高级植物生理学课件-ppt-word

第一章：植物的水分代谢一、植物对水分的需要For every gram of organic matter made by the plant, approximately 500 g of water is absorbed by the roots,水分在生命活动中的作用细胞内水分呈束缚水和自由水两种状态水分是细胞质的主要成分水分是代谢过程的反应物质水分是植物对物质吸收和运输的溶剂水分能保持植物的固有姿态二、植物对水分变化的反应及生态类型即水生植物和陆生植物（一）水生植物水生植物(hydrophite)指植株全部或至少根系可一直生长在水中的植物。

根据它们在水中的生长状态，可以把它们划分为沉水植物(submerged plant) 浮水植物(floating—leaf plant) 挺水植物(emerged plant)沉水植物(submerged plant)是指整个植物体都浸没在水中的植物其中一种类型是扎根于水底的土壤另一种类型则是悬浮于水中而根系退化的由于水中氧少光弱，因而植物的通气组织发达，构成连续的通气网络。

整个植株都可直接吸收水、矿质营养和水中的气体浮水植物(floating—leaf plant)指那些植物体完全漂浮在水面上或植物扎根于水底而叶子漂浮在水面上的植物浮水植物水下部分结构与沉水植物相似，但水面上部分由于直接与空气接触，表皮细胞常具薄的角质层，气孔一般只生于叶的上表皮，并有通气结构贯通整个植物挺水植物(emerged plant)指那些根、下部茎，有的还包括部分下部叶浸没于水中，而上部的茎叶挺伸出水面以上的植物挺水植物的维管组织、机械组织和保护组织在水生植物中是最发达的，并具有良好的通气组织，常能忍受一定时间限度的土壤干燥（二）陆生植物湿生植物(hygrophyte)中生植物(mesophyte)旱生植物(xerophytic plant)短命植物(short—1ife plant)避旱植物(drought—evading plant)耐旱植物(drought—enduring plant)抗旱植物(drought—resisting Plant)三、水分经植物从土壤到大气，水势T r e e s c a n g r o w m u c h t a l l e r t h a n10m•Suction tension(吸水压) in the xylem must be greater than that of a vacuum•Water potential (or pressure) in the xylem must be negative•How do we account for a negative water potential (pressure)?W a t e r m o v e m e n t b e t w e e n c o m p a r t m e n t syp = -RTc R: gas constant T: temperature (K) c: solute concentrationG e n e r a t i o n o f r o o t p r e s s u r e i n a n e x c i s e d p l a n t四、根系对水分的吸收water-channel proteins (aquaporins)E x o d e r m i s a n d e n d o d e r m i s•S u b e r i n in cell walls of exodermis and endodermis blocks a p o p l a s t i c w a t e r f l o w•Water must e n t e r the cells (symplasm)•Plasma membrane offers enormous resistance to water transport•How can water enter the symplast?•Through special pores in the plasma membrane: w a t e r-c h a n n e l p r o t e i n s (a q u a p o r i n s) Water transport across membrane is mediated by water channels (aquaporins)Water movement – ALWAYS PASSIVE !Water movement occurs either as diffusion or bulk flowDiffusion: driven by concentration gradient, permeability defined as diffusional water permeability (Pd: m s-1)Bulk flow: driven by pressure difference (hydrostatic or osmotic); defined as osmotic permeability (Pf: m s-1) or hydraulic conductivity (Lp:m s-1 MPa-1)Pf often greater than Pd, Why ?Water channels -Aquaporins•Transmembrane proteins;•Facilitate passive transport of water; 10-1000 fold higher than lipid permeability.•Can be highly selective to water (true aquaporins).•Some can be more selective for small neutral solutes (eg. glycerol; aquaglyceroporins)•Some animal aquaporins have recently been shown to create ion channels under certain conditions.Aquaporins can also mediate flux of other substances across cell membranesP I P1i n c h l o r o p l a s t sWater and CO2 conductivity•Plant aquaporins conduct water or CO2•Aquaporin CO2 conductivity is significant for photosynthesisActivities of aquaporins are reflected by permeability to water (P f or Lp).C h e m i c a l s,e.g.,H g t h a t i n h i b i t w a t e r-c h a n n e l p r o t e i n s r e d u c e t h e w a t e r f l o w t h r o u g h r o o t s;i t i sa r e v e r s ib l e e f f ec t五、植物吸收水分的来源W a t e r i n t h e s o i l:t y p i c a l s o i l w a t e r c o n t e n t s(%)o fd i f fe r e n t t y p e s of s o i lW a t e r p o t e n t i a l o f v a r i o u s s o i l sS o i l w a t e r p o t e n t i a l a n d s o i l w a t e r c o n t e n t a t d i f f e r e n t s o i l d e p t h六、干旱及植物的适应性反应当植物耗水大于吸水时，使组织内的水分亏缺。

过渡水分亏缺的现象称为干旱。

干旱可分大气干旱和土壤干旱。

大气干旱的特点是大气温度高而相对湿度低(10％－20％)，蒸腾大大加强，于是破坏水分平衡。

此外，大气温度高，阳光强，也会造成植物的热害。

大气干旱如果长期存在，会引起土壤干旱。

土壤干旱是指土壤中缺乏植物能吸收的水分的情况。

土壤干旱时，植物生长困难或完全停止，受害情况比大气干旱严重。

干旱对植物的伤害水分亏缺严重时，植物细胞失去紧张，叶片和茎的幼嫩部分下垂，这种现象称为萎蔫（wilting）。

萎蔫可分为暂时萎蔫和永久萎蔫两种。

在炎夏的白天，蒸腾强烈，水分暂时供应不及，叶片和嫩茎萎蔫;到晚间，蒸腾下降，而吸水继续，消除水分亏缺，即使不浇水也能恢复原状。

这种靠降低蒸腾即可消除水分亏缺以恢复原状的萎蔫，称为暂时萎蔫(temporary wilting)。

如果由于士壤已无可资植物利用的水，虽然降低蒸腾仍不能消除水分亏缺以恢复原状的萎蔫，称为永久萎蔫(pemanent wilting)。

永久萎蔫时间持续过久，植物就会死亡。

Growth rate of shoot and root of young corn plants in nutrient solution with a defined water potential Water loss is controlled by stomata on the leaf surfaceGuard cells are the only epidermal cells that contain chloroplastsStomatal guard cells control stomatal apertureStomata open when turgor increasesOrientation of the cellulose fibers in the cell walls allows guard cell expansion in longitudinaldirection only: the aperture increasesStomata may be on the leaf surface, or …hidden‟ in a stomatal crypt (…sunken stomata‟)ABA - a phytohormone involved in acclimation to water stressConcentration of ABA in xylem exudate and leaves of water stressed sunflower plants (closedsymbols: control; open symbols: water stress)L e a f c o n d u c t a n c e a s a f u n c t i o n o f t h e A B A c o n c e n t r a t i o n i n t h e x y l e m s a p o f f i e l d-g r o w n m a i z ep l a n t sDemonstration of 10µM ABA on 86Rb (K+) efflux from stomata cells .ABA was added 2min. or 40 min. of wash-out in the absence of ABA ABA-pathway in a drought stressed plant from root or leaf-mesophyll to the guard cellsThe mesophyll cell in the insert shows pH dependent ABA compartmentation.Only the protonated ABA can pass membranes.ABA通过影响保卫细胞的膨压来控制气孔的开关ABA对保卫细胞离子运输至少有四种效应：控制向内运输的K+通道，激活向外运输的K+通道，促进Ca2+通道的开放和膜的去极化。