Sub4Spring: finally, it’s spring!
Posted prepared by: Dryland Pastures Research Team – C. Teixeira; R. Lucas, S. Olykan, A. Mills and Prof. Derrick Moot
Spring is the most important period for most pastoral farms in New Zealand. It brings lambing, calving, milking, and rapid changes in plant growth rates.
The increase in air and soil temperatures and day length, during a period when soil moisture is generally non-limiting, accelerates plant growth and development. Planning for a successful spring period with our annual clover based pastures began back in March (autumn) when decisions were made to allow us to achieve sub clover dominant spring pastures for priority stock (see previous March post).
The vegetative phase (emergence, leaf production, branching) of the sub clover occurred through autumn/winter (Figure 1). Now the plants are rapidly producing leaf area and thriving in spring sunshine.
|Figure 1 Main vegetative stages of sub clover from cotyledon emergence (left) to rosette stage and leaf production (middle), and branching as runners appear (right, finger indicates a runner). Photos: D. Hollander (2015) and G. Cun (2016).|
Sub clover produces most of its herbage yield in early to mid-spring (Figures 2 and 3) and potential yield is affected by flowering time of the cultivar(1).
|Figure 2 Hoggets and ewes “enjoying” a nutritious clover rich paddock in winter. Lincoln University, Field Research Centre experimental area, 28 July 2017. Photo: C. Teixeira.|
|Figure 3 Ewes and lambs graze the sub clover experimental area at Poukawa Research Station. On-Farm Research, Hawkes Bay, 03 August 2017. Photo: N. Smith.|
How do the subs know when it is time to flower?
In temperate climates like New Zealand plants use seasonal cues to match their flowering with the environment. This is a result of evolution and adaptation of these annual plants to ensure reproduction occurs in the most favourable conditions. The two main seasonal signals plants use are temperature and daylength(2).
Temperature is the main driver for the majority of plant processes: from germination to leaf production and flowering(3, 4, 5, 6). Sub clover will not flower before it has grown for a certain number of days within a specific environment. The number of days from sowing, or seedling germination, to flowering is used to classify cultivars as early, mid or late flowering (see the sub clover guide for details). However, the chronological time (days) is not constant because days can be warmer or colder depending on the season, year and location. Therefore, plants development is faster or slower depending on the temperature experienced. For this reason, it is best to calculate heat units or thermal time to describe plant development phases (e.g. germination, time to first flower). Plants accumulate thermal time (temperature) during growth and this parameter is expressed as the degree Celsius days (°Cd) or growing degree days (GDD).
A sub clover plant must accumulate a minimum number of heat units before flowering. This requirement ensures that the plants have grown sufficient leaves and roots, for example, to support reproduction. Some sub clover cultivars also need to experience low temperatures before flowering in a process known as vernalisation(3). Other crops including grapevines, apples, kiwifruit and some cereals also require vernalisation. A period of chilling is also a signal to the plants that “hey, spring is coming!”
The growing point (meristem) of a plant contains cells that undergo continual division and expansion. After a specific amount of thermal time has accumulated the cells in the meristem start to differentiate and form specific plant tissues: leaves only or leaves plus flowers (Figure 4). The images below show close ups of shoot apical meristems which are still vegetative (only leaves are produced) and at the reproductive phase (leaves and floral buds are produced).
|Figure 4 Shoot apical meristem of sub clover cultivar ‘Monti’ at vegetative (left) and reproductive (right) phase. The stipules protect the meristem and have been removed to make structures visible. Scale bar = 2 mm. Photo: C. Teixeira, 2017.|
Sub clover is considered a long day plant. This means, after the thermal time requirements are satisfied, it flowers when exposed to light periods longer than a critical length (measured in hours). It is this blend of temperature (heat and cold) and daylength requirements which differentiate the cultivars in terms of maturity time(7).
The graph below shows the effect of the sowing date on two contrasting sub clover cultivars currently being studied at Lincoln: ‘Monti’ and ‘Antas’ (Figure 5). Our observations at this experimental site show that ‘Monti’ flowered before ‘Antas’ when sown in summer and autumn. But there was little difference in flowering time when sowing occurred in winter or early spring. When the seeds were sown in February, ‘Monti’ required about 170 days to flower (~ 1800 °Cd). In contrast ‘Antas’, needed more time (250 days after sowing, DAS) to accumulate 30% more thermal units (~2300 °Cd). Sub clover evolved to grow from seeds germinating in late summer and autumn in a Mediterranean environment.
