Responses of woody plants to flooding and salinity
Flooding affects soils by altering soil structure, depleting O2, accumulating CO2, inducing anaerobic decomposition of organic matter, and reducing iron and manganese. Flooding of soil with nonsaline or saline water adversely affects the distribution of many woody plants because it inhibits seed germination as well as vegetative and reproductive growth, alters plant anatomy, and induces plant mortality. In nonhalophytes, waterlogging suppresses leaf formation and expansion of leaves and internodes, causes premature leaf abscission and senescence, induces shoot dieback, and generally decreases cambial growth. However, flooding sometimes increases stem thickness because growth of bark tissues is increased more than production of xylem cells. In some plants, soil inundation induces formation of abnormal wood and increases the proportion of parenchymatous tissue in the xylem and phloem. Soil inundation inhibits root formation and branching, and growth of existing roots and mycorrhizae. Flooding also leads to decay of the root system. Root growth typically is reduced more than shoot growth. When the flood water drains away, plants may be less drought tolerant because of their low root/shoot ratios. Waterlogging of soil also inhibits initiation of flower buds, anthesis, fruit set, and fruit growth of nonhalophytes. Fruit quality is reduced by smaller fruit size, altered chemical composition, and appearance of fruits. Some fruits may crack following flooding of soil. Soil inundation induces multiple physiological dysfunctions in plants. Photosynthesis and transport of carbohydrates are inhibited. Absorption of macronutrients is decreased in flooded plants because of root mortality, loss of mycorrhizae, and suppression of root metabolism. Soil inundation alters hormonal balances in plants, usually by increasing the proportion of ethylene. Flood tolerance varies greatly among plant species, genotypes and rootstocks, and is influenced by plant age, time and duration of flooding, condition of the floodwater, and site characteristics. Flood-tolerant plants survive waterlogging by complex interactions of morphological, anatomical, and physiological adaptations. Important adaptations include production of hypertrophied lenticels, aerenchyma tissue, and adventitious roots. Salinity induces injury, inhibits seed germination and vegetative and reproductive growth, alters plant morphology and anatomy, and often kills nonhalophytes. Combined flooding and salinity decreases growth and survival of plants more than either stress alone. In angiosperms, salt injury includes leaf scorching or mottling, leaf shedding, and twig dieback. In gymnosperms, injury begins with necrosis of needle tips, spreads to the bases of the needles, and may be followed by needle shedding and shoot dieback. Injury also may include collapse of mesophyll cells, fragmentation of cuticles, and disintegration of chloroplasts and nuclei. Injury to cell membranes increases solute leakage. Salinity inhibits seed germination and adversely influences flowering, pollination, fruit development, yield and fruit quality, as well as seed production. Salinity inhibits vegetative growth of nonhalophytes, with shoot growth typically reduced more than root growth. Plant anatomy is often altered by salinity. Leaves become thicker and more succulent. The greater leaf thickness may reflect more layers of mesophyll cells, larger cells, or both. Salinity also may change the anatomy of xylem cells. In some normally diffuse-porous species, the xylem may become ringporous. Salinity stimulates suberization of the root hypodermis and endodermis. In nonhalophytes, salt-induced inhibition of plant growth is accompanied by metabolic dysfunctions, including decreased photosynthetic rates, and changes in protein and nucleic acid metabolism and enzymatic activity. In halophytes, physiological processes may be stimulated or not altered by salt concentrations that are inhibitory in nonhalophytes. The precise mechanisms by which salinity inhibits growth are complex and controversial. An attractive model incorporates a two-phased response of plants. Growth is first reduced by a water stress effect (a decrease in soil water potential) followed by a specific effect (namely salt injury in old leaves which die when their vacuoles cannot sequester any more salt). The loss of these leaves decreases the availability of carbohydrates or growth hormones to meristematic regions, thereby suppressing growth. Salt tolerance varies widely among species and genotypes. Plants adapt to salinity by tolerating or avoiding salt. In some plants salt tolerance is achieved by osmotic adjustment. This may involve absorption of ions from the soil followed by sequestering of ions in vacuoles, or it may result from synthesis of compatible solutes in the cytoplasm. Salt avoidance mechanisms include passive salt exclusion, active salt extrusion, and dilution of salt in the plant. Keywords: flood tolerance, morphological adaptations, physiological adaptations, salt tolerance.