Lack of Magnesium Causes Scientists Headaches
Citrus grown in Gisborne and to a lesser extent Hawkes Bay is sometimes less sweet than it needs to be for export, and this can cause substantial losses to growers.
Magnesium deficiency may be the problem, but checking whether that is the case is a mission because of many complicating soil and plant factors.
This partially completed project illustrates the complexity of biological systems and the dilemmas and frustrations scientists face as they try to unravel biological problems.
Low brix (sugar) levels in citrus fruit can cause headaches for growers in Gisborne. Two seasons ago one packhouse exported 100 tonnes of mandarins to Japan, but last season it exported only 10 tonnes because they simply couldn't get the fruit sweet enough in time. The 90% drop was of huge concern to them.
Growers don't really know what the problem is, but one anomaly in the trees is that their leaves are low in magnesium (Mg). The recommended minimum level from the overseas literature is 0.26%, but many Gisborne trees have only 0.14 0.18%. Could this be the problem?
The growers approached Crop & Food Research for help. In theory it should be simple to check out boost Mg levels in the trees and see if this fixes the problem.
In practice it isnt so straight forward, according to Crop & Food soil scientist Dr Stephen Trolove.
You can put up to a tonne of Mg fertiliser per hectare so that it looks like snow on the ground and the trees don't seem to take it up. Leaf tests show Mg is quite a bit below the minimum recommended levels, and no matter what they do growers can't seem to increase those levels, he says.
Foliar sprays dont seem to work either, probably because citrus leaves are waxy and will not absorb nutrients. Getting Mg into the trees is very difficult.
The dilemma for Stephen and the growers is that they dont know whether the fact that the leaves are below the recommended Mg level is actually causing the economic loss. Is it reducing yield or causing poorer quality fruit? And if it is, why is it happening and what can be done about it?
One of the effects of Mg deficiency is that it can stop starch and sugars being exported from leaves to the fruit. Low sugar in fruit has been a problem in some seasons in Gisborne, but at present there is no scientific proof that Mg deficiency is the cause.
We have done fertiliser trials, and put up to a ton of Mg on as fertiliser but we could only get a 0.02% increase in leaf levels, which is pathetic. We need about a 0.1% increase to get the levels up to what all the scientific literature recommends, says Stephen.
It seems that Gisborne soils are high in calcium and potassium, and they compete with Mg for uptake sites on the roots. We tried several forms of Mg but they didnt make any difference.
Stephen then looked at the measures used on other trees crops for forcing plants to take up nutrients by injecting a solution into the trunk.
We used a thing that looks like a grease gun but is actually a sort of pump that forces it in at high-pressure, and it is the kind of treatment they use in avocados for iron deficiency, and for controlling fungal diseases in grapes, he says.
We thought we could use it to compare a high Mg treatment with a low Mg treatment and with no Mg control, but even with that technique the Mg didn't seem to get into the leaves. It has got us stumped.
In the foliar spray trials magnesium sulphate was ineffective but magnesium nitrate had some effect, so just recently Stephen has tried injecting trees to compare magnesium sulphate with magnesium nitrate and with a couple of chelates to see what might work.
We are trying to get the Mg in there somehow in a form that the tree can mobilise and send around the plant.
Will it work? Time will tell.
Stephen is also undertaking small scale glasshouse trials, growing citrus in different nutrient culture solutions calcium, potassium and magnesium to get an indication of exactly which nutrient is the one that is causing the Mg uptake problem.
Gisborne soils are high in both potassium and calcium, and most research says that potassium is more competitive than calcium at reducing uptake, but this should give us a better understanding of exactly what is the problem, he says.
I am also looking at using solution culture to produce trees with different degrees of Mg deficiency, and measuring things like leaf starch and photosynthetic ability. I want to check the validity of the .26% Mg in leaves that the literature says is minimal.
Stephen says there doesnt seem to be any physiological basis for that figure, just a measurement taken on a healthy tree overseas that may not have been on the same rootstock as is used in NZ. By producing trees with a range of leaf Mg concentrations he hopes to be able to see at what level there is measurable stress on the plant that could affect fruit quality. It may be that low Mg levels are ok and do not cause stress.
Climate could also be a factor lower soil temperatures in some seasons may account for the difference in brix values.
The ideal experiment would be to compare high Mg trees in Gisborne with low Mg trees in Gisborne and see whether the high Mg trees produce fruit with a higher brix. Much to his frustration, Stephenss efforts to produce a high Mg tree have so far not been successful.
