Main image: Pea crop | Image credit: Geoff Dixon
Peas are a very rewarding garden crop. Husbandry is very straightforward, producing nutritious yields and encouraging soil health by building nitrogen reserves for future crops.
Rotations usually sequence cabbages and other nitrogen-demanding crops after peas. This is a sustainable way to use the organic nitrogen reserves left by pea roots resulting from their mutually beneficial association with benign bacteria. These microbes capture atmospheric nitrogen, producing ammonia, nitrites and nitrates in a sequence of natural steps.
Peas originated in the Mediterranean. They were cultivated continuously by ancient civilisations and through medieval times, and are now the seventh most popular vegetable.
Illustration 1: Pea seeds | Image credit: Geoff Dixon
In bygone centuries, peas provided a protein source for the general population as cooked meals of pea soup and pease pudding helped keep famine at bay before the introduction of potatoes. In the 18th century, French gardeners working for the aristocracy produced fresh peas using raised and protected beds of fermenting animal manure. The composting processes produced heat and released carbon dioxide, stimulating rapid growth.
Generally, however, eating fresh peas only gained popularity in the 20th century as canned and then quick-frozen foods were invented, and large-scale technological development enabled mechanised and automated commercial precision cropping. In recent times, retail market demand has returned for unshelled podded peas – a manually picked crop known colloquially as ‘pulling peas’.
Seeds can be sown directly (illustration 1) or transplants (illustration 2) can be raised under protection, giving an early boost for growth and maturity. Peas are cool season crops. They grow best at 13-18°C and mature about 60 days after sowing.
Illustration 2: Pea seedlings | Image credit: Geoff Dixon
Some cultivars such as Meteor can be grown over winter, preferably protected with cloches for very early cropping. The spring sown The Sutton cultivar group (CV) gives rapid but modest returns, and main crop CVs, such as Hurst Green Shaft, deliver the heaviest returns (illustration 2). This cultivar forms several long, well-filled pods at the fruiting nodes.
Sugar peas or mange tout – where the entire immature pod is eaten – is a popular fresh crop, while quick-growing pea shoots that mature in 20 days from sowing are excellent additions for salads or as garnishes for warm cuisine.
Human health benefits significantly by including peas in the diet. As well as being an excellent protein source, they produce a range of vitamins and nutrient elements. Their coumestrol content aids the control of blood sugar levels, helping combat diabetes, heart diseases and arthritis.
So, it’s certainly worth finding a spot for this versatile vegetable in your garden.
Written by Professor Geoff Dixon, author of Garden practices and their science.
Controlling when and how vigorously plants flower is a major discovery in horticultural science. Its use has spawned vast industries worldwide supplying flowers and potted plants out-of-season. The control mechanism was uncovered by two American physiologists in the 1920s. Temperate plants inhabit zones where seasonal daylength varies between extending light periods in spring and decreasing ones in autumn.
Those environmental changes result in plants which flower in long-days and those which flower in short-days. ‘Photoperiodism’ was coined as the term describing these events. Extensive subsequent research demonstrated that it is the period of darkness which is crucially important. Short-day plants flower when darkness exceeds a crucial minimum, usually about 12 hours which is typical of autumn. Long-day plants flower when the dark period is shorter than the crucial minimum.
Irises are long day flowers. Image: Geoffery R Dixon
A third group of plants usually coming from tropical zones are day-neutral; flowering is unaffected by day-length. Long-day plants include clover, hollyhock, iris, lettuce, spinach and radish. Gardeners will be familiar with the way lettuce and radish “bolt” in early summer. Short-day plants include: chrysanthemum, goldenrod, poinsettia, soybean and many annual weed species. Day-neutral types include peas, runner and green beans, sweet corn (maize) and sunflower.
Immense research efforts identified a plant pigment, phytochrome as the trigger molecule. This exists in two states, active and inactive and they are converted by receiving red or far-red wavelengths of light.
Sunflowers are day neutral flowers. Image: Geoffery R Dixon
In short-day plants, for example, the active form suppresses flowering but decays into the inactive form with increasing periods of darkness. But a brief flash of light restores the active form and stops flowering. That knowledge underpins businesses supplying cut-flowered chrysanthemums and potted-plants and supplies of poinsettias for Christmas markets. Identifying precise demands of individual cultivars of these crops means that growers can schedule production volumes gearing very precisely for peak markets.
