近日，華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院作物養(yǎng)分管理研究團隊在Plant, Cell & Environment和Journal of Experimental Botany上發(fā)表兩篇研究論文，系統(tǒng)揭示了鉀營養(yǎng)調(diào)控油菜葉片生長和葉片功能的生理機制，為油菜鉀肥的精準(zhǔn)施用提供了理論支撐。
我國長江流域冬油菜主產(chǎn)區(qū)土壤鉀素供應(yīng)普遍偏低，土壤有效鉀處于潛在缺乏及以上的比例高達80%。然而由于鉀在作物體內(nèi)的移動性較強，在田間生產(chǎn)中不易觀測到典型的缺鉀癥狀，從而導(dǎo)致油菜生產(chǎn)中出現(xiàn)輕視或忽視鉀肥施用的現(xiàn)象。研究團隊多年的田間觀測發(fā)現(xiàn)，油菜生產(chǎn)中的“隱性缺鉀”癥狀會掩蓋作物真實的鉀營養(yǎng)狀況：當(dāng)土壤鉀供應(yīng)略顯不足時，油菜早期并不表現(xiàn)出明顯的缺鉀癥狀，其葉片顏色、光合能力均不會發(fā)生明顯變化，但葉面積明顯減小。“隱性缺鉀”時油菜葉片生長和葉片功能呈現(xiàn)不同步現(xiàn)象。
作物葉片生長和葉片功能是密不可分的，葉片的光合作用和水分傳導(dǎo)為葉片擴張?zhí)峁┝吮匾奶妓衔锖臀锢砼驂海~片的擴張也為光合作用攔截更多的光、為水分傳輸提供基礎(chǔ)的水力系統(tǒng)。葉肉細胞結(jié)構(gòu)是關(guān)聯(lián)葉片生長和功能的核心，因為葉片生長受葉肉細胞分裂和擴張的調(diào)控，而細胞形態(tài)的改變亦會影響CO2和H2O在葉片內(nèi)的傳輸，鉀營養(yǎng)如何影響葉肉細胞形態(tài)協(xié)調(diào)葉片生長和葉片功能（光合能力和水分傳輸），以及葉片功能的變化如何反饋調(diào)節(jié)葉片生長均不清楚。
團隊首先研究了不同鉀營養(yǎng)條件下油菜葉面積、凈光合速率和水力學(xué)導(dǎo)度隨葉片生長的動態(tài)變化，發(fā)現(xiàn)缺鉀脅迫時油菜葉面積降低先于葉片功能，隨著缺鉀脅迫加重葉片的光合能力和葉片水力導(dǎo)度同步降低。不同鉀營養(yǎng)狀況下葉肉細胞顯微結(jié)構(gòu)的動態(tài)變化結(jié)果表明缺鉀脅迫下葉片厚度方向葉肉細胞投資增加，導(dǎo)致細胞間隙體積分?jǐn)?shù)占比（fias）增加，葉片擴張受阻。降低的fias減少了面向細胞間隙的葉肉細胞和葉綠體面積以及氣相H2O傳輸能力，進而引起CO2葉肉導(dǎo)度和木質(zhì)部外水力導(dǎo)度的同步降低。充足的鉀營養(yǎng)通過較低的柵欄組織細胞密度（松散的葉肉細胞排列）和小的海綿組織細胞提高了葉片fias，從而協(xié)調(diào)葉片擴張以及CO2、H2O的傳輸。研究揭示了鉀營養(yǎng)通過影響葉肉細胞結(jié)構(gòu)協(xié)調(diào)葉片生長過程中光合能力和水分傳輸?shù)慕Y(jié)構(gòu)生理機制。
進一步研究了不同鉀營養(yǎng)條件下油菜葉片晝夜生長特征、同化物合成和水力學(xué)特性等，結(jié)果發(fā)現(xiàn)鉀素營養(yǎng)主要通過調(diào)控夜間葉片生長來影響葉面積（>60%），且集中在葉片由庫向源轉(zhuǎn)換之前（葉片面積達到2/3總面積）。夜間植物蒸騰耗水量低，根壓作用使植物細胞水分充盈，充足的鉀營養(yǎng)保證了白天葉片較高的光合固碳效率和碳儲量，同時促進了成熟葉片碳水化合物往新葉的輸出，從而保證了夜間葉片生長充足的碳源。白天，植物維持氣孔開放以盡可能多的固定空氣中的CO2，但同時散失了大量的水分。適宜的鉀供應(yīng)提升葉片水分供應(yīng)效率的同時增加了葉肉細胞滲透調(diào)節(jié)物質(zhì)濃度（鉀離子和碳水化合物），從而保證了白天葉片的生長。研究揭示了鉀營養(yǎng)狀況通過影響葉片光合產(chǎn)物的合成和水分運輸，調(diào)控葉片晝夜節(jié)律生長的生理機制。
華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院博士研究生胡文詩和陸志峰副教授為論文的第一作者，任濤教授為論文通訊作者。作物養(yǎng)分管理研究團隊魯劍巍教授、李小坤教授、叢日環(huán)副教授等參與了相關(guān)的研究。該研究得到了國家自然科學(xué)基金、國家油菜產(chǎn)業(yè)技術(shù)體系和華中農(nóng)業(yè)大學(xué)自主創(chuàng)新基金項目的資助。
【英文摘要1】
Leaf growth relies on photosynthesis and hydraulics to provide carbohydrates and expansion power; in turn, leaves intercept light and construct organism systems for functioning. Under potassium （K） deficiency stress, leaf area, photosynthesis and hydraulics are all affected by alterations in leaf structure. However, the connection between changes in leaf growth and function caused by structure under K regulation is unclear. Consequently, the leaf hydraulic conductance （Kleaf） and photosynthetic rate （A） combined