6.2.7  Water

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Growth is a turgor-dependent process (Section 4.3), and later phases of leaf expansion that depend principally upon cell enlargement are especially sensitive to water stress. When plants encounter water stress, leaf area increase is either diminished or even ceases well ahead of any clear reduction in leaf gas exchange. NAR is thus less sensitive to water stress than RGRA, a distinction reported as early as 1943 for greenhouse tobacco plants at the Waite Institute. In a posthumous paper compiled by J.G. Wood, Petrie and Arthur (1943) subjected tobacco to four watering treatments, namely high-water range, low-water range, early temporary drought and late temporary drought. Growth indices were derived from nine sequential harvests and plant biomass analysed for total N, protein N, soluble sugars and crude fibre. NAR was expressed in terms of area, mass and protein content of leaves.

Total plant biomass at final harvest was greatly reduced by the low-water treatment due largely to early reductions in leaf expansion. NAR (area basis) was not affected to the same extent as final biomass but NAR (‘protein’ basis) was substantially reduced because leaf ‘protein’ was increased by water stress.

Especially significant, and perhaps paradoxically, J.G. Wood reported that ‘Both early and temporary drought cause an initial depression in growth rate due to a depression in net assimilation rate; this is followed by an increase in growth rate greater than that of the high-water plants. This increase is due to the greater protein content of the plants subjected to temporary drought.’ A single cycle of early drought and subsequent recovery resulted in whole-plant RGR that was still comparable to non-stressed controls. Since NAR (area basis) was relatively insensitive, significant reduction in final biomass must have been due to an initial reduction in leaf growth.

Early temporary drought (applied from day 64 to day 81 in a growing season of 175 d) enhanced growth of both shoots and roots subsequent to stress relief (rewatering). Total leaf area at 118 days was 7000 cm2 following early drought, compared with 5300 cm2 in unstressed controls, so that final size per leaf on upper nodes must have been considerably greater. A build up of ‘protein’ during drought was thought to have boosted expansion of later-formed leaves subsequent to rewatering, but in retrospect, accumulation of osmotically active materials during drought stress was almost certainly an added factor in this compensatory growth. For example, some sunflower cultivars respond to drought stress and recovery cycles by generating individual leaves that are as much as 60% larger than leaves on corresponding nodes of unstressed controls (Rawson and Turner 1982). Leaf-growth dynamics that underlie such a remarkable response are discussed below and are based on some earlier studies of Takami et al. (1981).

Takami et al. (1981) grew sunflowers in a greenhouse under natural light in Canberra (March–May 1980). Seedlings were initially well watered to ensure good establishment (first 15 d). After thinning to two plants per pot, irrigation was then withheld from some pots, and unstressed controls were maintained near field capacity. Drought stress developed slowly (as intended) and drought-stressed plants recovered fully within 4–6 d of irrigation. Just prior to rewatering, pre-dawn leaf turgor was actually higher in stressed plants (0.63 MPa) compared with controls (0.39) notwithstanding a rather lower bulk leaf water potential (Ψleaf = –0.47 and –0.16 MPa in stressed and control respectively).

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Table 6.8

Leaf growth dynamics (Table 6.8) are based on comparisons between mean data for control and stress-recovered plants, and apply to corresponding nodes, namely 5, 13, 17 and 23. Final leaf size varies with node number in sunflower (Figure 6.4) hence the need for strict correspondence. Leaves at node 5 (Table 6.8) encountered an intensifying stress soon after appearance. Stressed plants maintained similar r, and failed to reach the same final size (Ax) as well-watered controls. Taking r as indicative of cell division during the exponential phase of lamina expansion with subsequent growth driven mainly by enlargement, drought stress has restricted cell enlargement rather than cell division.

Leaves at node 13 on droughted plants (prior to stress relief on day 36) were similar in RGR (r = 0.24 d–1 cf. 0.26 d–1 in well-watered controls) but greatly restricted in final size (84 cm2 cf. 392 cm2 in controls), again emphasising the sensitivity of cell enlargement to moisture stress.

Phyllochron (Δt0 in Equation 6.14) was little affected up to node 13 (Table 6.8) but after-effects of previous stress became apparent on rate of leaf appearance from node 14 to node 17, resulting in Δt0 increasing from 1.4 to 3.2 d. Subsequent leaf appearance (node 17 to node 23) was even accelerated in stress-recovered plants, resulting in a Δt0 = 0.8 d cf. 1.1 d in non-stressed controls. r at node 23 was unchanged by stress-recovery treatments but final size was substantially greater (228 cm2) in stress-recovered compared with non-stressed controls (198 cm2). Such compensatory growth by individual leaves following stress relief would draw on N-based resources that accumulate during drought, while turgor-driven expansion to a greater final size could partly arise from drought-induced osmotic adjustment.

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