Editor’s note: the following is an excerpt from Dr. Norman Uphoff’s System of Rice Intensification FAQ Guide, used by permission of the author and available online at http://sri.cals.cornell.edu/aboutsri/SRI_FAQs_Uphoff_2016.pdf.
The profuse tillering of SRI plants can be better understood by considering the effects of phyllochrons, a little-known periodicity in rice plants’ growth that regulates and determines their ultimate number of tillers and roots. When Fr. Laulanié learned practically by accident in the 1983-84 season that transplanting very young seedlings can lead to more robust and productive rice plants, this was hard to explain; it was just an empirical observation (Laulanié 1993).
However, four years later, Laulanié learned about phyllochrons from reading a book on rice science that presented this concept (Moreau 1986). It derived from research that was done during the 1920s and 1930s by a crop scientist in Japan, T. Katayama. Unfortunately, his research findings were not published until after World War II (Katayama 1951), and they have never been translated into English, so they are not widely known outside Japan….
From his studies of rice, wheat and barley, Katayama discovered a regularity in the way tillers (and roots) emerge from the meristematic tissue at the plant base of these cereal crops. He documented a remarkable patterning in the way that grass-family species (gramineae/poaceae) grow, described below.
Understanding phyllochrons helps explain why transplanting rice seedlings before they are about 15 days old can give a different and greater growth response to all the other practices of crop management than seen with seedlings that are transplanted at an older age, i.e., after the start of the 4th phyllochron. The timing and length of phyllochrons is determined by multiple factors, as discussed below.
The term itself combines two Greek words phyllo + chron, which respectively mean leaf + time. Phyllochron refers to an interval of time during which a plant leaf, together with an associated root and tiller, emerges from the plant’s meristematic tissue, which produces new cells that create plant organs. This generative tissue derived from the rice plant’s seed is located at the base of the plant, at or near the soil surface, between the plant’s root system and its above-ground canopy.
Beyond the 4th phyllochron, multiple units of leaf and associated root and tiller will emerge at the same time, i.e., within a period of time that is designated as a phyllochron.
A synchronously emergent unit of a leaf, together with a tiller and a root, referred to collectively as a phytomer, grows both upward and downward from the plant’s meristem at the base of the visible plant. At the same time that a plant’s leaves and tillers grow upward into the air, its roots grow downward into the soil. Roots emanate from the same cell-division processes as do the leaves and the tillers.
The length of a phyllochron for rice can vary considerably, from:
• Perhaps 4 days if the conditions for growth are ideal, i.e., if the plant is encountering no stresses that will slow or impede its growth, to
• 8 to 10 days if growing conditions for the plant are unfavorable because the plant is subject to many stresses (temperature, water, compacted soil, etc.).
When growing conditions are good, with favorable temperatures, enough water and sunlight, adequate availability of nutrients in the soil, lots of space all around the roots and canopy, and friable soil for root growth, a phyllochron can be 5 or 6 days in length, and the plant can complete 10, 11 or 12 phyllochron periods of growth before it (a) comes to the end of its initial phase of vegetative growth and (b) switches into its reproductive phase, from panicle initiation to flowering and heading, and then (c) proceeds with grain forming and filling, ripening, and maturation, when the grains have become ready for harvesting….
Readers may have noticed that the pattern of tillering indicated in the table corresponds to what is known in mathematics and biology as a Fibonacci series. In such a series, the number that emerges in each period is the sum of the previous two periods: 1+1=2, 1+2=3, 2+3=5, 3+5=8 … The number of tillers produced in each period is approximately 2/3 more than emerged in the preceding period. Such mathematical regularity in nature is noteworthy.
A rice plant that can complete 12 phyllochrons of growth before the end of its vegetative phase and moves into its reproductive phase, starting with panicle initiation, can have as many as 84 tillers…. Such a plant would have a similarly profuse root system because its roots emanate from the same meristem cells that go through cell division and differentiation to create tillers and leaves, forming the plant’s canopy above-ground.
If rice plants are transplanted during the 4th phyllochron or during even later phyllochrons, it is seen that their subsequent production of phytomers is both decelerated and diminished. Accordingly, such rice plants, when they begin their reproductive phase, have fewer tillers and fewer leaves, and also fewer roots.
What is described here is a mechanistic presentation of a biological process. Specific plants do not necessarily grow as mathematically as the model above indicates. For one thing, the length of their phyllochrons is not always so uniform. Also, in practice, rice tillers increase according to a modified Fibonacci series, not a perfect one; 12 periods should hypothetically produce 89 tillers, rather than 84. This discrepancy is apparently due to physical congestion in the base of the rice plant, which keeps the emergence of tillers (and roots) from the meristematic tissue from reaching its hypothetical maximum.
Transplanting rice seedlings during their 2nd or 3rd phyllochron of growth — roughly between the 5th and 15th days — represents a window of opportunity for the best management of rice plants. Their roots will be less traumatized if they are transplanted during this relatively dormant period, and these plants, when they resume their growth after transplanting, will produce more phytomers (units of tiller, leave and root) in an accelerated way….
The concept underlying this presentation is that the rice plants’ growth proceeds according to some kind of ‘biological clock.’ This runs faster or more slowly depending on the totality of favorable and/or unfavorable growth conditions. It is regulated operationally by the speed with which the plant’s cells are growing, elongating and dividing, in turn growing, elongating and dividing….
More research remains to be done on phyllochrons and on their implications for rice crop growth. There has been considerable research on phyllochrons in wheat (see, e.g., a special issue of Crop Science, 35:1, 1995), and on phyllochrons in forage grasses, especially in Australia. However, there has been little consideration of rice phyllochrons except by rice scientists in Japan and China, where they are well known. In the English-reading world, phyllochrons do not figure much in plant science considerations, presumably because the original research on phyllochrons has not been translated into English.
Considerable research has been done along similar lines in terms of degree-days, but these are not linked to an understanding of plant physiology and morphology as closely as analysis done in terms of phyllochrons. For SRI, an understanding of phyllochrons helps to explain why the use of young seedlings has such a strong positive effect, validated empirically (Uphoff and Randriamiharisoa 2002). The rapid tillering and root growth which is possible when the full set of SRI practices are used together is not seen when older seedlings are used or when rice plants are grown under continuously flooded conditions with degenerating plant roots, which lengthens their phyllochrons. We hope that this area will become the focus of extensive research, such as that reported by Veeramani et al. (2012).
