HortResearch Publication - How do Rootstocks Affect Canopy Development?

HortResearch Publication - How do Rootstocks Affect Canopy Development?

Annette Richardson, Pauline Mooney, Peter Anderson, Ted Dawson and Maureen Watson - HortResearch, Kerikeri Research Centre

Introduction

In this article we examine the relationship between the roots and the canopy and how this may be modified by different rootstocks. Although citrus has been used as a basis for this article, the underlying principles apply to many orchard trees.

There is a close interaction between the root system and the canopy of the plant. The canopy produces carbohydrate to sustain and develop the plant, while the roots provide nutrients and water. One of the most important aspects of plant growth is the constant ratio maintained between the root system and canopy of the plant. Therefore the size of the root system and its level of activity is always proportional to the mass of shoots and their activity.

Any scion/rootstock combination has a specific root:shoot ratio and artificial attempts to modify this, by either shoot or root pruning, results in the plant changing its growth pattern in order to re-establish the balance. This means that pruning, which is necessary to control tree size and productivity, works against the natural growth habit of the tree. Valuable reserves which could be used to produce high quality fruit are used up maintaining the root:shoot ratio.

Interactions between rootstocks and scions occur both through the exchange of resources like water, nutrients and carbohydrate and also via hormonal messengers. The physical union between a rootstock and scion, or its degree of compatibility, influences the exchange of these materials and to a certain extent affects the relationship between scion and rootstock.

The need for new rootstock varieties

Significant efforts have been made to develop rootstocks which beneficially modify the performance of fruiting scions. While a large number of citrus rootstocks are available overseas, the New Zealand citrus industry has traditionally used only Poncirus trifoliata. This rootstock has important advantages like resistance to Phytophthora, tolerance of cool soil temperatures and resistance to Citrus Tristeza Virus. However, problems with poor fruit quality and vigorous scions have highlighted the need to investigate a range of new generation rootstocks for our main scion cultivars (Harty et al, 1993).

Citranges are now amongst the most important citrus rootstocks in the world, and six different selections have been included in rootstock evaluation trials at Kerikeri for satsumas and lemons. Further rootstock and scion combinations are also being trialled but all trials are in their infancy, and results will only be available in the year 2000. In order to examine the effects of rootstocks more closely we have studied the physiology of mature Silverhill satsuma mandarin trees on either Troyer citrange or Poncirus trifoliata rootstocks. The preliminary results from this trial presented in this article demonstrate the range of effects rootstocks can have on canopy development.

Trifoliata vs Troyer citrange

Trifoliata (Poncirus trifoliata) is a deciduous citrus relative which is rated as a semi-dwarfing stock, however trees will eventually get as big as those on vigorous stocks. Therefore it really has low vigour rather than a truly dwarfing habit and this can be a problem when combined with a low vigour scion like Miyagawa satsuma mandarin. A particular disadvantage of this rootstock for satsuma mandarins is high juice acidity and poor fruit size. Troyer citrange, which was derived from a cross between Ruby orange and trifoliate orange, has a semi-evergreen habit and confers more vigour and larger fruit size to scion cultivars.

Tree vigour

Silverhill trees with Troyer rootstocks were more vigorous resulting in taller, more spreading trees with a greater canopy volume. These trees carried considerably more fruit without affecting mean fruit size (Table 1). However large spreading trees are less efficient if production is compared on the basis of canopy volume. This is because more of the tree volume is shaded, and therefore produces only a few fruit compared with the exposed exterior of the canopy.

Rootstock	Tree Height (m)	Tree Width (m)	Canopy Volume (m³)	Yield (kg/tree)	Mean Fruit Weight	Yield Efficiency (kg/m³)

Troyer	2.8	3.4	22.7	80.6	47	3.5
Trifoliata	2.3	3.0	10.8	45.8	49	4.2

Table 1: Tree size and production of Silverhill mandarins on two rootstocks

Trees on the less vigorous trifoliata rootstock had a lower canopy volume with less non-productive wood and therefore have a higher yield efficiency (Table 1). However trees on Troyer citrange rootstocks still produce more fruit per hectare. If trees were pruned to minimise the amount of shaded non-productive wood, yields and fruit quality could be lifted further. The higher yield efficiency and low vigour of trees on Trifoliata is an advantage in high-density planting systems.

