To understand why citrus crops produce flushes we need to look closely at the physiology of the crop. Leaves on citrus plants have limited ‘shelf’ life whereby the ability to photosynthesise reduces dramatically as the leaves age. By the time leaves reach 40-50 days old they have lost most of there ability to photosynthesise and if not replaced with new leaves the tree can not produce enough food to support itself. This is why citrus plants have leaf flushes, to allow them to maintain photosynthesis at adequate levels.
Yield of all crops is directly related to the level of photosynthesis, so maintaining good levels of photosynthesis to power growth of the crop is very important. This presents citrus growers with some challenges as by nature the crop has major fluxes in photosynthesis. Helping the crop through dips and stretching the period of useful life from leaves can pay major dividends in yields.
To make new leaves the plant has to produce new cells, and this cell division is powered by the growth hormone auxin. This means that leaf flush is accompanied by large spike in production of auxins, which are synthesised in foliage.
This spike in auxin levels during leaf flush has negative effects on root function, effectively causing root growth to cease for week during each flush. It also creates a large sink for calcium in the foliage making developing fruit vulnerable to deficiency. The main driver in this is Nitrogen.
Research shows that the best form of nitrogen for foliar citrus applications is amine nitrogen (urea), but getting urea into citrus leaves is a wasteful process with as much as half of all foliar applied urea volatilising before the plant can absorb it.
Nitrogen form dictates where citrus crops allocate growth.Nitrogen form has an impact on where crops grow, with nitrates encouraging vegetative growth and amine forms encouraging reproductive growth and root development. Farmers do not get the benefits of this as the majority of nitrogen is taken up as nitrate as it changes form in the environment.
Levity’s research on citrus crops show that targeted applications of Amine-N can reduce excess shoot growth on citrus and allow the crop to produce bigger and better fruit yields.
During flushes there is a spike in calcium requirement, this needs management to avoid fruit drop and quality problems.
Calcium absorption is linked to polar auxin transport. This means that during leaf flush a large sink is created for calcium in the foliage leading to competition for the calcium available to the crop. This makes it very hard for flowers and fruit to access calcium during leaf flush and the main driver in fruit and flower shedding in citrus crops.
Secondly it makes it very difficult to maintain good calcium levels in fruit, as fruit are naturally low in auxins they can’t absorb calcium well. This is amplified during flushes where roots switch off (limiting supply) and demand is increased in foliage leaving the fruit struggling to access calcium.
Conventional Foliar calcium sprays are ineffective at raising fruit quality as applying calcium to fruit has limited impact because they have insufficient auxin levels to absorb calcium well no matter how much is applied.