced by nonstomatal factors.The photosynthetic rate of the whole plant was increased with the increase of root illumination intensities,which was significantly higher at PPFD60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1 than that of the control.It was consistent with the change trend of shoot dry weight and fresh weight.The root illumination had a significant effect on the photosynthetic pigment contents of leaf beet.The content of total chlorophyll in the leaves under root illumination at PPFD 60μmol·m^-2·s^-1 was significantly elevated than that of the control,but there was no significant difference between the other treatments and the control.The contents of chlorophyll a,total carotenoids and total chlorophyll had the same change trends.The chlorophyll a/b ratio was significantly higher under root illumination at PPFD 90μmol·m^-2·s^-1than that of the control.Compared with the control,the contents of chlorophyll a and chlorophyll b in roots were significantly higher under root illumination at PPFD 60μmol·m^-2·s^-1.The content of total chlorophyll in roots was significantly higher under root illumination at PPFD 30μmol·m^-2·s^-1and 60μmol·m^-2·s^-1 than that of the control.The above results indicated that photosynthetic pigment content was significantly increased by root illumination,which provided the basis for improving the photosynthetic rate of leaf beet.3.Compared with the control,the anthocyanin content in the middle leaves was significantly higher under root illumination at PPFD 60μmol·m^-2·s^-1,the anthocyanin content in the inner leaves was significantly lower under root illumination at PPFD 30μmol·m^-2·s^-1and 90μmol·m^-2·s^-1,and the anthocyanin content in the outer leaves was significantly lower under root illumination at PPFD 30μmol·m^-2·s^-1 than that of the control.The content of total phenols in leaves was significantly increased under root illumination at PPFD 30μmol·m^-2·s^-1,60μmol·m^-2·s^-1and 90μmol·m^-2·s^-1 and the content of total phenols in petioles was significantly increased under root illumination at PPFD 90μmol·m^-2·s^-1 than that of the control.The similar change trends were observed in total antioxidant activity,total phenol content in leaves and petioles.Therefore,the antioxidant activity and nutritional quality of leaf beet were significantly improved by root illumination.In conclusion,under root illumination,the plant growth of leaf beet was significantly increased,the contents of anthocyanins and total phenols were significantly elevated which resulted in the increase of total antioxidant capacity,resulting in improving the yield and nutritional quality.The present study provides a firm foundation for the optimization of the light mode of leaf beet production in the plant factory.