Soft Dry Mealy Fruit

Some members on our Facebook group have asked what's going on when their fruit trees sometimes produce unacceptably soft dry mealy fruit with little flavour? And is there anything we can do to address the problem? The short answer is that the phenomenon is far from being well understood & it remains an ongoing concern. For commercial growers it can mean significant proportions of fruit become un-marketable leading to loss of income, & for home growers it can compromise the reward & enjoyment of their efforts. In many instances unless characteristics of the fruit are well known, the problem is only revealed when consumed & may not be detected & removed before sale, leading to negative consumer reactions & future reluctance to either grow or buy the fruit. Only some aspects of the causes and possible remedies have emerged, but in most cases they remain elusive because there's considerable variation in responses between species and cultivars, the effects of variation in environmental conditions across seasons, fertilisation practices, pruning, thinning & fruit load through to maturity, harvest timing, post-harvest handling & storage conditions.

There are two parallel lines running through this story - fresh fruit eaten by home growers a short time after picking, & fruit that have been stored (usually at low temperature) for varying times before consumption. The latter is almost always the case with retail fruit as following harvest they have to be cleaned, sorted & packed, sometimes dipped & waxed, transported to central wholesale markets, distributed to retailers, & then put out for display at room temperature for varying periods of time before sale. It's easier for enthusiasts who aren't picking 10 tons of fruit to finesse the ideal maturity/ripening time on tree when eaten without storage, but we can still experience problems. Commercial growers are forced to pick fruit earlier than this to allow for the above delays before finally being sold to consumers, & this often has negative impacts, eg maintenance of stone fruit quality is very dependent on temperature being reduced as quickly as possible after harvest to avoid accelerated deterioration, but they may still never be as juicy as fruit eaten fresh. Fleshy fruit quality can be broadly described in 4 dimensions - colour & appearance, flavour (taste & aroma), texture, & nutrition (hidden but increasingly valued by consumers); safety is an additional one that should be kept in mind. We might judge texture initially by feel, eg slight softness of an avocado or mango, but the main sensations we experience are when eating them, with many subjective & personal descriptors such as hard, soft, crisp, crunchy, chewy, dry, juicy, melting, pasty, mealy, floury, smooth, gummy, fibrous, gritty etc. Several of these attributes can be measured objectively with instruments to guide commercial operations, but the ultimate judge is how we each perceive them. Apples have been the most studied on this topic but some comments will be made on other species. There can be considerable variation between species & cultivars in what is considered a desirable texture eg apples are usually preferred crisp, most people like their peaches to be soft & juicy, & other fruit like astringent persimmons & medlars are normally only eaten when they have lost structure & softened to jam consistency. Tree nuts that are naturally dry at the time of consumption represent a separate story outside the present discussion of fleshy fruit. It should be recognised at the outset that biology is not as precise as the physical sciences, & there are always variations & exceptions given the complexity of biology with many thousands of genes, plant age, vigour & health, rootstock effects in grafted plants, the environment & management factors all interacting.

Softening in many fruit tree species proceeds in a reverse sigmoidal pattern with three distinct stages, all being different developmental components progressing through to final senescence & decay. In the first slow stage after fruit set leading to early maturity, there is minimal change in firmness. If fruit are to be stored then they're best picked just before or slightly after entering the second stage, as this will enable maximal storage life while maintaining fruit quality, & they can later be allowed to ripen fully at room temperature for consumption before becoming unacceptably soft. The second faster stage is where the biggest decrease in firmness occurs, & once entered, further softening becomes increasingly irreversible & subsequent storage life more limited. A typical example is avocado; if stored at low temperature while still firm then storage may be possible for many weeks without greatly affecting quality, but once the fruit has entered stage two (slight softening) then further softening can't be arrested & storage life is reduced to a few days. How much softening occurs in the second stage after picking depends on whether fruit are climacteric or non-climacteric as the latter may progress very little. If you're going to eat the fruit fresh then the aim for maximum juiciness and flavour is to harvest when fully matured, & ripening in the second stage has progressed to the desired level. The maximum degree of softening that's possible in stage two varies between fruit species & cultivars when fully ripe, eg apples don't soften as much as pears that can become quite soft & pasty. The third terminal stage, whether reached on tree or after storage, is the one that progresses through to full senescence, beyond being enjoyable or even edible.

