15.3.4 Regulated deficits and fruit yield

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Contrary to most other agricultural outcomes, carefully managed water stress can increase fruit tree productivity. Irrigation deficits at critical stages in tree phenology can increase yields of peaches (Mitchell and Chalmers 1982) and pears (Mitchell et al. 1989). Irrigation scheduled in this way is called regulated deficit irrigation (RDI).

RDI may appear counter-intuitive because water deficits reduce transpiration and photosynthesis, thereby reducing total plant productivity, but in fruit production the harvested commodity (edible fruit) is only a modest fraction of biomass increase each season. RDI will reduce total biomass by significantly reducing shoot growth and tree size, but fruit yield will not necessarily decline in proportion.

RDI relies on tree phenology. In most fruit crops, vegetative growth predominates early in the season followed by a period of rapid fruit growth when shoot growth is suppressed or terminated. If water deficits are timed to coin-cide with rapid shoot growth, then vegetative vigour is contained but final fruit size is unaffected.

High-density orchards are designed to produce compact and highly productive trees (Figure 12.26). If soil moisture availability is not limiting then trees will become excessively vegetative and grow far too large. Winter pruning is used to limit overall size of the trees and to define the number of fruit and their position on the tree. RDI reduces canopy growth, thereby limiting overall tree size.

Poor light penetration throughout the canopy of fruit trees results from excessive shoot growth. This leads to poor flower bud initiation in lower and more accessible parts of the trees. Flower buds develop in the tops of trees where the fruit is difficult to reach and this becomes increasingly worse each year. This occurs in both close plantings and in widely spaced traditional orchards. Trees become out of balance, producing too much annual shoot growth and very few fruit.

Summer pruning of orchards is used to improve light conditions in the tree canopy (Figure 12.26). Repeated prunings of closely planted vigorous trees are necessary but become uneconomic over time. RDI is then used to complement summer pruning and increase fruitfulness. RDI also reduces the total irrigation requirement of orchards. Transpiration from peaches was reduced by 50% during RDI as a result of a reduction in both stomatal conductance and leaf area (Boland et al. 1993). Water use continued to be 30% less than that of control trees because of reduced leaf area. In other RDI experiments total irrigation for the season was reduced by 2 ML ha–1 (Mitchell et al. 1989).

Regardless of motivation, fruit growers need to be confident that final fruit size will not decrease. In experiments conducted at Tatura on both peaches and pears, final fruit size was either similar to (Mitchell et al. 1989) and in some situations larger than a fully irrigated control (Mitchell et al. 1984). How then can fruit size be increased when water deficits normally reduce plant growth?

Physiological effects of RDI

Osmotic adjustment (Morgan 1980a) may be responsible for the lack of a decline of fruit growth observed under RDI. If in the deficit tree, photoassimilate continues to translocate during the day and night despite low xylem water potential (y), then fruit osmotic pressure will be increased thereby attracting water and allowing fruit growth to continue.

Investigations by Jerie et al. (1989) support this osmotic adjustment hypothesis. Measuring pear fruit growth con-tinuously with transducers and a datalogger, they recorded similar diurnal fruit growth between RDI and full irrigation treatments. In both treatments fruit shrank during the day and then expanded rapidly in the late afternoon, followed by a slower steady rate of growth during the night. Fruit osmotic pressure was 2.4 MPa in trees on RDI, but only 1.7 MPa in trees on full irrigation. Such an increase in osmotic pressure could maintain sufficient turgor in RDI fruit for growth to be similar to those on full irrigation.


Figure 15.18 Diurnal growth of pear fruits over 4 d after changing from regulated deficit irrigation (RDI) to full irrigation (ex-RDI) compared with continuous full irrigation. Diurnal fluctuation on full irrigation agter RDI due to accumulation of osmotically active materials in fruit on trees previously subjected to RDI. (Based on Jerie et al. 1989)

Osmotic adjustment to RDI also explains enhanced growth in pear when RDI is discontinued (Figure 15.18). Harvest size in both peach and pear was larger due to accelerated fruit growth when RDI was discontinued during their rapid growth phase.

Linked with faster growth of fruits, shoot growth on pear trees diminishes during RDI in accordance with a decline in dawn leaf water potentials. Lower water potential is responsible for reducing shoot growth, stomatal conductance and photosynthesis (Boland et al. 1993). Since fruit growth is not compromised during RDI, the sink strength of fruit must outweigh that of shoot tips thereby allowing photoassimilate to accumulate in fruit faster than for a fully irrigated tree.

The use of RDI to increase fruit growth and yield is a highly specific and commercially valuable example of only one of the many responses that plants exhibit when exposed to drought. There are many other processes that enable plants to escape, avoid or tolerate drought. These are relevant to agriculture in environments where water availability is limiting productivity and are considered next.