| The agriculture production has been seriously affected by the increasing scarcity of water resources in China,especially in the arid regions of the northwest.Improving water use efficiency(WUE)has become a key alternative for sustainable development of agriculture,especially for cotton production in this area.Partial root-zone irrigation(PRI)is a water-saving irrigation technique that improves WUE without yield reduction,but the underlying mechanism is not well understood.In this study,PRI was simulated by using a split-root system and PEG6000 in a greenhouse,the deficit irrigation(DI)and full irrigation(control)treatments were also established.Physiological events or parameters investigated in this experiment included:water potential,water content and photosynthetic parameters of leaves;water uptake,hydraulic conductance(L)and the expression of GhPIP gene in roots;water content,the proline content and the expression of the osmotic stress response genes(P5CS and LEA protein genes)in the root;the content of MeJA,ABA and H2O2 in root,the expression of their biosynthesis and catabolism genes as well as their interactions in regulating the hydrated root L;the content of MeJA and the expression of its biosynthesis genes in leaves;the effect of phloem girdling of the hydrated side and VIGS-induced MeJA biosynthesis genes silencing in leaves on the hydrated roots L.In addition,a field experiment was conducted to study the effects of planting density(135000,180000 and 225000 plants·hm-2)and irrigation method(full irrigation,alternate partial root zone irrigation,deficit irrigation)on cotton growth,yield and water use efficiency in the west of Inner Mongolia.The main results and conclusions are as follows:1.Water balance in leaves under PRI was attributed to the increased water uptake of the hydrated roots.The expression of GhPIP genes in he hydrated roots under PRI significantly increased.The expression of GhPIP2;2 and GhPIP2;8 accounted for 84.8%of the total expression of all GhPIP2.They might play a major role in improving the hydrated roots L.The water uptake of the hydrated roots significantly increased,which accounted for 82.6%of the whole plant water uptake.There was no difference in total water uptake between PRI and full irrigation.The stomatal conductance and transpiration also decreased under PRI.The increased water uptake and reduced transpiration well maintained the water balance in leaves.There were no significant differences of water content and water potential in leaves between PRI and full irrigation,which might account for the unchanged photosynthesis under PRI.2.MeJA,ABA and H2O2 played an important role in regulating the hydrated root L under PRI.Proline content,the expression of P5CS and LEA protein gene and MDA content in the hydrated roots was not affected under PRI,suggesting an absence of osmotic stress in the hydrated roots and that the increased L may be induced by signals other than hydraulic signal.The contents of MeJA,ABA and H2O2 significantly increased in the hydrated roots under PRI.Exogenous MeJA,ABA and H2O2 significantly increased the hydrated roots L,while application of their inhibitor SA,fluridone and DPI reduced the hydrated roots L.Exogenous MeJA and H2O2 significantly increased the expression of GhPIP genes,while application of their inhibitor SA and DPI decreased them.Exogenous ABA and its inhibitor fluridone did not affect the the expression of GhPIP genes.These indicated that MeJA and H2O2 can regulate the root L by up-regulating the expression of GhPIP gene in the hydrated roots.ABA may increase the root L by decreasing the activation energy of PIP protein and increasing the activity of PIP protein.The effect of SA on the expression of GhPIP1;5?GhPIP2;2?GhPIP2;8 can be reversed by H2O2,while the effect of DPI on the expression of GhPIP1;5?GhPIP2;2?GhPIP2;8 can not be reversed by MeJA.The result indicated that H2O2 might be a key regulator in the pathway of MeJA regulating GhPIP gene expression.Exogenous MeJA increased the ABA and H2O2 content and their biosynthesis genes,while its inhibitors decreased the ABA and H2O2 content and their biosynthesis genes in the hydrated root under PRI.This indicated that MeJA can regulate ABA and H2O2 content by regulating the expression of their biosynthesis genes.Exogenous ABA and its inhibitor fluridone did not significant affect MeJA and H2O2 content and their biosynthesis genes,indicating that ABA did not regulate MeJA and H2O2 content in the hydrated root under PRI.Exogenous H2O2 