Regarding mechanisms, the best established & most widespread differences between juicy & soft mealy fruit with ripening involve cell & membrane structure. Primary cell walls in fruit are made up of microfibrils of cellulose imbedded in a matrix of hemicelluloses & pectins. Secondary cell walls are stronger, stiffer & less elastic usually because of thicker layers & variable amounts of lignins. The middle lamella is the 'glue' that holds cells together & consists primarily of pectins. All three walls normally break down in a controlled manner as part of the process of softening to make fruit edible. In ripe juicy non-dry fruit, the middle lamella holding the edible parenchymal cells together remains essentially intact & cells are tightly packed. Mastication breaks individual cells & they release their aqueous contents of sugars, acids etc to produce a moist, enjoyable taste & pleasant mouth feel. In dry fruit the middle lamella is swollen, more porous & partly broken down with more spaces between cells. The weaker intercellular binding & open structure reduces pulp firmness, & chewing then results in clumps of intact cells that are still enclosed in fibrous materials breaking apart without rupture or release of their contents. What is then sensed is a more fibrous consistency with a dry mouth feel & lack of juiciness. Intrinsic membrane differences between species have a strong genetic component. Our challenge is to optimise conditions to help ensure the middle lamella structure remains as intact & functional as possible for a given species, both during progress towards maturity & ripening before & after harvest, without accelerated progress through to various levels of senescence.

Calcium is a nutrient that's particularly important for lamella structure & strength through the formation of gel-like Ca pectates. If there is an inadequate supply, then fruit can become deformed, softer & mealy. Ca is taken up by the roots & moves to the leaves & stems in the xylem transpiration stream, with much smaller amounts being delivered to fruit as they have fewer stomata & a poorer xylem vasculature. It's pretty much immobile in the phloem. So if xylem-delivered Ca is inadequate due to drought, salinity, relative humidity, shoot & root temperatures, light, and mineral imbalance conditions, fruit quality suffers because any that's stored elsewhere in the plant largely can't be used. The corollary is that plants need a fairly continuous soil supply & root uptake of Ca to be delivered throughout growing periods. A relevant factor is that different rootstocks in grafted plants can affect the efficiency of soil Ca uptake & subsequent translocation. If you have a plant that often shows fruit deformity or soft mealy problems, you could consider using Ca chloride foliar sprays as an interim measure & explore what's causing the root uptake & delivery pathway to be inadequate. Unfortunately it's not a simple story as fruit can sometimes have normal Ca concentrations but still exhibit deformities and or softness. It's thought that the nature & location of storage sites is important, but generally good Ca supply will be positive. Imbalances of soil nutrients outside the optimal ranges can have negative impacts, eg low levels of Ca can increase the plant hormone ethylene which increases the rate of progress through fruit maturity, ripening & senescence; excess soil cationic nutrients (NH4+, K+, Mg2+, Zn2+ etc) can compete with & reduce Ca2+ uptake by roots; & excess N stimulates shoot growth which then competes with fruit for available Ca in the plant. A separate property influencing management is the effect of cuticle thickness on foliar absorption, eg multiple sprays can be effective with apples but less so with papaya which have a much thicker cuticle. Sprays may also be helpful to preserve quality in post-harvest fruit. Ca deficiencies on calcareous coastal Perth sands would not be expected, but nutrients always have to be in solution for uptake, & there is less in this form when pH is more alkaline. Roots acquire soluble Ca from the soil mainly by bulk flow & root growth, & dry periods or hot summers with inadequate water supply will reduce the former & subsequently the latter.

