Inter-Species Differences In Water Use Efficiency And Its Mechanisms In Malus

Water availability is a major determinate of agricultural productivity in arid and semi-arid regions of the world. Therefore, higher water use efficience should be center of the agricultural production in these regions. One solution to the problem is to improve water use efficiency (WUE) of plants themselves. In this study, we examined the differences of carbon isotope composition (δ¹³C), long-term water use efficiency (WUE_L), instantaneous water use efficiency (WUE_i), total water consumption and drought tolerance in ten Malus rootstocks, and elucidated morphological and physiological traits in relation to this variation in WUE. In addition, in order to gain a better understanding on the regulation of stomatal and photosynthetic capacity on WUE, M. sieverii and M. hupehensis were used to study the relationships of WUE (WUE_i) and aquaporin gene expression, and to investigate the effects of long-term exogenous abscisic acid (ABA) application and fertilization on WUE.The results were as follows:1. There was large variation in the WUE of Malus under both well-watered and drought-stressed conditions, the ranking and degree of response in WUE of the different species generally remained the same under both watering regimes. Analysis of WUE, drought resistance and total water consumption of different Malus rootstocks showed that M. prunifolia had high WUE, high drought resistance and large total water consumption, M. robusta had high WUE and total water comsuption but moderate drought tolerance, M. sieverii and M. micromalus had moderate WUE level, low total water consumption and high tolerance to drought stress, while M. hupehensis and M. toringoides had low WUE and drought resistant capability. Theδ¹³C value in Malus differed significantly. Foliarδ¹³C values increased significantly with the decreasing total soil water content. There was a positive correlation betweenδ¹³C and both WUE_i and WUE_L, while the species×treatment interaction was non-significant forδ¹³C, suggesting that it should be possible to useδ¹³C as a surrogate for WUE and to select Malus rootstock for high WUE.2. WUE of Malus was affected by morphological and physiological properties.δ¹³C did not correlated with total dry weight, root/shoot ratio and leaf area ratio either in the well-watered condition or in the dry treatment. This result opens a way for selecting Malus rootstock combining large WUE and high relative growth rate. In addition, these results showed that the specific activities of leaves and roots were more important in determining the water use of the plant than the allocation to leaves and roots. Under both well-watered and drought-stressed conditions,δ¹³C was negatively correlated with transpiration per plant weight, transpiration per leaf area, special leaf area, stomatal length, stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO2 concentration, and positively correlated with stomatal density and leaf nitrogen content. Pn was not primarily responsible for differences inδ¹³C. However, the positively relationship betweenδ¹³C and leaf nitrogen content could re?ect the fact that the diversity forδ¹³C may also be partly driven by Pn. Under both well-watered and drought-stressed conditions,δ¹³C were not correlated with ABA, ZR, IAA, GA and ABA/(ZR+IAA). Multiple stepwise regression analysis showed that stomatal density and Gs were key determinants of difference in WUE under well-watered condition, and that the diversity for WUE was mainly driven by special leaf area and Gs under drought-stressed condition.3. Under three conditions (1/2Hoagland solution, 20%PEG6000, 60μmol.L-1 ABA), WUE_i of seedlings of M. sieverii and M. hupehensis markedly decreased after HgCl2's inhibition to the activities of aquaporins. Under drought stress, WUE_i and gene expression of PIP1 in roots and leaves of potted apple plants of M. sieverii and M. hupehensis increased with the continuance of drought stress. WUE_i and gene expression of PIP1 of M. sieverii and M. hupehensis reached the highest at 4d and 2d, respectively, followed by an obvious decline with further increasing of drought level. At the mRNA level, the leaf and root PIP1 expression level of M. sieverii. were higher than that of M. hupehensis. After re-watering, WUE_i and PIP1 expression level began to reverse, but M. hupehensis still lower than control. ABA content in leaves and roots showed the similar changes as PIP1 expression levels. Our results showed that aquaporins may regulate WUE (WUE_i) of Malus rootstocks. The PIP1 expression of leaves and roots may be triggered by ABA.4. For M. sieverii and M. hupehensis, long-term exogenous ABA application significantly increased WUE_i, WUE_L,δ¹³C, endogenous ABA concentration, stomatal density, and root/shoot ratio, and markedly decreased total dry weight, stomatal length, Pn, Gs and Tr under both well-watered and drought-stressed conditions. However, M. sieverii was more responsive to exogenous ABA application than M. hupehensis, as indicated by the strong stomata closure and by greater plasticity of biomass allocation, as well as by higher WUE and ABA content. We concluded that sensitivity to exogenous ABA application is species dependent in Malus.Under both well-watered and drought-stressed conditions, maximal efficiency of PSâ…¡(FV/FM), electron transport rate (ETR), Chlorophyll content and RuBPcase activity of two contrasting Malus rootstocks were unaffected by exogenous ABA application. These results showed that ABA applied to leaf surfaces of two contrasting Malus rootstocks improved WUE and produced leaves with much-increased stomatal density and potential stomatal conductance, but had not a direct effect on photosynthetic apparatus. In addition, application of exogenous ABA appears to enhance the tolerance of two Malus species to drought-stress.5. Under both well-watered and drought-stressed conditions, fertilization increased the WUE_i, WUE_L andδ¹³C of M. sieverii and M. hupehensis. However, fertilization treatment had evident promoting effect on WUE under well-watered conditions. Under well-watered and fertilization treatment, the changes trends of total water consumption, total leaf area, total dry weight, Pn and Tr were consistent in M. sieverii and M. hupehensis seedlings, but the increased extent of them in M. hupehensis were more than that of M. sieverii. Under well-watered condition, treatment of fertilization increased Pn, Gs and Tr, but the increased extent of Gs and Tr were less than Pn. Fertilization treatment caused no significant changes in Gs and Tr under drought condition. However, under both well-watered and drought-stressed conditions, fertilization treatment significantly increased Pn, light saturation point, leaf nitrogen content, Chlorophyll content, RuBPcase activity, FV/FM, qP and ETR, and significantly decreased qN and light compensation point. These results indicated that application of fertilization could increase the light energy conversion efficiency, the potential activity of photosynthetic reaction center which can prevent leaf photosynthetic apparatus from damage of environmental stress. Fertilization treatment may improve WUE of Malus rootstocks through changes in carbon assimilation.