ISHS Acta Horticulturae 175: Symposium on Physiology of Productivity of Subtropical and Tropical Tree Fruits
TREE WATER RELATIONS |
| Author: | P.E. Kriedemann |
| Abstract:
Evergreen perennial trees such as citrus and avocado retain many general characteristics of ancestral shade plants from which they evolved viz. luxuriant foliage with high stomatal density and quantum efficiency, but limited photosynthetic capacity and low hydraulic conductivity. Moreover, horticultural plantings of these species make extensive use of semiarid regions which are subject to strong insolation, so that orchard trees are frequently subjected to environmental stresses where radiant load plus evaporative demand result in physiological strain. Short term responses include turgor loss and partial stomatal closure accompanied by reduction in photosynthesis and photoinhibitory damage. Longer term, and sometimes irreversible consequences of such cumulative strain then become manifest as impaired leaf expansion, lower productivity and disturbed patterns of growth and development. Alleviation of such stresses commonly depends upon supplementary watering where amount and timing become crucial for orchard viability in the long term. Leaf criteria which may contribute towards such irrigation schedules are clearly relevant for advanced management, and form a basis for this present paper. (Data in abstract refer exclusively to citrus.). Leaf gas exchange virtually ceases at values of leaf water potential ( Photosynthetic capacity seems inherently low with CO2 and light saturated laboratory rates c. 20 μmol CO2 m-2 s-1. Under humid orchard conditions (e.g. Florida) maximum values reach 14 to 17 (same units) depending upon leaf age, temperature, water vapour pressure deficit and rootstock. Notwithstanding an apparently low photosynthetic capacity, quantum efficiency of light energy conversion into biochemically useful forms remains high (shade plant characteristic) but such foliage is also more vulnerable to photoinhibitory damage in strong sunlight. Leaves so affected change their chlorophyll a fluorescence properties (esp. variable fluorescence Fv); such change can then serve as an index of | |
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l) below -2.5 MPa, but stomatal closure can be instigated at or below -1.0 MPa.
Conductance varies widely within that range according to tissue solute status, leaf turgor, incident radiation, leaf-air vapour pressure gradients, crop load and recent microclimate conditions.
Stomatal behaviour in citrus is characterized by spatial heterogeneity, but one stable feature pertains; maximum conductance (263 m mole m-2 s-1) is achieved under warm conditions (30°C) attenuated radiation (10–20 W m-2) and shallow vapour pressure gradient from leaf to air (7 mbar). Continuity of moisture supply from root-zone to transpiring leaf is crucial for maintenance of such fast gas exchange and 
