Transportasi Air, Nutrisi, dan Unsur Hara
Source dan Sink • Source: bagian di mana fotosintat memulai proses transportasi (tempat produksi atau menyimpan) • Sink: tempat di mana fotosintat di tempatkan (bagian yang membutuhkan maupun penyimpanan)
Xylem • Jaringan xylem memindahkan air dan unsur yang terkandung di dalamnya dari akar ke daun. • Saluran xylem tersusun atas selsel mati.
Phloem • Jaringan floem memindahkan fotosintat dari tempat produksi ke tempat yang membutuhkan atau penyimpanan. • Jaringan floem tersusun atas sel-sel hidup dan berdampingan dengan sel-sel pendamping
Transpirasi
Transportasi air terjadi di 3 bagian • Transpirasi adalah proses pemindahan air dan mineral dari akar menuju daun dan kemudian dilepaskan ke udara dalam bentuk uap. Proses ini melibatkan 3 langkah dasar : • Penyerapan di rambut akar. • Aktifitas kapiler di jaringan floem. • Penguapan di daun.
Bagian1: Perakaran • Akar menyerap air dan mineral melalui 4 proses: • Transport aktif mineral ke dalam rambut akar
• Difusi ke dalam perisikel • Transport aktif ke dalam silinder vaskuler • Difusi ke dalam xylem.
Penyerapan air dan mineral
Pita Kaspari
• Pita kaspari mengontrol jumlah air yang masuk ke dalam jaringan vaskuler pada akar
Step 2: Gaya Kapiler • Kohesi: molekul air cenderung berinteraksi satu sama lain melalui ikatan hidrogen
• Adhesi: molekul air cenderung untuk menempel pada permukaan benda yang dilalui
Gaya Kapiler • Gaya kohesi dan adhesi menyebabkan air dapat merambat melalui kolom yang sangat sempit. • Semakin sempit kolom, semakin tinggi air akan merambat.
Tekanan Air di Perakaran • Transpirasi menyebabkan perbedaan tekanan yang dapat menarik air dari akar dan sampai batang. • Tekanan air rendah di akar sehingga menarik lebih banyak air.
Part 3: Penguapan • Penguapan pada permukaan daun memicu pergerakan air di xylem. • Penguapan adalah pemicu proses transpirasi yang terkuat
Stomata control • Sel penjaga di stomata sangat sensitif terhadap cahaya, CO2, dan penguapan. • Stomata membuka saat cahaya dan CO2 pada tingkatan yang rendah, dan akan menutup saat terjadi defisiensi air k
Stomata • Ketika stomata terbuka, penguapan menarik air dari daun. Pertukaran gas juga dapat terjadi untuk menjaga berlangsungnya proses fotosintesis. • Ketika stomata tertutup, penguapan tidak dan pertukaran gas tidak terjadi, sehingga fotosintesis dan transpirasi melambat.
Transport Gula
Getah • Getah terdiri dari gula dilarutkan dalam air pada konsentrasi tinggi: biasanya antara 10% dan 25%.
• Karena ini sangat terkonsentrasi, tanaman harus menggunakan transportasi aktif untuk melawan gradien difusi sebagai bagian dari proses perpindahan getah.
Pressure-flow theory • Pergerakan gula dimulai dari sumber dan dipompa ke dalam sel tabung floem
• Osmosis memicu pergerakan air ke dalam sel-sel dan meningkatkan tekanan. • Tekanan kemudian memicu pergerakan getah getah
Pressure flow 1
• Daun merupakan sumber gula • Glukosa dan fruktosa disintesis melalui proses fotosintesis untuk pembentukan sukrosa.
Pressure-flow 2
• transpor aktif digunakan untuk memuat sukrosa ke dalam tabung floem terhadap gradien difusi.
Pressure-flow 3 • Konsentrasi tinggi sukrosa dalam sel floem menyebabkan pergerakan air oleh osmosis
• Fenomena ini meningkatkan tekanan dan menyebabkan getah bergerak.
Pressure-flow 4 • Buah merupakan contoh dari jaringan sink • Sukrosa secara aktif diangkut keluar dari floem ke dalam sel buah.
• Di akar, sukrosa diubah menjadi pati
Pressure-flow 5 • Saat konsentrasi gula di dalam sel tabung floem menurun, air bergerak keluar dari tabung melalui proses osmosis.
Pressure-flow 6 • Saat air bergerak keluar oleh osmosis, tekanan dalam sel tabung saringan turun.
• Perbedaan tekanan sepanjang kolom sel menyebabkan getah terus mengalir.
Pressure-flow: Review
CHAPTER XII Soil and Plant Water Relations
Air •
90% dari total masa tanaman disusun oleh air. Fungsi air 1. 2. 3. 4.
Diperlukan dalam proses perkecambahan. Sebagai bagian dari struktur tanaman Pembawa mineral ke dalam tanaman Memindahkan fotosintat dan produk biokimia ke seluruh tanaman 5. Mendinginkan suhu tanaman melalui evaporasi 6. Terlibat dalam proses fotosintesis.
