Dry mass distribution

Percentage values of dry mass partitioned into roots, stems, leaves, and fruit are presented in Table 7.3. Dry mass partitioning into root was similar among treatments. Higher dry mass was partitioned into stems and leaves in DI and PRD plants than in FI plants. However, dry mass partitioning into fruit was highest for FI plants (Table 7.3).

Table 7_3 Effect of irrigation treatments (ITs) on dry mass partitioning of tomato

plants. Means with same letters within columns are not significantly different using the LSD test at P � 0.05.

Dry mass distribution per plant (%)

ITs FI DI PRD Root l .4a 1 . 8a 1 .5a

7.3.5 Fruit quality assessment

Stems 1 6.6b 1 9.0a 1 804a Leaves 1 3 .2b 1 4.6ab 1 5 04a Fruit 68.8a 64. 6b 64. 7b

The fruit water content (FWC) was lower and skin colour in terms of hue angle (HAO) was redder in DI and PRO fruit than in FI fruit, but the total soluble concentration (TSSC) was higher in DI and PRO fruit than in FI fruit (Table 7 04).

Chapter Seven Comparing PRD and DJ for their effects on water relations and yield of tomatoes

Table 7.4 Effect of irrigation treatments (ITs) on fruit water content (FWC), total soluble solids concentration (TSSC), and fruit colour in terms of hue angle (HA 0). Different letters within columns indicate differences by the LSD test at P � 0.05.

Its FI 01 PRD 7.4 Discussion FWC TSSC (%) 95. 1 a 94.6b 94.5b (%) 4. 1 8b 4.66a 4.54a HAO 48.29a 46.46ab 46.03b

Here I focus on the effects of the irrigation treatments on plant water relations and gas exchange properties, fresh and dry mass of different parts of the plant, dry mass partitioning in the plant, and I provide an assessment of PRO as a management tool .

Values oPf'leaf, A, E, and gs remained unaffected by the treatments for the five

occasions I measured them. This experiment was carried out during the winter months in the glasshouse and the radiation levels were generally low on those days. In a

tomato leaf, photosynthesis is saturated at a PPF of approximately 400 /lmol m-2 S-I

(Venema et aI., 1 999). During a sunnier day the PPF values averaged at

456 /lmol m-2 S-I ( 1 60 DAS, Table 7. 1 ) resulting in a decreasing trend in \}'Iear for FI, 01, and PRD in this order. However, radiation was very variable on this day, hence the large value of standard deviation (SO) in Table 7. 1 , with no measurable effect of treatments on A, E, and gs. Behboudian et al. ( 1 994) showed that in Asian pear (Pyrus serotina Rehd.), low radiation overrides the effect of water deficit on photosynthesis, and their deficit-irrigated trees had the same low levels of photosynthesis as did their fully-watered control on a cloudy day.

Growth is the plant parameter most sensitive to water deficit (Hsiao, 1 973) and the decrease in fresh mass of the DI and PRO fruit, compared to the F I fruit (Table 7.2), indicates that a degree of water deficit did develop in the former treatments. Tomato is sensitive to water deficit during flowering and fruit set (Pulupol et aI., 1 996). Our treatments were applied before the first truss appeared and water deficit could have developed during the reproductive growth. The tomato fruit contains at least 92%

Chapter Seven Comparing PRD and DJ for their effects on water relations and yield of tomatoes

water most of which is transported to the fruit through the phloem and is reduced during a mild water deficit, although photoassimilates continue to be transported to the fruit (Ho, 1 999). This might have been a reason that the fruit fresh mass in the DI and PRD was lower than in the FI fruit, while the fruit dry mass was similar among treatments (Table 7 .2).

In the DI and PRD plants, a lesser proportion of dry mass was partitioned into the fruit than in the FI plants (Table 7.3). The DI and PRD fruit had significantly lower water content than the C fruit (Table 7.4). The DI and P RD fruit also had a higher

concentration of total soluble solids than the FI fruit (Table 7.4). Although we did not measure fruit water potential, lower water content and higher soluble solid

concentration in DI and PRD fruit than in FI fruit is indicative of lower water potential in the former treatments. In this case translocation of photoassimilates would be expected to have been higher into DI and PRD fruit than FI fruit, as demonstrated for the roots of Phaseo/us vulgaris by Lang and Thorpe ( 1 986). We therefore expect more partitioning of photo assimi lates into the fruit of the DI and PRD treatment. However, this could have been counteracted by higher respiration rate in the Dl and PRD fruit compared to the FI fruit as shown for ' Virosa' cultivar by Pulupol et aI. ( 1 996). Cantore et al. (2000), for a split-root experiment, reported a dry mass partitioning pattern for Capsicum annuum similar to our PRD treatment (Table 7.3).

