| Salt stress,its effects on plant growth and development are various,is one of the major factors limiting plant growth and yield.Citrus is sensitive to salt and thus its growth and physiological activities are altered under salt stress.Citrus is a grafting crop and its rootstock plays a key role in salt tolerance.It is know to all that boron is an essential nutrition element for citrus.In recent years,boron deficiency has caused extensive leaf chlorosis,leaf vein enlargement and corky split vein on citrus trees.As a result,more and more boron fertilizer,such as boric acid and borax,is used on citrus management practice.However,the suitable range of the boron content in soil for plants from deficiency to excess is too narrow.The misuse of boron fertilizer may cause boron toxicity on citrus if the application of boron is slightly excessive.The phenomenon of borate stress on citrus is consequently becoming prominent and some citrus orchards are probably to occur boron toxicity.Therefore,in order to solve the problem,this study was designed to explore citrus rootstock tolerance to sodium chloride and borate as well as the influence of different salt concentration on different rootstocks.Four citrus rootstock cultivars,trifoliate orange?Poncirus trifoliate?L.?Raf?,Ziyangxiangcheng?C.junos?Sieb.??,Carriao citrange?C.sinensis Osb.× P.trifoliate Raf?,red tangerine?C.tangerine?,were treated under salt stress and boron stress.For the salt stress,the concentrations of sodium chloride solution were set as 0 mmol/L?CK?25mmol/L,50mmol/L,100mmol/L,200mmol/L?Na Cl0,Na Cl25,Na Cl50,Na Cl100,Na Cl200?,respectively.For the boron stress,the parameters were 0.05mmol/L?CK?,0.2 mmol/L,0.8 mmol/L,3.2mmol/L?B0.05,B0.2,B0.8,B3.2?,respectively.The salt injury index,the cell membrane permeability,chlorophyll content of leaves,photosynthetic index and biochemical indices were measured for either salt or boron stress treatment.The principal component analysis was performed to evaluate the tolerance of the four citrus rootstocks to salt or boron stress.The results are presented as follows.1.Effects of salt stress on citrus rootstocks1.1.The salt injury symptom was showed obviously on the citrus rootstocks including leaf margin dying,brown spots on the leaf surface,shoot dying,leaf falling off and death of whole plant.Under the treatments of Na Cl100 or Na Cl200,according to the occurrence time of salt injury symptom and the salt injury index from high to low,the order of the 4 rootstocks was trifoliate orange> Ziyangxiangcheng> Carriao citrange> red tangerine.1.2.The tree height increment ratio of the 4 citrus rootstock seedlings declined generally in different salt treatments.Under the treatments of Na Cl50 to Na Cl200,the tree height increment ratio of red tangerine was stable while the index of trifoliate orange decreased significantly.Under low salt stress?Na Cl25?,compared with the control,both the fresh biomass and dry biomass of the seedlings were increased in 3 citrus rootstocks except trifoliate orange.However,with the further increase of salt stress,the biomass of the 4 rootstocks decreased significantly.1.3.With time prolonging and the salt concentration increasing,the relative conductivity in leaves of the 4 citrus rootstock seedlings increased in different degrees,and the growth of the relative conductivity negatively correlated with salt tolerance of the rootstocks.1.4.In the treatment of late period?49th-63 th day?,relative content of chlorophyll of the 4 rootstocks decreased obviously under the treatment of Na Cl100 and Na Cl200.According to the degree of the decline,the order of the 4 rootstocks was trifoliate orange> Ziyangxiangcheng> Carriao citrange> red tangerine.With the salt concentration increasing,the relative content of chlorophyll of the 4 rootstocks decreased remarkably.Under the high salt stress?Na Cl200?,the decline range of the chlorophyll content for the 4 rootstocks were 67.96%?57.65%?56.96% and 56.21% comparing to the control treatment,respectively.The variation regularity of Chlb and Chla+Chlb were similar with Chla.1.5.Under the long-period salt stress,the Pn and Gs of the 4 rootstocks decreased with the prolonged time of treatment and the increase of the salt concentration.The Pn and Gs showed a positive correlation with the tolerance of salt stress.The Ci fell first and then increased and it was no obvious correlation with the tolerance of salt stress.1.6.With the increase of the salt concentration,activity of the superoxide dismutase?SOD?and the contents of proline in leaf enhanced while the activities of peroxidase?POD?and soluble