Akar tanaman menyerap sejumlah air untuk menggantikan air yang hilang melalui transpirasi dan yang digunakan dalam aktifitas metabolik. Di bawah kondisi yang mudah menguap kehilangannya dapat mencapai 6 mm per hari (75.000 liter per Ha) selama awal musim panas. Tanaman menggunakan 300-500 kg air untuk setiap biomas kering. Air yang diperlukan mungkin lebih tinggi untuk tanaman pada tanah yang kuarang subur dari pada yang diperlukan pada tanah yang subur. Akar tanaman dapat secara efektif menyerap kelembaban tanah pada saat kelembaban tanah relativ rendah. Miselia mikoreza dapat memainkan peranan yang penting dalam pengambilan kelembaban tanah oleh pinus atau spesies lain yang mempunyai perakaran yang luas. Air tanah yang tertinggal hanya sebagian kecil disekitar area penyerapan akar spesies-spesies tersebut. Baik gerakan kapiler air tanah ke akar tanaman atau kecepatan pertumbuhan akar ke dalam kelembaban tanah tidak dapat dihitung untuk kapasitas penyerapan air yang sangat besar oleh tanaman pada kandungan kelembaban tanah yang rendah/
Meskipun demikian efisiensi sebagian besar sistem perakaran, penyebaran dan pertumbuhan pohon dikendalikan secara luas oleh suplai air. Di manapun pohon tumbuh perkembangannya dibatasi oleh ketersediaan air (terlalu banyak atau terlalu sedikit). Kelembaban relatif suatu area pada daerah temperatur, tanah hutan mungkin mengisi kapasitas lapang hanya pada musim dorman dalam waktu yang singkat setelah periode yang sangat basah. Curah hujan yang sangat tinggi ditahan oleh kanopi hutan atau hanya untuk mengisi lapisan air permukaan. Meskipun terisi kembali, lapisan tanah yang lebih atas yang mengandung sebagian besar sistem perakaran yang baik mungkin tereduksi mendekati titik layu permanen jauh seblum sisa profil mengering, akibatnya terjadi kekurangan air tanah selama musim pertumbuhan dalam jangka waktu yang lama dan pertumbuhan tanaman dapat kehilangan air dalam jumlah besar.
Pertumbuhan tanaman di reduksi karena kekurangan air, secara tidak langsung melalui aktifitas yang berkaitan dengan proses-proses fisiologis seperti fotosintesis, metabolisme nitrogen, absorbsi garam-garam dan translokasi secara langsung melalui efek menurunnya tekanan turgor pada pembesaran sel dan proses-proses lain yang lebih terlibat dalam pertumbuhan (Kramer, 1969). Untuk lebih meyakinkan bahwa pertumbuhan pohon dikendalikan oleh kesinambungan kelembaban dalam jaringan daripada kelembaban air tanah. Hal ini karena kekurangan air internal dikendalikan oleh kecepatan penyerapan relatif melalui akar dan hilangnya air melalui transpirasi, yang hanya secara tidak langsung dikendalikan oleh kandungan air tanah. Sepanjang siang transpirasi biasanya melebihi absorbsi dan air dikumpulkan kembali sepanjang malam ketika absorbsi lebih besar daripada transpirasi. Transpirasi secara luas diatur oleh lingkungan diatasnya dan struktur daun dan hanya absorbsi yang secara langsung dipengaruhi oleh air tanah. Absorbsi air dipengaruhi oleh air tanah Absorbsi air dipengaruhi oleh beberapa faktor tanah seperti tekanan, kelembaban tanah, suhu tanah, konsentrasi larutan tanah, aerasi tanah (seperti ukuran dan distribusi sistem perakaran).
Penyerapan air oleh tanaman dipengaruhi oleh dua kelompok faktor (Kramer.1969) Faktor tersebut mempengaruhi dalam perbedaan potensial air dari tanah ke akar tanaman dan resistensi terhadap pergerakan air melalui tanah dan dalam akar. Tekstur tanah dan daya hantar air mengendalikan pergerakan air ke permukaan akar. Aerasi, suhu dan derajat penggabusan akar mengubah resistensi terhadap pergerakan air ke dalam akar. Daya dorong merupakan perbedaan potensial air antara masa tanah dengan permukaan akar dengan Xylem.
