Effect of water and nitrogen on pear size and quality

By K. Shackel, Ph.D; D. Ramos; L. Schwankl, Ph.D; E. Mitcham, Ph.D; S. Weinbaum, Ph.D; R. Snyder; W. Biasi; and G. McGourty, University of California, Davis

The aboveground and belowground parts of a pear tree form a complex, interdependent production system. Roots depend on shoots for carbohydrates and other organic nutrients, and shoots depend on roots for water and mineral nutrients. Hence, any factor which has a direct impact on either of these systems in the short term, can indirectly cause changes in the plant as a whole, and in its productivity, over the long term.

Water is a particularly good example of a factor which is involved in both short- and long-term processes, because water plays a key role in almost all plant processes, from carbon fixation to fruit growth. Plants must pull water from the soil, and this pulling force (which is referred to as water potential) is best understood as a tension which exists in the xylem, and also in the walls of all the cells in the tree.

Just as more pressure is needed to increase the flow of water through irrigation pipes, more tension is needed to pull water through the plant as transpiration increases. Some amount of tension routinely occurs as part of daily and seasonal variation in evaporative demand and tree transpiration, even under wet soil conditions. As the soil becomes dryer however, the tree must exert an increasing level of pull to extract the water held by the soil particles.

Physiologists still do not know all of the consequences of the tension, or the level of water potential, within the tree, but one important consequence is that when the tension becomes too high, the pressure within the cells of the plant (called turgor pressure) is reduced.

Cell turgor is the primary force which keeps the soft parts of the plant erect, and the most obvious sign of a loss in cell turgor is wilting, such as of growing shoot tips, or flagging of leaves. A less obvious, but probably more important result of a loss in turgor is a reduced growth rate. Turgor is an important part of the cell expansion process in plant growth, and this is true for the growth of essentially all plant parts, including fruit. Hence, if turgor is low, we can expect growth to be reduced, and this is one of the primary reasons why overall tree growth, and, in some cases, fruit size and yield, are so sensitive to water stress.

A reduction in tree growth is in fact a beneficial physiological response of pear trees to water stress from a tree survival standpoint, because a small tree requires less water to survive than a large tree. Of course, reductions in growth, especially in the early years of orchard establishment, will slow down the development of the canopy and orchard yield. Mature orchards also need a certain amount of yearly growth to maintain active fruiting wood.

Since the roots are the point of water entry into the tree, in addition to dry soil, any factor which reduces the health of the root system can also cause symptoms of water stress. Trees with diseased root systems may show leaf flagging, reduced growth, and closed stomata, even though the soil may be wet, because roots are not functioning properly.

Nitrogen is the mineral nutrient required in the greatest quantity by plants, but excessive nitrogen availability is known to stimulate vegetative growth (shoots and leaves), increase shading, and reduce fruit quality in fruits such as apples. Since both high nitrogen levels and high water availability can stimulate vegetative growth, it is important to consider how the management of irrigation and nitrogen fertilizer will influence tree production and fruit quality in pears.

RESPONSES OF BARTLETT PEAR
TO DIFFERENCES IN IRRIGATION AND NITROGEN FERTILIZATION

Irrigation treatments approximating 100%, 85%, and 65% crop evapotranspiration (ET_c) and differential nitrogen fertilization treatments of 0 and 400 pounds nitrogen per acre were compared for three years (1992-1994) in a Bartlett pear orchard in Ukiah, California. It was anticipated that increasing irrigation and fertilization would lead to fruit that were larger in size, but poorer in postharvest holding quality. The results did demonstrate strong reductions in both tree and fruit growth as tree water stress increased (i.e., as tree water potential became more negative), but no relation to tree nitrogen fertilization or leaf nitrogen content was found for any production or fruit quality parameter.

