Effects of rice straw mulch on the diurnal leaf gaseous exchange and growth development of aerobic rice MRQ74 and MR253.
Incorporating straw usage in rice cultivation is highly favourable; it improves soil nutrient supply [1), soil organic carbon [2), microbial biomass, nitrogen mineralization [1, 3-4] and total soil nitrogen levels  hence resulting in higher nitrogen uptake . This thus augments panicle production per unit area  and grain yield . Moreover, effects on nitrogen availability and uptake, as well as on yield can last for the long term [8-10]. Mulching with straw has been reported to improve i] crop water use efficiency [11-12], ii] root biomass , root length density and root weight density , iii] stomatal conductance, transpiration, and photosynthesis characteristics  in addition to iv] grain yield .
Aerobic rice refers to high-yielding varieties  that are cultivated in well-drained , non-puddled and non-flooded fields [18-19] with adequate water inputs, either from irrigation and/or rainfall  as it is developed specifically to grow well with less water . However, the yield produced from aerobic cultivation is lower in comparison to those from the conventional practice [21-24], and this is exacerbated by yield decline when grown consecutively in the tropics . Yield decline cannot be amended by inorganic fertilizers . Similarly, urea application was found to improve plant growth under flooded conditions, but not under aerobic conditions . In this paper, rice straw mulching was applied on aerobic rice with the objective of assessing its effects on the growth development of aerobic rice, including diurnal leaf gaseous exchange.
MATERIALS AND METHODS
Research Location and Crop Management:
Aerobic rice was grown in a small plot (1m x 1m) located in the Malaysia Agricultural Research and Development Institute (MARDI) Seberang Prai, Penang (5[degrees]32'N, 100[degrees]28'E). Seeds of aerobic rice were grown in a plastic tray before being transplanted, with three seedlings per hill. The plants were watered daily and insecticides were sprayed once every fortnight for insect control, whereas fertilizers were applied based on conventional practice.
Straw mulch was applied one day after rice was transplanted and arranged in a complete randomized design with three replications. The data was subjected to an independent sample t-test to find significant means between varieties and between treatments in a variety using SPSS 19.0.
Tillers and Leaf Chlorophyll:
The number of tillers was counted manually by hand, similar to the number of weeds in a unit area. An average of five leaves with similar widths and lengths were selected and measured for leaf chlorophyll content by clamping leaves with a chlorophyll meter (SPAD-502, Soil-Plant Analysis Development (SPAD) Section, Minolta Camera Co., Osaka, Japan).
Measurement for Aci-curve was taken at DAT 105 around 1030 hour using the portable photosynthesis system LI-6400 XT (Licor Bioscience, Nebraska, USA). Photosynthetic active radiation (PAR) was set at 1500 [micro]mol [m.sup.-2] [s.sup.-1] while [C[O.sub.2]] was increased from 10, 20, 30, 40, 50, 100, 150, 300, 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2000 [micro]mol [m.sup.-2] [s.sup.-1].
Leaf Gaseous Exchange:
Five flag leaves from each treatment were measured for leaf photosynthesis rate (A, [micro]mol C[O.sub.2] [m.sup.-2][s.sup.-1]), transpiration rate (E, mmol [H.sub.2]O [m.sup.-2][s.sup.-1]) and stomatal conductance ([g.sub.s], mol C[O.sub.2][m.sup.-2][s.sup.-1]) using the portable photosynthesis system LI-6400 XT at DAT 107. The measurements were taken at the following hours: 0800, 1000, 1230, 1400, 1600 and 1830, starting from a flag leaf in MRQ74 with straw mulch, followed by MRQ7 without straw mulch, MR253 with straw mulch and lastly MR253 without straw mulch. IRGA was zeroed after each measurement. This was repeated with the subsequent flag leaves until all twenty flag leaves from all treatments were measured. Rotating measurements between treatments was undertaken to ensure better accuracy at a given time frame.
