TEMPERATURE AND RICE CROP DEVELOPMENT

The rice growth staging system

The rice growth staging system (RGS) was developed to provide a uniform, objective and adaptive system for describing rice plant development. ( see

http://www.uaex.edu/nerec/RTWG.htm for a description of the rice growth staging system).

The system is uniform in that two people assessing the same plant will arrive at the same growth stage determination using the system. The system is objective in that objective criteria are used to determine each stage of development. The system provides 4 seedling growth stages. The system is adaptive in that the enumeration of the vegetative growth stages, the largest part of the plant's developmental period, is adapted to the plant's unit of development - the phyllochron, the interval between developmental events on successive leaves. We chose to use the leaf collar appearance as the measure of the phyllochron. Consequently there are the number of vegetative (V stages) equal to the cumulative number of leaves with collars formed on the main stem. There are ten reproductive growth stages based on unique reproductive developmental events. The vegetative and reproductive growth stages overlap and several panicle developmental events are synchronous to specific Vstages. For instance, Growth Stage V_F-4 (V9 for a cultivar with 13 leaves produced on the main stem) is approximately 0 - ½ Vstage prior to R0. Growth Stage V_F-3 (V10 for a cultivar with 13 leaves produced on the main stem) is synchronous to R1. Go to this website:

http://www.uark.edu/depts/agronomy/facpage/counce/counceres.html

and see how to growth keep track of growth stages for experiments.

The importance of the R1 Growth Stage for Rice Management

In much of the southern U.S., several management practices are timed relative to ½ inch internode elongation. This event (½ inch internode elongation) is usually concurrent or nearly concurrent with the panicle branch differentiation which is the R1 Growth Stage and the V_F-3 Growth Stage. The 12 mm internode elongation is visible without magnification by splitting a growing rice culm lengthwise.

The R1 growth stage is critical to several management practices. The timing of these management practices depends upon the timing of the R1 Growth Stage. Application of several herbicides, straighthead control and midseason nitrogen fertilization are based on the timing of R1.

Several herbicides are applied in such a way to avoid applications on or after internode elongation. Internode elongation for the first elongating internode is concurrent with the R1 growth stage. The restriction on herbicide applications prior to R1 because damage, including physical damage such as leaf removal in hail damage, to the rice crop prior to R1 is relatively small while damage at R1 and later is relatively great. Molinate is applied after the flood but not later than R1. Application of 2,4-D is done at the R0 Growth Stage which occurs ½ to one V stage prior to R1. It is critical that 2,4-D is applied at this time because earlier application can reduce tillering and later application can cause damage to the developing panicle. The preferred latest application dates for propanil is the date of R1. Overall, for extensive reasons, many herbicides are applied relative to R1.

Straighthead is a condition of rice in which the grains do not develop normally. The straighthead is apparently caused by a failure of microsporogenesis. The pollen grains are therefore sterile and the egg cells are not fertilized. This leads to panicles remaining green longer than normal and the failure of top of the panicle to turn down due to the weight of the filled grain, thus the name "straight head" referring to the unbending panicle. To reduce the incidence of straighthead the flooded soil is drained and the soil is allowed to dry. To eliminate the possibility that the developing panicle is branching (R1) during the time the soil is dry. The land is drained early enough to allow reflooding by R1. Consequently, managing straighthead has required projecting the date of panicle branching (R1) to allow the soil to be drained, dried and reflooded prior to R1.

The third management practice requiring a date for panicle branching (R1) is midseason nitrogen fertilization. Nitrogen contents at the R1 Growth Stage partially determines panicle branching which, in turn, determines potential yield. If nitrogen is deficient at this stage of development, consequently, yields will be reduced. To allow farmers to plan for midseason nitrogen application, the date of panicle branching (R1) is projected.

Consequently, for herbicide application, draining for straighthead and midseason nitrogen application, the projected date for the R1 Growth Stage is needed for planning and timing. For these management practices, a "DD50" data base was developed and is maintained to project the dates of internode elongation (which is synonymous with panicle differentiation and the R1 Growth Stage) given a date of emergence and projected "DD50" thermal units.

Theory

Plant development is highly dependent and positively related to temperature. The plant has the characteristic repeating patterns it does because the development of the plant is brought about by the cytoskeleton including microtubules which guide cellulose microfibril formation which form cell walls. Consequently, development in plants is driven by enzymes. The key to developmental rate appears to be the temperature of the apical meristem (growing point for the main axis of the plant or the main stem in rice). Several attempts to model rice development to temperature have been made.

The DD50

The DD50 program in Arkansas, Missouri, Louisiana and Mississippi is an information delivery system which provides a broad range of projected dates for rice management practices. The data for the DD50 are generated for new cultivars grown in the southern U.S. as the cultivars are being developed. The DD50 program was initially based on Arkansas (USA) research to determine base temperatures for rice development. The calculation for the "DD50" program is straightforward while the formula for is more complicated .

The Formula for the DD50 is the same as the GDD in the graph (temperatures are in degrees Fahrenheit) (students need to know this formula for the DD50):

T_average= (Tmax + Tmin)/2 (1)

When Tmax > 94 then Tmax = 94

When Tmin > 70 then Tmin = 70

GDD = T_average - 50 (2)

If GDD < 0 then GDD = 0

The formua for thermal time, , is calculated as follows (you do not need to learn this formula it is provide for information only):

= 0 when T_b >T_average for t (3)

= 0 when T_average> T_x for t (4)

= (T_average - T_b)t when T_b < T_average < T_x for t (5)

= [(T_x - T_average) (T_p - T_b)/(T_x - T_p )]t when T_p < T_average < T_x for t (6)

See how effective the DD50 program works for predicting development compared to days in accompanying handout "thermal time and Vstages."

What determines development:

Several scientists have provided evidence that the apical meristem is the site of temperature sensing for plant development. Considering that leaf and panicle initiation are critical determinants of plant development, this argument is reasonable. A phenology prediction model may actually be quite accurate predictor but if the temperature provided is not accurate relative to the plants to be predicted, the model will not provide useful information. For most crop plants, the environment of the apical meristem is belowground, at the soil surface or aboveground. Prior to emergence of the meristem from the soil, development is more closely related to soil temperature while after emergence of the apical meristem from the soil surface, the plant development is better correlated to the air temperature. Consequently, while the apical meristem is belowground, using only the air temperature to predict phenology will be inaccurate to the degree the soil or the soil surface temperatures deviate from the air temperature. In lowland rice, the environment will be even more varied with the apical meristem (1) below ground in an unflooded soil or (2) below ground in a flooded soil, (3) at the soil surface in a flooded soil, (4) in the flood above the soil or (5) in the air above the flood. Suffice it to say, the temperature of the apical meristem may be quite different from the air temperature for the lowland rice crop compared to most other cereals. The lowland rice crop offers special challenges to modeling phenology compared to unflooded crops. Consequently, the actual temperature sensed by the apical meristem may be quite different from the air temperature and the thermal time calculations may result in distorted relationships between development and temperature. Usually, the air temperature provide sufficient information to predict development in the DD50 with excellent accuracy.