Boron is also a key nutrient essential for good fruit development. It's highly water soluble so many soils are deficient, particularly sandy soils with low water & nutrient retention. Addressing this problem requires care as the effective concentration window in plants between deficiency & toxicity is narrow. B has similar effects to Ca on cell walls & the middle lamella where the integrity of these are weakened with deficiency. Some studies have suggested membrane effects of B are mainly mediated by enhancing Ca binding to pectates. Unlike other plant nutrients taken up in charged form, it's the un-dissociated acid form of B that is absorbed by roots, when it is then delivered mainly to leaves & stems via transpiration flow; there is much less delivery by the xylem & phloem to fruit tissues. Accordingly like Ca, plants need a continuous supply of B throughout growing seasons. Deficiency can damage vegetative & reproductive tissues, including fruiting buds, flowers & fruit which need higher levels than vegetative tissues. B-deficient fruit may develop deformities & accelerate through maturity & ripening stages faster than normal, resulting in small low quality non-juicy pre- or post-harvest fruit. Commercial growers typically use leaf analysis to assess whether B (or other nutrient) levels are adequate, with recognition that the ideal range varies across species & cultivars. These tests are usually not feasible for home growers, so for diagnosis we have to rely on more subjective symptoms such as deformities & small dry fruit; unless extreme, B-induced defects in fruit are normally seen before vegetative. Soil administration of B (eg borax or solubor) is a good long term remedy; sprays may be useful in the shorter term but outcome depends on timing throughout the growth & reproductive cycle.

Another cause of textural changes in fruit apart from membrane defects involves cell turgor. Cells with higher turgor are firmer & this is directly influenced by the accumulation & metabolism of photosynthates during fruit maturation & ripening. The normal energy storage forms for fruits are macromolecular carbohydrates (polysaccharides) which have relatively low osmotic potential. With maturity & fruit ripening, many of these are converted to small soluble simple sugars, mainly mono- and disaccharides, with greater osmotic potential. This leads to cellular uptake of water, resulting in considerable expansion of cells. Indeed, many fruits such as figs can almost double in size in the last 1-2 weeks of ripening, solely by cell expansion rather than increased cell number. Larger cells mean there's more cell contents per tissue volume than fibrous cell wall material, & as a result when chewed there is a sweeter & pleasant mouth feel. But if soil moisture level is inadequate during summer, drought or high transpiration rates, then water delivery will be inadequate for this swelling to occur & fruit will be smaller, shrivelled & drier. Fruit can also lose moisture after harvest when stored in low humidity environments, leading to decreased turgor & firmness (looser cells). This is less of a problem while fruit are still on the tree as any loss of tissue moisture can be offset by sap inflow. Proper build-up of cuticle thickness & integrity during maturation influences moisture loss in storage, & if impaired then mealiness becomes more likely. Packing fruit cells full of photosynthates & maintaining good water supply comes down to good management. Whatever the challenges are on your property (soil pH, fertility & water retention, temperature, humidity, sunlight, root competition, tree structure ... .) you should endeavour to rectify these. Remember that in multifactorial situations, the biggest impact will be seen by addressing the most limiting factors, eg with plant nutrition, if all macro & micro nutrients satisfy minimal requirements, then the biggest changes you can achieve will be through N control. Again as always in biology, there are exceptions to the norm, eg starches in bananas are converted to sugars during ripening, but in some species & cultivars this conversion may not be extensive or complete even when fully ripe; this results in a dry mealy mouth feel & underlies why plantains normally need cooking.

So if you have a problem with soft dry mealy fruit, does it happen every year or only sometimes? If the former it could be that cultural conditions are not ideal for your species or cultivar as it may require continued special attention to make it work in our climate & soil conditions? We encourage members to try growing species from outside our sub-tropical climate, & it is amazing how often we can succeed provided we try & replicate native conditions as well as possible. If this is not done or achieved, plants may always suffer from eg too little moisture, infertile alkaline sands, intense sunlight during our summers or low temperatures in winter. If you get unacceptable fruit only in some seasons then for example was it abnormally hot & dry leading to fruit not filling out properly, or did you not harvest at the right time, or did a storm wreak havoc with branches & roots? Would it have been possible to anticipate the onset or consequences of these factors & pre-empt with appropriate counter measures - extra watering, fertiliser, pruning, spraying, earlier/later harvest etc?

Barry Madsen

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