Here is my question: why can't we use cross pollination between a heat-set cultivar as the male donor and a heat-intolerant cultivar as the pollen recipient to improve fruit set of the heat-intolerant cultivar during heat stress conditions? I recognize that this is a waste of time for commercial growers. I also recognize that you wouldn't be able to save seed for heirloom varieties,
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No "expert " here , but I think You've got a good approach.

Some people also use brushes to improve pollination/ pollen release. I am sure you will here from them as how to do it.

Some people even collect pollens and spray them, like light dusting to improve fruit setting.

Yet another Point: There are varieties that don't even need pollens to set fruits. Each pollen is supposed to fertilizes one ovule that will produces a seed (plants baby). So it is possible to grow tomatoes with very few or no seeds. This means you don't need too many pollen release to get fruits.
I know one such tomato called : LEGEND , developed by University of Oregon. I am growing few of them.

As I mentioned I am no expert here but I did a little brain storming for you.

seysonn

"I know that there are other factors that contribute to poor fruit set in heat stress, but perhaps this cross pollination technique will have a non-negligible effect .... Maybe some cultivars will be more amenable to this process than others? Anybody?"

Definitely will have a non-negligible effect for the pollen, but as for fruit set, the issue will likely be 10 times more viability than zero may still be near zero if the rest of the plumbing isn't working. If it is the ovule that will not be receptive, then pollen won't help.

Recognizing the differences among varieties is probably the way to go since each has its own particular physiology of what's going wrong. Varieties like Cherokee Purple hold together pretty well as do most Cherry Tomatoes. A combination approach will work best, but I tried a lot of things last summer including all kinds of manual + cross pollination and got about 10-15% set. I tried to give the pollen from the best setting (10-15% LOL) to the one that didn't at all.

My biggest problem was not necessarily with the pollen, but rather the blossoms dropped off before they matured, and there is nothing I could do about that except play with shade, shade cloth and water to lower my plants' temperatures, day and night. The flower stem (pedicel) just begins yellowing before I get the chance and it is all over before it even starts. But your conditions and set failure will be very environmentally specific. Where high heat and humidity and night temp may be my problem, you may have high day and drying temperatures at times, etc., so focusing only on pollen viability would not be enough here, I can say that.

PC

seysonn,

I am aware of some of the techniques you presented. Some good food for thought. Spraying is an interesting idea, although I fear my laziness may prevent such measures.

Bob

PC,

Yes! I was hoping someone had experience with this!

I understand that there's not much I can do to fix the female part of the reproductive cycle for a fixed cultivar other than doing my best to mitigate heat stress.

My hope was/is that there are some delicious cultivars (Cherokee Purple being a great example that you provided) that are primarily limited by pollen release and not ovule malfunction or flower abortion in heat stress. The natural hypothesis would be that using pollen donors from heat-tolerant plants could greatly improve fruit set in heat in these cultivars.

Your experience suggests that, at least for the heat stress in your garden, pollen release is only one factor. In the research literature that I've read, heat stress is usually defined as around 90 degree F daytime and 80+ degree F in the nighttime. Maybe that is a sweet spot where only the pollen release is affected. Once you get into extremes that you and I see (90+ days and hot nights for almost the entire summer, 95+F days in my case), everything just falls apart.

I think I will do some experimenting with this and I really appreciate you sharing your experience.

I will continue to try and find research articles on this topic at google scholar, but it's not my area so I lack the proper perspective to look in the right places and at the right data. Any help from others on good literature would be greatly appreciated.

Bob

One of the major problems in areas where it's not just high sustained heat that causes blossom drop and lack of fruit set is pollen clumping due to high sustained humidity and where both are present it's not good at all.

I think many have underestimated thepollen clumping problem.

Years ago I was referred to a tomato breeder from Petoseed who was in FL working on pollen clumping, I assume Jay Scott, the head of the U of FL tomato breeding program and his group were doing that as well.

To date I've seen no indication that the pollen clumping problem is anywheres near solved.

Above it was mentioned that there are varieties, many of them bred by Dr. Baggett at OSU, that do not need pollination of the blossoms to form fruit. They are called parthenocarpic varieties and were bred by him for the PNW where Spring temps are too cool to allow for good normal pollination Just the opposite of the high heat high humidity problems. The subsequent fruits, then, are not normal.

I've participated at many message sites since 1982 and I have to say that I have not been impressed with the feedback from those who have grown varieties said to be heat tolerant.

Carolyn

Carolyn,

I have not assessed the pollen clumping problem in my own garden (not sure I have the capability to do so).

My only empirical evidence wrt pollen quality is that there is a visual difference in the pollen released from the heat-set variety that I have planted this year (Summer Set) and some of the heirlooms that I have planted this year (Brandywine, Black Cherry, and Daetwyler). I also have Celebrity planted this year, but I did not check its pollen quality.

During the heat of the summer I extracted the stamen from many flowers (all in the same development stage) on the tomato plants in my garden. The only plants where I was able to observe "pollen dust" when I vibrated the stamen was the Summer Set plant. I'm not sure if this is due to clumping or pollen release. Or maybe my sample size was not large enough to draw any conclusions. I will say that our summers in Central TX are moderately dry (40-80% RH in the summer), so we're not humidity-limited like Florida.

As usual, you are correct that heat-set varieties leave much to be desired. They don't taste great (relatively) and their yield is still meager during heat stress (their is still a month long period in the peak of the summer where productivity is at a stand still), but since most tomatoes give me nothing for several months, to me they are worth putting in my garden. If I could replicate that productivity for tastier varieties I would be very happy.

Thanks for the tip on the parthenocarpic varieties.

Bob

I find it far easier to modify the growing conditions as much as possible to the needs of the plant rather than trying to modify the plant to the less-than-ideal growing conditions. Shade cloth, increasing air circulation with increased spacing, planting crosss dominant wind direction, can all help to varying degrees.

Dave

Dave,

Thank you for your input.

A few things:

(1) Those strategies help, and I certainly try to make the environment as cool as I can with good airflow, but the effect is limited for me. I have gotten feedback from a few Central TX gardening gurus...inevitably they always conclude that I'm just wasting my time trying to find a remedy ("tomatoes just don't like the heat...get your crop in as early as possible"). Good airflow and partial shade may be enough for your environment, but it is a different story for many of us. Most people around here just give up on new fruit being set in late June/July and hope that later in the summer it is mild enough for a fall harvest.

(2) I do have meager success with heat set varieties in the summer and I'd like to absorb those capabilities into better tasting varieties. I recognize that if scientists who breed tomatoes for a living haven't breed better tomatoes for the heat, there's not much hope for me trying to do this myself (although I do admit that most scientists are likely focused on commercial gardening).

(3) Even if I can't overcome tomato genetics, I'd like to understand the limiting factors better. I like data and the scientific method, but publicly-available scientific data on heat stress in tomatoes is hard for me to find...I've had some luck scouring the web for publications from large universities like NC State and UFL who pursue this kind of research. Some of it is only available via subscriptions to research journals.

(4) Cross pollinating manually is very simple. I don't see this as a huge burden for a backyard gardener.

Bob