THE RELATION OF CAMBIAL ZONE MECHANICAL STRENGTH TO GROWTH AND IRRIGATION OF ALMOND [PRUNUS-DULCIS (MILL) WEBB] TREES

Commercial harvesting of almonds [Prunus dulcis (Mill.) Webb.] with me chanical shakers is economical, but may also cause severe damage to th e cambial zone, leading to the establishment of a deadly cankerous dis ease, ceratocystis. Irrigation is often cut off for some period of tim e before harvest, anticipating a reduction in the damage as a result o f an increased strength of adhesion within the cambial zone between th e bark and the wood. Mechanical failure can occur in the cambial zone proper and in the differentiating cells on either side of the cambial zone. In this study the shear force per shear area in the cambial zone (cambial strength)was measured to represent mechanical failure due to shaker damage. Cambial strength of branches with intact bark tissues could be reversibly influenced by the level of tissue hydration, with high cambial strength associated with increased tissue hydration, pres umably a result of an increase in the turgor of cambial zone cells. Me thods of measuring cambial strength were developed for branches and tr unks to avoid the effects of reversible turgor changes. Irrigation reg imes imposed before harvest had a substantial and progressive influenc e on tree stem water potential, stomatal conductance, and growth rate of almond trees. Statistically significant levels of within seasonal d ifferences in rate of growth, stomatal conductance and tree water defi cits found under different irrigation regimes did not show any correla tion with the within seasonal fluctuations in cambial strength. Cambia l strength always showed a similar pattern and a similar magnitude of seasonal increase from spring to summer (as previous authors have repo rted), superimposed over the within seasonal fluctuations, despite sig nificant differences in tree water stress. Therefore, this study sugge sts that irrigation cut off may be an ineffective practice for the pur pose of increasing cambial strength.