Data from the 1993 season is representative for the level of stress achieved under the three irrigation regimes, and it shows (see Figure 1) that on the average, trees in the reduced ET treatments had lower water potential once irrigation treatments were imposed. There was however, substantial tree-to-tree variability in the levels of water stress experienced within each treatment, which was not due to differences in measured soil moisture (data not shown). Irrigation had only a small influence on fruit size when all of the trees from each irrigation treatment were averaged together, but when each tree was considered separately, a clear relation between increased fruit size and increased tree water potential was apparent (see Figure 2). Additional data collected in 1994 (not shown) indicated that fruit growth rate was reduced by water stress at all times during the growing season, and that the corresponding loss in fruit size could not be regained by overirrigation.

Fruit size distribution was also evaluated for selected trees in 1993 and showed that the average fruit size was a good indicator of the value of the crop on each tree (see Figure 3). For the fruit prices and tree spacings relevant to this study, a loss of about $720 per acre was associated with every 0.1 Megapascal (MPa) reduction in seasonal average tree water potential. Fruit size was not related to the total weight of the crop on the tree (data not shown), and hence these data demonstrate: 1) that as tree water stress increases, fruit size decreases, but 2) that irrigation may not be the only factor that determines the level of tree water stress.

If a tree exhibits low stress (i.e., good water status) even when irrigation is reduced, this may indicate that the tree's roots have access to a large and probably deep volume of soil. However, if a tree exhibits poor water status even when irrigated, this may indicate poor rooting or poor root function. These issues currently are being investigated.

In addition to reductions in fruit size, low tree water potential was strongly correlated with higher levels of fruit soluble solids (see Figure 4), and to a lesser extent yellower fruit color (see Figure 5) at harvest. A trend toward higher fruit firmness at harvest with low tree water potential was also statistically significant, but was not as strongly correlated as were the other fruit quality indices (data not shown). Despite these clear effects on fruit size, soluble solids, color, and firmness at harvest, no differences were found in any postharvest disorders of the fruit during storage or ripening (e.g., scald, internal breakdown), even after four months of storage (see Table 1). Hence, the effects of tree water stress appear to be limited to harvested fruit quality, with no particular impact on storage or ripening behavior.

T A B L E 1
Irrigation treatment means for selected storage fruit quality measures at harvest and after four months cold storage in 1993. None of the means are statistically different.

Irrigation treatment

Fruit quality measure 100% ET 80% ET 65% ET

Firmness at harvest 18.5 18.1 18.3

Firmness after 4 mo. 16.8 16.3 16.1

Harvest SS 12.0 12.2 13.0

Harvest TA 0.236 0.241 0.244

% Fruit with decay 4 2 5

% Fruit with IB <1 7 12

% Fruit with russet 42 35 46

Increasing applied nitrogen caused a statistically significant increase in leaf nitrogen levels in the second and third year of the study (see Table 2), but these levels were not related to any of the fruit quality factors measured (data not shown). In addition, it was anticipated that increased tree vegetative growth would be related to increased leaf nitrogen levels, but no relation between pruning weights and leaf nitrogen levels was found (see Figure 6). Tree water potential, however, was closely correlated to pruning weights (see Figure 7), indicating the importance of individual tree water status in determining overall tree growth and productivity.

T A B L E 2
Midsummer (July) leaf nitrogen content for three years under contrasting fertilizer programs.

Fertilizer treatment lbs. nitrogen/acre Leaf % dry weight N (year)

(1992) (1993) (1994)

400 2.31 2.02 2.13

0 2.26 1.90 1.98

Statistical Significance ns ** ***

In summary then, the level of water stress experienced by individual trees over the course of the growing season may be quite different, even when the trees are under the same irrigation conditions. Once individual tree water status is taken into account, clear effects are apparent on tree vegetative growth and on fruit size and quality, but not on any of the common fruit postharvest disorders. Pear trees have an apparently minimal response to high levels of applied nitrogen in terms of leaf nitrogen levels, and no apparent relation of leaf nitrogen levels to overall tree vegetative growth or fruit size or quality.

ACKNOWLEDGMENTS

The authors would like to thank the Steve Thomas company for their cooperation in this research project.

Departments of Pomology and Land, Air, and Water Resources, University of California, Davis, California

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Effect of water and nitrogen on pear size and quality

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