RESULTS AND DISCUSSIONS
Straw Mulch, Tillers, Leaf Chlorophyll and Weeds:
Mulch application in the main crop of aerobic rice decreased the number of tillers in MRQ74 at week 10 (Figure 1), but increased the leaf chlorophyll content during later weeks in both MRQ74 and MR253 (Figure 2). The number of weeds per pot was significantly decreased in MRQ74 at DAT47 and DAT61, and in MR253 at DAT 47, by 83%, 75% and 93%, respectively (Figure 3). The decrease in tiller development could be attributed to nitrogen immobilization [1, 27) which occurs during the early phase of straw application. However, the effect is only temporary [27-28] as at a later stage, nitrogen mineralization is enhanced under straw treatment [1, 3-4], thus enriching nitrogen availability in the soil [2, 8-10] and this may explain the increase in leaf chlorophyll content.
More importantly, mulching effectively suppressed weed growth, similar to the report of . Mulching covers the soil surface  thus delaying weed establishment , as seed production of weeds is associated closely and negatively with shading . Furthermore, weed growth in MRQ74 was suppressed up to 60 DAT [Figure 3]. Weeds in rice production are more threatening during the early growth period compared to later; late-emerging weed seedlings are generally less competitive, as well as produce less biomass and fewer seeds than early-emerging seedlings [33-35], as their growth is impaired by shading from rice plants [36-38]. A delay in weed emergence by only 15 days relative to the crop has led to at least 15% greater rice grain yield .
Leaf Gaseous Exchange:
Photosynthesis in MRQ74 and MR253 was found to be increased at higher [C[O.sub.2]], although the slope in MRQ74 is steeper than in MR253 (Figure 4). Despite this, the photosynthetic responses to rising [c.sub.i] were similar in both cultivars (Figure 5). By contrast, the compensation point and maximum photosynthesis (Amax) in MRQ74 and MR253 were at similar [C[O.sub.2]] but at different [c.sub.i]. Differences between MRQ74 and MR253 are made more apparent by comparing [c.sub.i] values in response to rising [C[O.sub.2]] (Figure 6). The photosynthetic response to [C[O.sub.2]] is affected by boundary layer and stomatal as well as mesophyll processes, while the response to [c.sub.i] depends solely on mesophyll processes (40]. The A[c.sub.i] curves in Figure 5 imply that MR253 has greater boundary layer and stomatal factors than MRQ74.
Diurnal Leaf Gaseous Exchange:
Photosynthesis increased from 0800 hour and peaked at 1230 hour before declining, exhibiting a single peak trend (Figure 7b). Likewise, transpiration and stomatal conductance increased from 0800 hour and peaked at 1400 hour before decreasing (Figure 7c and Figure 7d, respectively). These trends in transpiration and stomatal conductance are similar to those reported by  and by . By contrast, the trend in diurnal photosynthesis differs from the double-peaks trend reported by  and . The double-peaks trend is caused by mid-day depression which is a common occurrence in rice plants , especially at noon on a sunny day . Light intensity is one of the causal factors; photosynthesis is inhibited when light intensity exceeds light saturation point . However, high humidity increases the saturation point, from 1200 to 1800 [micro]mol [m.sup.-2] [s.sup.-1] , and thus, the double-peaks trend was induced into a single peak .
In Figure 7b, photosynthesis was shown to not be limited by high PAR, but was decreased when both vapour pressure deficit (VPD) and temperature were at their maximum. In concurrence,  reported that high VPD inhibits photosynthesis in the leguminous tree Prosopis juliflora. VPD also influences transpiration, as shown in Figure 7c and as concurred by ; plant water loss is enhanced by the dryness of the air . That aside, transpiration is primarily controlled by stomatal conductance [30, 47, 51], which is influenced by PAR, VPD and temperature (Figure 7d); stomatal conductance increase in the morning is in response to the rising PAR, while lower PAR, higher VPD and higher air temperature in the afternoon reduces stomatal conductance .
Although both aerobic cultivars showed similar trends in diurnal leaf gaseous exchange, the means in MRQ74 were higher than those in MR253 (Figure 7). This could perhaps be attributed to differences in boundary layer and stomatal factor, both of which were greater in MR253.