Karakter air: •Merupakan pelarut umum (universal solvent) • salah satu unsur alami yang paling stabil •Molekul air tidak simetris (membentuk dua kutub) Fenomena polaritas air mengakibatkan gaya tarik menarik antara molekul air. Air juga dapat menarik/ditarik oleh • Kation, seperti Na+, K+, and Ca++, or • Anion atau koloid tanah liat di tanah
Air Tanah Ketersediaan Walaupun air ada dalam tanah, bukan berarti bahwa air tersedia bagi tanaman Pori-pori tanah terisi oleh air, udara, atau gabungan keduanya. – Saat ruang pori terisi air, tanah akan menjadi jenuh. Kondisi tanah yang jenuh tidak bagus untuk tanaman karena oksigen yang diperlukan untuk respirasi tidak ada. – Saat ruang pori banyak diisi udara, tanah menjadi kering.
• The number and size of the soil pores vary with the soil's texture and structure. • Clay soils have smaller but more numerous pores than sandy soils. Thus, an equal volume of clay soil holds more water than a sandy soil when the pores are filled (Fig. 12-2).
• The ability of the soil to retain water is called its water-holding capacity. Fig. 12-3.
Water Movement and Retention in Soil Three forces are responsible for water movement within the soil. 1- Gravity causes water to move downward and is the principal force when a soil is saturated = percolation. 2- Adhesion is the force of attraction between unlike molecules (soil particles and water). 3- Cohesion is the force of attraction between like molecules (water and water).
• 2+ 3 = capillary motion • The latter two forces can cause water to move by capillarity in any direction—upward, downward, or laterally—and are the principal forces that move water in an unsaturated soil.
• The upward movement of water, called capillary rise, is responsible for the loss of water from the soil surface by evaporation (Fig. 12-4). • As soil dries, the water film surrounding each soil particle thins. Consequently, the adhesive and cohesive forces of attraction increase rapidly, making it more difficult for the plant to extract water that is held tightly in soil particles.
• Salts are present in soil water. The salts create osmotic energy, and if the salts are present in a sufficiently high concentration, the osmotic energy prevents water movement into the plant (Fig. 12-6) = Plasmolysis. • Remember that fertilizer is a salt.
• After a prolonged rain or irrigation, the air in the soil pores is displaced with water. In this condition, the soil is saturated, • When no more water is added, losses continue, first from the larger macropores=percolation and then from the smaller micropores=infiltration. Loss of water continues until the adhesive and cohesive forces equal gravity.
• At this moisture content, the soil is said to be at field capacity. The water has drained from the macropores but the micropores still contain water.
• If no water is added, eventually the soil reaches a moisture content that does not sustain plant life and the plants permanently wilt. • The soil moisture content at which a plant wilts and cannot recover when placed in an environment of 100 percent relative humidity is termed the permanent wilting point (PWP). • Soil texture determines the relationship between soil water content and soil moisture tension (Fig. 12-8).
Available water (AW) is defined as the soil moisture between field capacity (FC) and the permanent wilting point (PWP): AW = FC - PWP Water between FC and saturation is not considered available because it is lost through drainage. Water at tensions greater than PWP is held too tightly by the soil for plants to remove.
PLANT WATER Absorption and Conduction of Water The energy to move the water comes from the water potential gradient that develops in the soil, plant, and air continuum: – In most cases, the water potential of moist soils is greater than that of the roots, so water flows into the roots (some anatomical features of the root aid or deter this process). – Once inside the xylem of the root, however, the flow of water is almost strictly pressure dependent, with the pressure decreasing from the root to the stem, to the leaves, to the air (Fig. 12-10).
• The upward movement of water and dissolved minerals occur mainly in the xylem. The xylem extends from the root through the stem and other parts of the plant. • The upward movement is sometimes called the transpiration stream because transpiration is the primary cause of this movement. • The upward transpiration pull in the leaves is started by the evaporation of water molecules from the outer surfaces of the mesophyll cells.
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Most of the water is lost from the leaves by transpiration.
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Water loss in the plant is regulated to a certain extent by the opening and closing of stomata on the leaf surfaces.
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Not all water leaves the leaf: 1. Some water is part of the plant's structures or held in the cytoplasm. 2. Some is used for biochemical processes, 3. Some is stored in the tonoplast.
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The pressure of water inside a cell creates turgor pressure, which gives plants rigidity. When there is insufficient water to create turgor, the plant wilts.
Absorption and Transport of Mineral Nutrients The sap in root cells has concentrations 500 to 10,000 times higher than those of the same element in the soil solution. – If simple diffusion were the only mechanism involved in taking up soil nutrients, the mineral nutrients could not move into the roots against such a high concentration gradient. – In addition, the plasma membranes of cells are largely impermeable to the movement of ions.
• Energy is required to move ions – Against the concentration gradient and – Through the impermeable membranes.
• This energy is obtained from the respiration of starches and sugars that originated in the photosynthetic processes.
Translocation of Sugars
• Sugars that are synthesized during photosynthesis move throughout the plant, primarily in the phloem tissues. • The movement is – Mostly downward from leaves to roots, – Lateral or even upward movement from leaves to fruits or buds or other storage organs.
In woody perennials, when phloem tissue is severed by girdling: 1. The tissue above the cut proliferates =تتكاثر 2. The tissue below the cut is starved = تعانيfor photosynthates (Fig. 12-12). This can severely stunt and even kill a tree
The rate of translocation of sugars in the phloem is, in some instances, more than a thousand times faster than simple diffusion of sugar through water. The forces that create these high rates of movement are called active transport.
