The interactions of soil- and stem-water potentials with crop level, fruit size and stomatal conductance of field-grown 'Black Amber' Japanese plum

The interactions of crop level with soil and stem water potentials and water relations of field-grown Japanese plum were studied during two successive years. Four irrigation levels were applied in stage III of fruit growth, and their effects on soil- and stem water potentials and fruit size were examined. The relationships between stomatal conductance, leaf water potential and stem water potential were constructed from diurnal measurements of the three parameters. Yield of large fruit increased with increasing stern water potential, but did not level off towards the highest stem water potential values, indicating that additional increase in yield of large fruit is expected with further increase in stem water potential. Average fruit weight and relative yield >55 mm were better correlated with midday stem water potential than with soil matric potential due to the higher variability of soil matric potential compared with midday stem water potential readings. Yield of large fruit was more responsive to midday stem water potential in 2002 than in 2001 probably because of lower potential fruit size or a limitation of assimilates availability due to the higher temperatures in 2001 than in 2002. Midday stern water potential of the low irrigation treatments decreased with increasing crop level, indicating that stomatal conductance increased with crop level, probably due to limitation of assimilates. Stomatal conductance was highly correlated with stem water potential; the correlation was higher than that of stomatal conductance with leaf water potential in both years, irrespective of whether the leaves were covered by a plastic bag prior to, or immediately after, their detachment.