We seem to be understanding the problem better but not coming up with any final answers, he says.
Magnesium deficiency may be the problem, but checking whether that is the case is a mission because of many complicating soil and plant factors.
This partially completed project illustrates the complexity of biological systems and the dilemmas and frustrations scientists face as they try to unravel biological problems.
Low brix (sugar) levels in citrus fruit can cause headaches for growers in Gisborne. Two seasons ago one packhouse exported 100 tonnes of mandarins to Japan, but last season it exported only 10 tonnes because they simply couldn't get the fruit sweet enough in time. The 90% drop was of huge concern to them.
Growers don't really know what the problem is, but one anomaly in the trees is that their leaves are low in magnesium (Mg). The recommended minimum level from the overseas literature is 0.26%, but many Gisborne trees have only 0.14 0.18%. Could this be the problem?
The growers approached Crop & Food Research for help. In theory it should be simple to check out boost Mg levels in the trees and see if this fixes the problem.
In practice it isnt so straight forward, according to Crop & Food soil scientist Dr Stephen Trolove.
You can put up to a tonne of Mg fertiliser per hectare so that it looks like snow on the ground and the trees don't seem to take it up. Leaf tests show Mg is quite a bit below the minimum recommended levels, and no matter what they do growers can't seem to increase those levels, he says.
Foliar sprays dont seem to work either, probably because citrus leaves are waxy and will not absorb nutrients. Getting Mg into the trees is very difficult.
The dilemma for Stephen and the growers is that they dont know whether the fact that the leaves are below the recommended Mg level is actually causing the economic loss. Is it reducing yield or causing poorer quality fruit? And if it is, why is it happening and what can be done about it?
One of the effects of Mg deficiency is that it can stop starch and sugars being exported from leaves to the fruit. Low sugar in fruit has been a problem in some seasons in Gisborne, but at present there is no scientific proof that Mg deficiency is the cause.
We have done fertiliser trials, and put up to a ton of Mg on as fertiliser but we could only get a 0.02% increase in leaf levels, which is pathetic. We need about a 0.1% increase to get the levels up to what all the scientific literature recommends, says Stephen.
It seems that Gisborne soils are high in calcium and potassium, and they compete with Mg for uptake sites on the roots. We tried several forms of Mg but they didnt make any difference.
Stephen then looked at the measures used on other trees crops for forcing plants to take up nutrients by injecting a solution into the trunk.
We used a thing that looks like a grease gun but is actually a sort of pump that forces it in at high-pressure, and it is the kind of treatment they use in avocados for iron deficiency, and for controlling fungal diseases in grapes, he says.
We thought we could use it to compare a high Mg treatment with a low Mg treatment and with no Mg control, but even with that technique the Mg didn't seem to get into the leaves. It has got us stumped.
In the foliar spray trials magnesium sulphate was ineffective but magnesium nitrate had some effect, so just recently Stephen has tried injecting trees to compare magnesium sulphate with magnesium nitrate and with a couple of chelates to see what might work.
We are trying to get the Mg in there somehow in a form that the tree can mobilise and send around the plant.
Will it work? Time will tell.
Stephen is also undertaking small scale glasshouse trials, growing citrus in different nutrient culture solutions calcium, potassium and magnesium to get an indication of exactly which nutrient is the one that is causing the Mg uptake problem.
Gisborne soils are high in both potassium and calcium, and most research says that potassium is more competitive than calcium at reducing uptake, but this should give us a better understanding of exactly what is the problem, he says.
I am also looking at using solution culture to produce trees with different degrees of Mg deficiency, and measuring things like leaf starch and photosynthetic ability. I want to check the validity of the .26% Mg in leaves that the literature says is minimal.
Stephen says there doesnt seem to be any physiological basis for that figure, just a measurement taken on a healthy tree overseas that may not have been on the same rootstock as is used in NZ. By producing trees with a range of leaf Mg concentrations he hopes to be able to see at what level there is measurable stress on the plant that could affect fruit quality. It may be that low Mg levels are ok and do not cause stress.
Climate could also be a factor lower soil temperatures in some seasons may account for the difference in brix values.
The ideal experiment would be to compare high Mg trees in Gisborne with low Mg trees in Gisborne and see whether the high Mg trees produce fruit with a higher brix. Much to his frustration, Stephenss efforts to produce a high Mg tree have so far not been successful.
We seem to be understanding the problem better but not coming up with any final answers, he says.