NOTE: The experimental winter and spring sowing dates used here are for research purposes only and not recommended for commercial farming systems.
|Figure 5 Mean number of days and thermal time (°Cd) from sowing to 50% flowering for sub clover cultivars ‘Monti’ and ‘Antas’ when sown from February to December at the Lincoln University Field Research Centre.|
We discovered that sub clover cultivars sown in spring and summer (not the usual time of the year to sow sub clover!) flowered rapidly. ‘Monti’ flowered ~ 70 days after sowing (~800 °Cd). ‘Antas’ needed 100 days to flower (~ 1000 °Cd) after sowing at this lowland Canterbury site. Also, in practical terms for commercial farms, because the plants grew for a shorter period before flowering was initiated they were smaller and the end result is a) less high quality clover feed to grow grazing livestock to target weights fast and b) less viable seed produced to regenerate clover dominant pastures in subsequent years.
Sub clover produces flowers in the stem, which is called a runner (Figure 6). Note that each node in the runner produces a leaf and a flower (as shown in Figure 4). The runner displayed shown was taken from an ungrazed plant and therefore it is very long. Usually runners and nodes are short when sub is grazed.
|Figure 6 ‘Narrikup’ runner bearing flowers at different phenological phases: from an open flower (left side, youngest) to a burr (right side, oldest). Photo: D. Hollander, 2016.|
Note in Figure 6 that leaves and flowers are produced from the same node. While leaves grow upright the flowers develop downwards (positive geotropism). ’Narrikup’ and ‘Mount Barker’ have the characteristic flower red pigmentation.
Figure 7 shows key reproductive development phases of a sub clover flower. These development phases will occur during spring time and are driven mainly by temperature(7,8).
The Dryland Pastures Research Team have been assessing the floral morphology and patterns of sub clover cultivars in the North and South Island of New Zealand. The knowledge of flowering time for different cultivars is crucial for the graze management of sub clover based pastures in spring. To ensure adequate seed set and seedling regeneration in subsequent years it is important that sub clover plants are grazed properly during the flowering and seed maturation period. More about grazing management of the sub clovers will follow in our next post!
Spring is also the best time to identify common resident sub clovers. The Guide for Subterranean Clover Identification and Use in New Zealand contains substantial information to aid identification and cultivar selection. For more comprehensive information a list of references is provided below. Cold and long nights are almost over, spring is coming and sub clover is already being grazed by ewes and lambs!!
|Figure 7 Close up of ‘Narrikup’ sub clover phenological flower phases: from a floral bud (left) to seed fill stage (right, individual seeds were detached from inside the burr for visualisation). *Anthesis = flower is fully open. Scale bar = 2 mm. Photos: C. Teixeira, 2016.|
(1) Evans, P. M., Lawn, R. J., Watkinson, A. R. 1992. Use of linear models to predict the date of flowering in cultivars of subterranean clover (Trifolium subterraneum L.). Australian Journal of Agricultural Research, 43, 1547-1548.
(2) Bäurle, I., Dean, C. 2006. The timing of developmental transitions in plants. Cell, 125, 655-664.
(3) Aitken, Y. 1955b. Flower initiation in pasture legumes. II. Geographical implications of cold temperature requirements of varieties of Trifolium subterraneum L. Crop and Pasture Science, 6, 245-257.
(4) Moot, D. J., Scott, W. R., Roy, A. M., Nicholls, A. C. 2000. Base temperature and thermal time requirements for germination and emergence of temperate pasture species. New Zealand Journal of Agricultural Research, 43, 15-25.
(5) Moot, D., Black, A., Scott, W., Richardon, J. 2003. Leaf development and dry matter production of subterranean clover cultivars in relation to autumn sward management. Legumes for Dryland Pastures; New Zealand Grassland Association Symposium. 193-200.
(6) Nori, H., Moot, D. J., Monks, D. P., Black, A. D., Lucas, R. J. 2014. Reproductive development of four top flowering annual clovers. Crop and Pasture Science, 65, 388-399.
(7) Hay, K.M. and Porter, J.R. 2006.Development and Phenology. How crops fit their environment: concepts and case histories. In: The Physiology of Crop Yield, 2nd Ed.
(8) Hay, K.M., Porter, J.R. 2006. Phenology determined by events at axillary meristems: determinate and indeterminate soybean varieties. In: The Physiology of Crop Yield, 2nd Ed.