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Sunitha H. Gurusinghe and Kenneth A. Shackel

Commercial harvesting of almonds [Prunus dulcis (Mill.) Webb.] with mechanical shakers is economical, but may also cause severe damage to the cambial zone, leading to the establishment of a deadly cankerous disease, ceratocystis. Irrigation is often cut off for some period of time before harvest, anticipating a reduction in the damage as a result of an increased strength of adhesion within the cambial zone between the 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, presumably a result of an increase in the turgor of cambial zone cells. Methods of measuring cambial strength were developed for branches and trunks to avoid the effects of reversible turgor changes. Irrigation regimes imposed before harvest had a substantial and progressive influence on tree stem water potential, stomatal conductance, and growth rate of almond trees. Statistically significant levels of within seasonal differences in rate of growth, stomatal conductance and tree water deficits found under different irrigation regimes did not show any correlation with the within seasonal fluctuations in cambial strength. Cambial strength always showed a similar pattern and a similar magnitude of seasonal increase from spring to summer (as previous authors have reported), superimposed over the within seasonal fluctuations, despite significant differences in tree water stress. Therefore, this study suggests that irrigation cut off may be an ineffective practice for the purpose of increasing cambial strength.

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Sunitha H. Gurusinghe and Kenneth A. Shackel

Mechanized shaker harvesting of large acreages of almond [Prunus dulcis (Mill.) Webb.] trees leads to economical use of labor, general cost reduction, and speed of harvest. However, shaking can separate the bark (all tissues external to the vascular cambium) from the wood (all tissues internal to the vascular cambium) and exposes the vascular cambial zone to infection by the fungus Ceratocystis fimbriata Ell. & Halst. leading to mallet wound canker and loss of productivity. Treatment of ethephon caused a significant increase in the cambial strength of both branches and trunks, presumably by ethylene-induced anatomical and biochemical changes in the cambial zone. An increase in the ratio of total tangential area of the groups of ray initials to fusiform initials and the thickness of ray initial cell walls was observed as a response to ethephon treatment. Spraying an ethephon solution (500 μl-liter-1) to the runoff point on almond trunks caused significant increases in the cambial strength; ethephon, therefore, may be useful in reducing bark injury during shaker harvesting.

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Sunitha Gurusinghe, Ann L.T. Powell, and Kent J. Bradford

While seed priming (hydration in water or osmotic solutions followed by drying) enhances seed germination performance, the longevity of primed seeds in storage often is reduced. Postpriming treatments including a reduction in seed water content followed by incubation at 37 or 40 °C for 2 to 4 h can substantially restore potential longevity in tomato (Lycopersicon esculentum Mill.) seeds. These conditions might induce heat-shock proteins (hsp) that could be involved in the extension of seed longevity. The abundance of BiP (78 kD Binding Protein), hsp70 and class I small hsp in primed seeds subjected to postpriming treatments was examined to assess this possibility. BiP mRNA and protein amounts increased during postpriming heat treatments that extended longevity of tomato seeds. Treatment of primed seeds with the calcium ionophore calcimycin (A21387) enhanced BiP protein accumulation in the absence of heat treatment and also extended potential seed longevity. Changes in the abundance of hsp70 and class I small hsps were not consistently associated with potential seed longevity. Thus, enhanced BiP expression may contribute to the improved longevity of primed seeds following postpriming treatments.