protein content rose first and then fell.However,the activities of catalase?CAT?dropped all the time.In other words,their contents and activities of these indices were positive correlation with the tolerance of salt stress.1.7.Under high salt stress?Na Cl200?,the contents of Na+?Cl-in all organs of the 4 rootstocks increased significantly,and the accumulation of these two elements in leaves and stems was higher than those in the root.The contents of K+ in all organs of the 4 rootstocks were reduced.The contents of Na+?Cl-were negative correlation with the tolerance of salt stress while the contents of K+ played a positive role on the salt tolerance of citrus rootstocks.1.8.In order to evaluate the salt tolerance of the 4 citrus rootstocks,principal component analysis and subordinate function were performed based on the data of salt tolerance coefficients and other leaf indices.The result showed that the salt tolerance of the 4 citrus rootstocks was red tangerine > Carriao citrange > Ziyangxiangcheng > trifoliate orange.2.Effects of boron excess on citrus rootstocks2.1.The boron injury symptoms which were showed obviously on the citrus rootstocks include that the leaf appeared tip yellowing,yellow spots along the leaf margin,and then the whole leaf yellowing and dropping,the shoots dying back and death.Under the treatments of B0.8 or B3.2,according to the occurrence time of boron injury symptom and the boron injury index from high to low,the order of the 4 rootstocks was trifoliate orange> Ziyangxiangcheng> Carriao citrange> red tangerine.2.2.With the increment of the boron concentration,the tree height increment ratio,above ground and root fresh biomass,above ground and root dry biomass of the 4 citrus rootstock seedlings declined totally.Trifoliate orange was the most sensitive to excess boron stress among the 4 rootstocks.2.3.With time prolonging and the boron concentration increasing,the relative conductivity in leaves of the 4 rootstocks seedlings increased in different degrees.And the growth of the relative conductivity negatively correlated with the boron tolerance of rootstocks.2.4.In the treatment of the late period?35th-49 th day?,relative content of chlorophyll of the 4 rootstocks decreased obviously under the treatment of B0.8 and B3.2.According to the degree of the decline,the order of the 4 rootstocks was trifoliate orange> Ziyangxiangcheng> Carriao citrange> red tangerine.And the decline of Chl negatively correlated with boron tolerance of the rootstocks.2.5.Under the long-period boron stress,the Pn and Gs of the 4 rootstocks decreased with the prolonged time of treatment and the increase of the boron concentration.The Pn and Gs showed a positive correlation with the tolerance of boron stress.Under the high boron stress?B3.2?,the decline range of the content of chlorophyll for the 4 rootstocks were 98.75%?88.76%?56.50% and 76.86% at 45th days.2.6.With the increase of the boron concentration,SOD and POD activity increased,and CAT activity always dropped.The soluble protein content rose first and then fell.In a word,their contents and activities of these indices were positive correlation with the tolerance of boron stress.The proline content in leaves always rose and it played a negative role in the boron excess tolerance.2.7.Under high boron stress?B3.2?,the B contents in all organs of the 4 rootstocks increased significantly,and the B content in organs from high to low was leaf >root>stem.The B content was negative correlation with the tolerance of boron stress.2.8.In order to evaluate the boron excess tolerance of the 4 citrus rootstocks,principal component analysis and subordinate function were performed based on the data of boron tolerance coefficients and other leaf indices.The result showed that the boron tolerance of the 4 citrus rootstocks was red tangerine > Carriao citrange > Ziyangxiangcheng > trifoliate orange.3.Conclusion 3.1.The salt tolerant ability of the 4 rootstocks was red tangerine > Carriao citrange > Ziyangxiangcheng > trifoliate orange.3.2.The boron tolerant ability of the 4 rootstocks was red tangerine > Carriao citrange > Ziyangxiangcheng > trifoliate orange.3.3.Red tangerine had the highest salt and boron tolerance,followed by Carriao citrange and Ziyangxiangcheng,and the trifoliate orange was the weakest.Therefore,suitable rootstocks should be chosen according to local soil conditions in citrus production practice.At the same time,the boron fertilizer application should be controlled according the type of rootstocks to avoid the boron injury. |
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