Providing the appropriate photoperiods requires very substantial capital investment. Consequently, there has been a century-long quest for the ‘Holy Grail of Flowering’, a molecule which when sprayed onto crops initiates the flowering process.
Chrysanthemums are short day flowers. Image: Geoffery R Dixon
In 2006 the hormone, florigen, was finally identified and characterised. Biochemists and molecular biologists are now working furiously looking for pathways by which it can be used effectively and provide more efficient flower production in a wider range of species.
The IHNV virus has spread worldwide and is fatal to salmon and rainbow trout – costing millions in sales of lost farmed fish. The current vaccination approach requires needle injection of fish, one by one. Now, however, Seattle-based Lumen Bioscience has come up with a new technology to make recombinant vaccines in a type of blue-green algae called Spirulina that costs pennies to produce and can be fed to fish in their feed.
To be effective, oral vaccines have not only to survive the gut environment intact but must also target the appropriate gut-associated immune cells. The approach developed by Lumen overcomes many of the problems with complex and expensive encapsulation strategies attempted in the past, according to CEO Brian Finrow.
‘[It] focuses on a new oral-vaccine platform [using] engineered Spirulina to express high amounts of target antigen in a form that is both provocative to the immune system – ie generates a desirable immune response that protects against future infection – and can be ingested orally without purification, in an organism that has been used as a safe food source for both humans and fish for decades.’
To produce the new oral vaccine, the Lumen researchers first developed a strain of Spirulina that manufactures recombinant proteins in its cell walls that the salmon immune system recognises as IHNV viruses. They then rapidly grew the strain in a large-scale indoor production system – requiring only light, water, salt and trace nutrients – and harvested and dried all the raw Spirulina biomass. This dried powder can then be fed to the fish.
It’s well known that the oceans are becoming more acidic as they absorb increasing amounts of CO2 from the atmosphere. Now, German researchers say they have found the first evidence that this is happening in freshwaters, too, with potentially widespread effects on ecosystems.
‘Many current investigations describe tremendous effects of rising CO2 levels on marine ecosystems,’ says Linda Weiss at Ruhr-University Bochum: acidic oceans can have major impacts on marine food webs, nutrient cycles, overall productivity and biodiversity. ‘However, freshwater ecosystems have been largely overlooked,’ she adds.
Waters with high acidity have reduced biodiversity.
Weiss and colleagues looked at four freshwater reservoirs in Germany. Their analysis of data over 35 years – from 1981 to 2015 – confirmed a continuous increase in CO2, measured as the partial pressure or pCO2, and an associated decrease in pH of about 0.3, suggesting that freshwaters may acidify at a faster rate than the oceans.
In lab studies, the team also investigated the effects of higher acidity on two species of freshwater crustaceans called Daphnia, or water fleas. Daphnia found in lakes, ponds and reservoirs are an important primary food source for many larger animals.
Daphnia are an essential part of the freshwater food chain. Image: Faculty of Natural Sciences at Norwegian University of Science and Technology/Flickr
When Daphnia sense that predators are around, they respond by producing ‘helmets’ and spikes that make them harder to eat. Weiss found that high levels of CO2 reduce Daphnia’s ability to detect predators. ‘This reduces the expression of morphological defences, rendering them more vulnerable,’ she says.
The team suggest that CO2 alters chemical communication between species, which could have knock-on effects throughout the whole ecosystem. Many fish learn to use chemical cues from injured species to detect predatory threats and move away from danger, for example.
Ocean acidification - the evil twin of climate change | Triona McGrath | TEDxFulbrightDublin. Video: TEDx Talks
Cory Suski, an ecologist at the University of Illinois at Urbana-Champaign, US, says he is not aware of many other data sets showing trends in CO2 abundance in freshwater over an extended time. Also, he notes: ‘A lot of the work to date in this area has revolved around behavioural or physiological responses to elevated CO2, so a morphological change is novel.’
But he points out that it is difficult to predict how this change could impact aquatic ecosystems, or whether this may be a global phenomenon, simply because of the complex nature of CO2 in freshwater. The amount of CO2 in freshwater is driven by a number of factors including geology, land use, water chemistry, precipitation patterns and aquatic respiration.