Seringkali berbagai bagian dari sistem perakaran pohon berada dalam stratifikasi tanah yang memiliki perbedaan tingkat potensial air, dan mereka tidak hanya survive tetapi juga dengan hanya bagian perakarannya dalam tanah di atas presentase titik layu permanen. Minimum potensial air tanah pada saat absorbsi terus menerus dibatasi oleh potensial air minimum yang dapat dikembangkan di dalam akar. Air tanah akan menjadi pembatas terhadap tanaman yang mempunyai laju transpirasi tinggi pada potensial air tanah yang lebih tinggi daripada tanaman yang mempunyai laju transpirasi rendah.
Laju pemanjangan akar sangat dipengaruhi oleh tingkat kelembaban tanah selama semusim pertumbuhan. Pertumbuhan akar tanaman jarang dipengaruhi oleh defisit air selama semusim semi tetapi musim panas sters air dapat menyebabkan melambatnya dan berhentinya perluasan jaringan akar. Contohnya akar pirlus radiate terjad pemanjangan secara lengkap selama musim hangat pada periode kering. Akhir dari musim tersebut, jika terjadi pemanjangan pucuk (ahner, 1968)
Sistem perakaran yang luas berkembang pada bibit tanaman ketika ditumbuhkan pada tanah yang terpelihara dekat dengan kapasitas lapang, tetapi hanya perakaran yang tipis yang dapat berkembang pada tanah yang kering mendekati titik layu permanen sebelum dilakukan penyiraman. Zahner (1968) bahwa kekuatan tanah meningkat dengan tajam pada saat tanah kering dan menghasilkan resistensi fisik terhadap penetrasi oleh ujung akar yang mungkin tidak berhubungan dengan defisiensi air untuk absorbsi.
Perakaran pohon yang tua diketahui berkembang sedalam-dalamnya pada daerah profil tanah yang mengandung suplai air yang tersedia. Contohnya pembelahan akar yang kecil pada bagian akhir akar yang tertindih batu yang besar ditemukan diatas kapiler tepi dari neraca air. Disisi lain kelebihan air dan aerasi tanah yang buruk besar pengaruhnya terhadap morfologi akar sebagai akibat kurangnya air tanah atau karena penyebaran substrata yang impermeable. Tanah dengan pertukaran gas yang terbatas, baik karena tingginya kepadatan isi (bulk density) atau kelebihan air membantu tanaman bagian atas dengan sistem perakaran yang jarang.
GROUND WATER ABSORPTION BY PLANT ROOTS
Plant roots absorb some water to replace water lost through transpiration and used in metabolic activity. Under conditions of a volatile loss may reach 6 mm per day (75,000 liters per hectare) during the early summer. Plants use 300-500 kg of water for each dry biomass. Of water needed may be higher for plants on fertile soil kuarang than required on fertile soil. Plant roots can effectively absorb soil moisture at relatively low soil moisture. Mycelial mikoreza can play an important role in making soil moisture by pine or other species that have extensive roots. Ground water is left only a small area around the root absorption of these species. Both the capillary movement of soil water to plant roots or the speed of root growth into the soil moisture could not be calculated for water absorption capacity is very large by plants at low soil moisture content /
Nevertheless most of the root system efficiency, the spread and growth of trees is widely controlled by the supply of water. Wherever the trees grow its development is limited by the availability of water (too much or too little). Relative humidity is an area on the regional temperatures, forest land may fill field capacity only in the dormant season in a short time after a very wet period. Very high rainfall was arrested by the forest canopy or simply to fill the surface water layer. Although filled again, the upper soil layers that contain most of the good root system may be reduced close to permanent wilting point far seblum remaining dry profile, resulting in shortage of ground water during the growing season in a long time and can lose water plant growth in the number of large.