with leaf anatomical characteristics of Brassica napus were continuously observed during leaf growth under different K supply levels. The results showed that Kleaf and A decreased simultaneously after leaf area with the increasing K deficiency stress. K deficiency significantly increased longitudinal mesophyll cell investment, leading to reduced the volume fraction of intercellular air-space （fias） and decreased leaf expansion rate. Furthermore, reduced fias decreased mesophyll and chloroplast surfaces exposed to intercellular airspace and gas phase H2O transport, which induced coordinated changes in CO2 mesophyll conductance and hydraulic conductance in extra-xylem pathways. Adequate K supply facilitated higher fias through smaller palisade tissue cell density （loose mesophyll cell arrangement） and smaller spongy tissue cell size, which coordinated CO2 and H2O conductance and promoted leaf area expansion.
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論文鏈接：https://onlinelibrary.wiley.com/doi/10.1111/pce.14405
【英文摘要2】
Carbon and water are two main factors limiting leaf expansion. Restriction of leaf growth by low availability of carbon or water is among the earliest visible effects of potassium （K） deficiency. It is not known how K is involved in regulating the rhythmic supply of these two substrates, which differ remarkably across the day–night cycle, affecting leaf expansion. We investigated the effects of different K regimes on the time courses of leaf expansion, carbon assimilation, carbohydrates, and hydraulic properties of Brassica napus. Potassium supply increased leaf area, predominantly by promoting night-time leaf expansion （>60%）， which was mainly associated with increased availability of carbohydrates from photosynthetic carbon fixation and import from old leaves rather than improvement of leaf hydraulics. However, sufficient K improved leaf hydraulic conductance to balance diurnal evaporative water loss and increase the osmotic contribution of water-soluble carbohydrates, thereby maintaining leaf turgor and increasing the daytime expansion rate. The results also indicated an ontogenetic role of K in modifying the amplitude of circadian expansion; almost 80% of the increase in leaf area occurred before the area reached 66.9% of the mature size. Our data provide mechanistic insight into K-mediated diel coordination of rhythmic carbon supply and water balance in leaf expansion.
論文鏈接：https://academic.oup.com/jxb/article/73/11/3686/6530306
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