increased ABA content by increasing the expression of ABA biosynthesis genes GhNCED2 and GhNCED3 as well as reducing the expression of catabolism genes GhCYP707A1 and GhCYP707A2,while DPI decreased ABA content by decreasing the expression of ABA biosynthesis genes GhNCED2 and GhNCED3 as well as increasing the expression of catabolism genes GhCYP707A1 and GhCYP707A2.Exogenous H202 and its inhibitor DPI did not significant affect MeJA and H2O2 content as well as their biosynthesis genes.The results suggested that H2O2 can regulate ABA by regulating the expression of its biosynthesis genes and catabolism genes,but H2O2 can not regulate the MeJA content in the hydrated root under PRI.Therefore,the content of ABA and H2O2 can be regulated by MeJA and the content of ABA can also be regulated by H2O2 in the hydrated root under PRI.3.MeJA acted as a shoot-root long distance signal to increase the hydrated roots L under PRI.Compared with the normal irrigation,the MeJA content was increased but the expression of its biosynthesis genes GhOPR11,GhAOS6 and GhLOX3 did not increase in the hydrated roots under PRI.This suggested that the elevated MeJA content in the hydrated roots was derived from other organs.In order to determine the source of the increased MeJA in the hydrated roots,we applied exogenous MeJA and its inhibitor SA to the dehydrated roots and found that neither of them affected MeJA content in the hydrated roots.This indicated that MeJA content in the hydrated roots was not affected by the dehydrated roots.The MeJA content and the expression of its biosynthesis genes GhOPR11,GhAOS6 and GhLOX3 significantly increased in leaves under PRI.Foliar application of exogenous MeJA increased the MeJA content in leaves and in the hydrated roots,while that of SA decreased the MeJA content in leaves and in the hydrated roots.Phloem girdling of the hydrated side increased the MeJA content in leaves but decreased the MeJA content in the hydrated roots.VIGS-induced MeJA biosynthesis genes GhOPR11?GhAOS6 and GhLOX3 silencing in leaves decreased the MeJA content in leaves and the hydrated roots.The results showed that PRI increased the MeJA content in leaves by increasing the expression of GhOPR11,GhAOS6 and GhLOX3 genes.The shoot-sourced MeJA was transported to the hydrated roots via phloem,thereby increasing the L and water uptake in the hydrated root side.4.Field experiment showed that the highest yield was achieved with a combination of high plant density(225000 plants·hm-2)and alternate partial root zone irrigation(APRI)in all treatments,and irrigation water use efficiency(WUEi)was significantly improved.Plant density and irrigation mode and their interaction significantly affected the biomass and harvest index,biomass significantly increased with increasing of plant density,but the harvest index significantly reduced.APRI significantly decreased biomass but promoted distribution of assimilates to reproductive organs and then improved harvest index.The number of bolls per unit area was only affected by plant density and increased with the increase of plant density.Plant density and irrigation mode and their interaction significantly affected the boll weight.Boll weight decreased with the increase of plant density under full irrigation,but there was no significant difference of boll weight between high and medium plant density under APRI.Compared with full irrigation,APRI under 225000 plants·hm-2 reduced the amount of irrigation by 30%,increased pre-frost seed cotton by 22.5%and WUEi by 50%without yield reduction.In conclusion,PRI increased the MeJA content in leaves by increasing the expression of GhOPR11,GhAOS6 and GhLOX3 genes.The shoot-sourced MeJA was transported to the hydrated roots via phloem,thereby increasing the contents of H2O2 by enhancing the expression of GhRBOHC genes in the hydrated root side under PRI.The increased H2O2 raised the contents of ABA by increasing the expression of GhNCED2,GhNCED3 and GhNCED9 and decreasing the expression of GhCYP707A1 and GhCYP707A2 in the hydrated root side under PRI.The increased H2O2 further up-regulated GhPIP1;5,GhPIP2;2,and GhPIP2;8 genes and then increased the L and water uptake in the hydrated root side under PRI.Also,the increased ABA content increased the L and water uptake probably via decreasing the activity of PIP protein in the hydrated roots under PRI.In conclusion,MeJA and H2O2 play an important role in increasing the root L of the hydrated roots under PRI. |
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