| Many experiments in fruit trees revealed that root restriction inhibited vegetative growth and improved fruit quality, which suggested that nitrogen and carbon metabolisms might be changed. However, up to date, it is still unknown the key steps exactly where the alterations of nitrogen and carbon metabolism took place, and these are bottle-necked problems for well understanding the root restriction mechanism and the rules of nitrogen absorption, as well as the nitrogen and carbon assimilation and translocation. Therefore, the present study, with nitrogen nutrition as a cut-in point, aimed at the sugar accumulation in fruit, probed into the changes of nitrogen and carbon metabolisms in grapevines under root restriction and their relations with vegetative growth and fruit quality. The results showed as follows: 1. The analysis of vegetative growth, fruit quality and the grapevine nutrition of"Fujiminori"under root restriction showed that the shoot growth was inhibited and the shoot length was significantly shorter than that of control after little berry enlargement stage; the leaf area also showed the same trend as the shoot length; leaf photosynthetic rate was lower than that of control all the while and reached significant level at 8:00, 10:00 and 12:00; root restriction significantly increased the anthocyanin content in skins and total soluble solids in fruits, and decreased the acid content in fruits, thereby improved fruit quality; the concentrations of total N in new leaves and brown roots, NO3- in all leaves and roots, NH4+ and P in new leaves, K+ in new leaves and mature leaves and Mg2+ in mature leaves were lower than those of control, and the concentration of Ca2+ had no significant difference compared with control.2. Nitrate uptake kinetics of"Fenghou"grapevine under root restriction indicated that root restriction enhanced the nitrate uptake rate, decreased the Km value and increased Vmax value, that is to say, root restriction depressed the affinity of nitrate high-affinity transport system (HATS) and improved the nitrate uptake rates of both high (HATS) and low-affinity transport system (LATS). However, the amount of net nitrate uptake decreased significantly because of the reduction of root fresh weight under root restriction. 3. The daily variation of enzyme activities related to nitrogen metabolism in"Fujiminori"grapevine under root restriction showed that NR, NiR and NR activity displayed similar trend, the minimum appeared at 6:00 and reached the maximum at 10:00, then declined and with a little rise at 18:00; NR activity in mature leaves all the daytime, in brown roots from 10:00 to 18:00, in white roots after 10:00 were lower than those of control; NiR activity in new leaves, mature leaves, brown roots and white roots at 10:00, in mature leaves and brown roots at 14:00, in new leaves at 18:00 were also lower than control; and GS activity in new leaves from 6:00 to 14:00, in brown roots at 14:00, in white roots at 18:00 were lower compared with control. So, root restriction reduced the key enzyme activities related to nitrate metabolism, thereby decreased nitrate assimilation.4. By using"berry-cup"technique, the 24h variations of sugar phloem unloading in ripening grape berries were determined. Their variations exhibited three peaks and two bottoms. The peaks appeared at 9:00, 17:00 and 21:00, the peak value in the morning was about 1.7 and 1.2 fold higher than those in the afternoon and evening, respectively. The two bottoms appeared at 3:00 and 15:00, the bottom value in the afternoon was about 1.5 fold higher than that in the early morning. Moreover, the amount of sugar phloem unloading at daytime (6:00-19:00) was 1.7 (control)-2.0 (RR) fold higher than that at night (20:00-5:00), and that in the morning (6:00-11:00) were 1.7 (control)-3.1 (RR) fold, and before midnight (17:00-23:00) were 2.5 fold higher than that in the afternoon (12:00-16:00) and after midnight (0:00-5:00), respectively. Compared with the control, the amount of sugar phloem unloading per fresh weight of root-restricted berries was higher all the time (about 1.5 fold), therefore root restriction enhanced sugar phloem unloading of grape berries.5. Sugar phloem unloading of"Fenghou"grape berries was accelerated by external abscisic acid (ABA) application. However, the effect of external zeatin (ZT) treatment on sugar phloem unloading was determined by the concentrations of endogenous ABA and cytokinin (CTK) and their balance in grapevine. Treated with erythrosin B (EB) and ethyleneglycol-bis (β-aminoethyl ether)-N, N, N′, N′-tetraacetic acid (EGTA), the inhibitors of energy or transporters needed by sugar transport, the variations of sugar phloem unloading indicated that the pathway of sugar phloem unloading of ripening"Fenghou"grape berries did not changed and was the same as control, which belongs to apoplastic pathway and needs transporters and energy.From all above, root restriction decreased the uptake and assimilation of mineral elements such as NO3- etc and lowered mineral element content in grapevines, which it is maybe the main cause resulting in the inhibitory vegetative growth of grapevines. In addition, although the sugar phloem unloading pathway of root restricted grape ripening berries did not change (confirmed by EB and EGTA treatment experiments), however, it is the concentration alterations of endogenous ABA and CTK under root restriction that led to sugar phloem unloading being enhanced (confirmed by external ABA and ZT application experiments), which was the very important or main reason why root restriction could improve fruit quality significantly. The findings of the present study may provide a theoretical base for root restriction of fruit trees. |
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