Reduction in plant growth due to water shortages, indirectly through activities related to physiological processes such as photosynthesis, nitrogen metabolism, absorption and translocation of salts directly through turgor pressure decreasing effect on cell enlargement and other processes are more involved in growth (Kramer, 1969). To be sure that the tree growth is controlled by the continuity of the moisture in the network rather than soil moisture. This is because the lack of internal water is controlled by the speed of the relative absorption through the roots and water loss through transpiration, which only indirectly controlled by soil water content. Throughout the afternoon transpiration usually exceeds water absorption and collected back during the night when the absorption is greater than the transpiration. Transpiration is widely regulated by the environment above and leaf structure and only absorption which is directly influenced by ground water. Water absorption is affected by water absorption of ground water is influenced by several factors such as soil pressure, soil moisture, soil temperature, soil solution concentrations, soil aeration (such as size and distribution of root system).
Nevertheless most of the root system efficiency, the spread and growth of trees is widely controlled by the supply of water. Wherever the trees grow its development is limited by the availability of water (too much or too little). Relative humidity is an area on the regional temperatures, forest land may fill field capacity only in the dormant season in a short time after a very wet period. Very high rainfall was arrested by the forest canopy or simply to fill the surface water layer. Although filled again, the upper soil layers that contain most of the good root system may be reduced close to permanent wilting point far seblum remaining dry profile, resulting in shortage of ground water during the growing season in a long time and can lose water plant growth in the number of large.
Reduction in plant growth due to water shortages, indirectly through activities related to physiological processes such as photosynthesis, nitrogen metabolism, absorption and translocation of salts directly through turgor pressure decreasing effect on cell enlargement and other processes are more involved in growth (Kramer, 1969). To be sure that the tree growth is controlled by the continuity of the moisture in the network rather than soil moisture. This is because the lack of internal water is controlled by the speed of the relative absorption through the roots and water loss through transpiration, which only indirectly controlled by soil water content. Throughout the afternoon transpiration usually exceeds water absorption and collected back during the night when the absorption is greater than the transpiration. Transpiration is widely regulated by the environment above and leaf structure and only absorption which is directly influenced by ground water. Water absorption is affected by water absorption of ground water is influenced by several factors such as soil pressure, soil moisture, soil temperature, soil solution concentrations, soil aeration (such as size and distribution of root system).
The absorption of water by plants is influenced by two groups of factors (Kramer.1969) These factors affect the potential difference of water from soil to plant roots and resistance to water movement through soil and in roots. The texture of soil and water conductivity control the movement of water into the root surface. Aeration, temperature and degree of change penggabusan roots resistance to the movement of water into the roots. Thrust is the potential difference between the past ground water with surface roots with Xylem.
Often different parts of the tree root system is in the stratification of land that have different levels of water potential, and they not only survive but also with only the roots in the soil above the permanent wilting point percentage. Minimum soil water potential during continuous absorption is limited by the minimum water potential can be developed in the roots. Ground water would be limiting to plants that have a high transpiration rate on soil water potential is higher than the plants that have a low transpiration rate.
Root elongation rate is strongly influenced by the level of soil moisture during the growing season. The growth of plant roots are rarely affected by water deficit during the spring season but summer sters water can cause a slowing and cessation of root tissue expansion. For example root elongation terjad pirlus radiate completely during the warm season in the dry period. The end of the season, if there shoot elongation (ahner, 1968)
Extensive root system developed on seedlings when grown on land which maintained close to field capacity, but only a thin roots that can grow on dry soils near the permanent wilting point before watering. Zahner (1968) that soil strength increased sharply when the soil is dry and produces physical resistance against penetration by the end of the root that may not be associated with deficiencies of water for absorption.
Extensive root system developed on seedlings when grown on land which maintained close to field capacity, but only a thin roots that can grow on dry soils near the permanent wilting point before watering. Zahner (1968) that soil strength increased sharply when the soil is dry and produces physical resistance against penetration by the end of the root that may not be associated with deficiencies of water for absorption.
Old tree roots found growing profusely on the soil profile that contains the available water supply. For example cleavage of small roots at the end of the crushed root of a large stone was found on the edge of the capillary water balance. On the other hand the excess water and poor soil aeration greatest effect on root morphology as a result of lack of water or soil because of the spread of an impermeable substrate. Land with gas exchange limited, both because of the high density content (bulk density) or excess water to help plant the top of the root system is sparse.