Budbreak, fIowering and fruit maturity

During the dominant spring flush, trees on Troyer stocks began to produce new shoots earlier than those on trifoliata roots (Figure 2). Earlier bud break on trees with Troyer rootstocks was reflected in earlier flowering. However more new shoots developed on trees with trifoliata rootstocks. Trees on Troyer rootstocks also produced larger flowers than those on trifoliata stocks.

Figure 2: Bud break and flowering of Silverhill satsuma mandarins on two rootstocks

The appearance and maturity of fruit was also influenced by the rootstock (Figure 3). Fruit from trees on Troyer citrange coloured and reached minimum internal maturity standards earlier than those from trees on trifoliata.

Fruit from trees on Troyer citrange mature earlier than those on trifoliata

In 1993, fruit from trees on Troyer rootstocks reached the minimum local market Brix:acid ratio of 7.0 one week earlier and in 1994 fruit were three weeks earlier (Figure 4). This enhanced fruit maturity was a result of both higher brix and lower acid levels.

Differences in canopy and fruit development of trees on the two rootstocks demonstrates their different sensitivity to environmental triggers like soil and air temperature. There is also evidence that rootstocks can influence tree water relations.
Figure 3: Fruit from trees on Troyer citrange mature earlier than those on trifoliata

Leaf nutrient levels

Rootstocks have the ability to control both the supply and distribution of nutrients within the plant. As we have already demonstrated they also have a significant effect on the vigour and growth pattern of the canopy. All of these factors contribute to the balance of nutrients within the plant. Leaf analysis results obtained in this study show clear differences between the two rootstocks.

Leaves from trees on trifoliata rootstocks had higher magnesium and calcium levels, but lower phosphate and potassium levels than trees on Troyer rootstocks. It appears that calcium and potassium levels are inversely related (Figure 5a). Of the key trace elements, only boron was significantly affected by the choice of rootstock. Trees on trifoliata roots had consistently higher leaf boron levels than those with Troyer roots (Figure 5b).

These differences highlight one of the problems that can occur when interpreting leaf nutrient levels. Generally leaf nutrient levels from trees on both rootstocks fell within the optimal range. One exception was potassium levels which were optimal in trees with Troyer stocks but deficient in trees with trifoliata rootstocks. This demonstrates the need for care in interpreting leaf nutrient analyses as many factors including the choice of rootstock and scion can affect results.

Figure 4: Brix:acid ratio of fruit from trees on two rootstocks for 1993 and 1994

Photosynthesis and starch reserves

Overseas studies have documented the effect citrus and apple rootstocks may have on photosynthesis in the canopy. Generally leaf photosynthetic rates are related to the vigour of the tree. Therefore leaves from trees with either dwarfing or low vigour rootstocks tend to have lower photosynthetic rates than those from more vigorous trees.

In our study we have measured differences in the amount of starch and other carbohydrate reserves stored in plant organs. Preliminary results show that trees with trifoliata as a rootstock had higher starch reserves in the stem and branches but lower levels in the roots than-trees with Troyer rootstocks. These results must now be correlated to tree development and fruit production.

Seasonal trends in potassium, calcium and boron

Figure 5: Seasonal trends in (a) potassium and calcium and (b) boron on trees with two different rootstocks

Summary

Rootstocks provide growers with a useful tool to manipulate the vigour and performance of orchard trees. Effects on tree size, precocity, fruit production and maturity are achieved through complex interrelationships between the roots and canopy of the plant. Rootstocks directly affect the ability of plants to take up water and nutrients. They are also able to significantly alter the pattern of canopy development and functions such as photosynthesis.

In this study we have demonstrated that Troyer citrange significantly improved in tree vigour and yield as well as altering the development and nutrition of a Silverhill satsuma mandarin scion. This article suggests that Troyer citrange could prove an interesting stock for satsuma mandarins.

Reference

Harty, A.; Sutton, P.; Machin, T.; Jagiello, J. 1993. Citrus rootstocks. The Orchardist of New Zealand, September 1993: 43-46.

Source: The Orchardist, November 1994

Copyright © 1997 The Horticulture and Food Research Institute of New Zealand Ltd. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of The Horticulture and Food Research Institute of New Zealand Ltd is prohibited.