Mulching and Diurnal Leaf Gaseous Exchange:
Diurnal trends were similar between the mulch treatment and the control, indicating that applying straw mulch has no significant effect on leaf gaseous exchange (Figure 8, Figure 9 and Figure 10). This differs from the findings of others; stomatal conductance, transpiration, and photosynthesis in rice under non-flooded mulching cultivation were reported to be higher under mulch treatment in comparison to those without soil cover  while mulching was found to increase leaf transpiration in wheat crop . Applying mulch creates a barrier [54-55] that reduces soil surface evaporation [30, 56-57), thus improving soil moisture  and allowing higher transpiration . This also implies higher stomatal conductance and photosynthesis, as stomatal opening allows fluxes of water vapour and carbon dioxide .
However, mulch application in this study had no effect on diurnal leaf gaseous exchange, which could be attributed to different soil moistures. The mulch benefit of retaining soil moisture is futile under prolonged drought, the soil surface becomes completely dried . During measurement, leaf rolling was observed at around 1130 hours and lasted until sundown. Leaf rolling is commonly associated with water deficit [61-63], thus its occurrence during diurnal measurements indicates that aerobic rice was experiencing drought, therefore demonstrating the absence of mulch benefits. The absence of mid-day depression in Figure 7b allows aerobic rice to photosynthesize at optimum level throughout the day although this is a double-edge sword, as the plant simultaneously experiences water loss hence resulting in water deficit.
Leaf Gaseous Exchange in Rolling Flag Leaf:
The observation on leaf rolling led to measurement for gaseous exchange on the rolling flag leaf in tandem with diurnal measurements, which were taken on healthy flag leaves. Figure 11 showw that photosynthesis, transpiration and stomatal conductance in the tightly rolled flag leaves were lower than those in healthy flag leaves. The rice leaf rolls when the specialized bulliform cells of the upper epidermis experience water loss , causing them to lose their turgor . When these cells shrink, the leaf rolls, and when they are turgid, the leaf flattens . Other than affecting the bulliform cells, water deficit also causes the guard cells to lose their turgor pressure , thus leading to stomatal closure . However, for the Angiosperms and Gymnosperms, the subsidiary cells also lose their turgor pressure and the force from the subsidiary cells pulls the guard cells apart, resulting in stomatal opening . Thus, the leaf stomata remained open therefore allowing photosynthetic activity to resume despite water deficit.
The practice of mulching using rice straw as plant material yielded mixed results: ranging from decreasing the number of tillers to increasing the leaf chlorophyll content. This decrease, however, can be avoided if rice straw is incorporated into the soil for at least three months prior to the cropping season. The period of three months was determined by judging the trend of decreased tiller development to the increase in leaf chlorophyll content. Additionally, applying mulch a day after rice is transplanted offers an alternative to weed management as it suppressed weed emergence up to 60 days, depending on the variety used. Overall, proper mulch management in aerobic rice can be advantageous to crop growth development in aerobic rice.
Received 6 June 2015
Accepted 19 July 2015
Available online 1 August 2015
The authors would like to express gratitude to Dr. Othman Omar from MARDI Seberang Prai for his guidance and advice. This research was funded by University Research Grant no. PS149/2009A
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(1) Hilyah Mohd Khalid, (2) Syed Shahar Barakbah, (3) Normaniza Osman
(1) University of Malaya, Institute of Biological Sciences, Faculty of Science, 50603 Kuala Lumpur, Malaysia.
(2) Albukhary International University, 05200 Alor Setar, Kedah, Malaysia.
(3) University of Malaya, Institute of Biological Sciences, Faculty of Science, 50603 Kuala Lumpur, Malaysia.
Corresponding Author: Hilyah Mohd Khalid, University of Malaya, Institute of Biological Sciences, Faculty of Science, 50603 Kuala Lumpur, Malaysia.
E-mail: email@example.com; Phone (+60 012 7353639).
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|Author:||Khalid, Hilyah Mohd; Barakbah, Syed Shahar; Osman, Normaniza|
|Publication:||Advances in Environmental Biology|
|Date:||Aug 1, 2015|
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