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。
