Toxin Biosynthesis, Physiological Plasticity And Carbon Assimilation Under The Operation Of Root-sourced Signal In Grass Pea (Lathyrus Sativus L.)

Root to shoot communication is widely considered as the early-warning response to drought stress in plant, while its eco-physiological role and relevant regulatory mechanism are slightly reported in grass pea. On the basis of our previous studies, pot-culture experiments were conducted to investigate the root-sourced signal under exogenous application of10μmol/L absisic acid (ABA) and its inhibitors Sodium tunstate (0.25mmol/L) and fluridone (10μmol/L), by soil drenching application while13C labelling was also performed during vegetative stage in grass pea (Lathyrus sativus L.). In order to reveal the toxin biosynthesis, physiological plasticity and carbon assimilation in the regulation of non-hydraulic root-sourced signals (nHRS), the following parameters were measured:stomatal conductance, leaf water content, leaf water potential, antioxidant defense, flower and pod production and abortion, seed production, yield, water use efficiency (WUE), β-ODAP concentration and13C allocation. The major results achieved follow;1. Drenching the soil with exogenous10μmol/L ABA significantly altered the performance of root-souced signal in grass pea, leading to reduce stomatal conductance and water loss, meanwhile maintaining high leaf relative water content. The results indicated that addition of ABA increased the soil water content (SWC) threshold range of nHRS from69.1to45.6%FWC (Field water capacity) compared with65.5to49.0%FWC in control and63.9to48.5%FWC with ST (Sodium tunstate) application. Thus exogenous ABA resulted in early triggering of non-hydraulic root-sourced signals (nHRS) in grass pea, and enhanced drought tolerance in grass pea through increasing the SWC threshold range under nHRS.2. Exogenous ABA enhanced drought tolerance of grass pea by increasing the defense mechanisms and reducing the damage to lipid membrane peroxidation. The role of ABA was not so obvious in well-watered plants for all attributes while in other treatments it showed significant effects. Exogenous ABA increased proline content by44.2%and15.5%respectively than control when the SWC decreased to50%FWC and35%FWC, respectively. Exogenous ABA decreased H2O2levels by14.1%as compare to control under moderate drought stress (50%FWC, nHRS), but increased by15.2%under severe drought stress (35%FWC, SS). Exogenous ABA application decreased malondialdehyde (MDA) levels by17.7%and14.1%respectively under nHRS and SS. It also enhanced the antioxidant enzymes activity (SOD, CAT and POD) by16.5%,11.8%and13.2%than control respectively, under the regulation of nHRS, and enhanced by8.3%,28.2%and32.6%respectively under SS.3. ABA had no significant effect on yield, while biomass allocation pattern changed markedly and it also enhanced the WUE. Application of ABA and ST under three water treatments (WW, MS and SS) during P1(Period,1), P2(Period,2) and P3(Period,3), in grass pea promoted dry matter transferring from leaves and stems to seeds by reducing biomass allocation to leaves and stems, while no significant changes was observed under SS. Although exogenous ABA had no significant effect on plant height, branch diameter, effective and ineffective numbers of branches and pods, whereas had significant effect on dry matter allocation. Exogenous ABA inhibited the growth of leaves in plants under WW treatments, while promoted dry matter transferring from leaves and stems to seeds after PI treatment. The results indicated that drought stress during vegetative stage promoted the above-ground biomass transferring to roots and seeds while during reproductive stage reduced the yield (P<0.05).4. A non-linear trend was observed in the synthesis of β-ODAP in leaves and seeds of grass pea under the operation of nHRS, and there was no direct coorelation between ABA level and β-ODAP biosynthesis.β-ODAP concentration in leaves increased81.5% and135.3% respectively under nHRS and SS during seedling stage, while in seeds it increased by26.3% and43.5%respectively, under nHRS and SS after P1water treatment. Exogenously applied ABA and ST increased β-ODAP concentration both in leaves and seeds of grass pea, but had no other direct effects. β-ODAP concentration in leaves increased47.9% and36.2% respectively under nHRS and SS during seedling stage, though it increased in seeds by34.7%,12.2%and22.5%respectively, under WW, nHRS and SS after PI treatment. ST application during different treatments also increased β-ODAP concentration both in leaves and seeds, as well as ABA and ST application during P1, P2and P3showed the same results in seeds. However, fluridone which is another inhibitor of ABA showed no significant effect on β-ODAP concentration in seeds during another experiment. Therefore, the relationship between nHRS and β-ODAP needs to be further verified.5. nHRS regulated the flower, pod and seed production in grass pea. In the present study, the seeds production stopped when plant available soil water content (PAWC) was below55.5%, even though the flowers and pods still produced till PAWC was below40.1%and35.3%respectively. So the results indicated that nHRS controlled the seed and filled pod production while HRS controlled flower and pod production.6. The13C study showed that less than5%of seed carbon was retranslocated from vegetative tissues and the amount of carbon retranslocation from vegetative tissues was similar in both WW (well watered) and WS (water stress) treatments. Most of the labelled carbon was in leaves and stems at podding stage, so the storage of carbon was largely unavailable for retranslocation to the seeds. Presumably it was due to the carbon that was used for the structure of the leaves and stems, as most of the13C stored in the leaves during pre-podding appeared to be moved to the newly-developed leaves (post-podding leaves). Some carbon was retranslocated from the stems and roots to the seed during seed filling, but this was small amount and could not compensate for decreased current assimilation as a result of the water deficit.The study combined the research methods of ecology, physiology and biochemistry, with controlled experiment and13C labelling experiment in order to reveal drought tolerance of grass pea in the regulation of nHRS, toxin biosynthesis, carbon assimilation and translocation. These results indicated that nHRS played an important role in plant growth and yield production of grass pea. Moreover, the results indicated that ABA is the main signal substance that was not only associated with toxin biosynthesis and most importantly the carbon storage during vegetative stage played an important role in seed production. These results enrich the root to shoot communication theory, and provide the theoretical potential for the low toxicity and non-toxicity of grass pea breeding and cultivation management.