The higher yield, in terms of total fresh mass of fruit, in FI than in DI and PRD treatments indicates the importance of water quantity applied, while the similarity of total fresh mass of fruit between DI and PRD shows that it does not matter what

volume of soil is wetted with each irrigation (Table 7.2). Tan et aI. ( 1 98 1 ) reported that for tomato irrigating part of the rhizosphere (ca 50%) could be enough to meet plant's water requirements rather than irrigating the entire root system.

The FWC was lower in DI and PRD than in FI (Table 7 .4), and this is preferred by the processing industry because less energy would be needed to evaporate water from the fruit. The TSSC in fruit was higher in DI and PRD than in PI (Table 7.4), which is also important for processing industry (Mitchell et aI., 1 99 1 a). The TSSC and FWC were highly correlated (r -0. 80 and P � 0.000 1 ) and therefore the increased TSSC in the DJ

Chapter Seven Comparing PRD and DJ for their effects on water relations and yield of tomatoes

and PRD fruit might have been due to a lower fruit water content. Higher conversion of starch to sugars under water deficit (Kramer, 1 983, p. 364) could be another reason. Although differences in red fruit colour were not visible among treatments at harvest and fruit were picked based on visual colour uniformity, PRD fruit had the lowest hue angle (Table 7.4) and were therefore redder. A higher lycopene accumulation under water deficit has been speculated as a reason by Pulupol et a1. ( 1 996). The PRD fruit were ready for picking one week before the other treatments and this has positive implications in terms of marketing. This advancement in fruit maturity observed in PRD treatment deserves further study.

7.5 Conclusions

I _{did this experiment to assess DI and PRD as water-saving irrigation techniques. In}

both DI and PRD treatments total dry mass of fruit was maintained and irrigation water was saved by 50%, compared to FI plants. IUETFMF _increased 60% (Table 7.2). For processing tomatoes, a relative lower water content in fruit and higher total soluble solids concentration are important fruit quality attributes for marketing (Ho, 1 999). These attributes were improved by DI and PRD treatments. In both DI and PRD treatments, the dry mass concentration in the fruit was similar to FI fruit. Therefore

PRD and DI could both be considered as feasible irrigation strategies for the production of processing tomatoes. However, field research at a more stressful time of the year is needed to corroborate these results.

Chapter Eight Maintenance of yield and fruit quality in processing tomatoes by PRO

Chapter 8

Maintenance of yield and fruit quality in processing tomatoes by partial rootzone drying

Abstract

Drip irrigation is more efficient in water application and precision placement than other irrigation systems, but this has not been tested under partial rootzone drying (PRD) irrigation. Therefore, PRD and DI were assessed using drip irrigation for their effects on water relations, photosynthesis, yield, plant growth, and fruit quality. The

treatments were: daily full irrigation (Fl) on both sides of the root system (RS)

considered as the control, daily irrigation on one side of the RS with half the volume of water given to the control (PRD1), full irrigation every other day of both sides of the RS (01), and irrigation only on one side of the RS at a time every other day with half the volume of water given to the control (PRD2) . Photosynthetic rate (A), stomatal conductance (gs), leaf water potential ('l'leaf), total plant fresh mass, and total dry mass of fruit (TDMF) were lower in DI and PRD2 plants than in Fl and PRD1 plants.

Irrigation use efficiency was improved by 1 . 8 times in PR01 relative to FI. Dry matter partitioning into stems and leaves was increased, but total dry mass of fruit was reduced in DI and PRD2 plants relative to FI and PRD 1 plants. Fruit quality in terms of fruit water content (FWC), total soluble solids concentration (TSSC), and fruit background skin colour (HAO) were the same for PRD 1 and FI. As levels of water deficit increased, so did the percentage of blossom-end rot, while leaf calcium concentration decreased. PR01 can be seen as a feasible water-saving irrigation protocol for processing tomatoes and might be important for production in areas where water is scarce.

Chapter Eight M aintenance of yield and fruit quality in processing tomatoes by PRD

8. 1 I ntroduction

In Chapter 7 \f'leaf, _{A, E, and gs were studied in relation to DI and PRD. However, the}

results were not conclusive because most of the measurements had to be done in cloudy days and low radiation over-rode the possible effects on water deficit. TDMF was reduced by 1 1 % in PRD plants relative to FI plants. Fruit quality; in terms of FWC, TSSC, and HA 0; was improved in PRD and DI in comparison to FI plants. However, low radiation levels and low evaporative demand could have overestimated the benefits of PRD and DJ. Also, the plants were irrigated mimicking furrow irrigation. This experiment was conducted during the spring and early summer using drip irrigation which is a more effective water-saving irrigation strategy.

Tomato production under DI has saved considerable amounts of water and reduced production cost (May and Gonzales, 1 999). However, plant growth and fresh fruit yield have been reduced (Mitchell et ai., 1 99 1 a; Pulupol et ai., 1 996), but TDMF has not (Mitchell et ai . , 1 99 I b). Under Dl plants inevitably experience water deficit because transpiration rate exceeds water supplied long-term, and then, the plant water status is reduced to such an extent that the plant's normal growth and development are adversely affected (Kramer, 1 983, p. 343). This might not happen with PRD where part of the rhizosphere is kept moist.

Recent PRD experiments on grapes showed that stomatal conductance and transpiration rate decreased without changes in plant water status. This led to higher water use efficiency, but PRO's effect on yield was cultivar dependent (Loveys et ai., 2000). In fact, in a short-term split-root tomato experiment, Tan et al. ( 1 98 1 ) concluded that the entire root system of tomatoes does not need to be irrigated to maintain high rates of transpiration and photosynthesis as well as plant growth and yield comparable to fully irrigated plants. If this is true, DJ 's adverse effects on processing tomato might not happen under PRO. This could be possible if the well-watered part of the root

accomplishes equilibrium between water absorption and transpiration rates so that total plant water potential is maintained and therefore allowing normal metabolic activity between source and sink. Green and Clothier ( 1 999) have observed that when part of the apple root system is irrigated, water uptake is doubled by the wetted part.

Chapter Eight Maintenance of yield and fruit quality in processing tomatoes by PRO

The fruit quality of tomatoes, in terms of low FWC, HAo, and high TTSC, have been enhanced under DJ (Mitchell et aI., 1 99 1 a; Mitchell et aI., 1 99 1 b; Pulupol et aI., 1 996; May and Gonzales, 1 999). A reduced FWC and related increase in TSSC in processing tomatoes is desirable when paste production is the aim. The PRD plants might achieve this because less water would be supplied, but photosynthetic rate is not expected to reduce nor does the assimilate transport to the fruit. However, FWC could be reduced since less water will be applied. The objective of this study, therefore, was to compare the effect of PRD and DI on \f'leaf, A, _{gs, yield, yield components, dry mass partitioning,}

and fruit quality of ' Petopride' processing tomatoes. PRD might be more relevant for processing tomatoes, which are normally grown in the field, rather than in glasshouse tomatoes. I also carried out this experiment by using drip-irrigation to improve water application efficiency and water precision supply in good agreement with water conservation strategies.

8.2 M aterials and methods

8.2. 1 Experimental conditions and treatments

The general experimental conditions and plant material were the same as detailed in Chapter 7 (Section 7.2 . 1 ) . The experiment was conducted from July to December 200 1 . Seeds were sown on 3 1 July 200 1 and 40-day old individual seedlings were transplanted and spaced as detailed in Chapter 7 (Section 7.2. 1 ). In this experiment, twelve wooden boxes were used as described in Chapter 7 (Section 7.2. 1 ) each housing four compartments. The compartments were improved in two aspects. Firstly, to avoid lateral water displacement, a small piece of wood (600 mm x 50 mm x 26 mm) was placed centrally on the base of each compartment. Secondly, the compartments were lined with black polyethylene with a thickness of 1 25 /-lm and perforated laterally at the bottom to allow drainage.

Chapter Eight Maintenance of yield and fruit quality in processing tomatoes by PRD

on both sides of the root system (RS) considered as the control, daily irrigation on one side of the RS with half the volume of water given to the control (PRD I ), full irrigation every other day of both sides of the RS (DI), and irrigation only on one side of the RS at a time every other day with half the volume of water given to the control (PRD2). The experiment was conducted in a completely randomised design with the four treatments replicated three times. There were four plants per treatment for each replication.